Interannual variability of atmospheric ammonia over the Sichuan Basin, southwestern China: Trend, sources, and implications on particle matter control

Atmospheric ammonia (NH3) is an important constituent of biogeochemical cycle and excessive NH3 in the environment results in detrimental effects on ecosystems, air quality, and human health. However, interannual variability and emission changes of NH3 remain highly uncertain, particularly for NH3 hotspot areas. Here, we identify the emission hotspots of NH3 and reveal the trend of NH3 emissions over the Sichuan Basin (SCB) based on a high-resolution map from infrared Atmospheric Sounding Interferometer (IASI) satellite observations in combination with the MEIC inventory from 2008 to 2019. IASI observations indicate that NH3 columns persistent increase over time while MEIC inventory suggests that the past increase in NH3 emissions has indeed ended in 2017 and exhibit decreasing trend afterwards. The NH3 hotspots derived from IASI mainly concentrate in the Chengdu Plain and southern SCB city cluster, which mostly agrees with MEIC emission inventory. CMAQ model simulations well reproduce PM2.5 and ammonia loading and capture observed patterns of NH3. We find that ammonium makes up 15-35% of inorganic fine PM2.5 mass in winter over the SCB. Regulation efforts focused on joint reducing 30% of NOx, SO2, and NH3 emissions could lead to basin-wide wintertime PM2.5 reduce by 10.0 mu g/m3, while cutting 30% emissions of NH3 or NOx alone only yield PM2.5 reduced by 0.8 and 5.0 mu g/m3, respectively. This work contributes insights on the trend and spatial pattern of NH3 source over the SCB and reveals the critical role of regulating NH3 emissions in mitigating PM2.5 pollution, shedding insights on nitrogen management strategies