The Influence of Soil Nitrogen Oxide Emissions on Primary and Secondary Pollutant Formation

Anthropogenic fossil nitrogen oxide (NOx = NO + NO₂) emissions have exhibited a decreasing trend in recent decades, highlighting the need for an improved understanding of other NOx sources. Soils represent a substantial source of atmospheric reactive nitrogen, including NOx, which is a harmful air pollutant. Additionally, NOx contributes to the formation of harmful secondary atmospheric pollutants, including ozone (O₃) and particulate matter (PM). Soil NOx emissions are strongly influenced by environmental conditions, with soil moisture being a dominant driver due to its ability to regulate the amount of oxygen available to soil microbes that drive emissions. Despite this understanding, the specific relationship between soil NOx emissions and soil moisture content remains poorly constrained. Here, we incorporate an updated version of the Berkeley-Dalhousie Soil NOx Parameterization (BDSNP) into the Weather Research and Forecasting model coupled with chemistry (WRF-Chem) that accounts for a more spatially dynamic relationship between soil moisture and soil NOx emissions. We evaluate the influence of this updated emissions scheme on concentrations of surface pollutants, including NOx, O₃ and PM. We further evaluate the influence of this emissions change on tropospheric column concentrations of NO₂ for comparison with NO₂ columns from the TROPOspheric Monitoring Instrument (TROPOMI). We show that observed surface concentrations of nitrogen dioxide (NO₂) can exhibit a distinct relationship with soil moisture that is captured when using the updated soil NOx emissions parameterization. We further show that over fertilized, agricultural regions, the updated emissions scheme produces WRF-Chem NO₂ column concentrations that better reflect the seasonality observed in TROPOMI column NO₂ than the standard emissions scheme. This work highlights the need for improved representation of biogenic emissions sources in agricultural regions, as well as a need for a higher density of surface air quality monitors in agricultural and rural regions.