Assessment of Nitrogen Oxide Emissions and San Joaquin Valley PM_2.5 Impacts From Soils in California

Abstract

Abstract Soils are a source of atmospheric nitrogen oxides (NO x ), especially in regions with significant cropland where nitrogen (N) fertilizers are used to enhance crop yields. The magnitude of soil NO x emissions, however, varies substantially by region, depending on the local land use pattern and management activities. We estimated soil NO x emissions in California based on the DeNitrification‐DeComposition (DNDC) biogeochemical model, linked to a detailed spatial‐temporal differentiated California‐specific database. The DNDC‐generated surface fluxes were used in the Community Multiscale Air Quality (CMAQ) model to evaluate impacts of soil NO x emissions on formation of ambient particulate (PM 2.5 ) nitrate in the San Joaquin Valley (SJV) where cropland is the dominant land use. The DNDC‐generated soil NO x emissions contribute approximately 1.1% of total anthropogenic NO x emissions in California, at an emission rate of roughly 24 t day −1 (as NO 2 ) statewide and 9 t day −1 in the SJV. Cropland is the dominant source of soil NO x emissions in California, contributing nearly 60% of statewide soil NO x emissions, driven principally by fertilizer use. The PM 2.5 nitrate concentrations simulated by CMAQ using the DNDC‐generated soil NO x emissions are compatible with those observed in the SJV, suggesting that soil NO x emissions have limited impacts on PM 2.5 nitrate formation in the atmosphere. Our emission and air quality modeling results are further supported by long‐term ambient NO x ‐to‐carbon monoxide (CO) and satellite NO 2 data analyses in the SJV, which showed diurnal, monthly, and annual trends consistent with characteristics of NO x sources dominated by traffic combustion in both urban and agricultural regions.

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