Spatiotemporal evolution of urban–rural ozone disparities and the “pollution transfer” effect in rapidly urbanizing regions

Abstract

Rapid urbanization has intensified surface ozone (O₃) pollution, posing a serious threat to public health. This study investigated the spatiotemporal evolution of urban–rural O₃ disparities in the Yangtze River Delta (YRD), an emblematic urbanizing region in China, by integrating ground-based observations (2015–2023) with China High Air Pollutants (CHAP) data. Using Sen's slope, the Mann-Kendall test, and random forest modeling, we identified a 24.7 % increase in annual O₃ concentrations from 89 μg/m³ in 2015 to 111 μg/m³ in 2023, with dual peaks in May and September. Average O₃ levels in urban core, suburban, and rural area were 103.50 μg/m³, 104.63 μg/m³, and 99.67 μg/m³, respectively. The Nanjing-Wuhu-Maanshan's, characterized by dense heavy industry, emerged as a prominent pollution hotspot, exhibiting an average annual growth rate of 3.67 μg/m³/yr. Urban–rural disparities revealed the largest relative O₃ increase in rural areas (92.05 %), exceeding those in urban cores (87.57 %) and suburbs (85.89 %). MDA8 O₃ variations were best explained at rural scales. Meteorological factors dominated O₃ formation in urban cores and suburbs, contributing 37.52 %–39.24 %, while industrial plants density (IPD) was identified as a key factor influencing O₃ generation. In megacities, PM_2.5 and NO₂ were the primary emission contributors of ozone concentration changes, with contributions of 18.68 % and 23.22 %, respectively. Changes in industrial emissions of NO₂ and VOCs in urban and rural areas indicated that under rapid urbanization, O₃ pollution exhibited pronounced urban-rural differences, with pollution shifting from urban cores toward suburban and rural areas. Industrial spillover and precursor emission migration jointly shaped the spatial distribution of O₃, providing a scientific basis for understanding pollution transfer mechanisms and formulating differentiated governance strategies.