[Federal Register Volume 60, Number 194 (Friday, October 6, 1995)]
[Rules and Regulations]
[Pages 52315-52329]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 95-24042]



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ENVIRONMENTAL PROTECTION AGENCY
40 CFR Part 58

[FRL-5304-9]
RIN 2060-AF88


Ambient Air Quality Surveillance Siting Criteria for Open Path 
Analyzers

AGENCY: Environmental Protection Agency (EPA).

ACTION: Final rule.

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SUMMARY: EPA is amending its regulations to define the appropriate 
ambient air monitoring criteria for open path (long-path) analyzers. 
These revisions to the Ambient Air Quality Surveillance regulations 
define the siting requirements for open path analyzers used as State 
and Local Air Monitoring Stations (SLAMS), National Air Monitoring 
Stations (NAMS) and Photochemical Assessment Monitoring Stations 
(PAMS), as well as general quality assurance procedures for this 
technology. These changes provide the ambient air monitoring community 
with criteria needed to effectively use open path analyzers and 
associated data for regulatory purposes.

EFFECTIVE DATE: This final rule and all contained regulatory changes 
except for appendix D, section 2.2, are effective on October 6, 1995. 
The 40 CFR part 58, appendix D, section 2.2 requirements are not 
effective until the Office of Management and Budget approves the 
information requirements contained in them and the EPA publishes a 
document announcing their approval in the Federal Register.

ADDRESSES: Copies of the comments received on the notice of proposed 
rulemaking, supporting documentation, and the response to public 
comments document may be obtained from: Air Docket (LE-131), Attention: 
Docket Number A-93-44, U.S. Environmental Protection Agency, room M-
1500, 401 M Street, SW., Washington, D.C. 20460. Docket Number A-93-44, 
containing supporting information used in developing these revised 
regulations, is available for public inspection and copying between 
8:30 a.m. and 12 noon, and between 1:30 p.m. and 3:30 p.m., Monday 
through Friday, at the EPA's Air Docket Section at the address noted 
above. As provided in 40 CFR part 2, a reasonable fee may be charged 
for copying.

FOR FURTHER INFORMATION CONTACT: Lee Ann B. Byrd (919) 541-5367, 
Monitoring and Quality Assurance Group (MD-14), Office of Air Quality 
Planning and Standards, U.S. Environmental Protection Agency, Research 
Triangle Park, North Carolina 27711.

SUPPLEMENTARY INFORMATION:

Table of Contents

I. Authority
II. Background
III. Discussion of Regulatory Revisions and Major Comments on 
Proposal
    A. Section 58.1 Definitions
    B. Appendix A--Quality Assurance Requirements for State and 
Local Air Monitoring Stations (SLAMS)
    C. Appendix B--Quality Assurance Requirements for Prevention of 
Significant Deterioration (PSD) Air Monitoring
    D. Appendix D--Network Design Criteria for State and Local Air 
Monitoring Stations (SLAMS), National Air Monitoring Stations 
(NAMS), and Photochemical Assessment Monitoring Stations (PAMS)
    E. Appendix E--Probe and Path Siting Criteria for Ambient Air 
Quality Monitoring
IV. Administrative Requirements
    A. Administrative Designation
    B. Reporting and Recordkeeping Requirements
    C. Regulatory Flexibility Act
    D. Unfunded Mandates Reform Act of 1995

I. Authority

    Sections 110, 301(a), 313, and 319 of the Clean Air Act as amended 
42 U.S.C. 7410, 7601(a), 7613, 7619.

II. Background

    A new technique for monitoring pollutants in ambient air has been 
developed and introduced to the EPA. Instruments based on this new 
technique, called open path (or long-path) analyzers, use ultraviolet, 
visible, or infrared light to measure nitrogen dioxide (NO2), 
ozone (O3), carbon monoxide (CO), sulfur dioxide (SO2), and 
other gaseous pollutant concentrations over a path of several meters up 
to several kilometers. The measurements obtained by these open path 
analyzers are path-integrated values from which path-averaged 
concentrations are obtained. In contrast, traditional point analyzers 
measure pollutant concentrations at one specific point by extracting an 
air sample from the atmosphere through an inlet probe.
    Due to the fundamental difference in the measurement principles of 
open path and point analyzers, there may be tradeoffs in using each 
type of instrument for certain applications. Because of the ability of 
open path analyzers to measure pollutant concentrations over a path, 
these new techniques are expected to provide better spatial coverage, 
and thereby a better assessment of a general population's exposure to 
air pollutants for certain applications. However, due to this same 
path-averaging characteristic, open path analyzers could underestimate 
high pollutant concentrations at specific points within the measurement 
path for other ambient air monitoring situations. The applicability of 
either technique to a particular monitoring scenario is dependent on a 
number of factors including plume dispersion characteristics, 
monitoring location, pollutant of interest, population density, site 
topography, and monitoring objective. The EPA has considered these 
factors in evaluating the advantages and disadvantages of using open 
path analyzers for the various ambient air monitoring applications 
detailed in 40 CFR part 58.
    The EPA has assessed the performance of an open path analyzer as 
candidate equivalent methods for measuring ozone, sulfur dioxide, and 
nitrogen dioxide under part 53. This open path analyzer was formally 
designated as an equivalent method for each of the three pollutants in 
a Federal Register notice, volume 60, number 84 on May 2, 1995. In 
parallel with this effort, the EPA developed these part 58 siting and 
quality assurance criteria for open path analyzers, which were 
published on August 18, 1994 as a notice of proposed rulemaking.
    The intended purpose of these revisions to part 58 is to define 
first the conceptual framework of network design and siting which is 
equally relevant to open path and point types of ambient air monitoring 
sites, followed by the practical implications that flow from the 
conceptual approach. Comments received in response to the notice of 
proposed rulemaking have been carefully considered. Improvements to the 
network design and siting criteria were identified from these comments, 
and, as appropriate, 

[[Page 52316]]
were incorporated into the regulatory text as detailed in this action. 
Copies of the specific EPA responses to each comment received are 
available in the docket as noted previously.

III. Discussion of Regulatory Revisions and Major Comments on Proposal

A. Section 58.1  Definitions

    Today's action adds several new definitions to part 58 which are 
needed to clearly define the proposed new requirements for open path 
analyzers. Definitions for ``point analyzer'' and ``open path 
analyzer'' have been added to define these two types of automated 
instruments and to clarify the distinction between them, since the 
various new and existing requirements may apply to one or the other or 
both types of analyzers. A new definition for ``probe'' is added to 
specify the inlet where an air sample is extracted from the atmosphere 
for delivery to a sampler or point analyzer. Similarly, a new 
definition is added for ``monitoring path'' to describe the path in the 
atmosphere over which an open path analyzer measures and averages a 
pollutant concentration. Closely associated with the term ``monitoring 
path'' are new definitions for ``monitoring path length,'' to describe 
the scalar length of the monitoring path, and ``optical measurement 
path length,'' to describe the actual length of the optical beam of an 
open path instrument. The length of the optical beam may be two or more 
times the length of the monitoring path when one or more mirrors are 
used to cause the optical beam to pass through the monitoring path more 
than once. One public comment recommended changes to the language of 
the two former definitions to clarify the differences between path 
integrated values and path-averaged concentrations. The EPA concurs 
with this recommendation and clarifying language has been added.
    To help describe the new requirements for data quality assessment 
procedures, the term ``effective concentration'' is defined. This term 
refers to the ambient concentration of a pollutant over the monitoring 
path that would be equivalent to a much higher concentration of the 
pollutant contained in a short calibration cell inserted into the 
optical beam of an open path analyzer during a precision test or 
accuracy audit. Specifically, effective concentration is defined as the 
actual concentration of the pollutant in the test cell multiplied by 
the ratio of the optical measurement path length of the test cell to 
the optical measurement path length of the atmospheric monitoring path. 
Also, when a calibration cell is inserted into the actual atmospheric 
measurement beam of an open path analyzer for a precision or accuracy 
test, the resulting measurement reading would be the sum of the 
pollutant concentration in the calibration cell and the pollutant 
concentration in the atmosphere. The atmospheric pollutant 
concentration must be measured separately and subtracted from the test 
measurement to produce a ``corrected concentration,'' which would be 
the true test result. Thus, the term ``corrected concentration'' is 
defined as the result of such a precision or accuracy assessment test 
after correction of the test measurement by subtracting the atmospheric 
pollutant concentration.
    Finally, a formal definition of ``monitor'' is provided to clarify 
its use in the regulations as a generic term to refer to any type of 
ambient air analyzer or sampler that is acceptable for use in a SLAMS 
monitoring network under appendix C of this part. A monitor could thus 
be a point analyzer, an open path analyzer, or a sampler.

B. Appendix A--Quality Assurance Requirements for SLAMS

    Appendix A describes both general quality assurance requirements 
applicable to SLAMS air monitoring as well as specific procedures for 
assessing the quality of the monitoring data obtained in SLAMS 
monitoring networks. While the general quality assurance requirements 
(in section 2) are directly applicable to open path analyzers without 
change, the more specific data quality assessment procedures (in 
section 3) must be modified somewhat to apply to open path analyzers. 
Accordingly, changes to these procedures are provided to incorporate 
appropriate data quality assessment tests applicable to open path 
monitoring instruments. To the extent possible, the new requirements 
are similar or parallel to the existing requirements for point 
analyzers.
    For both the precision test (section 3.1) and the accuracy audit 
(section 3.2), the new requirements specify that an optical calibration 
or test cell containing a pollutant concentration standard must be 
inserted into the optical measurement beam of the open path analyzer. 
Both theory and testing indicate that the use of such a calibration or 
test cell is equivalent in accuracy to measurement of the equivalent 
pollutant concentration in air over the entire monitoring path of an 
open path analyzer. Each concentration standard must be selected such 
that it produces an ``effective concentration'' equivalent to a 
specified ambient concentration over the monitoring path. As noted 
previously, effective concentration is defined as the actual 
concentration of the pollutant in the test cell multiplied by the ratio 
of the optical measurement path length of the test cell to the optical 
measurement path length of the atmospheric monitoring path. The 
effective concentrations specified for the precision and accuracy tests 
for open path analyzers are the same as the test concentrations 
currently specified in these procedures for point analyzers.
    Ideally, precision and accuracy assessments should test a 
monitoring instrument in its normal monitoring configuration. 
Therefore, the new test procedures require that the test or calibration 
cell containing the test pollutant concentration standard be inserted 
into the actual atmospheric measurement beam of the open path analyzer. 
The resulting test measurement of the pollutant concentration is thus 
the sum of the test concentration in the cell and the pollutant 
concentration in the atmosphere, because the measurement beam would 
pass through both the test cell and the atmospheric monitoring path. 
Accordingly, a correction for the atmospheric concentration is required 
to obtain the true test result. In the new procedures, the atmospheric 
pollutant concentration is measured immediately before and again 
immediately after the precision or accuracy test, and the average of 
these two measurements is subtracted from the test concentration 
measurement to produce a ``corrected concentration,'' which is reported 
as the test result. One comment was received regarding the former 
correction procedure which indicated a concern that a second, point 
analyzer would be needed to complete the accuracy audit and precision 
check procedures described in the proposal. The accuracy audit and 
precision check procedures defined in this action do not require the 
use of a second point analyzer. It is intended that the ambient air 
concentration measurements needed to correct the test readings would be 
obtained by the open path analyzer under test. The language of the 
procedures has been changed to clarify this requirement.
    The corrected concentration reported for a precision or accuracy 
test may not be accurate if the atmospheric pollutant concentration 
changes during the test. When the ambient concentration is variable, 
the average of the pre- and post-test measurements may not be an 
accurate representation of the ambient pollutant concentration during 
the test. 

