Screening modeling encompasses a number of conservative analytical modeling techniques for estimating extreme upper bound concentrations. These "worst-case" estimates are based on simplified assumptions/representations of source-receptor geometries. The primary purpose of screening modeling is to quickly and easily eliminate sources, whose impacts are low enough that they will not pose a threat to ambient air quality standards or ambient increment standards, from further analysis. Another purpose of screening modeling is to identify the "worst-case" operating scenario or load case for a particular source(s) that require a more refined modeling analysis to demonstrate compliance with ambient air quality standards and ambient increment standards.
Screening modeling tends to be easy-to-run, quick and conservative (i.e., tends to overpredict air contaminant concentrations).
Screening modeling may be used by itself to demonstrate compliance with ambient air quality standards or to indicate the proposed operating scenario most likely to produce the maximum predicted air contaminant concentrations to be further analyzed via refined modeling.
The Guideline on Air Quality Models provides a complete list of available air quality screening models and modeling techniques. Some basic guidance on a few of the more commonly used models can be found later in this page, however, the "Guideline on Air Quality Models" should be consulted for further details on the models suggested in this guideline and for those modeling situations not specifically covered in this guidance.
AERSCREEN is a single-source screening version of AERMOD that will produce conservative impact estimates without the need for refined meteorological or detailed terrain data. Applicants should refer to the AERSCREEN User’s Guide to determine the proper use of the model.
All EPA-approved screening models can be downloaded at the SCRAM website.
Screening Modeling Receptors
Receptors for screening modeling should be selected so as to provide detailed horizontal and vertical resolution of the terrain surrounding the source being modeled.
It is important to note that screening receptors are to be selected to minimally 10 kilometers. This geographical extent is important to demonstrate that the maximum impact area is included in the analysis and also to determine the significant impact area (which will be used to determine the geographical extent for a refined modeling analysis, if applicable). MEDEP-BAQ believes a complete screening receptor grid provides valuable information relative to the analysis.
If screening modeling results show that concentrations are still increasing at the 10 kilometer receptor, then the grid must be extended beyond 10 kilometers. Receptors in areas not meeting the definition of "ambient air" (e.g., receptors within the "production area" of a source, see Chapter 116 of the MEDEP-BAQ Regulations) should be eliminated.
There are also certain cases where additional special purpose receptors may have to be selected for inclusion in the analysis. Some examples of special purpose receptors are:
- Fenceline receptors at the closest occurrence of "ambient air", as defined by Chapter 116;
- Receptors within the wake region of the controlling building (outside of the production area, if any);
- Receptors at state/international borders;
- Receptors at Class I areas; and
- Recepotors at any designated sensitive locations (typically requested by MEDEP - AQ).
For screening analyses where the stack height is less than its formula GEP height, the height and the maximum projected width of the overall controlling structure (as determined through the use of any BPIP program) is to be used. As stated previously, hand calculations will NOT be accepted for making this determination!
A cavity analysis is to be conducted for all stacks (whose cavity effects may possibly extend into ambient air as defined by Chapter 116) that are below 100% of full GEP height using any EPA approved model that correctly incorporates the PRIME downwash algorithm.
Regardless of the model used, if the cavity area does not or is not likely to extend into ambient air (as defined by Chapter 116), the cavity analysis may be omitted.
All cavity methodologies and techniques are to be included in the protocol and (if applicable) discussed with MEDEP-BAQ prior to performing the analysis.
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 ration, and surface roughness).
Ambient air quality standards, ambient increments, and significance levels cover a variety of time (or averaging) periods depending on the air pollutant in question.
AERMET produces 1-hour concentration values which will need to be converted to other averaging periods using the conversion factors listed in the table below (values are taken from the AERSCREEN User's Guide, page 3):
Multiply 1-hour result by:
To get the:
If predicted concentrations from screening modeling are below their respective significant impact levels (as defined in MEDEP-BAQ Regulations, Chapters 100 and 110) for a given regulated pollutant, then no additional modeling is necessary for that pollutant.
Significant Impact Area
The area which is significantly impacted by the source, based on screening modeling, shall be defined as a circular area surrounding the source with radius equal to the maximum distance to which screening no longer predicts concentrations that exceed significance levels. This area, which is significantly impacted by the source, may be redefined using the results of sequential modeling for the source only (see Refined Modeling section for methodologies).
If screening modeling results indicate significance levels being exceeded by the source being modeled, then other nearby sources may need to be included in the analysis. If applicable, MEDEP-BAQ will provide the applicant with a list of any sources that may have to be included in the multi-source modeling analysis and the model input data for these sources. The list will contain all data required for model input including source location(s), emission rates, stack parameters, and model-ready BPIP input files or necessary building dimensions for the applicant to determine direction-specific building parameters.
Although the results tend to be very conservative, it is possible for a combined source analysis to demonstrate compliance using only screening modeling. If the total predicted maximum concentrations of all sources, modeled separately, when added to background does not exceed ambient air quality standards or if the total predicted maximum ambient increment concentration does not exceed ambient increment limits, then compliance with the ambient air quality standards and ambient increment standards shall have been demonstrated and no further modeling will be required. This approach applies to simple, complex and intermediate terrain modeling.
If single and/or combined source modeling results cannot demonstrate compliance with MAAQS or increment standards, refined modeling will be necessary.
Further guidance on modeling contributions from other nearby sources using a screening model can be found in Screening Procedures for Estimating the Air Quality Impact of Stationary Sources.