Abstract

Differences of surface ozone levels between urban and non-urban areas provide a good indicator of local ozone formation regimes. However, trends in the urban vs. non-urban ozone differences over the past decades are unclear. Here, we construct 6361 pairs of urban and non-urban ozone measurement sites to assess the trends worldwide. We find that urban vs. non-urban ozone differences have greatly narrowed in the summers of Northern mid-latitude countries over 1990--2020. Analyses of satellite measurements of ozone precursors and statistical predictions driven by meteorology show that, the reduction in anthropogenic emissions of nitrogen oxides, which weakened the titration of ozone over urban areas, is probably the dominant driver of the narrowing difference. Climate change partly contributes to the narrowing trend. Our results indicate that surface ozone concentrations and ozone formation regimes become increasingly similar over urban and non-urban areas, and this shall be considered in future air pollution controls.