Two-decade spatiotemporal variations in ground-level ozone over Ontario, Canada

Zhou Zang¹Jane Liu^1*Erjia Ge²Yi Zhang¹

• ¹Department of Geography and Planning, Faculty of Arts and Science, University of Toronto, Toronto, Ontario, Canada
• ²Dalla Lana School of Public Health, University of Toronto, Toronto, Ontario, Canada

Ground-level ozone (O3) remains a persistent air quality concern in Ontario, Canada's most populous province. To investigate the long-term trend and spatially explicit details of O3 concentrations over Ontario, here we construct a dataset of maximum daily 8-hour average O3 (MDA8 O3) at high spatial and temporal resolutions (daily, 10 km) over 2004-2023, through development of a two-step model facilitated by machine learning technology. In the model development, we hypothesized and realized that accounting for transboundary influences can enhance the accuracy of O3 estimation in Ontario. Validation against in-situ measurements confirmed the hypothesized high accuracy of the dataset (R 2 = 0.82, RMSE = 4.99 ppb), outperforming the traditional model and two existing datasets. The dataset reveals pronounced spatiotemporal heterogeneity in surface MDA8 O3 concentration, which is low in northern Ontario but high in southern Ontario, especially in southwest Ontario. Seasonally, the provincial mean MDA8 O3 is in a peak in spring and in a trough in autumn, while the summertime MDA8 O3 in southwestern Ontario can be even higher, suggesting regional differences in the dominant O3 sources. From 2004 to 2023, MDA8 O3 shows no significant trend in the entire Ontario but a significantly (-0.1 ppb/year, p<0.05) across southern Ontario, over where, there is a clear seasonal contrast in MDA8 O3 trend that significantly increases in winter but decreases in summer. Our analysis suggests that in summer, reductions in local O3 precursor emissions have effectively lowered O3 across southern Ontario, which partially offsets the meteorological-driven increase in O3. Furthermore, the days exceeding the World Health Organization (WHO) O3 guideline range from 10-80 days in southern Ontario, with a frequency decreased by 1-4 days (up to 15%) per year over 2004-2023. This MDA8 O3 dataset offers a valuable resource for further research in environmental health, air quality policy, and O3 impact on agriculture.