[Federal Register Volume 68, Number 173 (Monday, September 8, 2003)]
[Notices]
[Pages 52934-52935]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 03-22766]



[[Page 52934]]

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ENVIRONMENTAL PROTECTION AGENCY

[FRL-7554-8]
RIN 2060-AF01


Availability of Additional Documents Relevant to Anticipated 
Revisions to Guideline on Air Quality Models Addressing a Preferred 
General Purpose (Flat and Complex Terrain) Dispersion Model and Other 
Revisions

AGENCY: Environmental Protection Agency (EPA).

ACTION: Notice of data availability.

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SUMMARY: We are providing notice that additional information in the 
form of two documents relevant to revisions of the Guideline on Air 
Quality Models--hereafter, the Guideline--have been placed in Docket 
No. A-99-05. The revisions would enhance the Guideline by incorporating 
a new, general purpose dispersion model called the AMS/EPA Regulatory 
MODel (AERMOD) to replace the existing Industrial Source Complex (ISC3) 
model in many air quality assessments and incorporate a new downwash 
algorithm--PRIME. An earlier version of AERMOD was proposed, and we 
have considered recommendations made both in public comment on that 
proposal and by beta testers of the model's computer code. The two 
documents discussed today provide information on the performance of 
AERMOD when the model is modified in a manner suggested by public 
comment. We invite comment on these documents.

DATES: Comments must be in writing and either postmarked or received at 
the address below by October 8, 2003.

ADDRESSES: Copies of both documents have been placed in Docket No. A-
99-05. These new documents are available for inspection at the EPA 
Docket Center, (EPA/DC) EPA West (MC 6102T), 1301 Constitution Ave., 
NW., Washington, DC. The EPA Docket Center Public Reading Room (B102) 
is open from 8:30 a.m. to 4:30 p.m., Monday through Friday, excluding 
legal holidays. The telephone number for the Air Docket is (202) 566-
1742.

FOR FURTHER INFORMATION CONTACT: Joseph A. Tikvart, Leader, Air Quality 
Modeling Group (D243-01), Office of Air Quality Planning and Standards, 
U.S. Environmental Protection Agency, Research Triangle Park, NC 27711; 
telephone (919) 541-5562.

