[Federal Register Volume 60, Number 153 (Wednesday, August 9, 1995)]
[Rules and Regulations]
[Pages 40465-40474]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 95-19057]
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ENVIRONMENTAL PROTECTION AGENCY
40 CFR Parts 51 and 52
[AH-FRL-5268-8; Docket No. A-92-65]
RIN 2060-AG04
Requirements for Preparation, Adoption, and Submittal of
Implementation Plans
AGENCY: Environmental Protection Agency (EPA).
ACTION: Final rule.
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SUMMARY: The ``Guideline on Air Quality Models (Revised)''
(hereinafter, the ``Guideline''), as modified by supplement A (1987)
and supplement B (1993), sets forth air quality models and guidance for
estimating the air quality impacts of sources and for specifying
emission limits for them. The Guideline, codified as appendix W to 40
CFR part 51, is referenced in the PSD (Prevention of Significant
Deterioration) regulations and is applied to SIP revisions for existing
sources and to all new source reviews. On November 28, 1994 EPA issued
a Notice of Proposed Rulemaking to augment the final rule that was
published on July 20, 1993. Today EPA takes final action that makes
several additions and changes as supplement C to the Guideline.
Supplement C does the following: incorporates improved algorithms for
treatment of area sources and dry deposition in the Industrial Source
Complex (ISC) model, adopts a solar radiation/delta-T (SRDT) method for
estimating atmospheric stability categories, adopts a new screening
approach for assessing annual NO2 impacts, and adds SLAB and
HGSYSTEM as alternative models. This action is responsive to public
comments received. Adoption of these new and refined modeling
techniques and associated guidance should significantly improve the
technical basis for impact assessment of air pollution sources.
EFFECTIVE DATE: This rule is effective September 8, 1995.
ADDRESSES: Docket Statement: All documents relevant to this rule have
been placed in Docket No. A-92-65, located in the Air Docket (6102),
Room M-1500, Waterside Mall, Attention: Docket A-92-65, U.S.
Environmental Protection Agency, 401 M Street SW., Washington, DC
20460. This docket is available for public inspection and copying
between 8:00 a.m. and 5:30 p.m., Monday through Friday, at the address
above.
Document Availability: Copies of supplement C to the Guideline may
be obtained by downloading a text file from the SCRAM (Support Center
for Regulatory Air Models) electronic bulletin board system by dialing
in on (919) 541-5742. Supplement C may also be obtained upon written
request from the Air Quality Modeling Group, U.S. Environmental
Protection Agency (MD-14), Research Triangle Park, NC 27711. The
``Guideline on Air Quality Models (Revised)'' (1986), supplement A
(1987), supplement B (1993), and supplement C (1995) are for sale from
the U.S. Department of Commerce, Technical Information Service (NTIS),
5825 Port Royal Road, Springfield, VA 22161. These documents are also
available for inspection at each of the ten EPA Regional Offices and at
the EPA library at 401 M Street SW., Washington, DC.
FOR FURTHER INFORMATION CONTACT: Joseph A. Tikvart, Leader, Air Quality
Modeling Group, Office of Air Quality Planning and Standards, U.S.
Environmental Protection Agency, Research Triangle Park, NC 27711;
telephone (919) 541-5561 or C. Thomas Coulter, telephone (919) 541-
0832.
SUPPLEMENTARY INFORMATION:
Background 1
\1\ In reviewing this preamble, note the distinction between the
terms ``supplement'' and ``appendix''. Supplements A, B and C
contain the replacement pages to effect Guideline revisions;
appendix A to the Guideline is the repository for preferred models,
while appendix B is the repository for alternate models justified
for use on a case-by-case basis.
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The purpose of the Guideline 2 is to promote consistency in
the use of modeling within the air management process. The Guideline
provides model users with a common basis for estimating pollution
concentrations, assessing control strategies and specifying emission
limits; these activities are regulated at 40 CFR 51.46, 51.63, 51.112,
51.117, 51.150, 51.160, 51.166, and 51.21. The Guideline was originally
published in April 1978. It was incorporated by reference in the
regulations for the Prevention of Significant Deterioration of Air
Quality
[[Page 40466]]
in June 1978 (43 FR 26380). The Guideline was subsequently revised in
1986 (51 FR 32176), and later updated with the addition of supplement A
in 1987 (53 FR 393). The last such revision was supplement B, issued on
July 20, 1993 (58 FR 38816). The revisions in supplement B included
techniques and guidance for situations where specific procedures had
not previously been available, and also improved several previously
adopted techniques.
\2\ Guideline on Air Quality Models ``(Revised)''(1986)[EPA-450/
2-78-027R], with supplement A (1987) and supplement B (1993),
hereinafter, the ``Guideline''. The Guideline is published as
appendix W of 40 CFR part 51. The text of appendix W will be
appropriately modified to effect the revisions incorporated as
supplement C.
During the public comment period for supplement B, EPA received
requests to consider several additional new modeling techniques and
suggestions for enhanced technical guidance. However, because there was
not sufficient time for the public to review the new techniques and
technical guidance before promulgation of supplement B, the new models
and enhanced technical guidance could not be included in the supplement
B rulemaking. Thus, in a subsequent regulatory proposal, EPA proposed
to revise the Guideline and sought public comment on the following four
items: incorporation of improved algorithms for treatment of area
sources and dry deposition in the Industrial Source Complex (ISC)
model, adoption of a solar radiation/delta-T (SRDT) method for
estimating atmospheric stability categories, adoption of a new
screening approach for assessing annual NO2 impacts, and addition
of SLAB and HGSYSTEM as alternative models.
Final Action
Today's action amends appendix W of 40 CFR part 51 to effect the
revisions known as supplement C, slightly modified in form since
proposal. All significant comments have been considered, and whenever
they revealed any new information or suggested any alternative
solutions, such were considered in EPA's final action.
As proposed, EPA is replacing the area source algorithm in the
Industrial Source Complex model with a new one based on a double
integration of the Gaussian plume kernel for area sources. This
replacement includes that of the finite line segment approximation
employed by the short term version of ISC and of the virtual point
source technique used in the long term version of ISC.
As proposed, EPA is replacing the dry deposition algorithm in ISC
with an improved technique that is more accurate for estimating
deposition for small (i.e., < 20m diameter) particles. Use the
deposition algorithm in modeling analyses in which particle settling is
considered important will remain optional.
EPA will adopt the solar radiation/delta-T (SRDT) method for
Pasquill-Gifford (P-G) stability classification discussed in section 9
of appendix W. However, instead of adopting the SRDT method as a
replacement for the currently accepted turbulence-based methods (i.e.,
and ), as proposed, SRDT will
join them as an ensemble of acceptable methods. Furthermore, while the
current hierarchy of acceptable methods is eliminated, the Turner
method using on-site wind speed and representative cloud cover
observations, remains the preferred classification method.
