Large contributions 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 the 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 soil NOx and soil HONO emissions (SNOx and SHONO) in North China during July 2018 using an updated soil Nr emissions scheme in a chemical transport model, Unified Inputs (initial and boundary conditions) for Weather Research and Forecasting coupled with Chemistry (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 the Fenwei Plain (FWP), known for high soil Nr and anthropogenic emissions. We find that the flux of SNOx is nearly double that of SHONO. The monthly contributions of SNOx and SHONO account, respectively, for 37.3 % and 13.5 % of anthropogenic NOx emissions in BTH and for 29.2 % and 19.2 % in the FWP during July 2018. Soil Nr emissions have a significant impact on surface O3 and nitrate, exceeding SNOx or SHONO effects alone. On average, soil Nr emissions increase maximum daily 8 h (MDA8) O3 by 16.9 % and nitrate concentrations by 42.4 % in BTH, and they increase MDA8 O3 by 17.2 % and nitrate concentrations by 42.7 % 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, through their role as precursors for secondary inorganic aerosols, can result in a slower increase rate of surface air temperature in future emissions reduction scenarios. This study suggests that mitigating O3 pollution and addressing climate change in China should consider the role of soil Nr emissions and their regional differences.