Temporal variations and trend of ground-level ozone based on long-term measurements in Windsor, Canada

Abstract

Abstract. This study investigates temporal variations and long-term (1996–2015) trends\nof ground-level O3 (ozone) and its precursors, NOx (nitrogen\noxides),\nand volatile organic compounds in Windsor, Ontario, Canada. During the\n20-year study period, NOx , non-methane hydrocarbon concentrations, and ozone\nformation potential decreased significantly by 58\u2009%, 61\u2009%, and 73\u2009%,\nrespectively, while O3 concentrations increased by 33\u2009% (20.3\u2009ppb in\n1996 vs.\xa027\u2009ppb in 2015). Our analysis revealed that the increased annual\n O3 concentrations in Windsor were due to (1)\xa0decreased O3 \ntitration (by 50\u2009% between 1996 and 2015) owing to declining nitric\noxide concentrations, which is suggested by a slightly decreasing trend of\nannual mean total O3 concentrations after the titration effect is\nremoved, (2)\xa0reduced local photochemical production of O3 because of\ndwindling precursor emissions, and (3)\xa0an increased background O3 level\nthat has a greater impact on the low-to-median concentrations. The net effect of\nthose factors is decreasing peak O3 levels during the smog season from\nMay to September but an overall increasing trend of annual means. These\nresults indicate that the emission control measures are effective in\nreducing peak ozone concentrations. However, challenges in lowering annual\n O3 levels call for long-term collaborative efforts in the region and\naround the globe.