Long-term measurements of ground-level ozone in Windsor, Canada and surrounding areas

This study investigated temporal variability of ground-level ozone (O₃) during smog season in Windsor, Canada and surrounding states of the US during 1996-2015. Cluster analysis classified six sites into two groups with similar features of O₃ concentrations. The first group consists of four urban/suburban sites, Windsor, Allen Park and Lansing in Michigan, and Erie in Ohio, and the second group includes two rural sites, Delaware and National Trail School in Ohio. The similarities among all six sites include (1) diurnal and seasonal variability of O₃ concentrations owing to similar weather conditions, and (2) decreasing peak O₃ (95^th percentile) concentrations due to reduced emissions of precursors, thus less photochemical O₃ formation. However, how O₃ levels changed with reduced NO_X emissions during the study period and on weekends differed between the two groups. Lower O₃ concentrations were recorded at urban/suburban sites (30 ppb) than at rural sites (34 ppb). At urban sites, annual smog season O₃ concentrations increased by 0.12-3.2 ppb/year. The increasing trends occurred at all percentile levels except for 95^th percentile and in most months, due to weakened NO titration effect. At the rural sites, smog-season O₃ concentrations decreased by 0.01-2.71 ppb/year. The decreasing trends were observed at 50-95^th percentile levels and in most months. Between the two groups, the urban/suburban group had a greater increment in weekend O₃ concentrations (3.3 vs. 1.6 ppb) due to a greater reduction of local NO emissions on weekend, thus, weakened NO titration effect. Overall, O₃ formation was more sensitive to VOCs during the study period; however, the O₃ formation regime gradually shifted toward more sensitive to NO_X during 1996-2007 then became more sensitive to VOCs during 2008-2015. Therefore, controlling anthropogenic VOC emissions is needed to effectively mitigate O₃ pollution in the study region.