Air quality modeling simulates the flow of air pollution in the atmosphere and is used to predict health and environmental impacts. By utilizing mathematical equations that replicate weather patterns and characteristics of the physical atmosphere, computer programs are able to simulate the release and transport of these pollutants from their sources. They can also incorporate chemical reactions that occur in the atmosphere, as well as account for the effects of precipitation and the gravitational settling of particles. The model then produces an estimation of pollution concentrations or depositions at specific locations of concern.

DES performs two types of air quality modeling: permit and regional modeling. Permit modeling assesses the impacts associated with individual stationary sources for all regulated air pollutants as part of the DES permitting process. Regional modeling looks at how all sources of air pollution throughout large portions of the United States affect air pollution in New Hampshire and New England. Regional modeling is also used to analyze the impacts of widespread air pollution prevention measures.

Permit Modeling

Permit modeling gives a conservative estimate on how stationary air pollution sources impact their surroundings. The modeling results are compared to state and federal health-based air quality standards for criteria and toxic air pollutants. If these concentrations are above the standards, the source may be required to take permit restrictions that could result in production limits, stack modifications, and/or the addition of control devices. For permit modeling to be performed, sources need to provide the amount of pollutant being emitted from each stack, the stack characteristics, a site plan (which includes the property line), stack locations and the dimensions of any nearby buildings. This information is processed with local meteorological data and site-specific terrain elevations in order to predict worst-case air pollution impacts. Updated background air quality and the contributions from nearby pollution sources are also incorporated into the analysis to determine area-wide impacts.

For facilities or consultants that perform their own modeling, we strongly encourage reviewing our recently-updated guidance manual, the Guidance and Procedure For Performing Air Quality Impact Modeling in New Hampshire (PDF). This document details all of the accepted procedures for performing permit modeling in New Hampshire and was updated in July, 2006 to include information on the AERMOD dispersion model, which will replace ISCST3 as the standard model beginning on November 9, 2006. New AERMOD users are encouraged to view DES's AERMOD Help Guide for a step-by-step tutorial on how to setup the model. More information on AERMOD, including EPA's AERMOD User's Guide, can be found on EPA's air quality modeling web page at http://www.epa.gov/scram001/dispersion_prefrec.htm.

AERMOD requires terrain data in Digital Elevation Model (DEM) format. Information on DEM files and how to obtain them can be found at http://erg.usgs.gov/isb/pubs/factsheets/fs04000.html. DES maintains a full set of DEM files for New Hampshire and will provide them to applicants at no charge as long as the files needed in the specific AERMOD analysis have been identified.

Meteorological data (2000 to 2004) are available from the DES read-only FTP server for refined (sequential) model runs using AERMOD. Model users should refer to DES's modeling guidance document to determine the appropriate data set to be used.

Refined 2000-2004 Meteorological Data for use with AERMOD:

• Concord/Gray five-year data set (Concord met data v5.zip)
• Portsmouth/Gray five-year data set (Portsmouth met data v5.zip)
• Keene/Gray five-year data set (Keene met data v5.zip)
• Whitefield/Gray five-year data set (Whitefield met data v5.zip)

The DES read-only FTP server can be accessed at ftp://199.192.6.23/DES/ARD/. A user name and password are NOT required in order to download files.  Please contact Jim Black at James.Black@des.nh.gov if you have problems accessing the data files.

Screening modeling analyses may be done with the SCREEN3 model until the proposed AERSCREEN model (with preset meteorology) becomes available.

Permit Modeling Updates

On January 9, 2008 EPA released a new version of the AERMOD Implementation Guide, which contains an updated procedure for determining surface characteristics for input into the AERMET processor. To facilitate this procedure, EPA has developed AERSURFACE (08009), which accesses 1992 land cover data to generate values of surface roughness, albedo and Bowen ratio. In order to be consistent with current guidance, DES has reprocessed the full set of New Hampshire meteorological data files, which is the version available on the FTP site (version 5).

All permit applicants who have submitted a modeling protocol or modeling analysis to DES prior to May 1, 2008 may use the previous version of the meteorological data set. All new permit applicants must use the revised data set processed using AERSURFACE.

Please check this section frequently for updates to air quality models, meteorological data and modeling guidance.

Regional Modeling

DES regional modeling efforts have addressed the immediate New England area as well as the states east of the Mississippi River. Regional models incorporate various types of emissions source categories, such as stationary (large industries), mobile (cars and trucks), biogenic (natural sources of emissions), and area sources. Area sources include all of the smaller, more widespread sources of emissions that are not included in other categories. Unlike permit modeling, regional modeling can encompass point, mobile, biogenic, and area sources from across the United States. Land use and terrain, chemistry of air pollution, and meteorology data are also important components of regional modeling. This type of modeling incorporates various control strategies and future regulations for stationary and mobile sources and predicts how they might improve the air quality in New Hampshire, New England, and the eastern United States.

Air Quality Forecasting and Analysis

In addition to air quality modeling, the DES modeling and analysis unit makes daily predictions of air quality and is involved in the analysis of air pollution events. Daily forecasts are available on this web site and at 1-800-935-SMOG.

Links to Forecasting and Analysis Tools