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.