Soil Moisture, Soil NOx and Regional Air Quality in the Agricultural Central United States.

Agricultural soils containing nitrogen‐rich fertilizers are a substantial source of reactive nitrogen to the atmosphere with potential to impact air quality. One form of reactive nitrogen, nitrogen oxides (NOx = NO + NO2), are a harmful air pollutant and form secondary pollutants, including particulate matter (PM) and ozone (O3). Soil nitrogen oxide emissions (SNOx) are heavily influenced by environmental conditions, however the understanding of the influence of environmental drivers on the behavior of SNOx is limited. Here, we implement a modified soil moisture‐dependent SNOx parameterization into the Weather Research and Forecasting model coupled with Chemistry (WRF‐Chem) and investigate the impact on regional air quality in the central U.S. Evaluating against TROPOspheric Monitoring Instrument (TROPOMI) column NO2 observations, WRF‐Chem columns better capture the TROPOMI column magnitudes earlier in the growing season when using the updated SNOx parametrization, with modeled column bias improved to −1.1% over the most heavily fertilized regions. Evaluating against Environmental Protection Agency (EPA) surface NO2 observations, the relationship between surface NO2 and soil moisture is better represented in agriculturally‐dominant regions when using the updated parameterization, with greatest surface NO2 concentrations at moderate soil moisture and lower concentrations at wetter or drier soil conditions. In simulations, these SNOx lead to increased O3 in select urban regions, with more than double the occurrences of O3 exceeding the EPA 8‐hr O3 standard of 70 ppb. Plain Language Summary: In cropland areas where large quantities of fertilizers are commonly used, emissions of nitrogen oxides (NOx) from soils can significantly impact air quality. While harmful on its own, NOx contributes to the formation of additional harmful air pollutants like particulate matter and ozone, posing risks to human and environmental health. Our understanding of the factors influencing soil NOx emissions remains limited. In this study, we integrated a new approach for estimating soil NOx emissions into a regional atmospheric model, with a focus on soil moisture impacts. By comparing model results with satellite and surface observations, we found that this new approach better captures the magnitudes and distribution of NOx concentrations, particularly in agricultural areas. Our simulations suggest that soil NOx emissions can exacerbate urban air pollution as well, increasing the frequency of pollutant levels surpassing regulatory standards. Additionally, we show that surface observations of NOx in cropland regions display a distinct relationship with soil moisture, with largest concentrations at moderate soil moisture values, as opposed to wet or dry conditions. Key Points: Simulated NO2 column bias is reduced over U.S. cropland in mid‐April to mid‐July when using a soil moisture‐dependent soil NOx schemeSurface air quality sites near cropland show higher NO2 at moderate soil moisture, and lower NO2 at relatively wet and dry soil moistureCropland soil NO emissions increase simulated surface O3 concentrations in downwind urban areas [ABSTRACT FROM AUTHOR]

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