[[Page 52317]]
The proposed test procedures recommend that these tests should be 
carried out, if possible, during periods when the atmospheric pollutant 
concentration is low and steady. The lower the atmospheric pollutant 
concentration, the steadier the concentration is likely to be and the 
better the pre- and post-test measurements will represent the actual 
atmospheric concentration during the test measurement. Further, the 
procedures provide that if the pre- and post-test measurements of the 
atmospheric concentration differ by more than 20 percent of the 
effective concentration of the test standard, the test result is 
discarded and the test repeated.
    Two comments were received regarding the recommendation that pre- 
and post-test measurements be taken when the atmospheric pollutant 
concentration is low and steady, such as during early morning or late 
evening hours. These comments illustrated a concern that it may be 
difficult for a monitoring agency to conduct the accuracy audits and 
precision checks at such specific times. In amending the monitoring 
regulations to permit the use of open path analyzers, the EPA is not 
suggesting that the use of open path analyzers is necessarily cost 
effective or even necessarily advantageous. The EPA is permitting their 
use, at the discretion of the monitoring agency, for whatever benefit 
the agency may believe to accrue. The recommendation cited is intended 
to point out that the precision and accuracy test results may be better 
if carried out during periods when concentration levels are more likely 
to be low and steady, and therefore the timing of these tests as to the 
time of day or the meteorological conditions of the day should be 
considered--to the extent practicable--by the monitoring agency 
scheduling these tests.
    A comment was received which recommended that accuracy limits on 
the measurement of the optical measurement path length be incorporated 
into the regulation. This issue of the determination of the optical 
measurement path length is particularly important because an error in 
this parameter would not normally be compensated for in the calibration 
or be evident in the results of the accuracy audit procedures for open 
path analyzers. Therefore, the accuracy audit procedure has been 
revised to include reverification of this parameter.
    It is recognized that the new tests for precision and accuracy for 
open path analyzers, as well as the existing tests for point analyzers, 
are described in very general terms, and that additional, more detailed 
information and guidance are usually necessary for an analyzer operator 
to carry out these tests properly. Accordingly, section 3 of appendix A 
is amended by adding an explicit indication that supplemental 
information and guidance to assist the analyst in conducting these 
tests may be available in the publication, ``Quality Assurance Handbook 
for Air Pollution Measurement Systems, Volume II'' (EPA-600/4-77-027a, 
identified as Reference 3 at the end of appendix A), or in the 
operation or instruction manual associated with the particular monitor 
being used.
    The techniques for precision and accuracy assessment of open path 
analyzers are based largely on consultations with the manufacturer, 
along with EPA tests, of the differential optical absorption 
spectrometer that is currently under consideration by the EPA for 
possible designation as equivalent methods under 40 CFR part 53. 
However, it is desirable that the techniques be generic in nature, if 
possible, so that they would be applicable to other types of open path 
monitoring instruments as well. In addition, for some types of open 
path instruments or for some installations or configurations, there may 
be technical reasons why the new techniques for precision and accuracy 
assessment may not be feasible, appropriate, or advisable. As a result, 
these procedures allow for the use of an alternate local light source 
or an alternate optical path that does not include the normal 
atmospheric monitoring path, if such an alternate configuration is 
permitted by the operation or instruction manual associated with the 
analyzer. Since the analyzer operation or instruction manual would be 
subject to approval as part of the requirements for EPA designation of 
an open path analyzer as an equivalent method, the EPA would thereby 
have control over the alternate configurations that would be allowable 
for the precision and accuracy assessment tests.
    One comment was received recommending more details be provided 
within the regulation defining the limitations and conditions under 
which an alternative light source could be used. Because it is 
impossible to anticipate the variety of open path analyzers and audit 
techniques that could eventually be used, it is difficult, if not 
impossible, to define specific limits and conditions under which an 
alternative light source could be permitted for accuracy audits and 
precision checks. The specific authorization to use an alternate light 
source will be determined on a case-by-case basis for each specific 
open path analyzer subject to an equivalent method determination under 
part 53. Then, if permitted, the analyzer-specific conditions and 
limitations for its use would be described in detail in the associated 
operation/instruction manual. This manual is approved as part of the 
formal designation of the analyzer as an equivalent method, and the EPA 
can make sure that the procedures and conditions are addressed 
adequately in the manual before a candidate method is designated as an 
equivalent method.

C. Appendix B--Quality Assurance Requirements for Prevention of 
Significant Deterioration (PSD) Air Monitoring

    Appendix B sets forth both general quality assurance requirements 
for PSD monitoring as well as specific procedures for assessing the 
quality of the monitoring data obtained in PSD monitoring networks. The 
amendments and procedures proposed for appendix B to extend the 
existing requirements to open path analyzers are essentially identical 
to the changes proposed for appendix A. Similarly, changes to the 
regulatory language resulting from public comments received on appendix 
A apply equally to appendix B.

D. Appendix D--Network Design for State and Local Air Monitoring 
Stations (SLAMS), National Air Monitoring Stations (NAMS), and 
Photochemical Air Monitoring Stations (PAMS)

    Changes to appendix D were not recommended with the original 
proposal associated with this action. Public comments indicated the 
need for the EPA to consider the comparability of data collected by 
point analyzers and data collected by open path analyzers, particularly 
in situations of nonuniform pollutant concentrations. This issue also 
raises an additional concern over introducing new ambient air 
monitoring technologies into the Nation's monitoring program which is 
currently based on traditional point-specific monitoring techniques, 
and its impact on existing air quality management programs.
    In response to these issues, the EPA has modified appendix D with 
criteria and requirements intended to help agencies determine what, if 
any, impacts the introduction of this technology may have on their 
local air quality management programs. These criteria include 
investigations into the specific technology selected for a chosen 
application, the site location with respect to the monitoring 
objective, and a requirement for concurrent 

[[Page 52318]]
monitoring when replacing an existing monitor with one using a 
different ambient air monitoring technique. The intent of the latter 
requirement is to provide a bridge between the two types of ambient air 
monitoring data (point and path-averaged values).
    The EPA recognizes that these appendix D requirements can be more 
effectively and efficiently used to improve an ambient air monitoring 
network if consideration for the particular monitoring site, objective, 
and related conditions is included in the network analysis. As a 
result, these requirements are presented in general terms, with waiver 
provisions provided as appropriate.

E. Appendix E--Probe and Path Siting Criteria for Ambient Air Quality 
Monitoring

    This action amends appendix E by adding new siting criteria 
applicable to open path analyzers for monitoring of SO2, O3, 
NO2, CO, and O3 precursors (defined in the PAMS program as 
volatile organic compounds, oxides of nitrogen, and selected 
carbonyls). Because of the substantial similarity in the siting 
criteria for SO2, O3, and NO2 (both the existing 
criteria for point monitors and proposed new criteria for open path 
analyzers), the siting requirements for these three pollutants are 
combined, consolidated, and set forth in section 2 of appendix E. As a 
result, the existing criteria for SO2, O3, and NO2 in 
sections 3, 5, and 6 are deleted, and those sections are reserved. As 
noted below, the criteria for CO monitoring are somewhat different, so 
they are retained in a separate section 4. Siting criteria for 
measuring O3 and its precursors as part of a PAMS network are 
included in section 10. In all cases, the new open path provisions have 
been incorporated into the existing provisions, as appropriate.
    The new open path siting requirements largely parallel the existing 
requirements for point analyzers, with the revised provisions 
applicable to either a ``probe'' (for point analyzers), a ``monitoring 
path'' (for open path analyzers), or both, as appropriate. Accordingly, 
criteria for the monitoring path of an open path analyzer are described 
for horizontal and vertical placement, spacing from minor sources, 
spacing from obstructions, spacing from trees, and spacing from 
roadways. The open path requirements apply to most of the monitoring 
pathgenerally 80 or 90 percentbut not to the entire monitoring path, 
to allow some needed flexibility in siting open path analyzers. For 
example, using the proposed 80 percent requirement, a monitoring path 
may be sited across uneven terrain, where up to 20 percent of the 
monitoring path may not fall within the proposed 3- to-15 meter 
specification for height above ground.
    Two comments were received on the optical obstructions, or physical 
interferences (e.g., rain, snow, fog) criteria discussed in sections 
2.3, 4.2, and 10.2 of the proposed rule. The specific open path 
analyzer currently under consideration for designation as an equivalent 
method calculates the level of uncertainty for each data value obtained 
based on several factors including diminished light levels due to 
optical obstructions. These uncertainty levels may be used to 
invalidate data that are outside of established error acceptance 
levels. Invalidating these data will have an effect on the data capture 
percentages, and potentially, on the database's ability to properly 
characterize air quality for a given region. Because of this 
possibility, recommendations for conducting analyses of obscuration 
potential and its resulting effect on the representativeness of the 
data record have been included in sections 2.3, 4.2, and 10.2 of 
appendix E.
    In addition to the criteria common to both point and open path 
analyzers mentioned above, two new provisions, applicable only to open 
path analyzers, are included which limit the maximum length of the 
monitoring path and the cumulative interferences on the path. The 
maximum monitoring path length limit helps to ensure that open path 
monitoring data represent the air volume that they are intended to 
measure according to the monitoring objectives of the spatial scale 
identified for the site. Similarly, the limit for the cumulative 
interferences on the monitoring path controls the total amount of 
interference from minor sources, roadways, obstructions, and other 
factors that might unduly influence the monitoring data collected by an 
open path analyzer. This limit is necessary because a long monitoring 
path presents a much greater opportunity to be affected by multiple 
interferences.
    In the consolidation of current sections 3, 5, and 6 to section 2, 
Tables 2 and 3, which list the minimum separation distances between 
O3 and NO2 stations and nearby roadways, are combined and 
redesignated as Table 1. As a result, Table 1 (in section 3), Table 4 
(in section 7), Table 5 (in section 10), and Table 6 (in section 12) 
are renumbered as Tables 2, 3, 4, and 5, respectively. Finally, the 
summary of all the general siting requirements in renumbered Table 5 is 
modified to include the new criteria for monitoring paths.