As proposed, EPA revises the annual NO2 screening technique
described in section 6 of appendix W. The new technique, known as the
Ambient Ratio Method (ARM), is simpler and less conservative than the
Ozone Limiting Method (OLM) it replaces.
As proposed, EPA adds two new models, namely SLAB and HGSYSTEM, as
alternative models for use on a case-by-case basis.
Discussion of Public Comments and Issues
All comments submitted to Docket No. A-92-65 are filed in Docket
Category IV-D. EPA has summarized these comments, developed detailed
responses, and drawn conclusions on appropriate actions for this Notice
of Final Rulemaking in an external Agency document.3 In this
document, all significant comments have been considered and discussed.
Whenever the comments revealed any new information or suggested any
alternative solutions, such were considered in EPA's final action.
\3\ ``Summary of Public Comments and EPA Responses on the
Proposal for Supplement C to the Guideline of Air Quality Models
(Revised)''; August 1995 (Air Docket A-92-65, Item V-C-1).
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Major issues raised by the commenters, along with EPA responses,
are summarized below. Guidance and editorial changes associated with
the resolution of these issues are adopted in the appropriate sections
of the Guideline and are promulgated as supplement C (1995) to the
``Guideline on Air Quality Models (Revised)'' (1986) (Docket Item V-B-
1). See the ADDRESSES section of this Notice (above) for general
availability.
Although a more detailed summary of the comments and EPA's
responses are contained in the aforementioned response-to-comments
document (Docket Item V-C-1), the remainder of this preamble section
overviews the primary issues encountered by the Agency during the
public comment period. This overview also serves to explain the changes
to the Guideline from today's action, and the main technical and policy
concerns addressed by the Agency. In our view, all of the changes being
made reasonably implement the mandates of the Clean Air Act, and are in
fact beneficial to both EPA and the regulated community. While modeling
by its nature involves approximation based on scientific methodology,
and entails utilization of advanced technology as it evolves, EPA
believes these changes respond to recent advances in the area so that
the Guideline continues to be comprised of the best and most proven of
the available models and analytical techniques, as well as reflect
reasonable policy choices.
1. Enhancements to the Industrial Source Complex (ISC2) Model
While for clarification these enhancements are discussed
separately, EPA will integrate these enhancements into one model for
actual use. Several conforming Guideline revisions will be made: (a)
the latest version of ISC that integrates the revised algorithms will
be called ISC3, and will hereafter be specified only in main references
(section 12) and in its description in appendix A; (b) the term
``ISC2'' (the version of ISC currently in use) in all but appendix A
(i.e., in sections 7.1, 7.2.2, 7.2.5, 7.2.8, 8.2.5 and 8.2.7) will be
revised to the more generic ``ISC'' to make future Guideline revisions
more manageable; and (c) section 4.2.1 will be amended to say that the
latest version of SCREEN (i.e., SCREEN3), a screening model that uses
ISC algorithms, will be specified in the main references, and
``SCREEN2'' in section 4.2.1 and 5.2.1.1 will be changed to ``SCREEN''.
A. Area Source Algorithm
There was general public support for adoption of the proposed area
source algorithm. Some concern, however, was expressed over the
evaluation of the algorithm's performance being based on wind tunnel
simulations. A commenter urged the Agency to evaluate the algorithm
using a particular ``available field data'' set. EPA had been aware of
the value of such data for evaluation purposes generally but the use of
the specific data set cited by the commenter was recommended against by
EPA's contractor. And since other such data sets were unavailable, EPA
feels that the wind tunnel evaluation was the best possible. EPA will
therefore adopt the algorithm, as proposed.
[[Page 40467]]
B. Dry Deposition Algorithm
No comments were received about the proposed algorithm's
performance in ISCST. Regarding ISCLT, however, concern was expressed
over the algorithm's 50-fold increase in deposition estimates for small
particles from near-surface releases compared with the current
algorithm. As explained in the response-to- comments document, EPA
investigated the commenter's perception and explained the apparent
disparity in performance is explicable in terms of a series of
independent effects related to the improvements made in the new
algorithm. EPA will adopt the algorithm, as proposed.
In the proposal, EPA solicited public comment on whether it would
be appropriate to require that the new dry deposition algorithm be used
for all ISC analyses involving particulate matter in any of the
programs for which Guideline usage is required under 40 CFR parts 51
and 52. No comments were received. EPA will continue to allow optional
use of the algorithm on a case-by-case basis, depending on the
application and on the availability of source specific, fractionated
emissions data.
2. Enhancements to On-Site Stability Classification
Much of the expressed public concern was based on a perception of
substantial added costs the SRDT method would add to meteorological
monitoring programs. As stated in the response-to-comments document,
investigation of the cost factors associated with instrumenting a
meteorological tower to implement the SRDT method (i.e., T and
insolation) showed that such would add approximately $2500-$3500.
Relative to the cost of all the monitoring equipment, including data
acquisition systems, tower, etc., the added instrumentation costs for
implementing the SRDT method are approximately 25 to 45 percent of the
total costs (depending on tower height). Thus, as was pointed out in
public comment, there is a capital cost associated with implementation
of the SRDT method, but EPA believes that cost is not excessive,
particularly in relation to the total monitoring program.
While no analyses were offered to directly refute the viability of
the SRDT method on a technical basis, there was general concern over
the SRDT method's proposed replacement of the currently acceptable
turbulence based methods (i.e., or
), particularly given that the evaluation report for
the SRDT method did not demonstrate its superiority over the latter
methods.
Therefore, in an effort to balance an array of concerns, consistent
with the intent and motivation for the proposal, EPA will adopt the
SRDT method but revise the current hierarchical system of stability
classification in Guideline section 9.3.3.2. Specifically, the Turner
method using site-specific wind speed and representative cloud cover
and ceiling height will be preferred for estimating P-G stability
categories. This preference is founded in the fundamental radiation
basis for P-G categories. In the absence of requisite data to implement
the Turner method, however, the SRDT method or one of the turbulence
based methods may be used. Regarding the collection of requisite
representative cloud cover data for implementing the preferred Turner
method, it should be noted that the operative word is representative.
The previous distinction made for ``off-site'', associated with the
last choice in the current hierarchy, is semantic. ``On-site'' is a
preferable ideal; what is important is representativeness. As aptly
pointed out in public comments, when representative off-site'' cloud
cover data are judiciously used, there can be good P-G category
correspondence with what would have been obtained using strictly on-
site observations. The emphasis on representativeness, inherent in
EPA's final action, should obviate the historical contention over this
semantic issue. As stated in the proposal, the on-site guidance 4
will be revised by addendum to reflect the new stability classification
system, including the SRDT methodology. The document will also be
revised to add some additional guidance on considerations of
representativeness with respect to the Turner method.
\4\ Environmental Protection Agency, 1987. On-Site
Meteorological Program Guidance for Regulatory Modeling
Applications. EPA Publication No. EPA-450/4-87-013. U.S.
Environmental Protection Agency, Research Triangle Park, NC.
