[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

* * * * *

Table 4-1.--Preferred Models for Selected Applications in Simple Terrain
------------------------------------------------------------------------
                                 Land use                 Model 1       
------------------------------------------------------------------------
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.                                                        
                                                                        
                  *        *        *        *        *                 
------------------------------------------------------------------------
\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