Quantifying the Impact of Fertilizer-Induced Reactive Nitrogen Emissions on Surface Ozone Formation in China: Insights from FEST-C* and CMAQ Simulations

The emissions of reactive nitrogen (Nr) from cropland links the pedosphere and atmosphere, playing a crucial role in the Earth's nitrogen cycle while significantly impacting regional climate change, air quality, and human health. Among various Nr species, nitrogen oxide (NO) and nitrous acid (HONO) have garnered increasing attention as critical precursors to surface ozone (O3) formation due to their participation in photochemical reactions. While most studies focus on Nr emissions from soils, the specific contributions of cropland Nr emissions considering planting activities to regional O3 pollution remain insufficiently investigated. This study applied the enhanced process-based agroecological model (FEST-C*) coupled with the air quality (CMAQ) model to quantify cropland Nr emissions and assess their contributions to regional O3 formation across China in June 2020. The simulated results indicated that the fertilizer-induced total Nr emission was estimated at 1.26 Tg in China, with NO emissions accounting for 0.66 Tg and HONO emissions for 0.60 Tg. North China was identified as a hotspot for cropland Nr emissions, contributing 43% of the national total. The peak emissions of cropland NO and HONO occurred in June, with emissions of 169 and 192 Gg, respectively. Cropland Nr emissions contributed approximately 8% to the national monthly mean MDA8 O3 concentration, with localized enhancements exceeding 9% in agricultural hotspots in summer. North China experienced the largest MDA8 O3 increase, reaching 11.71 mu g m-3, primarily due to intensive fertilizer application and favorable climatic conditions. Conversely, reductions in nighttime hourly O3 concentrations were observed in southern North China and northern Southeast China due to the rapid titration of O3 via NO. In this study, the contributions of cropland Nr emissions to MDA8 O3 concentrations across different regions of China have been further constrained. Incorporating cropland Nr emissions into the CMAQ model improved simulation accuracy and reduced mean biases in MDA8 O3 predictions. This study offers a detailed quantification of the contribution of cropland Nr emissions in regional ozone formation across China and highlights the critical need to address cropland NO and HONO emissions in air quality management strategies.