IV. Administrative Requirements

A. Administrative Designation

1. Executive Order 12866
    Under Executive Order 12866 (58 FR 51735 (October 4, 1993)) the 
Agency must determine whether the regulatory action is ``significant'' 
and therefore subject to Office of Management and Budget (OMB) review 
and to the requirements of the Executive Order. The Order defines 
``significant regulatory action'' as one that is likely to result in a 
rule that may:
    (1) Have an annual effect on the economy of $100 million or more or 
adversely affect in a material way the economy, a sector of the 
economy, productivity, competition, jobs, the environment, public 
health or safety, or State, local, or tribal governments or 
communities;
    (2) Create a serious inconsistency or otherwise interfere with an 
action taken or planned by another Agency;
    (3) Materially alter the budgetary impact of entitlements, grants, 
user fees, or loan programs or the rights and obligations or recipients 
thereof; or
    (4) Raise novel legal or policy issues arising out of legal 
mandates, the President's priorities, or the principles set forth in 
the Executive Order.
    It has been determined that this rule is not a ``significant 
regulatory action'' under the terms of Executive Order 12866 and is 
therefore not subject to OMB review.
2. Enhancing the Intergovernmental Partnership Under Executive Order 
12875
    In compliance with Executive Order 12875, we have involved State 
and local governments in the development of this rule. To accomplish 
this effort, we have presented information on the new open path 
analyzer technology at various national and international technical 
symposiums, such as the Air and Waste Management Association specialty 
conferences, which were attended by several State and local agencies. 
We have presented information and solicited comment from State and 
local ambient air monitoring agencies on the use of this new technology 
and the contents of this rule through forums such as the Standing Air 
Monitoring Work Group. This work group, which consists of various State 
and local agency and EPA representatives, is designed to provide a 
strategic vision and direction for the ambient air monitoring programs 
within the nation. 

[[Page 52319]]
In evaluating open path analyzers, we have conducted joint methodology 
experiments in various locations with the States of Connecticut, 
Georgia, Florida, and Texas.

B. Reporting and Recordkeeping Requirements

    The information collection requirements contained in this rule have 
been submitted for approval to OMB under the Paperwork Reduction Act, 
44 U.S.C. 3501 et seq. An Information Collection Request document has 
been prepared by the EPA (ICR No. 0940.12) and a copy may be obtained 
from Sandy Farmer, Information Policy Branch, EPA, 401 M Street S.W., 
Mail Code 2136, Washington, D.C. 20460; or by calling (202) 260-2740. 
These requirements are not effective until OMB approves them and a 
technical amendment to that effect is published in the Federal 
Register.
    This collection of information has an estimated reporting burden 
averaging 300 hours per response and an estimated annual recordkeeping 
burden averaging 24 hours per respondent. These estimates include time 
for reviewing instructions, searching existing data sources, gathering 
and maintaining the data needed, and completing and reviewing the 
collection of information.
    Send comments regarding the burden estimate or any other aspect of 
this collection of information, including suggestions for reducing this 
burden to Chief, Information Policy Branch, EPA, 401 M Street S.W., 
Mail Code 2136, Washington, D.C. 20460, and to the Office of 
Information and Regulatory Affairs, Office of Management and Budget, 
Washington, D.C. 20503, marked ``Attention: Desk Officer for EPA.

C. Regulatory Flexibility Act

    Pursuant to section 605(b) of the Regulatory Flexibility Act, 5 
U.S.C. 605(b), the Administrator certifies that this rule will not have 
a significant economic impact on a substantial number of small 
entities. This rulemaking package does not impose any additional 
requirements on small entities, rather, it is this action's intent to 
provide all entities with the option to choose the most suitable 
ambient air method for their particular application. This proposal 
provides the appropriate siting and quality assurance criteria for a 
new ambient air monitoring technology (open path analyzers) as they are 
used in various applications. The criteria listed in this rulemaking 
package parallel existing requirements and vary only as necessary due 
to technological differences between measurement techniques. It is 
possible that a beneficial impact may be encountered by some small 
entities that use this new technology in certain scenarios.

D. Unfunded Mandates Reform Act of 1995

    Under sections 202, 203 and 205 of the Unfunded Mandates Reform Act 
of 1995 (``Unfunded Mandates Act''), signed into law on March 22, 1995, 
the EPA must undertake various actions in association with proposed or 
final rules that include a Federal mandate that may result in estimated 
costs of $100 million or more to the private sector, or to State, 
local, or tribal governments in the aggregate.
    The EPA's final action does not impose any federal 
intergovernmental mandate, as defined in section 101 of the Unfunded 
Mandates Act, upon any State, local, or tribal government. This action 
gives these entities an opportunity to choose the most suitable ambient 
air quality monitoring method for their program, but does mandate any 
particular method. Finally, the EPA has determined that this action 
does not include a mandate that may result in estimated costs of $100 
million or more to State, local, or tribal governments in the 
aggregate. This action does not directly affect the private sector.

List of Subjects in 40 CFR Part 58

    Environmental Protection, Air pollution control, Ambient air 
monitoring, Ambient air pollutant measurements, Ambient air monitoring 
networks and siting criteria, Ambient data, Intergovernmental 
relations, National ambient air monitoring program, Open path 
analyzers, Optical sensing, Quality assurance requirements, Reporting 
and recordkeeping requirements, State and local agency ambient air 
monitoring programs.

    Dated: September 21, 1995.
Carol M. Browner,
Administrator.
    For reasons set forth in the preamble, title 40, chapter I, part 58 
of the Code of Federal Regulations is amended as follows:

PART 58--[AMENDED]

    1. The authority citation for part 58 continues to read as follows:

    Authority: 42 U.S.C. 7410, 7601(a), 7613, and 7619.

    2. In Sec. 58.1 paragraphs (aa) through (ii) are added to read as 
follows:


Sec. 58.1  Definitions.

* * * * *
    (aa) Point analyzer is an automated analytical method that measures 
pollutant concentration in an ambient air sample extracted from the 
atmosphere at a specific inlet probe point and that has been designated 
as a reference or equivalent method in accordance with part 53 of this 
chapter.
    (bb) Probe is the actual inlet where an air sample is extracted 
from the atmosphere for delivery to a sampler or point analyzer for 
pollutant analysis.
    (cc) Open path analyzer is an automated analytical method that 
measures the average atmospheric pollutant concentration in situ along 
one or more monitoring paths having a monitoring path length of 5 
meters or more and that has been designated as a reference or 
equivalent method under the provisions of part 53 of this chapter.
    (dd) Monitoring path for an open path analyzer is the actual path 
in space between two geographical locations over which the pollutant 
concentration is measured and averaged.
    (ee) Monitoring path length of an open path analyzer is the length 
of the monitoring path in the atmosphere over which the average 
pollutant concentration measurement (path-averaged concentration) is 
determined. See also, optical measurement path length.
    (ff) Optical measurement path length is the actual length of the 
optical beam over which measurement of the pollutant is determined. The 
path-integrated pollutant concentration measured by the analyzer is 
divided by the optical measurement path length to determine the path-
averaged concentration. Generally, the optical measurement path length 
is:
    (1) Equal to the monitoring path length for a (bistatic) system 
having a transmitter and a receiver at opposite ends of the monitoring 
path;
    (2) Equal to twice the monitoring path length for a (monostatic) 
system having a transmitter and receiver at one end of the monitoring 
path and a mirror or retroreflector at the other end; or
    (3) Equal to some multiple of the monitoring path length for more 
complex systems having multiple passes of the measurement beam through 
the monitoring path.
    (gg) Effective concentration pertains to testing an open path 
analyzer with a high-concentration calibration or audit standard gas 
contained in a short test cell inserted into the optical measurement 
beam of the instrument. Effective concentration is the equivalent 
ambient-level concentration that would produce the same spectral 
absorbance 

[[Page 52320]]
over the actual atmospheric monitoring path length as produced by the 
high-concentration gas in the short test cell. Quantitatively, 
effective concentration is equal to the actual concentration of the gas 
standard in the test cell multiplied by the ratio of the path length of 
the test cell to the actual atmospheric monitoring path length.
    (hh) Corrected concentration pertains to the result of an accuracy 
or precision assessment test of an open path analyzer in which a high-
concentration test or audit standard gas contained in a short test cell 
is inserted into the optical measurement beam of the instrument. When 
the pollutant concentration measured by the analyzer in such a test 
includes both the pollutant concentration in the test cell and the 
concentration in the atmosphere, the atmospheric pollutant 
concentration must be subtracted from the test measurement to obtain 
the corrected concentration test result. The corrected concentration is 
equal to the measured concentration minus the average of the 
atmospheric pollutant concentrations measured (without the test cell) 
immediately before and immediately after the test.
    (ii) Monitor is a generic term for an instrument, sampler, 
analyzer, or other device that measures or assists in the measurement 
of atmospheric air pollutants and which is acceptable for use in 
ambient air surveillance under the provisions of appendix C to this 
part, including both point and open path analyzers that have been 
designated as either reference or equivalent methods under part 53 of 
this chapter and air samplers that are specified as part of a manual 
method that has been designated as a reference or equivalent method 
under part 53 of this chapter.
    3. Appendix A is amended as follows:
    a. The fourth paragraph of section 3 introductory text is revised.
    b. Section 3.1 is revised.
    c. The text preceding the table in the second paragraph, and the 
seventh, and eighth paragraphs of section 3.2 are revised; and a new 
paragraph is added between the seventh and eighth paragraphs.
    d. Table A-1 is revised.

