Impact of photochemistry on wintertime haze in the Southern Sichuan Basin, China

Nitrate and organic matters (OM) have become dominant components in fine particles (PM_2.5) during winter haze in recent years. Based on continuous observations of gaseous pollutants, the chemical composition of PM_2.5, and other relevant data collected over a one-month period (December 1–31, 2021), we investigated the main controlling factors contributing to the formation of wintertime haze in Yibin, located in the southern Sichuan Basin. Our observations reveal that two major haze episodes occurred during the campaign. Nitrate and OM were the dominant components in PM_2.5, with an overall contribution of more than 50%. Nitrate and OM concentrations nearly quadrupled and more than tripled, respectively, from the non-pollution phase to the pollution phase. Furthermore, the mixing ratios of high-activity VOCs also noticeably increased during the pollution period, particularly OVOCs mixing ratios increased by 123.83%. PM_2.5 concentrations were positively correlated with O_x concentrations, with a stronger relationship observed when O_x concentrations exceeded 80 μg m⁻³. There were also significant positive correlations between nitrate and O_x concentrations, as well as between OVOCs and OM concentrations. Furthermore, the pollution period showed a much higher degree of photochemical aging compared to the non-pollution period. Potential Source Contribution Function (PSCF) analysis revealed that, in addition to local emissions, regional transport, particularly air pollutants from Chengdu and Chongqing, significantly contributed to winter haze in Yibin. Our findings suggest that intense atmospheric photochemical oxidation and pronounced photochemistry contributed greatly to the occurrence of severe winter haze events.