Large contribution of soil emissions to the atmospheric nitrogen budget and their impacts on air quality and temperature rise in North China

Abstract. Soil emissions of nitrogen compounds, including NO and HONO, play a significant role in atmospheric nitrogen budget. However, HONO has been overlooked in previous research on soil reactive nitrogen (Nr) emissions and their impacts on air quality in China. This study estimates both soil NOx and HONO emissions (SNOx and SHONO) in North China with an updated soil Nr emissions scheme in a chemical transport model, the Unified Inputs for WRF-Chem (UI-WRF-Chem). The effects of soil Nr emissions on O3 pollution, air quality and temperature rise are also studied, with a focus on two key regions, Beijing-Tianjin-Hebei (BTH) and Fenwei Plain (FWP), known for high soil Nr and anthropogenic emissions. We find that the flux of SNOx is nearly doubled those of SHONO; the monthly contributions of SNOx and SHONO account for 37.3 % and 13.5 % of anthropogenic NOx emissions in the BTH, and 29.2 % and 19.2 % in the FWP during July 2018, respectively. Soil Nr emissions have a significant impact on surface O3 and nitrate, exceeding SNOx or SHONO effects alone. On average, soil Nr emissions increase MDA8 O3 by 16.9 % and nitrate concentrations by 42.4 % in the BTH, 17.2 % for MDA8 O3 and 42.7 % for nitrate in the FWP. Reducing anthropogenic NOx emissions leads to a more substantial suppressive effect of soil Nr emissions on O3 mitigation, particularly in BTH. Soil Nr emissions, via their role as precursors for secondary inorganic aerosols, can result in a slower increase rate of surface air temperature. This study suggests that mitigating O3 pollution and addressing climate change in China should consider the role of soil Nr emission, and their regional differences.