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3. Screening Approaches for Assessing Annual NO2 Impact
Public comments were generally supportive of the proposed NO2
screening approach: the ARM. Some, however, recommended the retention
of OLM that ARM was proposed to replace. As stated in EPA's response,
this recommendation would imply that OLM, applied on an hourly basis as
a tertiary screening method, would yield a better estimation of annual
NO2 impact. EPA believes, however that application of OLM in this
manner is affected by several technical and logistical problems.
Because the oversimplified OLM approach does not necessarily result in
more accurate estimates, adding OLM as a third tier screening method to
be implemented on a hourly basis for screening is unnecessary.
Therefore, EPA will adopt the Ambient Ratio Method, as proposed.
4. Modeling Techniques for Toxic Air Pollutants
There was support for EPA's proposal to adopt two new models for
treating dense gas releases. Therefore, as proposed, EPA will add these
models, SLAB and HGSYSTEM Version 3.0, to the Guideline where they will
accompany DEGADIS, another appendix B model for treating dense gas
releases for use on a case-by-case basis.
Administrative Requirements
A. Executive Order 12866
Under Executive Order (E.O.) 12866 [58 FR 51735 (October 4, 1993)],
the Agency must determine whether the regulatory action is
``significant'' and therefore subject to review by the Office of
Management and Budget (OMB) and 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 of the rights and obligations of
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 Order.
It has been determined that this rule is not a ``significant
regulatory action'' under the terms of E.O. 12866 and is therefore not
subject to OMB review.
B. Paperwork Reduction Act
This final rule does not contain any information collection
requirements subject to review by OMB under the Paperwork Reduction Act
on 1980, 44 U.S.C. 3501 et seq.
C. Regulatory Flexibility Act
The Regulatory Flexibility Act (5 U.S.C. 601 et seq.) requires EPA
to consider potential impacts of regulations on small ``entities''. The
final action taken today is a supplement to the notice of final
rulemaking that was published on July 20, 1993 (58 FR 38816). As
described earlier in this
[[Page 40468]]
preamble, the revisions here promulgated as supplement C to the
Guideline encompass the use of new model algorithms and techniques for
using those models. This rule merely updates existing technical
requirements for air quality modeling analyses mandated by various
Clean Air Act programs (e.g., prevention of significant deterioration,
new source review, SIP revisions) and imposes no new regulatory
burdens. As such, there will be no additional impact on small entities
regarding reporting, recordkeeping, compliance requirements, as stated
in the notice of final rulemaking (aforementioned). Furthermore, this
final rule does not duplicate, overlap, or conflict with other federal
rules. Thus, pursuant to the provisions of 5 U.S.C. 605(b), EPA hereby
certifies that the attached final rule will not have a significant
impact on a substantial number of such entities.
D. Unfunded Mandates
Under Section 202 of the Unfunded Mandates Reform Act of 1995
(``Unfunded Mandates Act''), signed into law on March 22, 1995, EPA
must prepare a budgetary impact statement to accompany any proposed or
final rule that includes a Federal mandate that may result in estimated
costs to State, local, or tribal governments in the aggregate; or to
the private sector, of $100 million or more. Under Section 205, EPA
must select the most cost-effective and least burdensome alternative
that achieves the objectives of the rule and is consistent with
statutory requirements. Section 203 requires EPA to establish a plan
for informing and advising any small governments that may be
significantly or uniquely impacted by the rule.
EPA has determined that the action promulgated today does not
include a Federal mandate that may result in estimated costs of $100
million or more to either State, local, or tribal governments in the
aggregate, or to the private sector. Therefore, the requirements of the
Unfunded Mandates Act do not apply to this action.
List of Subjects
40 CFR Part 51
Administrative practice and procedure, Air pollution control,
Intergovernmental relations, Reporting and recordkeeping requirements,
Ozone, Sulfur oxides, Nitrogen dioxide, Lead, Particulate matter,
Hydrocarbons, Carbon monoxide.
40 CFR Part 52
Air pollution control, Ozone, Sulfur oxides, Nitrogen dioxide,
Lead.
Authority: This rule is issued under the authority granted by
sections 110(a)(2), 165(e), 172 (a) & (c), 173, 301(a)(1) and 320 of
the 1990 Clean Air Act Amendments, 42 U.S.C. 7410(a)(2), 7475(e),
7502 (a) & (c), 7503, 7601(a)(1) and 7620, respectively.
Dated: July 25, 1995.
Carol M. Browner,
Administrator.
Parts 51 and 52, chapter I, title 40 of the Code of Federal
Regulations are amended as follows:
PART 51--REQUIREMENTS FOR PREPARATION, ADOPTION, AND SUBMITTAL OF
IMPLEMENTATION PLANS
1. The authority citation for part 51 continues to read as follows:
Authority: 42 U.S.C. 7410(a)(2), 7475(e), 7502 (a) and (b),
7503, 7601(a)(1) and 7620.
Sec. 51.112 [Amended]
2. In Sec. 51.112, paragraphs (a)(1) and (a)(2) are amended by
revising ``and supplement B (1993)'' to read ``, supplement B (1993)
and supplement C (1995)''.
Sec. 51.160 [Amended]
3. In Sec. 51.160, paragraphs (f)(1) and (f)(2) are amended by
revising ``and supplement B (1993)'' to read ``, supplement B (1993)
and supplement C (1995)''.
Sec. 51.166 [Amended]
4. In Sec. 51.166, paragraphs (l)(1) and (l)(2) are amended by
revising ``and supplement B (1993)'' to read ``, supplement B (1993)
and supplement C (1995)''.
5. Appendix W to part 51, section 4.2.1 is amended by removing
``SCREEN2, is available.19, 20'' in the last sentence of the first
paragraph and adding ``SCREEN2, is available.19, 20 For the
current version of SCREEN, see reference 20.''
6. Appendix W to part 51, section 4.2.2 is amended by revising
Table 4-1 to read as follows:
Appendix W to Part 51--Guideline on Air Quality Models
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Table 4-1.--Preferred Models for Selected Applications in Simple Terrain
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Land use Model 1
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Short Term (i.e., 1-24
hours):
Single Source........ Rural.................. CRSTER
Urban.................. RAM
Multiple Source...... Rural.................. MPTER
Urban.................. RAM
Complicated Sources 2 Rural/Urban............ ISCST 3
Buoyant Industrial Rural.................. BLP
Line Sources.
Long Term (i.e.,
monthly, seasonal or
annual):
Single Source........ Rural.................. CRSTER
Urban.................. RAM
Multiple Source...... Rural.................. MPTER
Urban.................. CDM 2.0 or RAM 4
Complicated Sources 2 Rural/Urban............ ISCLT3
Buoyant Industrial Rural.................. BLP
Line Sources.
* * * * *
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\1\ The models as listed here reflect the applications for which they
were originally intended. Several of these models have been adapted to
contain options which allow them to be interchanged. For example,
ISCST could be substituted for ISCLT. Similarly, for a point source
application, ISCST with urban option can be substituted for RAM. Where
a substitution is convenient to the user and equivalent estimates are
assured, it may be made.
