### MEMORANDUM UNITED STATES ENVIRONMENTAL PROTECTION AGENCY RESEARCH TRIANGLE PARK, NC 27711



# APR 11 

# 2Un OFFICE OF AIR QUALITY PLANNING AND STANDARDS

SUBJE CT: AERSCRE EN Released as the EPA Recommended Screening Model
FROM : Ty ler Fox,

# Lead**~ r~**  Air Quality Modeling Group, C439-0 1
TO: EPA Regional Modeling Contacts



### INTROD U CTION
In August 2010, E PA released a beta version of AERSCRE EN 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 ver sion 11060 of AERSCREEN on March 11, 2010 with a subsequent update, version 11076 , on March 1 7 , 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 A ER SCREEN test cases. AERSCRE EN uses the A E RMOD 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 Guidelin e on Air Quality Models (Guideline, published as Appendix W to 40 CFR Part 51) has been SCREE N3. However , AERSCREEN (the sing le source screening version of AERMOD) is no w available as a full release or non-beta version. This memorandum clarifies the replacement of SCREEN3 with AERSCRE E N as the recommended screening model.
With respect to AERSCRE EN replacing SCREEN3 , the preamble of the 2005 rule promulgating A E RMOD as the preferred Guideline model for a wide range of regulatory app lications in all types of terrain states (See 70 FRat 68221 ):

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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 public ly 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 beli eve 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 b ecome the recommended screening model once it is released. In addition, since AERS CREEN is the screening version of AERMOD, EPA’s preferred model for near-field disper sion, it follows that AERSCREEN would become the recommended screening model once availab le. 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 entryof 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 enhancement s relative to the SCREEN3 model. For example, AERSCREEN generates application-specif ic 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 downw ash algorithms that are part of the AERMOD refined model and utilizes the BPIPPRIM toolto provide a detailed analysis of downwash influences on a direction-specific basis. AERSCREEN also incorporates AERMOD’s complex terrain algorithms and utilizes th e AERMAP terrain processor to account for the actual terrain in the vicinity of the sourc e on a direction-specific basis. The question has also arisen about the role of scre ening modeling and refined dispersion modeling under Appendix W. Section 2.2 of the Guideline , explains that: “[t]he purpose of such [screening] techniques is toeliminate 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 Amb ient Air Quality Standards (NAAQS) or the allowable prevention of significant deterior ation (PSD) concentration increments. If a screening technique indicates that the concentration contributed by the source exceeds the PSD increment or the increment remainin g to just meet the NAAQS, then the second level of more sophisticated models should beapplied.” In recent years, the use of screening models has be en 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 requiredfor a proposed source if no other representative meteorological data are readily avai lable. 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 theSCRAM web site.

• AERSCREEN is based on AERMOD, EPA’s preferred near- field dispersion model, and replaces SCREEN3 as the recommended screening modelbased on the Guideline on Air Quality Models

. If there are any questions regarding AERSCREEN, ple ase 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