Appendix A--Quality Assurance Requirements for State and Local Air 
Monitoring Stations (SLAMS)

* * * * *
3. Data Quality Assessment Requirements
* * * * *
    Assessment results shall be reported as specified in section 4. 
Concentration and flow standards must be as specified in sections 2.3 
or 3.4. In addition, working standards and equipment used for accuracy 
audits must not be the same standards and equipment used for routine 
calibrations. Additional information and guidance in the technical 
aspects of conducting these tests may be found in Reference 3 or in the 
operation or instruction manual associated with the analyzer or 
sampler. Concentration measurements reported from analyzers or 
analytical systems (indicated concentrations) should be based on stable 
readings and must be derived by means of the same calibration curve and 
data processing system used to obtain the routine air monitoring data 
(see Reference 1 and Reference 3, section 2.0.9.1.3(d)). Table A-1 
provides a summary of the minimum data quality assessment requirements, 
which are described in more detail in the following sections.
3.1  Precision of Automated Methods
    A one-point precision check must be carried out at least once every 
two weeks on each automated analyzer used to measure SO2, 
NO2, O3, and CO. The precision check is made by challenging 
the analyzer with a precision check gas of known concentration 
(effective concentration for open path analyzers) between 0.08 and 0.10 
ppm for SO2, NO2, and O3 analyzers, and between 8 and 10 
ppm for CO analyzers. To check the precision of SLAMS analyzers 
operating on ranges higher than 0 to 1.0 ppm SO2, NO2, and 
O3, or 0 to 100 ppm for CO, use precision check gases of 
appropriately higher concentration as approved by the appropriate 
Regional Administrator or the Regional Administrator's designee. 
However, the results of precision checks at concentration levels other 
than those specified above do not need be reported to the EPA. The 
standards from which precision check test concentrations are obtained 
must meet the specifications of section 2.3.
    Except for certain CO analyzers described below, point analyzers 
must operate in their normal sampling mode during the precision check, 
and the test atmosphere must pass through all filters, scrubbers, 
conditioners, and other components used during normal ambient sampling 
and as much of the ambient air inlet system as is practicable. If 
permitted by the associated operation or instruction manual, a CO point 
analyzer may be temporarily modified during the precision check to 
reduce vent or purge flows, or the test atmosphere may enter the 
analyzer at a point other than the normal sample inlet, provided that 
the analyzer's response is not likely to be altered by these deviations 
from the normal operational mode.
    If a precision check is made in conjunction with a zero or span 
adjustment, it must be made prior to such zero or span adjustments. 
Randomization of the precision check with respect to time of day, day 
of week, and routine service and adjustment is encouraged where 
possible.
    Open path analyzers are tested by inserting a test cell containing 
a precision check gas concentration into the optical measurement beam 
of the instrument. If possible, the normally used transmitter, 
receiver, and, as appropriate, reflecting devices should be used during 
the test, and the normal monitoring configuration of the instrument 
should be altered as little as possible to accommodate the test cell 
for the test. However, if permitted by the associated operation or 
instruction manual, an alternate local light source or an alternate 
optical path that does not include the normal atmospheric monitoring 
path may be used. The actual concentration of the precision check gas 
in the test cell must be selected to produce an ``effective 
concentration'' in the range specified above. Generally, the precision 
test concentration measurement will be the sum of the atmospheric 
pollutant concentration and the precision test concentration. If so, 
the result must be corrected to remove the atmospheric concentration 
contribution. The ``corrected concentration'' is obtained by 
subtracting the average of the atmospheric concentrations measured by 
the open path instrument under test immediately before and immediately 
after the precision check test from the precision test concentration 
measurement. If the difference between these before and after 
measurements is greater than 20 percent of the effective concentration 
of the test gas, discard the test result and repeat the test. If 
possible, open path analyzers should be tested during periods when the 
atmospheric pollutant concentrations are relatively low and steady.
    Report the actual concentration (effective concentration for open 
path analyzers) of the precision check gas and the corresponding 
concentration measurement (corrected concentration, if applicable, for 
open path analyzers) indicated by the analyzer. The percent differences 
between these concentrations are used to assess the precision of the 
monitoring data as described in section 5.1.
3.2  Accuracy of Automated Methods
* * * * * 

[[Page 52321]]

    The audit is made by challenging the analyzer with at least one 
audit gas of known concentration (effective concentration for open path 
analyzers) from each of the following ranges that fall within the 
measurement range of the analyzer being audited: * * *
* * * * *
    For point analyzers, the audit shall be carried out by allowing the 
analyzer to analyze the audit test atmosphere in its normal sampling 
mode such that the test atmosphere passes through all filters, 
scrubbers, conditioners, and other sample inlet components used during 
normal ambient sampling and as much of the ambient air inlet system as 
is practicable. The exception provided in section 3.1 for certain CO 
analyzer does not apply for audits.
    Open path analyzers are audited by inserting a test cell containing 
the various audit gas concentrations into the optical measurement beam 
of the instrument. If possible, the normally used transmitter, 
receiver, and, as appropriate, reflecting devices should be used during 
the audit, and the normal monitoring configuration of the instrument 
should be modified as little as possible to accommodate the test cell 
for the audit. However, if permitted by the associated operation or 
instruction manual, an alternate local light source or an alternate 
optical path that does not include the normal atmospheric monitoring 
path may be used. The actual concentrations of the audit gas in the 
test cell must be selected to produce ``effective concentrations'' in 
the ranges specified in this section 3.2. Generally, each audit 
concentration measurement result will be the sum of the atmospheric 
pollutant concentration and the audit test concentration. If so, the 
result must be corrected to remove the atmospheric concentration 
contribution. The ``corrected concentration'' is obtained by 
subtracting the average of the atmospheric concentrations measured by 
the open path instrument under test immediately before and immediately 
after the audit test (or preferably before and after each audit 
concentration level) from the audit concentration measurement. If the 
difference between the before and after measurements is greater than 20 
percent of the effective concentration of the test gas standard, 
discard the test result for that concentration level and repeat the 
test for that level. If possible, open path analyzers should be audited 
during periods when the atmospheric pollutant concentrations are 
relatively low and steady. Also, the monitoring path length must be 
reverified to within 3 percent to validate the audit, since 
the monitoring path length is critical to the determination of the 
effective concentration.
    Report both the audit test concentrations (effective concentrations 
for open path analyzers) and the corresponding concentration 
measurements (corrected concentrations, if applicable, for open path 
analyzers) indicated or produced by the analyzer being tested. The 
percent differences between these concentrations are used to assess the 
accuracy of the monitoring data as described in section 5.2.
* * * * *

                                Table A-1.--Minimum Data Assessment Requirements                                
----------------------------------------------------------------------------------------------------------------
        Method           Assessment method           Coverage          Minimum frequency     Parameters reported
----------------------------------------------------------------------------------------------------------------
Precision:                                                                                                      
  Automated methods    Response checks at     Each analyzer........  Once per 2 weeks.....  Actual concentration
   for SO2, NO2, O3,    concentration                                                        \2\ & measured     
   and CO.              between .08 & .10                                                    concentration.\3\  
                        ppm (8 & 10 ppm for                                                                     
                        CO) \2\.                                                                                
  Manual methods       Collocated samplers..  1 site for 1-5 sites;  Once per week........  Two concentration   
   including lead.                             2 sites for 6-20                              measurements.      
                                               sites; 3 sites> 20                                               
                                               sites (sites with                                                
                                               highest conc.).                                                  
Accuracy:                                                                                                       
  Automated methods    Response checks at:    1. Each analyzer. 2.   1. Once per year.....  Actual concentration
   for SO2, NO2, O3,    .03-.08 ppm; 1,2 .15-  25% of analyzers (at  2. Each calendar        \2\ & measured     
   and CO.              .20 ppm; 1,2 .35-.45   least 1).              quarter.               (indicated)        
                        ppm; 1,2 .80-.90                                                     concentration \3\  
                        ppm; 1,2 (If                                                         for each level.    
                        applicable).                                                                            
  Manual methods for   Check of analytical    Analytical system....  Each day samples are   Actual concentration
   SO2 and NO2.         procedure with audit                          analyzed, at least     & measured         
                        standard solutions.                           twice per quarter.     (indicated)        
                                                                                             concentration for  
                                                                                             each audit         
                                                                                             solution.          
  TSP, PM-10.........  Check of sampler flow  1. Each sampler......  1. Once per year.....  Actual flow rate and
                        rate.                 2. 25% of samplers     2. Each calendar        flow rate indicated
                                               (at least 1).          quarter.               by the sampler.    
  Lead...............  1. Check sample flow   1. Each sampler......  1. Include with TSP..  1. Same as for TSP. 
                        rate as for TSP. 2.   2. Analytical system.  2. Each quarter......  2. Actual           
                        Check analytical                                                     concentration &    
                        system with Pb audit                                                 measured           
                        strips.                                                              (indicated)        
                                                                                             concentration of   
                                                                                             audit samples      
                                                                                             (g Pb/    
                                                                                             strip).            
----------------------------------------------------------------------------------------------------------------
\1\ Concentration times 100 for CO.                                                                             
\2\ Effective concentration for open path analyzers.                                                            
\3\ Corrected concentration, if applicable, for open path analyzers.                                            

* * * * *
    4. Appendix B is amended as follows:
    a. The first paragraph of section 3 is revised.
    b. Section 3.1 is revised.
    c. The text preceding the table in the first paragraph, and the 
third, and fourth paragraphs of section 3.2 are revised. A new 
paragraph is added between the third and fourth paragraphs.
    d. Table B-1 is revised.

[[Page 52322]]


Appendix B--Quality Assurance Requirements for Prevention of 
Significant Deterioration (PSD) Air Monitoring