\2\ Complicated sources are those with special problems such as
aerodynamic downwash, particle deposition, volume and area sources,
etc.
\3\ For the current version of ISC, see reference 58 and note the model
description provided in Appendix A of this document.
\4\ If only a few sources in an urban area are to be modeled, RAM should
be used.
* * * * *
7. Appendix W to Part 51, section 5.2.1.1 is amended by removing
``SCREEN2'' in the third paragraph and by adding ``SCREEN''.
8. Appendix W to Part 51, section 6.2.3 is revised to read as
follows:
Appendix W to Part 51--Guideline on Air Quality Models
* * * * *
6.2.3 Models for Nitrogen Dioxide (Annual Average)
a. A tiered screening approach is recommended to obtain annual
average estimates of NO2 from point sources for New Source Review
analysis, including PSD, and for SIP planning purposes. This multi-
tiered approach is conceptually shown in Figure 6-1 below:
[[Page 40469]]
Figure 6-1.--Multi-Tiered Screening Approach for Estimating Annual NO2
Concentrations From Point Sources
Tier 1:
Assume Total Conversion of NO to NO2
Tier 2:
Multiply Annual NOX Estimate by Empirically Derived NO2 / NOX Ratio
b. For Tier 1 (the initial screen), use an appropriate Gaussian
model from appendix A to estimate the maximum annual average
concentration and assume a total conversion of NO to NO2. If the
concentration exceeds the NAAQS and/or PSD increments for NO2,
proceed to the 2nd level screen.
c. For Tier 2 (2nd level) screening analysis, multiply the Tier 1
estimate(s) by an empirically derived NO2 / NOX value of 0.75
(annual national default).\36\ An annual NO2 / NOX ratio
differing from 0.75 may be used if it can be shown that such a ratio is
based on data likely to be representative of the location(s) where
maximum annual impact from the individual source under review occurs.
In the case where several sources contribute to consumption of a PSD
increment, a locally derived annual NO2 / NOX ratio should
also be shown to be representative of the location where the maximum
collective impact from the new plus existing sources occurs.
d. In urban areas, a proportional model may be used as a
preliminary assessment to evaluate control strategies to meet the NAAQS
for multiple minor sources, i.e. minor point, area and mobile sources
of NOX; concentrations resulting from major point sources should
be estimated separately as discussed above, then added to the impact of
the minor sources. An acceptable screening technique for urban
complexes is to assume that all NOX is emitted in the form of
NO2 and to use a model from appendix A for nonreactive pollutants
to estimate NO2 concentrations. A more accurate estimate can be
obtained by: (1) calculating the annual average concentrations of
NOX with an urban model, and (2) converting these estimates to
NO2 concentrations using an empirically derived annual NO2 /
NOX ratio. A value of 0.75 is recommended for this ratio. However,
a spatially averaged annual NO2 / NOX ratio may be determined
from an existing air quality monitoring network and used in lieu of the
0.75 value if it is determined to be representative of prevailing
ratios in the urban area by the reviewing agency. To ensure use of
appropriate locally derived annual NO2 / NOX ratios,
monitoring data under consideration should be limited to those
collected at monitors meeting siting criteria defined in 40 CFR part
58, appendix D as representative of ``neighborhood'', ``urban'', or
``regional'' scales.
Furthermore, the highest annual spatially averaged NO2 /
NOX ratio from the most recent 3 years of complete data should be
used to foster conservatism in estimated impacts.
e. To demonstrate compliance with NO2 PSD increments in urban
areas, emissions from major and minor sources should be included in the
modeling analysis. Point and area source emissions should be modeled as
discussed above. If mobile source emissions do not contribute to
localized areas of high ambient NO2 concentrations, they should be
modeled as area sources. When modeled as area sources, mobile source
emissions should be assumed uniform over the entire highway link and
allocated to each area source grid square based on the portion of
highway link within each grid square. If localized areas of high
concentrations are likely, then mobile sources should be modeled as
line sources with the preferred model ISCLT2.
f. More refined techniques to handle special circumstances may be
considered on a case-by-case basis and agreement with the reviewing
authority should be obtained. Such techniques should consider
individual quantities of NO and NO2 emissions, atmospheric
transport and dispersion, and atmospheric transformation of NO to
NO2. Where they are available, site-specific data on the
conversion of NO to NO2 may be used. Photochemical dispersion
models, if used for other pollutants in the area, may also be applied
to the NOX problem.
* * * * *
9. Appendix W to part 51, section 7.1 is amended by removing
``ISC2'' in the fourth paragraph and by adding ``ISC''.
10. Appendix W to part 51, section 7.2.2 is amended by removing
``ISC2'' in the third paragraph and by adding ``ISC''.
11. Appendix W to part 51, section 7.2.5 is amended by removing
``ISC2'' in the second paragraph and by adding ``ISC''.
12. Appendix W to part 51, section 7.2.8 is amended by removing
``ISC2'' in the second paragraph and by adding ``ISC''.
13. Appendix W to part 51, section 8.2.5 is amended by removing
``ISC2'' in the second paragraph and by adding ``ISC''.
14. Appendix W to part 51, section 8.2.7 is amended by removing
``total suspended particulate'' in the first paragraph and by adding
``particle''.
15. Appendix W to part 51, section 8.2.7 is amended by removing
``At least one'' in the second paragraph and by adding ``One''.
16. Appendix W to part 51, section 9.3.3.2, is revised to read as
follows:
* * * * *
9.3.3.2 Recommendations.
a. Site-specific Data Collection. The document ``On-Site
Meteorological Program Guidance for Regulatory Modeling Applications''
\66\ provides recommendations on the collection and use of on-site
meteorological data. Recommendations on characteristics, siting, and
exposure of meteorological instruments and on data recording,
processing, completeness requirements, reporting, and archiving are
also included. This publication should be used as a supplement to the
limited guidance on these subjects now found in the ``Ambient
Monitoring Guidelines for Prevention of Significant
Deterioration''.\63\ Detailed information on quality assurance is
provided in the ``Quality Assurance Handbook for Air Pollution
Measurement Systems: Volume IV''.\67\ As a minimum, site-specific
measurements of ambient air temperature, transport wind speed and
direction, and the parameters to determine Pasquill-Gifford (P-G)
stability categories should be available in meteorological data sets to
be used in modeling. Care should be taken to ensure that meteorological
instruments are located to provide representative characterization of
pollutant transport between sources and receptors of interest. The
Regional Office will determine the appropriateness of the measurement
locations.
b. All site-specific data should be reduced to hourly averages.
