Drivers of the Changing Urban and Rural Surface Ozone Differences in China: Reducing Emissions or Warming Cities

The long-term evolution and underlying drivers of urban-rural ozone (O3) differences across China remain unclear. Here, we identify a national-scale reversal in the urban-rural Maximum Daily 8 h Average (MDA8) O3 difference (urban minus rural), shifting from higher O3 in rural areas (−2.3 μg m–3 in 2013) to higher O3 in urban areas (1.2 μg m–3 in 2020) based on the reanalysis data, indicating increasingly intensified O3 exposure in densely populated cities. Using chemical transport model simulations, we demonstrate that changing meteorological conditions account for a notable positive shift in the urban-rural O3 difference across nearly half of the Chinese provinces during 2013–2019. Meanwhile, larger reductions in urban nitrogen oxides (NOx) emissions relative to rural areas emerge as the dominant anthropogenic driver, contributing 4.9 μg m–3 in summer and 3.7 μg m–3 in winter to increasing urban-rural O3 difference. This transition has reshaped ozone–NOx–volatile organic compounds (VOC) sensitivity, shifting optimal O3 production zones from rural to urban areas across most regions in summer. Our findings highlight the need for strengthened urban NOx mitigation strategies while accounting for the growing ozone risk associated with intensifying UHI effects.