Spatio-temporal variations of PM_2.5 and O₃ in China during 2013-2021: Impact factor analysis

Fine particulate matter (PM_2.5) and ozone (O₃) pollution are regarded as significant secondary air pollutants. The PM_2.5 in most regions in China declined, and the decreasing rate in January was lower than the annual average. However, O₃ concentration showed a steady increasing trend in most regions, and the increasing rate in July was slightly higher than the annual average. In particular, the annual average PM_2.5 concentration and excess rate showed an increasing trend on the northern slope of the Tianshan Mountains. Conversely, O₃ concentrations had shown a consistent increasing trend, exceeding the annual average limit of 100 μg/m³. Surface pressure exhibited positive correlations with PM_2.5 in winter and O₃ in summer across urban agglomerations. Moreover, soil temperature at different depths explained over 30% of the variations in PM_2.5 and O₃ in the Chengdu-Chongqing, Beijing-Tianjin-Hebei, and Lanzhou-Xining urban agglomerations. In winter, relative humidity demonstrated a positive correlation with urban agglomerations in northeast and northwest China, regions characterized by dry climates. During the COVID-19 period, the impacts of meteorological factors and soil temperature on PM_2.5 and O₃ differed significantly compared to preceding and subsequent periods. Notably, during the winter of 2020, the Harbin-Changchuan urban agglomeration exhibited a notable transition, as O₃ and PM_2.5 concentrations shifted from a strong negative correlation to a robust positive correlation. This remarkable shift, with deviations explained up to 60%, represents a unique phenomenon worth emphasizing in the study's findings.