Table 9-3 lists the wind related parameters and the averaging time
requirements.
c. Solar Radiation Measurements. Total solar radiation should be
measured with a reliable pyranometer, sited and operated in accordance
with established on-site meteorological guidance.\66\
d. Temperature Measurements. Temperature measurements should be
made at standard shelter height (2m) in accordance with established on-
site meteorological guidance.\66\
e. Temperature Difference Measurements. Temperature difference
(T) measurements for use in estimating P-G stability
categories using the SRDT methodology (see Stability Categories) should
be obtained using two matched
[[Page 40470]]
thermometers or a reliable thermocouple system to achieve adequate
accuracy.
f. Siting, probe placement, and operation of T systems
should be based on guidance found in Chapter 3 of reference 66, and
such guidance should be followed when obtaining vertical temperature
gradient data for use in plume rise estimates or in determining the
critical dividing streamline height.
g. Wind Measurements. For refined modeling applications in simple
terrain situations, if a source has a stack below 100m, select the
stack top height as the wind measurement height for characterization of
plume dilution and transport. For sources with stacks extending above
100m, a 100m tower is suggested unless the stack top is significantly
above 100m (i.e., 200m). In cases with stack tops
200m, remote sensing may be a feasible alternative. In some
cases, collection of stack top wind speed may be impractical or
incompatible with the input requirements of the model to be used. In
such cases, the Regional Office should be consulted to determine the
appropriate measurement height.
h. For refined modeling applications in complex terrain, multiple
level (typically three or more) measurements of wind speed and
direction, temperature and turbulence (wind fluctuation statistics) are
required. Such measurements should be obtained up to the representative
plume height(s) of interest (i.e., the plume height(s) under those
conditions important to the determination of the design concentration).
The representative plume height(s) of interest should be determined
using an appropriate complex terrain screening procedure (e.g.,
CTSCREEN) and should be documented in the monitoring/modeling protocol.
The necessary meteorological measurements should be obtained from an
appropriately sited meteorological tower augmented by SODAR if the
representative plume height(s) of interest exceed 100m. The
meteorological tower need not exceed the lesser of the representative
plume height of interest (the highest plume height if there is more
than one plume height of interest) or 100m.
i. In general, the wind speed used in determining plume height is
defined as the wind speed at stack top.
j. Specifications for wind measuring instruments and systems are
contained in the ``On-Site Meteorological Program Guidance for
Regulatory Modeling Applications''.\66\
k. Stability Categories. The P-G stability categories, as
originally defined, couple near-surface measurements of wind speed with
subjectively determined insolation assessments based on hourly cloud
cover and ceiling height observations. The wind speed measurements are
made at or near 10m. The insolation rate is typically assessed using
observations of cloud cover and ceiling height based on criteria
outlined by Turner.\50\ It is recommended that the P-G stability
category be estimated using the Turner method with site-specific wind
speed measured at or near 10m and representative cloud cover and
ceiling height. Implementation of the Turner method, as well as
considerations in determining representativeness of cloud cover and
ceiling height in cases for which site-specific cloud observations are
unavailable, may be found in section 6 of reference 66. In the absence
of requisite data to implement the Turner method, the SRDT method or
wind fluctuation statistics (i.e., the E and
A methods) may be used.
l. The SRDT method, described in section 6.4.4.2 of reference 66,
is modified slightly from that published by Bowen et al. (1983) \136\
and has been evaluated with three on-site data bases.\137\ The two
methods of stability classification which use wind fluctuation
statistics, the E and A methods, are also
described in detail in section 6.4.4 of reference 66 (note applicable
tables in section 6). For additional information on the wind
fluctuation methods, see references 68-72.
m. Hours in the record having missing data should be treated
according to an established data substitution protocol and after valid
data retrieval requirements have been met. Such protocols are usually
part of the approved monitoring program plan. Data substitution
guidance is provided in section 5.3 of reference 66.
n. Meteorological Data Processors. The following meteorological
preprocessors are recommended by EPA: RAMMET, PCRAMMET, STAR, PCSTAR,
MPRM,\135\ and METPRO.\24\ RAMMET is the recommended meteorological
preprocessor for use in applications employing hourly NWS data. The
RAMMET format is the standard data input format used in sequential
Gaussian models recommended by EPA. PCRAMMET \138\ is the PC equivalent
of the mainframe version (RAMMET). STAR is the recommended preprocessor
for use in applications employing joint frequency distributions (wind
direction and wind speed by stability class) based on NWS data. PCSTAR
is the PC equivalent of the mainframe version (STAR). MPRM is the
recommended preprocessor for use in applications employing on-site
meteorological data. The latest version (MPRM 1.3) has been configured
to implement the SRDT method for estimating P-G stability categories.
MPRM is a general purpose meteorological data preprocessor which
supports regulatory models requiring RAMMET formatted data and STAR
formatted data. In addition to on-site data, MPRM provides equivalent
processing of NWS data. METPRO is the required meteorological data
preprocessor for use with CTDMPLUS. All of the above mentioned data
preprocessors are available for downloading from the SCRAM BBS.\19\
* * * * *
17. Appendix W to Part 51, section 12.0, is amended by:
a. Revising references 20, 36, 58 and 90; and
b. Adding references 136 through 138.
The revisions and additions read as follows:
Appendix W to Part 51--Guideline on Air Quality Models
* * * * *
12.0 * * *
* * * * *
20. Environmental Protection Agency, 1995. SCREEN3 User's Guide. EPA
Publication No. EPA-454/B-95-004. U.S. Environmental Protection Agency,
Research Triangle Park, NC. (NTIS No. PB 95-222766)
* * * * *
36. Chu, S. H. and E. L.Meyer, 1991. Use of Ambient Ratios to Estimate
Impact of NOX Sources on Annual NO2 Concentrations.
Proceedings, 84th Annual Meeting & Exhibition of the Air & Waste
Management Association, Vancouver, B.C.; 16-21 June 1991. (16 pp.)
(Docket No. A-92-65, II-A-7)
* * * * *
58. Environmental Protection Agency, 1995. User's Guide for the
Industrial Source Complex (ISC3) Dispersion Models, Volumes 1 and 2.
EPA Publication Nos. EPA-454/B-95-003a & b. U.S. Environmental
Protection Agency, Research Triangle Park, NC. (NTIS Nos. PB-95-222741
and PB 95-222758, respectively)
* * * * *
90. Environmental Research and Technology, 1987. User's Guide to the
Rough Terrain Diffusion Model (RTDM), Rev. 3.20. ERT document No.
PD535-585. Environmental Research and Technology, Inc.,
[[Page 40471]]
Concord, MA (NTIS No. PB 88-171467)
* * * * *
136. Bowen, B.M., J.M. Dewart and A.I. Chen, 1983. Stability Class
Determination: A Comparison for One Site. Proceedings, Sixth Symposium
on Turbulence and Diffusion. American Meteorological Society, Boston,
MA; pp. 211-214. (Docket No. A-92-65, II-A-5)
137. Environmental Protection Agency, 1993. An Evaluation of a Solar
Radiation/Delta-T (SRDT) Method for Estimating Pasquill-Gifford (P-G)
Stability Categories. EPA Publication No. EPA-454/R-93-055. U.S.
Environmental Protection Agency, Research Triangle Park, NC. (NTIS No.
