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 NO_x and soil HONO emissions (SNO_x 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 SNO_x is nearly double that of SHONO. The monthly contributions of SNO_x and SHONO account, respectively, for 37.3 % and 13.5 % of anthropogenic NO_x 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 SNO_x 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 NO_x 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.

Due to the lockdown during the COVID-19 pandemic in China from late January to early April in 2020, a significant reduction in primary air pollutants, as compared to the same time period in 2019, has been identified by satellite and ground observations. However, this reduction is in contrast with the increase of surface ozone (O₃) concentration in many parts of China during the same period from 2019 to 2020. The reasons for this contrast are studied here from two perspectives: emission changes and inter-annual meteorological variations. Based on top-down constraints of nitrogen oxide (NO_x) emissions from TROPOMI measurements and GEOS-Chem model simulations, our analysis reveals that NO_x and volatile organic compound (VOC) emission reductions as well as meteorological variations lead to 8 %, −3 % and 1 % changes in O₃ over North China, respectively. In South China, however, we find that meteorological variations cause ∼ 30 % increases in O₃, which is much larger than −1 % and 2 % changes due to VOC and NO_x emission reductions, respectively, and the overall O₃ increase in the simulations is consistent with the surface observations. The higher temperature associated with the increase in solar radiation and the decreased relative humidity are the main reasons that led to the surface O₃ increase in South China. Collectively, inter-annual meteorological variations had a larger impact than emission reductions on the aggravated surface O₃ pollution in China during the lockdown period of the COVID-19 pandemic.