Long-term variations and formation drivers of PM2.5 nitrate in a megacity of Guanzhong Plain, China: Implications for air quality management.

The reduction in nitrate (NO3-) is crucial for further mitigating atmospheric PM2.5 pollution. Guanzhong Plain is a significant region of PM2.5 control in China. To reveal the variation characteristics and formation drivers of NO3- in this region, a comprehensive investigation had been conducted by combining field observations in the representative city namely Xi'an with MCM-CAPRAM multi-phase chemical box model. Over 2003-2023, NO3- concentrations showed an initial increase followed by a decline after inflection points, while its ratios to PM2.5 and SO42- persistently rose. Recent observations in 2023 presented the largest contributions of NO3- to PM2.5 by 24.0 %, 17.9 %, 23.6 % and 20.9 % during four seasons, respectively. Case statistics and model simulations collectively indicated that NO3- formations exhibited positive sensitivity to O3 concentrations and VOCs emissions during winter, and positive responses to NO2 concentrations and NOx emissions in other seasons. Furthermore, NO3- formations were sensitive to particulate NH4+ available for neutralizing HNO3 and NH3 emissions. The simulation results, based on various emission reduction scenarios, indicate the necessity of a coordinated multi-pollutant reduction scheme, which is subject to seasonal differentiation. Unlike the long-term air pollution controls in Beijing, a greater reduction in NOx emissions failed to achieve proportional decrease in NO3- concentration in Xi'an due to accelerated atmospheric oxidation intensity from an excessive emission reduction ratio of NOx to VOCs and insufficient decreases in NH3 emissions. Overall, this study provides a scientific basis for formulating targeted nitrate control policies, offering global reference for PM2.5 mitigation strategies.