This white paper describes the EPA exploration of a reported discrepancy between estimates of NOX and reactive nitrogen (NOY) concentrations from the Community Multiscale Air Quality (CMAQ) model and ambient measurements. The paper summarizes analysis that has been published in Toro et al. (2021) and Appel et al. (2017). The data used in those publications is publicly available from Zenodo and data.gov (links below).

Figures 1-2: Appel, et. al, 2017, STICS ORD-018177 Appel, K.W., Napelenok, S.L, Foley, K.M., Pye, H.O.T., Hogrefe, C., Luecken, D.J., Bash, J.O., Roselle, S.J., Pleim, J.E., Foroutan, H., Hutzell, W.T., Pouliot, G.A., Sarwar, G., Fahey, K.M., Gantt, B., Gilliam, R.C., Heath, N.K., Kang, D., Mathur, R., Schwede, D.B., Spero, T.L., Wong, D.C., Young, J.O. (2017) Description and evaluation of the Community Multiscale Air Quality (CMAQ) modeling system version 5.1. Geoscientific Model Development, 10, 1703-1732. https://doi.org/10.5194/gmd-10-1703-2017 DATA: https://catalog.data.gov/dataset/site-compare-scripts-and-output

Figures 4-10: Toro et. al, 2021, STICS ORD-039261 Toro, C., Foley, K., Simon, H., Henderson, B., Baker, K.R., Eyth, A., Timin, B., Appel, K.W., Luecken, D., Beardsley, M., Sonntag, D., Possiel, N., Roberts, S. (2021) Evaluation of 15 years of modeled NOX across the contiguous United States. Elementa: Science of the Anthropocene, 9(1). DOI: https://doi.org/10.1525/elementa.2020.20.00158. DATA: https://zenodo.org/record/4633517#.YZ0mkNDMJPY

Figure 11 - US EPA 2021 MOVES Technical Report U.S. EPA (2021c). Overview of EPA’s MOtor Vehicle Emission Simulator (MOVES3). EPA-420-R-21-004. Office of Transportation and Air Quality, Ann Arbor, MI, March 2021, https://www.epa.gov/moves/moves-technical-reports.

This dataset is associated with the following publication: Simon, H., P. Dolwick, A. Eyth, B. Henderson, S. Koplitz, R. Owen, S. Phillips, N. Possiel, V. Rao, B. Timin, J. Vukovich, C. Bailey, M. Beardsley, J. Choi, J. Han, H. Michaels, S. Roberts, D. Sonntag, J. Warila, M. Zawacki, K. Appel, J. Bash, K. Foley, G. Pouliot, H. Pye, L. Valin, S. Ballare, C. Toro, and S. Kimbrough. EPA-454/R-21-008: Overview of Progress and Findings from the Cross-EPA Coordination Effort for Understanding and Evaluating NOx Emissions Discrepancies. U.S. Environmental Protection Agency, Washington, DC, USA, 2021.