* * * * *
3. Data Quality Assessment Requirements
    All ambient monitoring methods or analyzers used in PSD monitoring 
shall be tested periodically, as described in this section 3, to 
quantitatively assess the quality of the data being routinely 
collected. The results of these tests shall be reported as specified in 
section 6. Concentration standards used for the tests must be as 
specified in section 2.3. Additional information and guidance in the 
technical aspects of conducting these tests may be found in Reference 3 
or in the operation or instruction manual associated with the analyzer 
or sampler. Concentration measurements reported from analyzers or 
analytical systems must be derived by means of the same calibration 
curve and data processing system used to obtain the routine air 
monitoring data. Table B-1 provides a summary of the minimum data 
quality assessment requirements, which are described in more detail in 
the following sections.
3.1  Precision of Automated Methods
    A one-point precision check must be carried out at least once every 
two weeks on each automated analyzer used to measure SO2, 
NO2, O2, and CO. The precision check is made by challenging 
the analyzer with a precision check gas of known concentration 
(effective concentration for open path analyzers) between 0.08 and 0.10 
ppm for SO2, NO2, and O3 analyzers, and between 8 and 10 
ppm for CO analyzers. The standards from which precision check test 
concentrations are obtained must meet the specifications of section 
2.3. Except for certain CO analyzers described below, point analyzers 
must operate in their normal sampling mode during the precision check, 
and the test atmosphere must pass through all filters, scrubbers, 
conditioners and other components used during normal ambient sampling 
and as much of the ambient air inlet system as is practicable. If 
permitted by the associated operation or instruction manual, a CO point 
analyzer may be temporarily modified during the precision check to 
reduce vent or purge flows, or the test atmosphere may enter the 
analyzer at a point other than the normal sample inlet, provided that 
the analyzer's response is not likely to be altered by these deviations 
from the normal operational mode.
    Open path analyzers are tested by inserting a test cell containing 
a precision check gas concentration into the optical measurement beam 
of the instrument. If possible, the normally used transmitter, 
receiver, and, as appropriate, reflecting devices should be used during 
the test, and the normal monitoring configuration of the instrument 
should be altered as little as possible to accommodate the test cell 
for the test. However, if permitted by the associated operation or 
instruction manual, an alternate local light source or an alternate 
optical path that does not include the normal atmospheric monitoring 
path may be used. The actual concentration of the precision check gas 
in the test cell must be selected to produce an ``effective 
concentration'' in the range specified above. Generally, the precision 
test concentration measurement will be the sum of the atmospheric 
pollutant concentration and the precision test concentration. If so, 
the result must be corrected to remove the atmospheric concentration 
contribution. The ``corrected concentration'' is obtained by 
subtracting the average of the atmospheric concentrations measured by 
the open path instrument under test immediately before and immediately 
after the precision check test from the precision test concentration 
measurement. If the difference between these before and after 
measurements is greater than 20 percent of the effective concentration 
of the test gas, discard the test result and repeat the test. If 
possible, open path analyzers should be tested during periods when the 
atmospheric pollutant concentrations are relatively low and steady.
    If a precision check is made in conjunction with a zero or span 
adjustment, it must be made prior to such zero or span adjustment. The 
difference between the actual concentration (effective concentration 
for open path analyzers) of the precision check gas and the 
corresponding concentration measurement (corrected concentration, if 
applicable, for open path analyzers) indicated by the analyzer is used 
to assess the precision of the monitoring data as described in section 
4.1. Report data only from automated analyzers that are approved for 
use in the PSD network.
3.2  Accuracy of Automated Methods
    Each sampling quarter, audit each analyzer that monitors for 
SO2, NO2, O3, or CO at least once. The audit is made by 
challenging the analyzer with at least one audit gas of known 
concentration (effective concentration for open path analyzers) from 
each of the following ranges that fall within the measurement range of 
the analyzer being audited: * * *
* * * * *
    For point analyzers, the audit shall be carried out by allowing the 
analyzer to analyze the audit test atmosphere in the same manner as 
described for precision checks in section 3.1. The exception given in 
section 3.1 for certain CO analyzers does not apply for audits.
    Open path analyzers are audited by inserting a test cell containing 
an audit gas concentration into the optical measurement beam of the 
instrument. If possible, the normally used transmitter, receiver, and, 
as appropriate, reflecting devices should be used during the audit, and 
the normal monitoring configuration of the instrument should be 
modified as little as possible to accommodate the test cell for the 
audit. However, if permitted by the associated operation or instruction 
manual, an alternate local light source or an alternate optical path 
that does not include the normal atmospheric monitoring path may be 
used. The actual concentrations of the audit gas in the test cell must 
be selected to produce ``effective concentrations'' in the range 
specified in this section 3.2. Generally, each audit concentration 
measurement result will be the sum of the atmospheric pollutant 
concentration and the audit test concentration. If so, the result must 
be corrected to remove the atmospheric concentration contribution. The 
``corrected concentration'' is obtained by subtracting the average of 
the atmospheric concentrations measured by the open path instrument 
under test immediately before and immediately after the audit test (or 
preferably before and after each audit concentration level) from the 
audit concentration measurement. If the difference between these before 
and after measurements is greater than 20 percent of the effective 
concentration of the test gas standards, discard the test result for 
that concentration level and repeat the test for that level. If 
possible, open path analyzers should be audited during periods when the 
atmospheric pollutant concentrations are relatively low and steady. 
Also, the monitoring path length must be reverified to within 
3 percent to validate the audit, since the monitoring path 
length is critical to the determination of the effective concentration.
    The differences between the actual concentrations (effective 
concentrations for open path analyzers) of the audit test gas and the 
corresponding concentration measurements (corrected 

[[Page 52323]]
concentrations, if applicable, for open path analyzers) indicated by 
the analyzer are used to assess the accuracy of the monitoring data as 
described in section 4.2. Report data only from automated analyzers 
that are approved for use in the PSD network.
* * * * *

                              Table B-1.--Minimum PSD Data Assessment Requirements                              
----------------------------------------------------------------------------------------------------------------
        Method           Assessment method           Coverage              Frequency         Parameters reported
----------------------------------------------------------------------------------------------------------------
Precision:                                                                                                      
  Automated Methods    Response check at      Each analyzer........  Once per 2 weeks.....  Actual concentration
   for SO2, NO2, O3,    concentration                                                        2 & measured       
   and CO.              between .08 & .10                                                    concentration.3    
                        ppm (8 & 10 ppm for                                                                     
                        CO) 2.                                                                                  
  TSP, PM10, Lead....  Collocated samplers..  Highest concentration  Once per week or       Two concentration   
                                               site in monitoring     every 3rd day for      measurements.      
                                               network.               continuous sampling.                      
Accuracy:                                                                                                       
  Automated Methods    Response check at:     Each analyzer........  Once per sampling      Actual              
   for SO2, NO2, O3,    .03-.08 ppm;1,2 .15-                          quarter.               concentration2 &   
   and CO.              .20 ppm;1,2 .35-.45                                                  measured           
                        ppm;1,2 .80-.90                                                      (indicated)        
                        ppm;1,2 (if                                                          concentration3 for 
                        applicable).                                                         each level.        
  TSP, PM10..........  Sampler flow check...  Each sampler.........  Once per sampling      Actual flow rate and
                                                                      quarter.               flow rate indicated
                                                                                             by the sampler.    
  Lead...............  1. Sample flow rate    1. Each sampler......  1. Once/quarter......  1. Same as for TSP. 
                        check..               2. Analytical system.  2. Each quarter Pb     2. Actual           
                       2. Check analytical                            samples are analyzed.  concentration &    
                        system with Pb audit                                                 measured           
                        strips.                                                              concentration of   
                                                                                             audit samples      
                                                                                             (g Pb/    
                                                                                             strip).            
----------------------------------------------------------------------------------------------------------------
\1\ Concentration shown times 100 for CO.                                                                       
\2\ Effective concentration for open path analyzers.                                                            
\3\ Corrected concentration, if applicable, for open path analyzers.                                            

* * * * *
    5. Appendix D is amended as follows:
    a. The second, third, and fourth paragraphs of section 1 are 
revised; and a new paragraph is added between Table 1 and the last 
paragraph of section 1.
    b. Section 2.2 is added.

Appendix D--Network Design for State and Local Air Monitoring 
Stations (SLAMS), National Air Monitoring Stations (NAMS), and 
Photochemical Assessment Monitoring Stations (PAMS)

1. SLAMS Monitoring Objectives and Spatial Scales
* * * * *
    The network of stations which comprise SLAMS should be designed to 
meet a minimum of four basic monitoring objectives. These basic 
monitoring objectives are: (1) To determine highest concentrations 
expected to occur in the area covered by the network; (2) to determine 
representative concentrations in areas of high population density; (3) 
to determine the impact on ambient pollution levels of significant 
sources or source categories; and (4) to determine general background 
concentration levels. Of these four basic ambient air monitoring 
network design objectives, attempts to measure in areas of maximum 
concentrations and maximum population exposures (these can be exclusive 
or coincident) are primary due to the combination of prevailing needs 
and constraints.
    It should be noted that this appendix contains no criteria for 
determining the total number of stations in SLAMS networks, except that 
a minimum number of lead SLAMS is prescribed. The optimum size of a 
particular SLAMS network involves tradeoffs between data needs and 
available resources which the EPA believes can best be resolved during 
the network design process.
    This appendix focuses on the relationship between monitoring 
objectives and the geographical location of monitoring stations. 
Included are a rationale and set of general criteria for identifying 
candidate station locations in terms of physical characteristics which 
most closely match a specific monitoring objective. The criteria for 
more specifically siting the monitoring station, including spacing from 
roadways and vertical and horizontal probe and path placement, are 
described in appendix E of this part.
* * * * *
    Open path analyzers can often be used effectively and 
advantageously to provide better monitoring representation for 
population exposure monitoring and general or background monitoring in 
urban and neighborhood scales of representation. Such analyzers may 
also be able to provide better area coverage or operational advantages 
in high concentration and source-impact monitoring in middle scale and 
possibly microscale areas. However, siting of open path analyzers for 
the latter applications must be carried out with proper regard for the 
specific monitoring objectives and for the path-averaging nature of 
these analyzers. Monitoring path lengths need to be commensurate with 
the intended scale of representativeness and located carefully with 
respect to local sources or potential obstructions. For short-term/
high-concentration or source-oriented monitoring, the monitoring path 
may need to be further restricted in length and be oriented 
approximately radially with respect to the source in the downwind 
direction, to provide adequate peak concentration sensitivity. 
Alternatively, multiple (e.g., orthogonal) paths may be used 
advantageously to obtain both wider area coverage and peak 
concentration sensitivity. Further discussion on this topic is included 
in section 2.2 of this appendix.
* * * * *
2. SLAMS Network Design Procedures
* * * * * 

[[Page 52324]]

