Long-term measurements of ground-level ozone in Windsor, Canada n Part I. temporal variations and trends

Abstract. This study investigates temporal variations and long-term (1996–2015) trends of ground-level O 3 (ozone) and its precursors, NO x (nitrogen oxides) and volatile organic compounds in Windsor, Ontario, Canada. During the 20-year study period, NO x , non-methane hydrocarbon concentrations and ozone formation potential decreased significantly by 58 %, 61 %, and 73 %, respectively, while O 3 concentrations increased by 33 % (20.3 ppb in 1996 vs. 27 ppb in 2015). Our analysis revealed that the increased annual O 3 concentrations in Windsor were due to (1) decreased O 3 titration (by 50 % between 1996 and 2015) owing to declining nitric oxide concentrations, which is suggested by a slightly decreasing trend of annual mean total O 3 concentrations after the titration effect is removed, (2) reduced local photochemical production of O 3 , because of dwindling precursor emissions, and (3) increased background O 3 level that has more impact on the low-to-median concentrations. The net effect of those factors is decreasing peak O 3 levels during the smog season from May to September, but an overall increasing trend of annual means. These results indicate that the emission control measures are effective in reducing peak ozone concentrations. However, challenges in lowering annual O 3 levels call for long-term collaborative efforts in the region and around the globe.