ARB Research Seminar

This page updated June 10, 2019

Particulate Nitrate Modeling in the San Joaquin Valley

Photo of Michael J. Kleeman, Ph.D.

Michael Kleeman, Ph.D., Professor of Civil and Environmental Engineering , University of California, Davis

June 21, 2019
Cal EPA Headquarters, 1001 "I" Street, Sacramento, CA



Particulate nitrate accounts for a significant fraction of the maximum 24-hr average PM2.5 mass concentrations in California's San Joaquin Valley (SJV). Two decades ago regional chemical transport models (CTMs) were able to predict maximum 24-hr average PM2.5 nitrate concentrations with reasonable accuracy. More recently, predicting maximum PM2.5 nitrate concentrations during contemporary air pollution episodes became more challenging, especially with lower ambient NOₓ levels. This project evaluated emission inventories, meteorological conditions, and regional chemical regimes to determine pathways to improving nitrate predictions in the SJV.

Predicted concentrations of total reactive nitrogen were consistently under-predicted in January of the years 2010, 2013, and 2015 in the SJV. Under-predictions for total reactive nitrogen became progressively more severe with years past 2010 suggesting a need to further evaluate our understanding of various NOₓ emission sources. These trends are consistent with continued reductions in mobile source NOₓ emissions combined with some unknown source of NOₓ emissions that is not currently represented in the emissions inventory.

The inclusion of additional NOₓ emissions in the SJV partially addressed the gap in reactive nitrogen emissions in central California. The case study with additional NOₓ emissions strongly increase predicted levels of total reactive nitrogen in rural areas and contribute approximately 20% to concentrations of total reactive nitrogen in urban locations of Fresno and Bakersfield. The conversion efficiency of NOₓ (from sources such as agricultural cropland) to particulate nitrate is higher than the conversion efficiency of urban NOₓ emissions to particulate nitrate due to the diffuse nature of the these NOₓ emissions which yields more favorable mixing ratios with background ozone. The additional NOₓ emissions that were evaluated improved the accuracy of predictions for wintertime particulate nitrate at ground level, wintertime particulate nitrate vertical profiles, summertime ozone concentrations at ground level, and NOₓ concentrations during both summer and winter.

The tests conducted in the current study suggest that updating NOₓ emissions in the SJV could improve the performance of CTMs. Future measurements should be made in the rural portions of the SJV to further test the hypothesis that additional area-wide NOₓ emissions are a significant factor in the air quality cycles within the region.

Speaker Biography

Michael Kleeman is a Professor of Civil and Environmental Engineering at the University of California at Davis. His research is focused on measurements and modeling of urban and regional air pollution problems. Dr. Kleeman has published over 130 peer-reviewed research articles on topics ranging from ozone production from agricultural sources to interactions between climate-energy-air quality to characterization & source apportionment of atmospheric ultrafine particles.

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