Although air quality in Québec is generally good, COVID-19 confinement measures, which have resulted in a decrease in road transportation, have had a positive impact on air quality. Consequently, air concentrations of  criteria contaminants This hyperlink will open in a new window. associated with road transportation have decreased.

The Henri-IV highway in Québec City as an example

A monitoring station alongside the Henri-IV highway in Québec City assesses the impact of road transportation on ambient air quality This hyperlink will open in a new window.. The station is equipped with a radar instrument that counts the number of vehicles on the highway.

During the first lockdown, from March 13 to April 24, 2020, highway traffic greatly diminished during the first weeks while subsequently and progressively returning to levels that were comparable to previously observed values. At the high point of the lockdown measures, the number of vehicles on the Henri-IV highway dropped by 60% from 500,000 to 200,000.

ThThe figure shows the number of vehicles traveling on the Henri-IV highway each week between January6 and May 4, 2020. Traffic is fairly stable and ranges from 450,000 to 550,000 vehicles per week between January 6 and March 2, 2020. Between the week of March 9 (the first week that confinement measures went into effect) and March 30, traffic decreased to fewer than 200,000 vehicles. Between March 30 and May 4, the number increased slightly to 350,000 vehicles.e figure shows the number of vehicles traveling on the Henri-IV highway each week between January6 and May 4, 2020. Traffic is fairly stable and ranges from 450,000 to 550,000 vehicles per week between January 6 and March 2, 2020. Between the week of March 9 (the first week that confinement measures went into effect) and March 30, traffic decreased to fewer than 200,000 vehicles. Between March 30 and May 4, the number increased slightly to 350,000 vehicles.

Several contaminants associated with road transportation can affect air quality. Some of them have decreased during the lockdown, the main changes being as follows:

  • Black carbon: - 45%
  • Nitrogen monoxide: - 65%
  • Nitrogen dioxide: - 60%
  • Nitrogen oxides: - 60%
  • Carbon monoxide: - 35%
  • Ultrafine particulate matter (PM0.1): -80%

The following figure shows the weekly concentrations of these contaminants.

The figure shows weekly concentrations from January 6 to May 4, 2020, of six contaminants measured at the Henri-IV station, namely carbon monoxide, nitrogen monoxide, black carbon, nitrogen dioxide, ultrafine particulate matter, and nitrogen oxides. Carbon monoxide concentrations range from 0.4 to 0.55 ppm prior to confinement measures and decrease to between 0.2 and 0.33 ppm following the start of the measures. Nitrogen monoxide concentrations range from 7 to 16 ppb prior to confinement measures and decrease to between 2 and 5.5 ppb following the start of the measures. Black carbon concentrations range from 0.25 to 0.46 µg/m3 prior to confinement measures and decrease to between 0.10 and 0.28 µg/m3 following the start of the measures. Nitrogen dioxide concentrations range from 9.5 to 13 ppb prior to confinement measures and decrease to between 1.8 and 10 ppb following the start of the measures. Ultrafine particulate matter concentrations range from 10,000 to 50,000 particles/cm3 prior to confinement measures and decrease to between 2,500 and 10,000 particles/cm3 following the start of the measures. Nitrogen oxide concentrations range from 18 to 30 ppb prior to confinement measures and decrease to between 3 and 15 ppb following the start of the measures.

The following figure illustrates the average concentrations of nitrogen monoxide (NO) measured at the Henri-IV station from Monday to Friday, based on the time of day.

Graph showing the average concentration of nitrogen monoxide, in parts per billion (ppb), before and during COVID-19. Over a 24-hour period, the curve representing the average concentration of nitrogen monoxide during COVID-19 continues to be at least 5 ppb lower than it was before the pandemic. The difference further increases at 7 a.m. and peaks around 9 a.m. It subsequently narrows until about noon, with the gap remaining closer to 10 ppb until 5 p.m., when the curve resumes its 5 ppb difference until the next morning.

As shown in the above two illustrations, the post-lockdown concentrations of nitrogen oxides were lower than they were prior to the outbreak of COVID-19. This situation is normal and not the result of lower highway traffic, but rather due to the natural ozone cycle and ultraviolet (UV) radiation. It turns out that the end of the lockdown coincided with the time of the year when ozone concentrations are higher. Ozone reacts rapidly with NO when the two gases are in contact, forming nitrogen dioxide (NO2). Additionally, UV radiation in turn reacts with NO2, creating ozone, which in the end lowers the concentrations of NOX.

Concentrations of other contaminants measured at the Henri-IV station, including, sulphur dioxide (SO2) and fine particulate matter (PM2.5), fluctuated less during the lockdown since they to a greater extent come from sources other than transportation, such as industry and wood heating.

Air quality monitoring elsewhere in Québec

The decrease in road traffic also appears to have had a positive impact on air quality elsewhere in Québec. The following figure illustrates the average weekly concentrations of nitrogen dioxide (NO2), another contaminant associated with road transportation, in parts per billion (ppb) in urban areas in Québec during the first lockdown period.

Graph showing the weekly concentrations of nitrogen dioxide (NO2), in parts per billion (ppb), between January 1 and May 4, 2020, in Gatineau, Brossard, Laval, Montréal, and Québec City. NO2 concentrations decreased on average by more than half starting the week of March 9, 2020. The decrease is even greater starting the week of March 30, 2020. As an example, in Montréal, NO2 concentrations averaged between 10 and 20 ppb before the pandemic, compared to between 5 and 10 ppb as of the week of March 9, 2020.

During the pandemic, Québec experienced lockdown episodes. The first three affected Québec from:

  • March 13 to April 24, 2020;
  • December 25, 2020 to March 7, 2020;
  • April 1, 2021 to May 28, 2021.

Statistical analysis made it possible to calculate the drop in NO2 concentrations in ambient air attributable to these three lockdowns due to COVID-19. The lockdown from December 20, 2021 to January 17, 2022 cannot be compared to the previous episodes, since the measures were implemented gradually.

Each of the three episodes caused significant declines in urban areas, but the first is the one that had the greatest impact. The declines of the second and third lockdowns are 40 to 50% lower than those of the first lockdown. The different levels observed in Gatineau can be explained by the fact that the health measures differed from those of its Ontario neighbour, Ottawa.

The impact of the curfew

During the second and third lockdown periods, the Québec population was subject to a curfew from 8 p.m. to 5 a.m. This restriction had a positive impact on urban air quality. The following figure shows that NO2 concentrations between 8 p.m. and 5 a.m. decreased compared to the average of the last five years.

To learn more

To learn more about air quality in Québec, go to the webpage of the Québec air quality monitoring network.

To get real-time information on air quality in your region, go to the Air Quality Index site This hyperlink will open in a new window..