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Modeled Ambient Concentrations

Information provided for informational purposes onlyNote: EPA no longer updates this information, but it may be useful as a reference or resource.
Summary of Results
Data Tables
Bar Charts
Comparison to Monitored Values
About the Model

Summary of Results
This link provides an overview of what the results show.

Ambient Concentrations Maps
These maps allow you to view 1996 ambient concentration estimates (in micrograms per cubic meter) based on the median concentration in each county. You can select the entire U.S. or any State in the United States (except Alaska and Hawaii) plus Puerto Rico and the Virgin Islands. The maps are color-coded (by percentile breakdown relative to the rest of the country) to show how each county's median concentration compares to the rest of the U.S. The median concentration is the value for which 50% of the census tracts in the county have ambient concentrations less than the median, and 50% of the census tracts in the county have ambient concentrations greater than the median. In other words, it is the midpoint for the ambient concentration values in that county. The median is expected to be more representative than the average of the estimated "typical" 1996 concentration within a county since it is less affected by outliers (i.e., very high or very low concentrations).

Ambient Concentrations Bar Charts
This link allows you to view two types of bar charts for each air pollutant:
  1. A comparison of statewide estimates
  2. A representation of the contribution of each of the four major source types as well as background estimates to the statewide concentration.
Note that you need Adobe Acrobat to be able to correctly view and print these bar charts. Information and downloading of Adobe Acrobat (PDF) files

Ambient Concentrations Data Tables
There are two types of tables you will be able to view from this link: one type is organized so you can see ambient concentrations specific to an individual pollutant, and the other is organized so you can see the ambient concentrations for all pollutants for each county in a State. All of these tables are available as either a downloadable file in Excel Spreadsheet format or as an Adobe Acrobat pdf file. Both types of tables contain information about ambient concentration distributions (i.e., as percentiles) as well as a breakdown of ambient concentrations into major, area, onroad mobile and nonroad mobile and background contributions. The county information includes a designation as urban or rural.

Comparison to Monitored Concentrations
This link provides details about EPA's efforts to compare available 1996 monitored air toxics concentrations to estimates from the dispersion model. This comparison helps EPA evaluate and refine its air quality models. Note that many of the ambient concentrations estimated for 1996 are below detectable limits of most available ambient monitoring equipment. In addition, EPA does not have methods to monitor for 6 of the 33 pollutants included in the assessment (acrolein, acrylonitrile, ethylene oxide, hydrazine, coke oven emissions and quinoline). EPA was able to compare available 1996 monitored air toxics concentrations to estimates from the dispersion model for seven pollutants: benzene, perchloroethylene, formaldehyde, acetaldehyde, cadmium, chromium and lead.

About the Model
To develop nationwide estimates of annual average ambient concentrations of air toxics, EPA is using the Assessment System for Population Exposure Nationwide (ASPEN) model (developed and used in EPA's Cumulative Exposure Project). The scope of this national modeling effort is the contiguous United States (i.e., excluding Alaska and Hawaii for this initial assessment), Puerto Rico and the Virgin Islands. The ASPEN model simulates the impacts of atmospheric processes (winds, temperature, atmospheric stability, etc.) on pollutants after they are emitted. The output of this air dispersion model is an estimate of the annual average ambient concentration of each air toxic pollutant at the centroid of each census tract within the geographic scope of the assessment ( 1996 census tract data). For more information about the model, see the ASPEN model.

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