Overview

The Air Pollution Emissions Experiments and Policy version 4 (AP4) reduced-form air quality model (RFM) was created by colleagues at Carnegie Mellon University (CMU) under the direction of N. Muller, PhD, and his graduate student L. Dennin, funded by the National Energy Technology Laboratory (2020–2024).

AP4 is a MATLAB program (built on AP3¹) that employs an RFM framework to analyze the marginal effects of air pollution throughout the contiguous United States at the county and census tract levels. Users can adjust the emissions in any particular region by any amount, which allows for both the spatial distribution and the magnitude of damages to be analyzed (i.e., who is being impacted and by how much). Marginal damages are determined by an increment in emissions of one (short) ton to a given baseline (e.g., 2002 observed emissions); the difference is the marginal damage per ton of pollutant from a selected source (there are approximately 10,000 sources in the United States). Emissions data (provided by the US EPA) are converted from their reported units (tons per year) to source-receptor units (grams per source), where sources are the counties, electric generating units (EGUs) and industries or manufacturing plants. Receptors are either counties or census tracts (i.e., AP4T version). As data are reported on the county level, a spatial interpolation method is employed to increase the resolution of the modeled result. Regarding our interpolation approach, facilities responsible for heavy emissions typically install smokestacks designed to disperse pollution and better enable local ground-level concentrations to achieve air quality standards, also allowing for concentrations to spread and smooth out; therefore, it is assumed that county-to-tract modeling via spatial interpolation is sufficient for NH3, NOx, and SO2 from all bins and all pollutants from point sources². Resulting ambient concentration estimates are in grams per cubic meter, which are converted to parts per billion and parts per million (ppb and ppm, respectively).

Footnotes

1. Muller, N.Z. and R. Mendelsohn (2007). Measuring the damages of air pollution in the United States, Journal of Environmental Economics and Management, 54, 1–14. ↩

2. Dennin, L. et al. (2025). Socially vulnerable communities face disproportionate exposure and susceptibility to U.S. wildfire and prescribed burn smoke. Commun. Earth Environ., 6:190, 1–19. ↩