Sites in the TOAR-Surface Ozone database with urban sites (red) and non-urban sites (blue). Urban and non-urban sites based on warm season average ozone data for 2010–2014. See section 4.3 for TOAR site classifications. There are 1,453 urban sites, 3,348 non-urban sites and 4,801 sites in total. DOI: https://doi.org/10.1525/elementa.273.f2