Daily Cropland Soil NOx Emissions Identified by TROPOMI and SMAP

We use TROPOMI (TROPOspheric Monitoring Instrument) tropospheric nitrogen dioxide (NO2) measurements to identify cropland soil nitrogen oxide (NOx = NO + NO2) emissions at daily to seasonal scales in the U.S. Southern Mississippi River Valley. Evaluating 1.5 years of TROPOMI observations with a box model, we observe seasonality in local NOx enhancements and estimate maximum cropland soil NOx emissions (15-34 ng N m(-2) s(-1)) early in growing season (May-June). We observe soil NOx pulsing in response to daily decreases in volumetric soil moisture (VSM) as measured by the Soil Moisture Active Passive (SMAP) satellite. Daily NO2 enhancements reach up to 0.8 x 10(15) molecules cm(-2) 4-8 days after precipitation when VSM decreases to similar to 30%, reflecting emissions behavior distinct from previously defined soil NOx pulse events. This demonstrates that TROPOMI NO2 observations, combined with observations of underlying process controls (e.g., soil moisture), can constrain soil NOx processes from space. Plain Language Summary Soils are a known source of atmospheric nitrogen oxides (NOx = NO + NO2), a pollutant that contributes to poor air quality. In cropland regions, where nitrogen-rich fertilizers are applied to soils, NOx emissions can be significantly enhanced. We use satellite observations of nitrogen dioxide (NO2) from TROPOMI (TROPOspheric Monitoring Instrument) to quantify the soil-driven contribution to the amount of NOx in the atmosphere in a cropland region in Mississippi, USA. At the daily level, we use TROPOMI measurements together with soil moisture observations from the SMAP (Soil Moisture Active Passive) satellite to show that soil moisture plays an important role in regulating the amount of NOx that cropland soils release. At the seasonal level, we see the largest NOx contribution from soils toward the beginning of the growing season (May-June).