Characterizing soil NOx emission in western U.S. using WRF-Chem and satellite measurement: rain-induced emission puffs and the impact on O3 air quality

Abstract:

Nitrogen oxides (NO_x) is a key precursor in the formation of O₃ and secondary aerosols. To improve air quality, the US Environmental Protection Agency (EPA) has implemented increasingly stringent NO_x emission controls since the early 2000s. Somewhat surprising, the flattening trend of the OMI NO₂ has been found after the year of 2009, which is inconsistent with the sustained decrease in NO_x emissions from anthropogenic sources reported by the National Emission Inventory (NEI). Fertilized soils are important sources of NO_x pollution, which account for ~1/4 of the NO₂ production over the continental United States and contribute even more in high-temperature rural areas. Numerical models at regional scale are often used to evaluate the emission control strategies in the U.S. However, estimates of soil NO_x emissions (SNO_x) vary widely among different models and rain-induced soil emission pulses are currently neglected in the standard WRF-Chem model. Here we implement an advanced SNO_x scheme in WRF-Chem, the Berkley Dalhousie Soil NO Parameterization with modification (hereafter the Berkley Dalhousie Iowa Soil NO Parameterization or WRF-Chem-BDISNP) and compare the model results with TROPOMI NO₂ columns. The implementation is tested for the state of California in 2018. The updated SNO_x scheme increases the modeled SNO_x flux by a factor of 8.7 (6.3) over the cropland (the whole domain) compared to the default (MEGAN) one. The WRF-Chem-BDISNP can reproduce the observed rain-induced pulse event of SNO_x from TROPOMI and increase the modeled NO₂ tropospheric columns by 37.3% (0.56 molec/m²) over the cropland. We also investigate the impact of SNO_x on the air quality in high-temperature agricultural region of California, and find that the elevated SNO_x flux both increases the surface NO₂ and O₃ concentrations by 30.6% (1.1 ppbv) and 11.0% (4.7 ppbv) in the rural area, respectively, which highlight the sensitivity of air quality to soil NO_x emissions in this region and confirm that this air shed is NO_x limited. Our results suggest that the soil NO_x emissions have large potential impacts on air quality and should be considered in the future emission control strategies.