Daily Cropland Soil NO(x) Emissions Identified by TROPOMI and SMAP

We use TROPOMI (TROPOspheric Monitoring Instrument) tropospheric nitrogen dioxide (NO(2)) measurements to identify cropland soil nitrogen oxide (NO(x) = NO + NO(2)) 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 NO(2) enhancements reach up to 0.8 × 10(15) 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 NO(2) observations, combined with observations of underlying process controls (e.g., soil moisture), can constrain soil NO(x) processes from space.