Variation characteristics and impact factors of the co-occurrence of surface ozone and particulate matter over the Southern Sichuan region, China

To elucidate the variance in the correlation between PM2.5 and O3 concentrations and their influencing factors in the southern Sichuan region, we conducted an analysis of PM2.5 concentrations in the cities of Yibin, Luzhou, Zigong, and Neijiang across different seasons in 2021. This analysis was juxtaposed with the daily maximum 8-hour average of O3 (MDA8 O3) and the atmospheric total oxidant concentration Ox (O3 + NO2). Augmenting this with reanalyzed data on the primary chemical constituents of PM2.5, we further explored the key chemical components predominantly influencing the correlations between PM2.5-O3 and PM2.5-Ox. Additionally, we elucidated the influence of meteorological conditions on these relationships using meteorological data. Our findings indicate that PM2.5 concentrations in the four cities exhibited a consistent seasonal trend, peaking in winter and reaching their lowest levels in summer, whereas O3 and Ox displayed an inverse trend compared to PM2.5. Organic matter (OM) and nitrate (NO3–) constituted the primary components of PM2.5. Except for autumn, positive correlations between PM2.5 and O3 were observed in each city across the seasons, with the strongest positive correlation manifesting during summer. Notably, OM and sulfate (SO42–) were dominant chemical components during summer, while NO3– and ammonium (NH4+) were influential in autumn, and OM in spring and winter, impacting the PM2.5-O3 correlations. Comparatively, PM2.5-Ox exhibited notably stronger positive correlations, particularly prominent in winter. Correlation coefficients between the main chemical components and Ox surpassed those with O3, indicating that atmospheric oxidation more effectively stimulated secondary components in PM2.5. The dominant chemical components across all seasons were similar to those observed in the PM2.5-O3 relationship. Meteorological conditions primarily weakened the positive synergistic relationships between PM2.5-O3 and PM2.5-Ox. Temperature (T) emerged as the dominant factor in spring, summer, and autumn, attenuating these correlations, while humidity (RH) dominated in winter.