SUPPLEMENTARY INFORMATION: We have placed the two documents described 
below in Docket No. A-99-05:
    1. USEPA, ``AERMOD: Latest Features and Evaluation Results.'' 
Office of Air Quality Planning and Standards, Research Triangle Park, 
NC 27711; EPA Report No. EPA-454/R-03-003, July 2003.
    2. USEPA, ``Comparison of Regulatory Design Concentrations: AERMOD 
vs. ISCST3, CTDMPLUS, ISD-PRIME.'' Office of Air Quality Planning and 
Standards, Research Triangle Park, NC 27711; EPA Report No. EPA-454/R-
03-002, July 2003.
    These reports are also available on our modeling Web site (http://www.epa.gov/scram001) and provide technical details on AERMOD revisions 
since it was proposed in the Federal Register (65 FR 21506) on April 
21, 2000. On April 15, 2003 (68 FR 18440), we promulgated proposed 
changes and additions to the Guideline (Appendix W to 40 CFR part 51) 
that were supported by public comments and that we deemed ready to 
finalize. Components of the proposal that we did not act on include: 
(1) Adopting AERMOD to replace ISCST3 in many assessments, (2) revising 
ISCST3 by incorporating a new downwash algorithm (PRIME) and renaming 
the model ISC-PRIME, and (3) updating the Emissions and Dispersion 
Modeling System (EDMS 3.1) in appendix A of the Guideline.
    Nearly every commenter on the April 2000 proposal urged us to 
integrate the aerodynamic downwash PRIME algorithm into AERMOD (i.e., 
not to require two models for some analyses), and no comments were 
received which contradicted these requests. In response to our request 
that this comment be addressed, AERMIC (the American Meteorological 
Society (AMS)/EPA Regulatory Model Improvement Committee) successfully 
revised AERMOD (version 02222), incorporating the PRIME algorithm and 
making other incidental modifications to respond to public comments and 
issues identified by beta testers of the code. Documentation of AERMOD 
(02222) and its computer code has since been available on our Web site 
(http://www.epa.gov/scram001/tt26.htm#aermod).
    Also proposed in April 2000 was an EDMS upgrade to version 3.1. 
Since that proposal, the model developer--Federal Aviation 
Administration (FAA)--decided to further upgrade EDMS to incorporate 
AERMOD in a version 4.0. Performance evaluation and adequate 
documentation was requested in public comments (A-99-05), and in our 
April 15, 2003, notice we said that this new information would be 
forthcoming. Recently, however, FAA has decided to withdraw EDMS from 
the Guideline's appendix A. No new information is therefore provided in 
this action; we support this removal from appendix A and will address 
the details more fully in a future promulgation of the Guideline.
    The most significant changes made to AERMOD in response to public 
comments include the following:
    [sbull] addition of the PRIME downwash algorithms;
    [sbull] modifications of the complex terrain algorithms to make 
AERMOD less sensitive to the selection of the domain of the study area;
    [sbull] modification of (a) urban dispersion for low-level emission 
sources, such as area sources, to produce more realistic urban 
dispersion and (b) minimum mixing layer depths used to calculate the 
effective dispersion parameters for all dispersion settings;
    [sbull] addition of plume meander to all stable and unstable 
conditions; and
    [sbull] upgrades of AERMOD to include all the newest features that 
exist in the latest version of ISCST3 such as FORTRAN 90 compliance, 
allocatable arrays, EVENTS processing, and the TOXICS option.
    The effect of these changes is now documented in the two reports 
cited above.
    The performance analysis of model accuracy is summarized in: 
``AERMOD: Latest Features and Evaluation Results.'' Office of Air 
Quality Planning and Standards, Research Triangle Park, NC 27711, EPA 
Report No. EPA-454/R-03-003, July 2003. That analysis provides 
comparisons of model estimates with measured air quality concentrations 
for a variety of source types and locations. Based on this analysis, we 
have concluded that (1) the performance of the revised version of 
AERMOD (02222) is slightly better than the April 2000 proposal and both 
versions of AERMOD significantly outperform ISCST3 and (2) AERMOD 
(02222) with PRIME performs slightly better than ISC-PRIME for 
aerodynamic downwash cases.
    The consequence analysis of effects on design concentrations is 
summarized in: ``Comparison of Regulatory Design Concentrations: AERMOD 
vs. ISCST3, CTDMPLUS, ISD-PRIME.'' Office of Air Quality Planning and 
Standards, Research Triangle Park, NC 27711, EPA Report No. EPA-454/R-
03-002, July 2003. That analysis provides comparisons of design 
concentrations (on which emission control limits might be based) for a 
wide variety of source configurations and settings. The analysis 
indicates that:
    [sbull] for non-downwash settings, the revised version of AERMOD 
(02222), on average, tends to predict concentrations closer to ISCST3 
with somewhat smaller

[[Page 52935]]

variations than the April 2000 proposal of AERMOD;
    [sbull] where downwash is a significant factor in the air 
dispersion analysis, the revised version of AERMOD predicts maximum 
concentrations that are very similar to ISC-PRIME;
    [sbull] for those source scenarios where maximum 1-hour cavity 
concentrations are calculated, the average AERMOD predicted cavity 
concentration tends to be about the same as the average ISC-PRIME 
cavity concentrations; and
    [sbull] in general, the consequences of using the revised AERMOD, 
instead of the older model ISCST3, in complex terrain remained 
essentially unchanged, although they varied in individual 
circumstances.
    Based on evaluations of the revisions described above, it appears 
that the modified AERMOD is ready to be incorporated into the 
Guideline, and we intend to promulgate the modified AERMOD (02222). 
This Notice of Data Availability concerning performance studies of the 
modified model is being provided to inform the public about the model 
performance and range of impacts which the improved version of AERMOD 
could have on estimated air quality concentrations. We invite public 
comment on the new studies (see DATES). Comments on the documents 
noticed today should be sent to the Docket Office (see ADDRESSES), and 
should clearly reference this Notice of Data Availability and Docket 
No. A-99-05.

    Dated: August 26, 2003.
Henry C. Thomas,
Acting Director, Office of Air Quality Planning and Standards.
[FR Doc. 03-22766 Filed 9-5-03; 8:45 am]
BILLING CODE 6560-50-P