MEMORANDUM 

UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

RESEARCH TRIANGLE PARK, NC 27711 

APR 11 2Un OFFICE OF 

AIR QUALITY PLANNING 

AND STANDARDS 

SUBJECT: AERSCREEN Released as the EPA Recommended Screening Model 

FROM: Tyler Fox, Lead~ r~ Air Quality Modeling Group, C439-0 1 

TO: EPA Regional Modeling Contacts 

INTRODUCTION 

In August 2010, EPA released a beta version of AERSCREEN with draft user's guide 

and test cases, taking public comment until September 30, 2010. These comments ranged from 

"bug" fixes to suggested AERSCREEN enhancements. After incorporating "bug" fixes and user 

comments, EPA released version 11060 of AERSCREEN on March 11, 2010 with a subsequent 

update, version 11076, on March 17, 2010. Version 11076 corrected an error found in version 

11060. The release package includes AERSCREEN (Fortran source code and executable), a 

User's Guide, the MAKEMET meteorological data generator, and AERSCREEN test cases. 

AERSCREEN uses the AERMOD executable, ensuring consistency with the refined model, and 

also utilizes the BPIPPRM building processor and AERMAP terrain preprocessor as needed to 

account for building downwash and terrain effects. AERSCREEN can be found on the Support 

Center for Regulatory Atmospheric Modeling (SCRAM) website: 

http:/ /www.epa.gov/ttn/scram/dispersion _ screening.htm#aerscreen 

RECOMMENDATION OF AERSCREEN AS SCREENING MODEL 

The recommended simple terrain screening model in The Guideline on Air Quality 

Models (Guideline, published as Appendix W to 40 CFR Part 51) has been SCREEN3. 

However, AERSCREEN (the single source screening version of AERMOD) is now available as 

a full release or non-beta version. This memorandum clarifies the replacement of SCREEN3 with 

AERSCREEN as the recommended screening model. 

With respect to AERSCREEN replacing SCREEN3, the preamble of the 2005 rule 

promulgating AERMOD as the preferred Guideline model for a wide range of regulatory 

applications in all types of terrain states (See 70 FRat 68221 ): 

Recycled/Recyclable • Printed wfth Vegetable Oil Based Inks on 1 00".4 Recycled Paper (40% Postconsumer) 
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“With respect to a screening version of AERMOD, a tool called AERSCREEN is being 

developed with a beta version expected to be publicly available in Fall 2005. SCREEN3 

is the current screening model in the Guideline, and since SCREEN3 has been 

successfully applied for a number of years, we believe that SCREEN3 produces an 

acceptable degree of conservatism for regulatory applications and may be used until 

AERSCREEN or a similar technique becomes available and tested for general 

application.” 

This language clearly implies that AERSCREEN will become the recommended screening 

model once it is released. In addition, since AERSCREEN is the screening version of 

AERMOD, EPA’s preferred model for near-field dispersion, it follows that AERSCREEN would 

become the recommended screening model once available. The SCREEN3 model is essentially 

a screening version of the ISCST3 model, which was replaced by AERMOD, and is subject to 

the same limitations as ISCST3. 

Similar to SCREEN3, AERSCREEN allows for user entry of emission inputs, source 

coordinates, building information (for downwash), receptor information, and meteorological 

information in a quick and easy fashion, either through an input file, or interactive prompts. 

However, AERSCREEN incorporates several enhancements relative to the SCREEN3 model. 

For example, AERSCREEN generates application-specific worst-case meteorology, via 

MAKEMET, that takes full advantage of the boundary layer scaling algorithms implemented in 

the AERMET meteorological processor using representative minimum and maximum ambient 

air temperatures, and site-specific surface characteristics (albedo, Bowen ratio, and surface 

roughness). AERSCREEN incorporates the PRIME downwash algorithms that are part of the 

AERMOD refined model and utilizes the BPIPPRIM tool to provide a detailed analysis of 

downwash influences on a direction-specific basis. AERSCREEN also incorporates 

AERMOD’s complex terrain algorithms and utilizes the AERMAP terrain processor to account 

for the actual terrain in the vicinity of the source on a direction-specific basis. 

The question has also arisen about the role of screening modeling and refined dispersion 

modeling under Appendix W. Section 2.2 of the Guideline, explains that: 

“[t]he purpose of such [screening] techniques is to eliminate the need of more detailed 

modeling for those sources that clearly will not cause or contribute to ambient 

concentrations in excess of either the National Ambient Air Quality Standards (NAAQS) 

or the allowable prevention of significant deterioration (PSD) concentration increments. 

If a screening technique indicates that the concentration contributed by the source 

exceeds the PSD increment or the increment remaining to just meet the NAAQS, then the 

second level of more sophisticated models should be applied.” 

In recent years, the use of screening models has been largely replaced with refined dispersion 

modeling because of advancements in computing power and the wider availability of 
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representative meteorological data that are needed to apply refined models. In this context, the 

primary regulatory purpose for application of a screening model would be to determine whether 

site-specific meteorological data would be required for a proposed source if no other 

representative meteorological data are readily available. However, a screening model such as 

AERSCREEN can also be a useful tool to estimate potential impacts during the design and 

planning stages of a project. 

SUMMARY 

In summary, 

• AERSCREEN has been released and is available on the SCRAM web site. 

• AERSCREEN is based on AERMOD, EPA’s preferred near-field dispersion model, and 

replaces SCREEN3 as the recommended screening model based on the Guideline on Air 

Quality Models. 

If there are any questions regarding AERSCREEN, please contact James Thurman of EPA’s Air 

Quality Modeling Group at (919) 541-2703 or thurman.james@epa.gov. 

cc: Richard Wayland, C304-02 

Scott Mathias, C504-01 

Raj Rao, C504-01 

Dan deRoeck, C504-03 

Elliot Zenick, OGC 

Brian Doster, OGC 

George Bridgers, C439-01 

Roger Brode, C439-01 

James Thurman, C439-01 

Air Division Directors 

Air Program Managers