Ozone levels in European and USA cities are increasing more than at rural sites, while peak values are decreasing

•We compared trends over 1990–2010 at paired urban and rural sites from USA and Europe.•Peaks decreased at both station types, with no differences between types.•Annual averages increased at both site types, with a faster rate at urban centers.•The overall trend was for convergence between urban and rural data.•Criteria for protection of people and vegetation were exceeded at both site types.

Ground-level ozone (O3) levels are usually lower in urban centers than nearby rural sites. To compare trends in O3 levels during the period 1990–2010, we obtained monitoring data from paired urban and rural sites from the European Environment Agency and the US Environmental Protection Agency. Ozone peaks decreased at both station types, with no significant differences between urban and rural stations. Ozone annual averages increased at both urban and rural sites, with a faster rate of increase for urban centers. The overall trend was for convergence between urban and rural O3 data. Ozone levels exceeded the criteria established for the protection of human and vegetation health at both urban and rural sites.