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 (NO_x) 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–NO_x–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 NO_x mitigation strategies while accounting for the growing ozone risk associated with intensifying UHI effects.