2.2  Substantive Changes in SLAMS/NAMS Network Design Elements
    Two important purposes of the SLAMS monitoring data are to examine 
and evaluate overall air quality within a certain region, and to assess 
the trends in air pollutant levels over several years. The EPA believes 
that one of the primary tools for providing these characterizations is 
an ambient air monitoring program which implements technically 
representative networks. The design of these networks must be carefully 
evaluated not only at their outset, but at relatively frequent 
intervals thereafter, using an appropriate combination of other 
important technical tools, including: dispersion and receptor modeling, 
saturation studies, point and area source emissions analyses, and 
meteorological assessments. The impetus for these subsequent 
reexaminations of monitoring network adequacy stems not only from the 
need to evaluate the effect that changes in the environment may pose, 
but also from the recognition that new and/or refined tools and 
techniques for use in impact assessments are continually emerging and 
available for application.
    Substantiative changes to an ambient air monitoring network are 
both inevitable and necessary; however, any changes in any substantive 
aspect of an existing SLAMS network or monitoring site that might 
affect the continuity or comparability of pollutant measurements over 
time must be carefully and thoroughly considered. Such substantive 
changes would include cessation of monitoring at an existing site, 
relocation of an existing site, a change in the type of monitoring 
method used, any change in the probe or path height or orientation that 
might affect pollutant measurements, any significant changes in 
calibration procedures or standards, any significant change in 
operational or quality assurance procedures, any significant change in 
the sources or the character of the area in the vicinity of a 
monitoring site, or any other change that could potentially affect the 
continuity or comparability of monitoring data obtained before and 
after the change.
    In general, these types of changes should be made cautiously with 
due consideration given to the impact of such changes on the network/
site's ability to meet its intended goals. Some of these changes will 
be inevitable (such as when a monitoring site will no longer be 
available and the monitor must be relocated, for example). Other 
changes may be deemed necessary and advantageous, after due 
consideration of their impact, even though they may have a deleterious 
effect on the long-term comparability of the monitoring data. In these 
cases, an effort should be made to quantify, if possible, or at least 
characterize, the nature or extent of the effects of the change on the 
monitoring data. In all cases, the changes and all information 
pertinent to the effect of the change should be properly and completely 
documented for evaluation by trends analysts.
    The introduction of open path methods to the SLAMS monitoring 
network may seem relatively straightforward, given the kinds of 
technical analyses required in this appendix. However, given the 
uncertainties attendant to these analyses and the critical nature and 
far-reaching regulatory implications of some sites in the current SLAMS 
network composed of point monitors, there is a need to `bridge' between 
databases generated by these different candidate methods to evaluate 
and promote continuity in understanding of the historical 
representativeness of the database.
    Concurrent, nominally collocated monitoring must be conducted in 
all instances where an open path analyzer is effectively intended to 
replace a criteria pollutant point monitor which meets either of the 
following:
    1. Data collected at the site represents the maximum concentration 
for a particular nonattainment area; or
    2. Data collected at the site is currently used to characterize the 
development of a nonattainment area State implementation plan.
    The Regional Administrator, the Administrator, or their appropriate 
designee may also require collocated monitoring at other sites which 
are, based on historical technical data, significant in assessing air 
quality in a particular area. The term of this requirement is 
determined by the Regional Administrator (for SLAMS), Administrator 
(for NAMS), or their appropriate designee. The recommended minimum term 
consists of one year (or one season of maximum pollutant concentration) 
with a maximum term indexed to the subject pollutant NAAQS compliance 
interval (e.g., three calendar years for ozone). The requirement 
involves concurrent monitoring with both the open path analyzer and the 
existing point monitor during this term. Concurrent monitoring with 
more than one point analyzer with an open path analyzer using one or 
more measurement paths may also be advantageous to confirm adequate 
peak concentration sensitivity or to optimize the location and length 
of the monitoring path or paths.
    All or some portion of the above requirement may be waived by the 
Regional Administrator (for SLAMS), the Administrator (for NAMS), or 
their designee in response to a request, based on accompanying 
technical information and analyses, or in certain unavoidable instances 
caused by logistical circumstances.
    These requirements for concurrent monitoring also generally apply 
to situations where the relocation of any SLAMS site, using either a 
point monitor or an open path analyzer, within an area is being 
contemplated.
* * * * *
    6. Appendix E is amended as follows:
    a. The heading of appendix E is revised.
    b. Section 1 is revised.
    c. Section 2 is added and sections 3, 5, and 6 are removed and 
reserved.
    d. Section 4 is revised.
    e. In section 7, table 4 is redesignated as table 3.
    f. The first paragraph of section 9 is revised.
    g. Section 10 is revised.
    h. Section 12 is revised.

Appendix E--Probe and Monitoring Path Siting Criteria for Ambient 
Air Quality Monitoring

1. Introduction
    This appendix contains specific location criteria applicable to 
ambient air quality monitoring probes and monitoring paths after the 
general station siting has been selected based on the monitoring 
objectives and spatial scale of representation discussed in appendix D 
of this part. Adherence to these siting criteria is necessary to ensure 
the uniform collection of compatible and comparable air quality data.
    The probe and monitoring path siting criteria discussed below must 
be followed to the maximum extent possible. It is recognized that there 
may be situations where some deviation from the siting criteria may be 
necessary. In any such case, the reasons must be thoroughly documented 
in a written request for a waiver that describes how and why the 
proposed siting deviates from the criteria. This documentation should 
help to avoid later questions about the validity of the resulting 
monitoring data. Conditions under which the EPA would consider an 
application for waiver from these siting criteria are discussed in 
section 11 of this appendix.
    The spatial scales of representation used in this appendix, i.e., 
micro, middle, neighborhood, urban, and regional, are defined and 
discussed in 

[[Page 52325]]
appendix D of this part. The pollutant-specific probe and monitoring 
path siting criteria generally apply to all spatial scales except where 
noted otherwise. Specific siting criteria that are phrased with a 
``must'' are defined as requirements and exceptions must be approved 
through the waiver provisions. However, siting criteria that are 
phrased with a ``should'' are defined as goals to meet for consistency 
but are not requirements.
2. Sulfur dioxide (SO2), Ozone (O3), and Nitrogen Dioxide 
(NO2)
    Open path analyzers may be used to measure SO2, O3, and 
NO2 at SLAMS/NAMS sites for middle, neighborhood, urban, and 
regional scale measurement applications. Additional information on 
SO2, NO2, and O3 monitor siting criteria may be found in 
references 11 and 13.
2.1  Horizontal and Vertical Placement
    The probe or at least 80 percent of the monitoring path must be 
located between 3 and 15 meters above ground level. The probe or at 
least 90 percent of the monitoring path must be at least 1 meter 
vertically or horizontally away from any supporting structure, walls, 
parapets, penthouses, etc., and away from dusty or dirty areas. If the 
probe or a significant portion of the monitoring path is located near 
the side of a building, then it should be located on the windward side 
of the building relative to the prevailing wind direction during the 
season of highest concentration potential for the pollutant being 
measured.
2.2  Spacing from Minor Sources (Applicable to SO2 and O3 
Monitoring Only)
    Local minor sources of SO2 can cause inappropriately high 
concentrations of SO2 in the vicinity of probes and monitoring 
paths for SO2. Similarly, local sources of nitric oxide (NO) and 
ozone-reactive hydrocarbons can have a scavenging effect causing 
unrepresentatively low concentrations of O3 in the vicinity of 
probes and monitoring paths for O3. To minimize these potential 
interferences, the probe or at least 90 percent of the monitoring path 
must be away from furnace or incineration flues or other minor sources 
of SO2 or NO, particularly for open path analyzers because of 
their potential for greater exposure over the area covered by the 
monitoring path. The separation distance should take into account the 
heights of the flues, type of waste or fuel burned, and the sulfur 
content of the fuel. It is acceptable, however, to monitor for SO2 
near a point source of SO2 when the objective is to assess the 
effect of this source on the represented population.
2.3  Spacing From Obstructions
    Buildings and other obstacles may possibly scavenge SO2, 
O3, or NO2. To avoid this interference, the probe or at least 
90 percent of the monitoring path must have unrestricted airflow and be 
located away from obstacles so that the distance from the probe or 
monitoring path is at least twice the height that the obstacle 
protrudes above the probe or monitoring path. Generally, a probe or 
monitoring path located near or along a vertical wall is undesirable 
because air moving along the wall may be subject to possible removal 
mechanisms. A probe must have unrestricted airflow in an arc of at 
least 270 degrees around the inlet probe, or 180 degrees if the probe 
is on the side of a building. This arc must include the predominant 
wind direction for the season of greatest pollutant concentration 
potential. A sampling station having a probe located closer to an 
obstacle than this criterion allows should be classified as middle 
scale rather than neighborhood or urban scale, since the measurements 
from such a station would more closely represent the middle scale. A 
monitoring path must be clear of all trees, brush, buildings, plumes, 
dust, or other optical obstructions, including potential obstructions 
that may move due to wind, human activity, growth of vegetation, etc. 
Temporary optical obstructions, such as rain, particles, fog, or snow, 
should be considered when siting an open path analyzer. Any of these 
temporary obstructions that are of sufficient density to obscure the 
light beam will affect the ability of the open path analyzer to 
continuously measure pollutant concentrations.
    Special consideration must be devoted to the use of open path 
analyzers due to their inherent potential sensitivity to certain types 
of interferences, or optical obstructions. While some of these 
potential interferences are comparable to those to which point monitors 
are subject, there are additional sources of potential interferences 
which are altogether different in character. Transient, but significant 
obscuration of especially longer measurement paths could be expected to 
occur as a result of certain prevailing meteorological conditions 
(e.g., heavy fog, rain, snow) and/or aerosol levels that are of a 
sufficient density to prevent the open path analyzer's light 
transmission. If certain compensating measures are not otherwise 
implemented at the onset of monitoring (e.g., shorter path lengths, 
higher light source intensity), data recovery during periods of 
greatest primary pollutant potential could be compromised. For 
instance, if heavy fog or high particulate levels are coincident with 
periods of projected NAAQS-threatening pollutant potential, the 
representativeness of the resulting data record in reflecting maximum 
pollutant concentrations may be substantially impaired despite the fact 
that the site may otherwise exhibit an acceptable, even exceedingly 
high overall valid data capture rate.
    In seeking EPA approval for inclusion of a site using an open path 
analyzer into the formal SLAMS/NAMS or PSD network, monitoring agencies 
must submit an analysis which evaluates both obscuration potential for 
a proposed path length for the subject area and the effect this 
potential is projected to have on the representativeness of the data 
record. This analysis should include one or more of the following 
elements, as appropriate for the specific circumstance: climatological 
information, historical pollutant and aerosol information, modeling 
analysis results, and any related special study results.
2.4  Spacing From Trees
    Trees can provide surfaces for SO2, O3, or NO2 
adsorption or reactions and obstruct wind flow. To reduce this possible 
interference, the probe or at least 90 percent of the monitoring path 
should be 20 meters or more from the drip line of trees. If a tree or 
trees could be considered an obstacle, the probe or 90 percent of the 
monitoring path must meet the distance requirements of Section 2.3 and 
be at least 10 meters from the drip line of the tree or trees. Since 
the scavenging effect of trees is greater for O3 than for other 
criteria pollutants, strong consideration of this effect must be given 
to locating an O3 probe or monitoring path to avoid this problem.
2.5  Spacing From Roadways (Applicable to O3 and NO2 Only)
    In siting an O3 analyzer, it is important to minimize 
destructive interferences from sources of NO, since NO readily reacts 
with O3. In siting NO2 analyzers for neighborhood and urban 
scale monitoring, it is important to minimize interferences from 
automotive sources. Table 1 provides the required minimum separation 
distances between a roadway and a probe and between a roadway and at 
least 90 percent of a monitoring path for various ranges of daily 
roadway traffic. A sampling station having a point analyzer probe 
located closer to a roadway than allowed by the Table 1 requirements 

[[Page 52326]]
should be classified as middle scale rather than neighborhood or urban 
scale, since the measurements from such a station would more closely 
represent the middle scale. If an open path analyzer is used at a site, 
the monitoring path(s) must not cross over a roadway with an average 
daily traffic count of 10,000 vehicles per day or more. For those 
situations where a monitoring path crosses a roadway with fewer than 
10,000 vehicles per day, one must consider the entire segment of the 
monitoring path in the area of potential atmospheric interference from 
automobile emissions. Therefore, this calculation must include the 
length of the monitoring path over the roadway plus any segments of the 
monitoring path that lie in the area between the roadway and the 
minimum separation distance, as determined from Table 1. The sum of 
these distances must not be greater than 10 percent of the total 
monitoring path length.