PB 94-113958)
138. Environmental Protection Agency, 1993. PCRAMMET User's Guide. EPA
Publication No. EPA-454/B-93-009. U.S. Environmental Protection Agency,
Research Triangle Park, NC.
18. Appendix A to Appendix W of Part 51, is amended:
a. The Table of Contents is revised by removing ``ISC2'' and by
adding ``ISC3'';
b. Section A.5 is amended by revising the Heading and Reference;
c. Section A.5 Abstract is amended by removing ``ISC2'' and by
adding ``ISC3'';
d. Section A.5.a is amended by removing ``ISC2'' in the first line
and by adding ``ISC3'';
e. Section A.5.b is amended by removing ``ISCST2'' and ``ISCLT2 in
the second paragraph and by adding ``ISCST3'';
f. Section A.5.d is revised;
g. Section A.5.e is amended by removing ``ISC2'' in the first line
and by adding ``ISC3'';
h. Section A.5.f is amended by removing ``ISC2'' in the first line
and by adding ``ISC3'';
i. Section A.5.g is amended by removing ``ISC2'' in the first line
and by adding ``ISC3'';
j. Section A.5.m is revised;
k. Section A.5.n is amended by adding four references in
alphabetical order; and
l. Section A.REF is amended by adding a reference at the end.
The revisions and additions read as follows:
Appendix W to Part 51--Guideline on Air Quality Models
* * * * *
Appendix A to Appendix W of Part 51--Summaries of Preferred Air
Quality Models
* * * * *
A.5 INDUSTRIAL SOURCE COMPLEX MODEL (ISC3)
Reference
Environmental Protection Agency, 1995. User's Guide for the
Industrial Source Complex (ISC3) Dispersion Models, Volumes 1 and 2.
EPA Publication Nos. EPA-454/B-95-003a & b. Environmental Protection
Agency, Research Triangle Park, NC. (NTIS Nos. PB-95-222741 and PB 95-
222758, respectively)
* * * * *
d. Type of Model
ISC3 is a Gaussian plume model. It has been revised to perform a
double integration of the Gaussian plume kernel for area sources.
* * * * *
m. Physical Removal
Dry deposition effects for particles are treated using a resistance
formulation in which the deposition velocity is the sum of the
resistances to pollutant transfer within the surface layer of the
atmosphere, plus a gravitational settling term (EPA, 1994), based on
the modified surface depletion scheme of Horst (1983).
* * * * *
n. Evaluation Studies
* * * * *
Environmental Protection Agency, 1992. Comparison of a Revised Area
Source Algorithm for the Industrial Source Complex Short Term Model and
Wind Tunnel Data. EPA Publication No. EPA-454/R-92-014. U.S.
Environmental Protection Agency, Research Triangle Park, NC. (NTIS No.
PB 93-226751)
Environmental Protection Agency, 1992. Sensitivity Analysis of a
Revised Area Source Algorithm for the Industrial Source Complex Short
Term Model. EPA Publication No. EPA-454/R-92-015. U.S. Environmental
Protection Agency, Research Triangle Park, NC. (NTIS No. PB 93-226769)
Environmental Protection Agency, 1992. Development and Evaluation
of a Revised Area Source Algorithm for the Industrial Source Complex
Long Term Model. EPA Publication No. EPA-454/R-92-016. U.S.
Environmental Protection Agency, Research Triangle Park, NC. (NTIS No.
PB 93-226777)
Environmental Protection Agency, 1994. Development and Testing of a
Dry Deposition Algorithm (Revised). EPA Publication No. EPA-454/R-94-
015. U.S. Environmental Protection Agency, Research Triangle Park, NC.
(NTIS No. PB 94-183100)
* * * * *
A.REF (REFERENCES)
* * * * *
Horst, T. W., 1983. A Correction to the Gaussian Source-depletion
Model. In Precipitation Scavenging, Dry Deposition and Resuspension. H.
R. Pruppacher, R. G. Semonin, and W. G. N. Slinn, eds., Elsevier, NY.
19. Appendix B to appendix W of part 51 is amended by:
a. Adding two entries to the Table of Contents in numerical order;
and
b. Adding sections B.32 and B.33 immediately following section
B.31.
The additions read as follows:
Appendix B to Appendix W of Part 51--Summaries of Alternative Air
Quality Models
Table of Contents
* * * * *
B.32 HGSYSTEM
B.33 SLAB
* * * * *
B.32 HGSYSTEM: Dispersion Models for Ideal Gases and Hydrogen Fluoride
References
Post, L. (ed.), 1994. HGSYSTEM 3.0 Technical Reference Manual. Shell
Research Limited, Thornton Research Centre, Chester, United Kingdom.
(TNER 94.059)
Post, L., 1994. HGSYSTEM 3.0 User's Manual. Shell Research Limited,
Thornton Research Centre, Chester, United Kingdom. (TNER 94.058)
Availability
The PC-DOS version of the HGSYSTEM software (HGSYSTEM: Version 3.0,
Programs for modeling the dispersion of ideal gas and hydrogen fluoride
releases, executable programs and source code can be installed from
floppy diskettes. These diskettes and all documentation are available
as a package from API [(202) 682-8340] or NTIS (see Section B.0).
Technical Contacts
Doug N. Blewitt, AMOCO Corporation, 1670 Broadway / MC 2018, Denver, CO
80201, (303) 830-5312
Howard J. Feldman, American Petroleum Institute, 1220 L Street,
Northwest, Washington, D.C. 20005, (202) 682-8340
Abstract
HGSYSTEM is a PC-based software package consisting of mathematical
models for estimating of one or more consecutive phases between
spillage and near-field and far-field dispersion of a pollutant. The
pollutant can be either
[[Page 40472]]
a two-phase, multi-compound mixture of non-reactive compounds or
hydrogen fluoride (HF) with chemical reactions. The individual models
are:
Database program:
DATAPROP generates physical properties used in other HGSYSTEM
models
Source term models:
SPILL transient liquid release from a pressurized vessel
HFSPILL SPILL version specifically for HF
LPOOL evaporating multi-compound liquid pool model
Near-field dispersion models:
AEROPLUME high-momentum jet dispersion model
HFPLUME AEROPLUME version specifically for HF
HEGABOX dispersion of instantaneous heavy gas releases
Far-field dispersion models:
HEGADAS(S,T) heavy gas dispersion (steady-state and transient
version)
PGPLUME passive Gaussian dispersion
Utility programs:
HFFLASH flashing of HF from pressurized vessel
POSTHS/POSTHT post-processing of HEGADAS(S,T) results
PROFILE post-processor for concentration contours of airborne
plumes
GET2COL utility for data retrieval
The models assume flat, unobstructed terrain. HGSYSTEM can be used
to model steady-state, finite-duration, instantaneous and time
dependent releases, depending on the individual model used. The models
can be run consecutively, with relevant data being passed on from one
model to the next using link files. The models can be run in batch mode
or using an iterative utility program.
a. Recommendations for Regulatory Use
HGSYSTEM can be used as a refined model to estimate short-term
ambient concentrations. For toxic chemical releases (non-reactive
chemicals or hydrogen fluoride; 1-hour or less averaging times) the
expected area of exposure to concentrations above specified threshold
values can be determined. For flammable non-reactive gases it can be
used to determine the area in which the cloud may ignite.
b. Input Requirements
1. HFSPILL input data: reservoir data (temperature, pressure,
volume, HF mass, mass-fraction water), pipe-exit diameter and ambient
pressure.
