Present day ozone (2010–2014 average) for SOMO35 (ppb day) for non-urban and urban sites. Sample sizes vary according to the data requirements for the calculation of each metric and are shown for each panel. Annual metrics requiring data from all 12 months have smaller sample sizes than the warm season metrics (April–September in the Northern Hemisphere) because many sites in the US only operate during April–September. DOI: https://doi.org/10.1525/elementa.273.f4c