Enhanced nitrate contribution during winter haze events in a megacity of Sichuan Basin, China: Formation mechanism and source apportionment

Chengdu, a typical basin city in China, still suffers from severe haze pollution in recent years. To gain a comprehensive understanding of the chemical causes and sources of fine particle matter (PM2.5) in this city, online observations of PM2.5 species and other pollutants were conducted at an urban supersite in winter of 2018. The average PM2.5 concentration during the observation period was 71.7 μg/m3, of which nearly half of the measurement days exceeded China's National Air Quality Standard (75 μg/m3). The most abundant species in PM2.5 was organic carbon (20.5%), followed by nitrate (16.0%), ammonium (13.2%), and sulfate (12.8%). Among them, nitrate formation contributed substantially to haze pollution, a phenomenon enhanced by the ammonia-rich environment and heterogeneous reactions. The positive matrix factorization model analysis showed that the largest contributor to PM2.5 was secondary aerosol formation (43.4%), while industrial process contributed the least (3.3%). In addition, two combustion sources, coal combustion and biomass burning/fireworks, accounted for 14.1% and 12.5%, respectively, and their contributions increased significantly with PM2.5 concentration. Air mass trajectory clustering analysis and concentration-weighted trajectory analysis showed that southeastern part of Chengdu was the hotspot for major pollution sources. These results suggest that sustained haze pollution improvement requires continuous strengthening control of precursors such as NOx and NH3, and strict control of combustion sources may be the key to eliminate heavy pollution, and various joint prevention and control measures should be implemented for specific pollution areas.