2. EVAP input data: spill rate, liquid properties, and evaporation
rate (boiling pool) or ambient data (non-boiling pool).
3. HFPLUME and PLUME input data: reservoir characteristics,
pollutant parameters, pipe/release data, ambient conditions, surface
roughness and stability class.
4. HEGADAS input data: ambient conditions, pollutant parameters,
pool data or data at transition point, surface roughness, stability
class and averaging time.
5. PGPLUME input data: link data provided by HFPLUME and the
averaging time.
c. Output
1. The HGSYSTEM models contain three post-processor programs which
can be used to extract modeling results for graphical display by
external software packages. GET2COL can be used to extract data from
the model output files. HSPOST can be used to develop isopleths,
extract any 2 parameters for plotting and correct for finite release
duration. HTPOST can be used to produce time history plots.
2. HFSPILL output data: reservoir mass, spill rate, and other
reservoir variables as a function of time. For HF liquid, HFSPILL
generates link data to HFPLUME for the initial phase of choked liquid
flow (flashing jet), and link data to EVAP for the subsequent phase of
unchoked liquid flow (evaporating liquid pool).
3. EVAP output data: pool dimensions, pool evaporation rate, pool
mass and other pool variables for steady state conditions or as a
function of time. EVAP generates link data to the dispersion model
HEGADAS (pool dimensions and pool evaporation rate).
4. HFPLUME and PLUME output data: plume variables (concentration,
width, centroid height, temperature, velocity, etc.) as a function of
downwind distance.
5. HEGADAS output data: concentration variables and temperature as
a function of downwind distance and (for transient case) time.
6. PGPLUME output data: concentration as a function of downwind
distance, cross-wind distance and height.
d. Type of Model
HGSYSTEM is made up of four types of dispersion models. HFPLUME and
PLUME simulate the near-field dispersion and PGPLUME simulates the
passive-gas dispersion downwind of a transition point. HEGADAS
simulates the ground-level heavy-gas dispersion.
e. Pollutant Types
HGSYSTEM may be used to model non-reactive chemicals or hydrogen
fluoride.
f. Source-Receptor Relationships
HGSYSTEM estimates the expected area of exposure to concentrations
above user-specified threshold values. By imposing conservation of
mass, momentum and energy the concentration, density, speed and
temperature are evaluated as a function of downwind distance.
g. Plume Behavior
1. HFPLUME and PLUME: (1) are steady-state models assuming a top-
hat profile with cross-section averaged plume variables; and (2) the
momentum equation is taken into account for horizontal ambient shear,
gravity, ground collision, gravity-slumping pressure forces and ground-
surface drag.
2. HEGADAS: assumes the heavy cloud to move with the ambient wind
speed, and adopts a power-law fit of the ambient wind speed for the
velocity profile.
3. PGPLUME: simulates the passive-gas dispersion downwind of a
transition point from HFPLUME or PLUME for steady-state and finite
duration releases.
h. Horizontal Winds
A power law fit of the ambient wind speed is used.
i. Vertical Wind Speed
Not treated.
j. Horizontal Dispersion
1. HFPLUME and PLUME: Plume dilution is caused by air entrainment
resulting from high plume speeds, trailing vortices in wake of falling
plume (before touchdown), ambient turbulence and density
stratification. Plume dispersion is assumed to be steady and momentum-
dominated, and effects of downwind diffusion and wind meander
(averaging time) are not taken into account.
2. HEGADAS: This model adopts a concentration similarity profile
expressed in terms of an unknown center-line ground-level concentration
and unknown vertical/cross-wind dispersion parameters. These quantities
are determined from a number of basic equations describing gas-mass
conservation, air entrainment (empirical law describing vertical top-
entrainment in terms of global Richardson number), cross-wind gravity
spreading (initial gravity spreading followed by gravity-current
collapse) and cross-wind diffusion (Briggs formula).
[[Page 40473]]
3. PGPLUME: It assumes a Gaussian concentration profile in which
the cross-wind and vertical dispersion coefficients are determined by
empirical expressions. All unknown parameters in this profile are
determined by imposing appropriate matching criteria at the transition
point.
k. Vertical Dispersion
See description above.
l. Chemical Transformation
Not treated.
m. Physical Removal
Not treated.
n. Evaluation Studies
1. PLUME has been validated against field data for releases of
liquified propane, and wind tunnel data for buoyant and vertically-
released dense plumes. HFPLUME and PLUME have been validated against
field data for releases of HF (Goldfish experiments) and propane
releases. In addition, the plume rise algorithms have been tested
against Hoot, Meroney, and Peterka, Ooms and Petersen databases.
HEGADAS has been validated against steady and transient releases of
liquid propane and LNG over water (Maplin Sands field data), steady and
finite-duration pressurized releases of HF (Goldfish experiments;
linked with HFPLUME), instantaneous release of Freon (Thorney Island
field data; linked with the box model HEGABOX) and wind tunnel data for
steady, isothermal dispersion.
2. Validation studies are contained in the following references:
McFarlane, K., Prothero, A., Puttock, J.S., Roberts, P.T. and Witlox,
H.W.M., 1990. Development and validation of atmospheric dispersion
models for ideal gases and hydrogen fluoride, Part I: Technical
Reference Manual. Report TNER.90.015. Thornton Research Centre, Shell
Research, Chester, England. [EGG 1067-1151] (NTIS No. DE 93-000953)
Witlox, H.W.M., McFarlane, K., Rees, F.J., and Puttock, J.S., 1990.
Development and validation of atmospheric dispersion models for ideal
gases and hydrogen fluoride, Part II: HGSYSTEM Program User's Manual.
Report TNER.90.016. Thornton Research Centre, Shell Research, Chester,
England. [EGG 1067-1152] (NTIS No. DE 93-000954)
B.33 SLAB
Reference
Ermak, D.L., 1990. User's Manual for SLAB: An Atmospheric
Dispersion Model for Denser-than-Air Releases (UCRL-MA-105607),
Lawrence Livermore National Laboratory.
Availability
1. The computer code is available on the Support Center for
Regulatory Air Models Bulletin Board System (Upload/Download Area; see
page B-1), and can also be obtained from: Energy Science and Technology
Center, P.O. Box 1020, Oak Ridge, TN 37830, (615) 576-2606.
2. The User's Manual (NTIS No. DE 91-008443) can be obtained from:
Computer Products, National Technical Information Service, U.S.
Department of Commerce, Springfield, VA 22161, (703) 487-4650.