  Table 1.--Minimum Separation Distance Between Roadways and Probes or  
Monitoring Paths for Monitoring Neighborhood--and Urban--Scale Ozone and
                            Nitrogen Dioxide                            
------------------------------------------------------------------------
                                                               Minimum  
                                                             separation 
      Roadway average daily traffic, vehicles per day       distance,\1\
                                                               meters   
------------------------------------------------------------------------
 10,000........................................            10
   15,000.................................................            20
   20,000.................................................            30
   40,000.................................................            50
   70,000.................................................           100
110,000........................................           250
------------------------------------------------------------------------
\1\ Distance from the edge of the nearest traffic lane. The distance for
  intermediate traffic counts should be interpolated from the table     
  values based on the actual traffic count.                             

2.6  Cumulative Interferences on a Monitoring Path
    The cumulative length or portion of a monitoring path that is 
affected by minor sources, obstructions, trees, or roadways must not 
exceed 10 percent of the total monitoring path length.
2.7  Maximum Monitoring Path Length
    The monitoring path length must not exceed 1 kilometer for 
analyzers in neighborhood, urban, or regional scale. For middle scale 
monitoring sites, the monitoring path length must not exceed 300 
meters. In areas subject to frequent periods of dust, fog, rain, or 
snow, consideration should be given to a shortened monitoring path 
length to minimize loss of monitoring data due to these temporary 
optical obstructions. For certain ambient air monitoring scenarios 
using open path analyzers, shorter path lengths may be needed in order 
to ensure that the monitoring station meets the objectives and spatial 
scales defined for SLAMS in appendix D. Therefore, the Regional 
Administrator or the Regional Administrator's designee may require 
shorter path lengths, as needed on an individual basis, to ensure that 
the SLAMS meet the appendix D requirements. Likewise, the Administrator 
or the Administrator's designee may specify the maximum path length 
used at monitoring stations designated as NAMS or PAMS as needed on an 
individual basis.
* * * * *
4. Carbon Monoxide (CO)
    Open path analyzers may be used to measure CO at SLAMS/NAMS sites 
for middle or neighborhood scale measurement applications. Additional 
information on CO monitor siting criteria may be found in reference 12.
4.1  Horizontal and Vertical Placement
    Because of the importance of measuring population exposure to CO 
concentrations, air should be sampled at average breathing heights. 
However, practical factors require that the inlet probe be higher. The 
required height of the inlet probe for CO monitoring is therefore 
3\1/2\ meters for a microscale site, which is a compromise 
between representative breathing height and prevention of vandalism. 
The recommended 1 meter range of heights is also a compromise to some 
extent. For consistency and comparability, it would be desirable to 
have all inlets at exactly the same height, but practical 
considerations often prevent this. Some reasonable range must be 
specified and 1 meter provides adequate leeway to meet most 
requirements.
    For the middle and neighborhood scale stations, the vertical 
concentration gradients are not as great as for the microscale station. 
This is because the diffusion from roads is greater and the 
concentrations would represent larger areas than for the microscale. 
Therefore, the probe or at least 80 percent of the monitoring path must 
be located between 3 and 15 meters above ground level for middle and 
neighborhood scale stations. The probe or at least 90 percent of the 
monitoring path must be at least 1 meter vertically or horizontally 
away from any supporting structure, walls, parapets, penthouses, etc., 
and away from dusty or dirty areas. If the probe or a significant 
portion of the monitoring path is located near the side of a building, 
then it should be located on the windward side of the building relative 
to both the prevailing wind direction during the season of highest 
concentration potential and the location of sources of interest, i.e., 
roadways.
4.2  Spacing from Obstructions
    Buildings and other obstacles may restrict airflow around a probe 
or monitoring path. To avoid this interference, the probe or at least 
90 percent of the monitoring path must have unrestricted airflow and be 
located away from obstacles so that the distance from the probe or 
monitoring path is at least twice the height that the obstacle 
protrudes above the probe or monitoring path. A probe or monitoring 
path located near or along a vertical wall is undesirable because air 
moving along the wall may be subject to possible removal mechanisms. A 
probe must have unrestricted airflow in an arc of at least 270 degrees 
around the inlet probe, or 180 degrees if the probe is on the side of a 
building. This arc must include the predominant wind direction for the 
season of greatest pollutant concentration potential. A monitoring path 
must be clear of all trees, brush, buildings, plumes, dust, or other 
optical obstructions, including potential obstructions that may move 
due to wind, human activity, growth of vegetation, etc. Temporary 
optical obstructions, such as rain, particles, fog, or snow, should be 
considered when siting an open path analyzer. Any of these temporary 
obstructions that are of sufficient density to obscure the light beam 
will affect the ability of the open path analyzer to continuously 
measure pollutant concentrations.
    Special consideration must be devoted to the use of open path 
analyzers due to their inherent potential sensitivity to certain types 
of interferences, or optical obstructions. While some of these 
potential interferences are comparable to those to which point monitors 
are subject, there are additional sources of potential interferences 
which are altogether different in character. Transient, but significant 
obscuration of especially longer measurement paths could be expected to 
occur as a result of certain prevailing meteorological conditions 
(e.g., heavy fog, rain, snow) and/or aerosol levels that are of a 
sufficient density to prevent the open path analyzer's light 
transmission. If certain compensating measures are not otherwise 
implemented at the onset of monitoring (e.g., shorter path lengths, 
higher light source intensity), data recovery during periods of 
greatest primary pollutant potential could be compromised. For 
instance, if heavy fog or high particulate levels are coincident 

[[Page 52327]]
with periods of projected NAAQS-threatening pollutant potential, the 
representativeness of the resulting data record in reflecting maximum 
pollutant concentrations may be substantially impaired despite the fact 
that the site may otherwise exhibit an acceptable, even exceedingly 
high overall valid data capture rate.
    In seeking EPA approval for inclusion of a site using an open path 
analyzer into the formal SLAMS/NAMS or PSD network, monitoring agencies 
must submit an analysis which evaluates both obscuration potential for 
a proposed path length for the subject area and the effect this 
potential is projected to have on the representativeness of the data 
record. This analysis should include one or more of the following 
elements, as appropriate for the specific circumstance: climatological 
information, historical pollutant and aerosol information, modeling 
analysis results, and any related special study results.
4.3  Spacing From Roadways
    Street canyon and traffic corridor stations (microscale) are 
intended to provide a measurement of the influence of the immediate 
source on the pollution exposure of the population. In order to provide 
some reasonable consistency and comparability in the air quality data 
from microscale stations, a minimum distance of 2 meters and a maximum 
distance of 10 meters from the edge of the nearest traffic lane must be 
maintained for these CO monitoring inlet probes. This should give 
consistency to the data, yet still allow flexibility of finding 
suitable locations.
    Street canyon/corridor (microscale) inlet probes must be located at 
least 10 meters from an intersection and preferably at a midblock 
location. Midblock locations are preferable to intersection locations 
because intersections represent a much smaller portion of downtown 
space than do the streets between them. Pedestrian exposure is probably 
also greater in street canyon/corridors than at intersections. Also, 
the practical difficulty of positioning sampling inlets is less at 
midblock locations than at the intersection. However, the final siting 
of the monitor must meet the objectives and intent of appendix D, 
sections 2.4, 3, 3.3, and appendix E, section 4.
    In determining the minimum separation between a neighborhood scale 
monitoring station and a specific line source, the presumption is made 
that measurements should not be substantially influenced by any one 
roadway. Computations were made to determine the separation distance, 
and table 2 provides the required minimum separation distance between 
roadways and a probe or 90 percent of a monitoring path. Probes or 
monitoring paths that are located closer to roads than this criterion 
allows should not be classified as a neighborhood scale, since the 
measurements from such a station would closely represent the middle 
scale. Therefore, stations not meeting this criterion should be 
classified as middle scale.

  Table 2.--Minimum Separation Distance Between Roadways and Probes or  
   Monitoring Paths for Monitoring Neighborhood Scale Carbon Monoxide   
------------------------------------------------------------------------
                                                               Minimum  
                                                              separation
                                                               distance 
                                                               \1\ for  
      Roadway average daily traffic, vehicles per day         probes or 
                                                               90% of a 
                                                              monitoring
                                                                 path   
---------------------------------------------------------------(meters)-
10,000..........................................           10
  15,000...................................................           25
  20,000...................................................           45
  30,000...................................................           80
  40,000...................................................          115
  50,000...................................................          135
  60,000........................................          150
------------------------------------------------------------------------
\1\ Distance from the edge of the nearest traffic lane. The distance for
  intermediate traffic counts should be interpolated from the table     
  values based on the actual traffic count.                             

