Statistical analysis of air pollution levels in Ontario during COVID-19 lockdown

Feb. 23, 2021

By Hind Al-Abadleh, Professor, Department of Chemisty

Our study published in the Journal of Hazardous Materials analyzed the statistical significance of short-term events like COVID-19 lockdowns on the pollutants levels that are used to assess air quality at 16 air-monitoring sites across southern Ontario. These pollutants include nitrogen oxides, carbon monoxide, ozone and fine particulate matter. 

During the period of April to June 2020, which included much of the first COVID-19-related lockdown in Ontario that began on March 17, after accounting for natural variability, we found:

• statistically significant and temporary drops in nitrogen dioxide and carbon monoxide, with pollutant levels 20 per cent lower than in the previous three years
• no statistically significant drops in ozone and fine particulate matter at the majority of sites

Ten out of the 16 sites also experienced drops in nitrogen dioxide levels between July and October which ranged from six to 37 per cent. Statistically significant decreases in carbon monoxide were also observed at the Downtown Hamilton site in September (16 per cent) and at the Toronto West site in August and September (eight and 11 per cent, respectively). This data might also reflect the influence of less “vacation traffic” during this time period. 

As a result of these temporary pollutant reductions, we calculated notably fewer days with air quality health index (AQHI) levels greater or equal to four – which is indicative of moderate risk – in April and May 2020 than during the same time periods in 2017 through 2019.

The city-specific long- and short-term trends in pollutant levels provided in our study will be very useful for informing the public about the status of air quality because we can calculate the AQHI in response to future government measures and regulations aimed at limiting the spread of new diseases or reducing air pollution and smog episodes. Our approach also highlights the benefits of using rigorous statistical analysis to assess pollution trends and to inform future policies aimed at mitigating carbon emissions in Ontario.

Our analysis addressed issues that were not raised in previously published work on this topic, including the effects of natural variability. We examined the 11-year trend in pollutant levels to assess progress based on regulations in the Clean Air Act. We found that the 11-year decline in nitrogen dioxide appeared to be leveling off for the majority of cities between 2017 and 2019. We also found that the 11-year trend in carbon monoxide appeared to vary by site: there is a clear downward trend at the Hamilton Downtown, Ottawa Downtown and Toronto West sites, and an upward trend for Windsor Downtown. The 11-year trend in ozone levels indicates either no change or a slight upward trend depending on the site. As for fine particulate matter, a downward trend is observed overall in the 11-year trend with magnitudes varying per site. 

Upon examining the effects of natural variability on pollutant levels, we found that seasonal meteorology, transboundary pollution and local industry account for large differences in air quality across years.

The research team was led by Hind Al-Abadleh, professor of Chemistry at Wilfrid Laurier University, and Martin Lysy from the University of Waterloo. They collaborated with three Laurier students and Lucas Neil from Hemmera Envirochem Inc.  This work was funded by the Natural Sciences and Engineering Research Council of Canada.