A smog day is a day in which air emissions and weather conditions result in the formation of high concentrations of fine particulates and/or ozone over a large area, lasting many hours. The following three criteria are used to define and determine smog days:
Number of fog days caused by fine particulates and ozone, by administrative region (2004–2020)
ND = no data
Average annual number of smog days by administrative region (2004–2020)
In 2020, the number of smog days varied by region, with none recorded in many regions and up to a maximum of 19 days in the Capitale-Nationale region.
As is the case each year, cold temperatures had a major impact on results, with 80% of smog episodes observed in the winter months (December, January, February and March). If we compare 2020 data to figures for earlier years, we see that the average number of smog days reached the highest value since 2013 (4.9).
The number of high smog days in 2020 can be explained in part by a multiple-day episode in seven regions that included four consecutive days in the Montréal, Laval, Montérégie, Lanaudière and Mauricie regions between January 31 and February 3. In addition, the Capitale-Nationale region experienced its second-highest number of smog days (19) since 2004, including a series of eight smog days in December, seven of which were consecutive. The addition of a new measuring station in (Québec - Charlesbourg) may have also influenced the result since it covered a new sector of the city.
The result for the Capitale-Nationale region is a good example of the impact of residential wood heating on air quality. Out of 19 smog days, 16 were observed in the coldest months of the year (January, February, November and December), matching the typical wood heating pattern. Wood stoves are usually operated between 6 p.m. and 9 a.m. For the 16 smog days, 90% of the « poor air quality» smog-causing data was measured during this period of the day. Moreover, this pattern is solely responsible for the seven consecutive smog day episode. During the period in question, smog episodes essentially occurred at night. Moreover, for the 16 days of smog, only two reached beyond the boundaries of Québec City itself, which shows that most of the episodes were local in origin.
The Rivière-Ouelle peat-bog fire of June 20-21, 2020, also contributed to smog days in the Montréal, Lanaudière, Montérégie, Centre-du-Québec, Mauricie, Chaudière-Appalaches and Capitale-Nationale regions.
The COVID-19 pandemic lockdown measures may also have contributed to the rise in some types of emissions, such as those stemming from residential wood stove heating. However, air dispersion conditions remain the main influence on the number of smog days.
Ozone has not been a smog day formation factor since 2017.
Learn more at Poor air quality statistics.