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Overview of Progress and Findings from the Cross-EPA Coordination Effort for Understanding and Evaluating NOx Emissions Discrepancies
Citation:
Simon, H., Patrick Dolwick, A. Eyth, B. Henderson, S. Koplitz, Robert Owen, S. Phillips, Norman Possiel, V. Rao, B. Timin, J. Vukovich, C. Bailey, M. Beardsley, D. Choi, J. Han, H. Michaels, S. Roberts, D. Sonntag, J. Warila, M. Zawacki, Keith Appel, J. Bash, K. Foley, G. Pouliot, H. Pye, L. Valin, S. Ballare, C. Toro, AND S. Kimbrough. 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, EPA-454/R-21-0084, 2021.
Impact/Purpose:
Several external groups have published analyses based on the EPA's 2011 emissions modeling platform that suggest that emissions of nitrogen oxides (NOX) from EPA’s mobile emissions model MOVES were too high by a factor of 2. Such conclusions have the potential to put into question the credibility of EPA’s emissions inventory and air quality modeling tools, as well as the benefits of additional NOX reductions. This white paper describes the EPA exploration of a reported discrepancy between modeled estimates of NOX and reactive nitrogen (NOY) concentrations and ambient measurements, particularly summertime overestimates. The criticism and subsequent analysis focused on results from modeling systems using 2011-based inventories and centered on NOX emissions from onroad mobile sources. The analysis in this summary report demonstrates that bias in NOX estimates from EPA emissions and air quality modeling has decreased substantially in more recent years, due in part to decreasing NOX levels, but also because of important improvements in more recent versions of the EPA's emissions modeling platform and the Community Multiscale Air Quality (CMAQ) modeling system.
Description:
During 2013-2017, external groups published analyses suggesting that mobile source nitrogen oxide (NOx) emission estimates developed by the U.S. Environmental Protection Agency (U.S. EPA) were too high by a factor of up to two. From 2015 to 2020, a cross-EPA workgroup met to coordinate complementary ongoing efforts in the Office of Air Quality Planning and Standards (OAQPS), the Office of Transportation and Air Quality (OTAQ) and the Office of Research and Development (ORD) to evaluate NOx emissions and modeling. The workgroup’s aim was to understand discrepancies between modeled estimates of atmospheric NOX and total reactive nitrogen (NOY) concentrations and ambient measurements and determine whether these discrepancies were driven by mobile source emissions estimates or other model processes. The workgroup identified key parameters and processes impacting NOx and NOy model predictions including: MOtor Vehicle Emission Simulator (MOVES) inputs and results, spatial and temporal allocation of emissions, meteorology, mixing, and dispersion treatment in air quality models, air quality model chemistry, and air quality model treatment of deposition. Smaller teams were formed to investigate key hypotheses addressing each of the parameters listed above. In a related effort, the Technical discussions on Emissions and Atmospheric Modeling (TEAM) team was formed as part of a cross-agency coordination effort between the U.S. EPA, the National Oceanic and Atmospheric Administration (NOAA) and the National Aeronautics and Space Administration (NASA). Several major findings came out of the analyses undertaken by the workgroup. First, the overestimates were most common in the summer, with distinct morning and evening peaks. EPA’s modeling system tended to underestimate NOx in winter. While the analyses did not find a unique explanation for the summer over-prediction of NOX and NOY concentrations, they identified several plausible hypotheses, while ruling out others, for the NOX positive biases seen at certain times and locations in the modeling. Model over-predictions were likely due to multiple compounding factors that each contributed to a portion of the bias. EPA members of this workgroup have shared results and engaged with the scientific and regulated communities through continued participation in scientific conferences, workshops, journal articles and other outreach opportunities. Members of the workgroup chaired four special sessions focused on this topic at conferences which included 14 EPA presentations and 26 relevant presentations from outside groups (See Table 2). Findings from this work have also been shared in 24 conference presentations at 11 conferences (See Table 3). Analyses from this workgroup have resulted in five journal articles led by EPA authors (Referenced in Section 7.4). Although the NOx evaluation workgroup is no longer active, work continues among EPA offices and staff to update and improve aspects of the emissions and modeling systems. This document serves to summarize our efforts and understanding of the NOx evaluation effort at the present time.