Climate-Chemistry Synergy Is Reversing Global Urban-Rural Ozone Differences.

Health-damaging surface ozone tends to be lower in populated urban areas than in the surroundings, yet this urban-rural difference is narrowing. While evidence from observations have attributed this narrowing to the reduced inhibition effect of urban nitrogen oxides, we lack clarity on how intensified humid-heat modulates precursor sensitivity in ways that reconfigure the urban-rural gradient under climate warming. Here, we integrated high-resolution air pollutant reanalysis with interpretable machine learning to quantify the global spatial patterns and temporal evolution of paired urban-rural ozone gradients. We show that meteorology-chemistry synergy in urban areas, encompassing weakened inhibition effects of NO2 and dewpoint temperature and an intensified positive effect of radiation, contributes significantly to the reversal of urban-rural ozone disparities in tropical regions and to the long-term global convergence (0.1 μg m-3 year-1, 2005-2023). Although heatwaves have become more frequent, ozone anomalies have diminished driven by moist atmospheric conditions and stringent emission cuts. Yet urban anomalies exceed rural ones, with the differences during moist heatwaves nearly 2-fold those observed in dry events. Our study urges targeted and proactive policies to prevent climate warming synergies from disproportionately worsening urban ozone burdens.