Abstract
The SLAB model is a computer model, PC-based, that simulates the
atmospheric dispersion of denser-than-air releases. The types of
releases treated by the model include a ground-level evaporating pool,
an elevated horizontal jet, a stack or elevated vertical jet and an
instantaneous volume source. All sources except the evaporating pool
may be characterized as aerosols. Only one type of release can be
processed in any individual simulation. Also, the model simulates only
one set of meteorological conditions; therefore direct application of
the model over time periods longer than one or two hours is not
recommended.
a. Recommendations for Use
The SLAB model should be used as a refined model to estimate
spatial and temporal distribution of short-term ambient concentration
(e.g., 1-hour or less averaging times) and the expected area of
exposure to concentrations above specified threshold values for toxic
chemical releases where the release is suspected to be denser than the
ambient air.
b. Input Requirements
1. The SLAB model is executed in the batch mode. Data are input
directly from an external input file. There are 29 input parameters
required to run each simulation. These parameters are divided into 5
categories by the user's guide: source type, source properties, spill
properties, field properties, and meteorological parameters. The model
is not designed to accept real-time meteorological data or convert
units of input values. Chemical property data are not available within
the model and must be input by the user. Some chemical and physical
property data are available in the user's guide.
2. Source type is chosen as one of the following: evaporating pool
release, horizontal jet release, vertical jet or stack release, or
instantaneous or short duration evaporating pool release.
3. Source property data requirements are physical and chemical
properties (molecular weight, vapor heat capacity at constant pressure;
boiling point; latent heat of vaporization; liquid heat capacity;
liquid density; saturation pressure constants), and initial liquid mass
fraction in the release.
4. Spill properties include: source temperature, emission rate,
source dimensions, instantaneous source mass, release duration, and
elevation above ground level.
5. Required field properties are: desired concentration averaging
time, maximum downwind distance (to stop the calculation), and four
separate heights at which the concentration calculations are to be
made.
6. Meteorological parameter requirements are: ambient measurement
height, ambient wind speed at designated ambient measurement height,
ambient temperature, surface roughness, relative humidity, atmospheric
stability class, and inverse Monin-Obukhov length (optional, only used
as an input parameter when stability class is unknown).
c. Output
1. No graphical output is generated by the current version of this
program. The output print file is automatically saved and must be sent
to the appropriate printer by the user after program execution. Printed
output includes in tabular form:
2. Listing of model input data;
3. Instantaneous spatially-averaged cloud parameters--time,
downwind distance, magnitude of peak concentration, cloud dimensions
(including length for puff-type simulations), volume (or mole) and mass
fractions, downwind velocity, vapor mass fraction, density,
temperature, cloud velocity, vapor fraction, water content, gravity
flow velocities, and entrainment velocities;
4. Time-averaged cloud parameters--parameters which may be used
externally to calculate time-averaged concentrations at any location
within the simulation domain (tabulated as functions of downwind
distance);
5. Time-averaged concentration values at plume centerline and at
five off-centerline distances (off-centerline distances are multiples
of the effective cloud half-width, which varies as a function of
downwind distance) at four user-specified heights and at the height of
the plume centerline.
[[Page 40474]]
d. Type of Model
As described by Ermak (1989), transport and dispersion are
calculated by solving the conservation equations for mass, species,
energy, and momentum, with the cloud being modeled as either a steady-
state plume, a transient puff, or a combination of both, depending on
the duration of the release. In the steady-state plume mode, the
crosswind-averaged conservation equations are solved and all variables
depend only on the downwind distance. In the transient puff mode, the
volume-averaged conservation equations are solved, and all variables
depend only on the downwind travel time of the puff center of mass.
Time is related to downwind distance by the height-averaged ambient
wind speed. The basic conservation equations are solved via a numerical
integration scheme in space and time.
e. Pollutant Types
Pollutants are assumed to be non-reactive and non-depositing dense
gases or liquid-vapor mixtures (aerosols). Surface heat transfer and
water vapor flux are also included in the model.
f. Source-Receptor Relationships
1. Only one source can be modeled at a time.
2. There is no limitation to the number of receptors; the downwind
receptor distances are internally-calculated by the model. The SLAB
calculation is carried out up to the user-specified maximum downwind
distance.
3. The model contains submodels for the source characterization of
evaporating pools, elevated vertical or horizontal jets, and
instantaneous volume sources.
g. Plume Behavior
Plume trajectory and dispersion is based on crosswind-averaged
mass, species, energy, and momentum balance equations. Surrounding
terrain is assumed to be flat and of uniform surface roughness. No
obstacle or building effects are taken into account.
h. Horizontal Winds
A power law approximation of the logarithmic velocity profile which
accounts for stability and surface roughness is used.
i. Vertical Wind Speed
Not treated.
j. Vertical Dispersion
The crosswind dispersion parameters are calculated from formulas
reported by Morgan et al. (1983), which are based on experimental data
from several sources. The formulas account for entrainment due to
atmospheric turbulence, surface friction, thermal convection due to
ground heating, differential motion between the air and the cloud, and
damping due to stable density stratification within the cloud.
k. Horizontal Dispersion
The horizontal dispersion parameters are calculated from formulas
similar to those described for vertical dispersion, also from the work
of Morgan, et al. (1983).
l. Chemical Transformation
The thermodynamics of the mixing of the dense gas or aerosol with
ambient air (including water vapor) are treated. The relationship
between the vapor and liquid fractions within the cloud is treated
using the local thermodynamic equilibrium approximation. Reactions of
released chemicals with water or ambient air are not treated.
m. Physical Removal
Not treated.
n. Evaluation Studies
Blewitt, D.N., J.F. Yohn, and D.L. Ermak, 1987. An Evaluation of
SLAB and DEGADIS Heavy Gas Dispersion Models Using the HF Spill Test
Data, Proceedings, AIChE International Conference on Vapor Cloud
Modeling, Boston, MA, November, pp. 56-80.
Ermak, D.L., S.T. Chan, D.L. Morgan, and L.K. Morris, 1982. A
Comparison of Dense Gas Dispersion Model Simulations with Burro Series
LNG Spill Test Results, J. Haz. Matls., 6: 129-160.
Zapert, J.G., R.J. Londergan, and H. Thistle, 1991. Evaluation of
Dense Gas Simulation Models. EPA Publication No. EPA-450/4-90-018. U.S.
Environmental Protection Agency, Research Triangle Park, NC.
PART 52--APPROVAL AND PROMULGATION OF IMPLEMENTATION PLANS
1. The authority citation for part 52 continues to read as follows:
Authority: 42 U.S.C. 7401-7671q.
Sec. 52.21 [Amended]
2. In Sec. 52.21, paragraphs (l)(1) and (l)(2) are amended by
revising ``and supplement B (1993)'' to read ``, supplement B (1993)
and supplement C (1994)''.
[FR Doc. 95-19057 Filed 8-8-95; 8:45 am]
BILLING CODE 6560-50-P