Requirements for Preparation, Adoption, and Submittal of Implementation Plans

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., m 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

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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