Daily Cropland Soil NO_x Emissions Identified by TROPOMI and SMAP

We use TROPOMI (TROPOspheric Monitoring Instrument) tropospheric nitrogen dioxide (NO₂) measurements to identify cropland soil nitrogen oxide (NO_x = 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 NO_x enhancements and estimate maximum cropland soil NO_x emissions (15-34 ng N m^-2 s^-1) early in growing season (May-June). We observe soil NO_x 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 × 10¹⁵ molecules cm^-2 4-8 days after precipitation when VSM decreases to ~30%, reflecting emissions behavior distinct from previously defined soil NO_x pulse events. This demonstrates that TROPOMI NO₂ observations, combined with observations of underlying process controls (e.g., soil moisture), can constrain soil NO_x processes from space.