Indicator
Figure 1. Seasonal means and 30-year trendlines of ground-level ozone at monitoring sites across Ontario for the period 1987–2017 (Source: OMOECC 2008 and OMECP 2019).
Figure 2. Annual peak (fourth-highest) daily maximum 8-hour average ground-level ozone concentration at sites across southern Ontario 2002–2016 (n = 38) (Source: Environment and Climate Change Canada 2018) (Note: The red widely spaced horizontal dashed line represents the value of the Canadian Ambient Air Quality Standards (CAAQS) for 2020 and is shown for indicative purposes only, and not for evaluation of the achievement status of the standard. The green more closely spaced horizontal dashed line represents the environmental threshold above which impacts are likely on vegetation (Royal Society 2008).
Status:
- Between 1987 and 2017, there was an increasing trend in seasonal means of ground-level ozone in Ontario, with summer average concentrations consistently higher than winter average concentrations. Over the 30-year period, average summer concentrations increased by approximately 12% while average winter concentrations increased by nearly 68%. However, more recently—between 2007–2017—average summer ground-level ozone concentrations in Ontario have been declining while concentrations in winter have been increasing at a much lower rate than in previous 10-year periods. The continued increase in winter ozone averages are mainly attributed to rising global background concentrations.
- A decreasing trend was detected in the annual peak concentrations of ground-level ozone from 2002 to 2016, representing a decrease of 23% (or an average of 1.6% per year) over that period. A reduction in emissions of ground-level ozone precursor gases (nitrogen oxides and volatile organic carbons) from Canada and the United States is likely an important factor in this downward trend. The annual peak ground-level ozone concentrations were above both the 2020 Canadian Ambient Air Quality Standard and the environmental threshold for all years.
Last Updated: June 3, 2021