4.4  Spacing From Trees and Other Considerations
    Since CO is relatively nonreactive, the major factor concerning 
trees is as obstructions to normal wind flow patterns. For middle and 
neighborhood scale stations, trees should not be located between the 
major sources of CO, usually vehicles on a heavily traveled road, and 
the monitor. The probe or at least 90 percent of the monitoring path 
must be 10 meters or more from the drip line of trees which are between 
the probe or the monitoring path and the road and which extend at least 
5 meters above the probe or monitoring path. For microscale stations, 
no trees or shrubs should be located between the probe and the roadway.
4.5  Cumulative Interferences on a Monitoring Path
    The cumulative length or portion of a monitoring path that is 
affected by obstructions, trees, or roadways must not exceed 10 percent 
of the total monitoring path length.
4.6  Maximum Monitoring Path Length
    The monitoring path length must not exceed 1 kilometer for 
analyzers used for neighborhood scale monitoring applications, or 300 
meters for middle scale monitoring applications. In areas subject to 
frequent periods of dust, fog, rain, or snow, consideration should be 
given to a shortened monitoring path length to minimize loss of 
monitoring data due to these temporary optical obstructions. For 
certain ambient air monitoring scenarios using open path analyzers, 
shorter path lengths may be needed in order to ensure that the 
monitoring station meets the objectives and spatial scales defined for 
SLAMS in appendix D. Therefore, the Regional Administrator or the 
Regional Administrator's designee may require shorter path lengths, as 
needed on an individual basis, to ensure that the SLAMS meet the 
appendix D requirements. Likewise, the Administrator or the 
Administrator's designee may specify the maximum path length used at 
monitoring stations designated as NAMS or PAMS as needed on an 
individual basis.
* * * * *
Table 3--Separation Distance Between Pb Stations and Roadways (Edge of 
Nearest Traffic Lane)
* * * * *
9. Probe Material and Pollutant Sample Residence Time
    For the reactive gases, SO2, NO2, and O3, special 
probe material must be used for point analyzers. Studies20-24 have 
been conducted to determine the suitability of materials such as 
polypropylene, polyethylene, polyvinyl chloride, Tygon, aluminum, 
brass, stainless steel, copper, Pyrex glass and Teflon for use as 
intake sampling lines. Of the above materials, only Pyrex glass and 
Teflon have been found to be acceptable for use as intake sampling 
lines for all the reactive gaseous pollutants. Furthermore, the 
EPA25 has specified borosilicate glass or FEP Teflon as the only 
acceptable probe materials for delivering test atmospheres in the 
determination of reference or equivalent methods. Therefore, 
borosilicate glass, FEP Teflon, or their equivalent must be used for 
existing and new NAMS or SLAMS.
* * * * *
10. Photochemical Assessment Monitoring Stations (PAMS)
10.1  Horizontal and Vertical Placement
    The probe or at least 80 percent of the monitoring path must be 
located 3 to 15 

[[Page 52328]]
meters above ground level. This range provides a practical compromise 
for finding suitable sites for the multipollutant PAMS. The probe or at 
least 90 percent of the monitoring path must be at least 1 meter 
vertically or horizontally away from any supporting structure, walls, 
parapets, penthouses, etc., and away from dusty or dirty areas.
10.2  Spacing From Obstructions
    The probe or at least 90 percent of the monitoring path must be 
located away from obstacles and buildings such that the distance 
between the obstacles and the probe or the monitoring path is at least 
twice the height that the obstacle protrudes above the probe or 
monitoring path. There must be unrestricted airflow in an arc of at 
least 270 deg. around the probe inlet. Additionally, the predominant 
wind direction for the period of greatest pollutant concentration (as 
described for each site in section 4.2 of appendix D) must be included 
in the 270 deg. arc. If the probe is located on the side of the 
building, 180 deg. clearance is required. A monitoring path must be 
clear of all trees, brush, buildings, plumes, dust, or other optical 
obstructions, including potential obstructions that may move due to 
wind, human activity, growth of vegetation, etc. Temporary optical 
obstructions, such as rain, particles, fog, or snow, should be 
considered when siting an open path analyzer. Any of these temporary 
obstructions that are of sufficient density to obscure the light beam 
will affect the ability of the open path analyzer to continuously 
measure pollutant concentrations.
    Special consideration must be devoted to the use of open path 
analyzers due to their inherent potential sensitivity to certain types 
of interferences, or optical obstructions. While some of these 
potential interferences are comparable to those to which point monitors 
are subject, there are additional sources of potential interferences 
which are altogether different in character. Transient, but significant 
obscuration of especially longer measurement paths could be expected to 
occur as a result of certain prevailing meteorological conditions 
(e.g., heavy fog, rain, snow) and/or aerosol levels that are of a 
sufficient density to prevent the open path analyzer's light 
transmission. If certain compensating measures are not otherwise 
implemented at the onset of monitoring (e.g., shorter path lengths, 
higher light source intensity), data recovery during periods of 
greatest primary pollutant potential could be compromised. For 
instance, if heavy fog or high particulate levels are coincident with 
periods of projected NAAQS-threatening pollutant potential, the 
representativeness of the resulting data record in reflecting maximum 
pollutant concentrations may be substantially impaired despite the fact 
that the site may otherwise exhibit an acceptable, even exceedingly 
high overall valid data capture rate.
    In seeking EPA approval for inclusion of a site using an open path 
analyzer into the formal SLAMS/NAMS or PSD network, monitoring agencies 
must submit an analysis which evaluates both obscuration potential for 
a proposed path length for the subject area and the effect this 
potential is projected to have on the representativeness of the data 
record. This analysis should include one or more of the following 
elements, as appropriate for the specific circumstance: climatological 
information, historical pollutant and aerosol information, modeling 
analysis results, and any related special study results.
10.3  Spacing From Roadways
    It is important in the probe and monitoring path siting process to 
minimize destructive interferences from sources of NO since NO readily 
reacts with O3. Table 4 below provides the required minimum 
separation distances between roadways and PAMS (excluding upper air 
measuring stations):

         Table 4.--Separation Distance Between Pams and Roadways        
                     [Edge of Nearest Traffic Lane]                     
------------------------------------------------------------------------
                                                               Minimum  
                                                              separation
                                                               distance 
                                                               between  
      Roadway average daily traffic, vehicles per day          roadways 
                                                                 and    
                                                             stations in
                                                              meters \1\
------------------------------------------------------------------------
 <10,000...................................................          >10
 15,000....................................................           20
 20,000....................................................           30
 40,000....................................................           50
 70,000....................................................          100
 >110,000..................................................         250 
------------------------------------------------------------------------
\1\ Distance from the edge of the nearest traffic lane. The distance for
  intermediate traffic counts should be interpolated from the table     
  based on the actual traffic flow.                                     

10.4  Spacing From Trees
    Trees can provide surfaces for adsorption and/or reactions to occur 
and can obstruct normal wind flow patterns. To minimize these effects 
at PAMS, the probe or at least 90 percent of the monitoring path should 
be placed at least 20 meters from the drip line of trees. Since the 
scavenging effect of trees is greater for O3 than for the other 
criteria pollutants, strong consideration of this effect must be given 
in locating the PAMS probe or monitoring path to avoid this problem. 
Therefore, the probe or at least 90 percent of the monitoring path must 
be at least 10 meters from the drip line of trees.
* * * * *
12. Summary
    Table 5 presents a summary of the general requirements for probe 
and monitoring path siting criteria with respect to distances and 
heights. It is apparent from Table 5 that different elevation distances 
above the ground are shown for the various pollutants. The discussion 
in the text for each of the pollutants described reasons for elevating 
the monitor, probe, or monitoring path. The differences in the 
specified range of heights are based on the vertical concentration 
gradients. For CO, the gradients in the vertical direction are very 
large for the microscale, so a small range of heights has been used. 
The upper limit of 15 meters was specified for consistency between 
pollutants and to allow the use of a single manifold or monitoring path 
for monitoring more than one pollutant.

                                                                        

[[Page 52329]]
                         Table 5.--Summary of Probe and Monitoring Path Siting Criteria                         
----------------------------------------------------------------------------------------------------------------
                                                           Horizontal and                                       
                                         Height from     vertical distance                                      
                     Scale [maximum    ground to probe    from supporting     Distance from      Distance from  
    Pollutant       monitoring path       or 80% of       structures B to   trees to probe or  roadways to probe
                    length, meters]   monitoring path A   probe or 90% of   90% of monitoring    or monitoring  
                                           (meters)      monitoring path A   path A (meters)    path A (meters) 
                                                              (meters)                                          
----------------------------------------------------------------------------------------------------------------
SO2 C,D,E,F......  Middle [300m]      3-15.............  >1...............  >10..............  N/A.             
                    Neighborhood,                                                                               
                    Urban, and                                                                                  
                    Regional [1km].                                                                             
CO D,E,G.........  Micro Middle       30.5;  >1...............  >10..............  2-10; See Table 2
                    [300m]             3-15.                                                    for middle and  
                    Neighborhood                                                                neighborhood    
                    [1km].                                                                      scales.         
O3 C,D,E.........  Middle [300m]      3-15.............  >1...............  >10..............  See Table 1 for  
                    Neighborhood,                                                               all scales.     
                    Urban, and                                                                                  
                    Regional [1km].                                                                             
Ozone precursors   Neighborhood and   3-15.............  >1...............  >10..............  See Table 4 for  
 (for PAMS) C,D,E.  Urban.                                                                      all scales.     
                   [1 km]...........                                                                            
NO2 C,D,E........  Middle [300m]      3-15.............  >1...............  >10..............  See Table 1 for  
                    Neighborhood and                                                            all scales.     
                    Urban [1km].                                                                                
Pb C,D,E,F,H.....  Micro; Middle,     2-7 (Micro); 2-15  >2 (All scales,    >10 (All scales).  5-15 (Micro); See
                    Neighborhood,      (All other         horizontal                            Table 3 for all 
                    Urban and          scales).           distance only).                       other scales.   
                    Regional.                                                                                   
PM-10 C,D,E,F,H..  Micro; Middle,     2-7 (Micro); 2-15  >2 (All scales,    >10 (All scales).  2-10 (Micro); See
                    Neighborhood,      (All other         horizontal                            Figure 2 for all
                    Urban and          scales).           distance only).                       other scales.   
                    Regional.                                                                                   
----------------------------------------------------------------------------------------------------------------
N/A--Not applicable.                                                                                            
A Monitoring path for open path analyzers is applicable only to middle or neighborhood scale CO monitoring and  
  all applicable scales for monitoring SO2, O3, O3 precursors, and NO2.                                         
B When probe is located on a rooftop, this separation distance is in reference to walls, parapets, or penthouses
  located on roof.                                                                                              
C Should be >20 meters from the dripline of tree(s) and must be 10 meters from the dripline when the tree(s) act
  as an obstruction.                                                                                            
D Distance from sampler, probe, or 90% of monitoring path to obstacle, such as a building, must be at least     
  twice the height the obstacle protrudes above the sampler, probe, or monitoring path. Sites not meeting this  
  criterion may be classified as middle scale (see text).                                                       
E Must have unrestricted airflow 270 deg. around the probe or sampler; 180 deg. if the probe is on the side of a
  building.                                                                                                     
F The probe, sampler, or monitoring path should be away from minor sources, such as furnace or incineration     
  flues. The separation distance is dependent on the height of the minor source's emission point (such as a     
  flue), the type of fuel or waste burned, and the quality of the fuel (sulfur, ash, or lead content). This     
  criterion is designed to avoid undue influences from minor sources.                                           
G For microscale CO monitoring sites, the probe must be >10 meters from a street intersection and preferably at 
  a midblock location.                                                                                          
H For collocated Pb and PM-10 samplers, a 2-4 meter separation distance between collocated samplers must be met.


* * * * *
[FR Doc. 95-24042 Filed 10-5-95; 8:45 am]
BILLING CODE 6560-50-P