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Results: 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

You can view information about the 1996 modeled ambient concentrations of the 33 air toxics plus diesel particulate matter (diesel PM) used in the National-Scale Assessment through the links below as a text summary of results, a map format, bar charts, or in the more detailed data tables. The "Comparison to Monitored Values" section below describes EPA's efforts to evaluate the performance of the dispersion model and "About the Model" provides a description of 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) for each of the 33 hazardous air pollutants and diesel PM 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 to show how each county's median concentration compares to the rest of the country. 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 of the 33 air toxics and diesel PM:
  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.

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 33 air toxics and diesel particulate matter 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. The EPA is conducting this type of comparison for seven pollutants: benzene, perchloroethylene, formaldehyde, acetaldehyde, cadmium, chromium and lead. To date, EPA has completed this comparison for benzene. EPA plans to complete the "model-to-monitor comparisons" for the other six pollutants by the end of September and will update this website once these comparisons have been evaluated.

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), based on emissions data for the 33 urban HAPs. 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 within the geographic scope of the assessment. Census tracts are areas that vary in size but typically contain about 4,000 residents each. For more information about the model, see the ASPEN model.

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