Modeling Protocol

The development of a modeling protocol is the first step in a dispersion modeling analysis performed as part of a regulatory permitting analysis. The modeling protocol describes how the modeling will be carried out. It is usually necessary to submit a modeling protocol to the regulatory agency in order to receive agency approval for the modeling prior to performance of the modeling. The modeling protocol contains the following elements:

• Dispersion model selection
• Modeling methodology and assumptions
• Source input selection and preparation
• Meteorological data selection and processing
• Basis for ambient pollutant concentrations
• Receptor grid development

Table 4.10 Regression equations for dispersion coefficients.

a. Rural release, neutral atmosphere, x < 500 m:	σ y σ z = 0.010 82 x 1.78
b. Rural release, neutral atmosphere, x > 500 m:	σ y σ z = 0.044 87 x 1.56
c. Rural release, stable atmosphere, x < 2000 m:	σ y σ z = 0.004 9 x 1.66
d. Rural release, stable atmosphere, x > 2000 m:	σ y σ z = 0.019 01 x 1.46
e. Urban release, neutral atmosphere, x < 500 m:	σ y σ z = 0.022 4 x 2
f. Urban release, neutral atmosphere, x > 500 m:	σ y σ z = 0.39 4 x 1.54
g. Urban release, stable atmosphere, x < 500 m:	σ y σ z = 0.008 x 2
h. Urban release, stable atmosphere, x > 500 m:	σ y σ z = 0.34 x 1.37

Dispersion Model Selection

AQIAs are carried out using Gaussian dispersion models – models that use the basic assumption of Gaussian dispersion. There are several such models, and different models are applicable to different source types and terrain.

The primary reference for selection of models is EPA’s Guideline on Air Quality Models. It identifies the models that have been developed and validated by EPA and are considered reliable for use in permitting. In particular, there are two models included in the Guideline that are most commonly used. They are

• AERSCREEN
• AERMOD

AERSCREEN

Rather than writing a separate screening model that would require updating whenever the refined model is updated, AERSCREEN is an interactive wrapper for AERMOD and associated auxiliary programs where the user supplies responses to program prompts. Those responses are used to write an input file for AERMOD and other required programs specific to the problem at hand.

AERSCREEN is the recommended screening model based on AERMOD. The model will produce estimates of “worst‐case” one‐hour concentrations for a single source, without the need for hourly meteorological data and also includes conversion factors to estimate “worst‐case” 3‐hour, 8‐hour, 24‐hour, and annual concentrations. AERSCREEN is intended to produce concentration estimates that are equal to or greater than the estimates produced by AERMOD with a fully developed set of meteorological and terrain data, but the degree of conservatism will vary depending on the application.

Before running AERSCREEN, users should consult and become familiar with the most recent versions of the following:

• AERMOD User’s Guide (USEPA 2016a)
• AERMAP User’s Guide (USEPA 2018b)
• AERMET User’s Guide (USEPA 2019b)
• AERSCREEN User’s Guide (USEPA 2016b)
• AERSURFACE User’s Guide (USEPA 2008)
• Screening Procedures for Estimating the Air Quality Impact of Stationary Sources (USEPA 1992)

AERMOD

The AERMOD atmospheric dispersion modeling system is an integrated system that includes three modules (Prater and Midgley 2008; Turner 1994; USEPA 2004b):

1. A steady‐state dispersion model designed for short‐range (up to 50 km) dispersion of air pollutant emissions from stationary industrial sources.
2. A meteorological data preprocessor (AERMET) that accepts surface meteorological data, upper air soundings, and optionally data from on‐site instrument towers. It then calculates atmospheric parameters needed by the dispersion model, such as atmospheric turbulence characteristics, mixing heights, friction velocity, Monin–Obukhov length, and surface heat flux.
3. A terrain preprocessor (AERMAP) whose main purpose is to provide a physical relationship between terrain features and the behavior of air pollution plumes. It generates location and height data for each receptor location. It also provides information that allows the dispersion model to simulate the effects of air flowing over hills or splitting to flow around hills.

AERMOD also includes PRIME (Plume Rise Model Enhancements) (Turner 1994; USEPA 2004b) which is an algorithm for modeling the effects of downwash created by the pollution plume flowing over nearby buildings.