Investigating Freeze/Thaw soil emissions of nitric oxide using in situ and Tropomi observations

Agricultural emissions of nitric oxide (NO) from soils can cause formation of tropospheric ozone and particulate matter pollution, and in the presence of ozone NO is rapidly transformed nitrogen dioxide (NO2), itself an air pollutant. Emissions of NO from soils can be highly episodic, with a large proportion of annual emissions occurring in pulses following fertilization or wetting of dry soils. Freeze-thaw events may also be an important source of NO pulses, but the magnitude of these emissions is poorly understood, as are the mechanisms underlying freeze-thaw NO pulses and their influence on atmospheric composition and air quality.  Here we use daily observations of NO₂ and air temperature for 2018-2021 from atmospheric monitoring stations in the Corn Belt of the midwestern United States to evaluate the potential influence of freeze/thaw events on atmospheric NO_x.  We supplement these data with retrievals of NO₂ from the Tropospheric Pollution Monitoring Instrument (TROPOMI) screened to include acceptable retrievals over snow, retrievals of soil freeze/thaw status from the Soil Moisture Active Passive project (SMAP), and observed and reanalyzed soil temperature.  We find evidence for elevated NO₂ concentrations during winter months, including instances of elevated concentrations at the onset of spring thaw.  Freezing degree days—the accumulation of average daily temperature for days with soil temperature maxima of 0°C or less—fail to act as a clear predictor of the magnitude of post-thaw concentrations.  These results will be integrated with complementary high temporal-resolution eddy covariance flux measurements at a long-term agricultural research station in Nebraska, along with soil core incubations involving biogeochemical and molecular analyses, to provide insights into the magnitude of freeze/thaw fluxes and their underlying mechanisms.