The Government of Ontario declared a provincial state of emergency on March 17, 2020, in response to the global COVID‑19 pandemic. Various measures were put in place including a temporary stay-at-home order which reduced transportation and industrial activities across the province. The stay-at-home-order in 2020 offered a unique opportunity to evaluate how reduced transportation and industrial emissions impacted ambient levels of common air pollutants in Ontario. Air quality measurements collected through the ministry’s ambient and roadside air monitoring networks were assessed for three different time periods in 2020 to determine the impact of the stay-at-home-order on ambient air quality including 1) pre-COVID period, 2) stay-at-home period, and 3) re-opening period. Historical data collected from 2017 to 2019 was used as baseline information to assess the 2020 measurements.

2020 time periods assessed
PeriodDate
Pre-COVIDJanuary 1 to March 16
Stay-at-homeMarch 17 to May 18
Re-openingMay 19 to December 31

Impact on traffic volumes at the Highway 401 roadside air monitoring station

During the pre-COVID period, traffic volumes of cars and trucks in 2020 measured at the ministry’s Highway 401 roadside research air monitoring station were similar to the baseline years; however, traffic volumes were greatly reduced during the stay-at-home period in 2020 for both cars and trucks. Traffic volumes decreased by 33% for cars and 22% for trucks during the stay-at-home period. Traffic volumes gradually increased after some of the stay-at-home measures were lifted. During the re-opening period, car volumes were 14% lower while truck volumes were similar in comparison to the pre-COVID period.

Changes in traffic volumes at the Highway 401 roadside station

This is a bar graph showing change percentages in 2020 relative to the 2017-2019 baseline years for traffic volumes of cars and trucks during the stay-at-home and re-opening periods at the ministry’s Highway 401 roadside station.

Note:

  • Baseline years: 2017 to 2019.
  • Pre-COVID period: January 1 - March 18; Stay-at-home period: March 19 - May 18; Re-opening period: May 19 - December 31.

A summary of the traffic volumes and air pollution concentrations is detailed in the Appendix: Traffic volumes and air pollution concentrations in baseline years and changes in 2020 at the Highway 401 roadside station.

Impact on air pollutants at the Highway 401 roadside air monitoring station

During the stay-at-home period, concentrations of common air pollutants measured at the ministry Highway 401 roadside research air monitoring station decreased by 27% for black carbon (BC), 21% for ultrafine particle (UFP), 10% for nitrogen dioxide (NO2) and 35% for sulphur dioxide (SO2). Reduced traffic-related emissions due to less vehicular traffic in the near road environment was responsible for the observed decrease in pollutant concentrations. As traffic volumes increased during the re-opening period, levels of these air pollutants remained lower than the baseline period but to a lesser degree than during the stay-at-home period.

In contrast, concentrations of fine particulate matter (PM2.5) and ozone (O3) changed little during the stay-at-home period and were higher during the re-opening period in comparison to the baseline years (2017-2019). Fine particulate matter is emitted to air directly and formed as a secondary pollutant, and ozone is formed as a secondary pollutant. Both PM2.5 and O3 concentrations are impacted by local emissions and their regional background levels. The little change in levels of PM2.5 and O3 during the stay-at-home period and the increased levels of PM2.5 and O3 during the re-opening period reflect the influence of local/regional PM2.5 sources (e.g., forest fires) and its secondary formation in air, as well as reduced O3 titration effects due to the reduction in nitric oxide emissions (e.g., transportation emissions).

Changes in pollutant concentrations at the Highway 401 roadside station

This is a bar graph showing change percentages of pollution concentrations for BC, NO2, PM2.5 and O3 in 2020 relative to the baseline years during the stay-at-home and re-opening periods at Highway 401.

Note:

  • Baseline years: 2017 to 2019.
  • Pre-COVID period: January 1 - March 18; Stay-at-home period: March 19 - May 18; Re-opening period: May 19 - December 31.

Data summaries of pollution concentrations in the baseline years and changes in 2020 are presented in the Appendix: Baseline concentrations of black carbon and changes in 2020; Baseline concentrations of nitrogen dioxide and changes in 2020; Baseline concentrations of fine particulate matter and changes in 2020; and Baseline concentrations of ozone and changes in 2020.

Impact on air pollutants across the province

Concentrations of common and traffic-related air pollutants collected through the ministry’s Highway 401 roadside research air monitoring station and select ambient air monitoring stations located in close proximity to major roads were assessed against the baseline years (2017-2019).

Average change in pollutant concentrations during stay-at-home and re-opening periods in 2020
PollutantsStay-at-home periodRe-opening period
BC-27% (-34% to -20%)-18% (-25% to -4%)
NO2-10% (-16% to -1%)2% (-12% to 19%)
PM2.55% (-5% to 17%)10% (-1% to 23%)
O3-2% (-4% to 1%)6% (-8% to 13%)

This table provides a summary of the average changes in common and traffic related air contaminants at the ministry’s Highway 401 roadside air monitoring station and select ambient air monitoring stations during the stay-at-home and re-opening periods.

Note:

  • Ranges of changes are in parentheses.
  • Baseline years: 2017 to 2019.
  • Pre-COVID period: January 1 - March 18; Stay-at-home period: March 19 - May 18; Re-opening period: May 19 - December 31.

Changes in black carbon concentrations across the province

  • 2020 stay-at-home period: Average change percentage of BC concentrations was -27%, ranging from -34% at Windsor Downtown to -20% at Etobicoke South.
  • 2020 re-opening period: Average change percentage of BC concentrations was -18%, ranging from -25% at Windsor Downtown to -4% at Etobicoke South.
  • The reduced BC concentrations in the stay-at-home and re-opening periods are attributable to the stay-at-home measures and associated decrease in traffic emissions.

Changes in NO2 concentrations across the province

  • 2020 stay-at-home period: Average change percentage of NO2 concentrations was -7%, ranging from -16% at Etobicoke South to -1% at Windsor Downtown and Toronto North.
  • 2020 re-opening period: Average change percentage of NO2 concentrations was 2%, ranging from -12% at Etobicoke South to 19% at North Bay.
  • The decreases in NO2 concentrations during the stay-at-home and re-opening periods reflect reduced traffic emissions, especially at stations close to major roads like the Highway 401 roadside station. At a few stations (e.g., North Bay), the decrease in NO2 concentrations during the re-opening period is less than that observed during the pre-COVID period, likely reflective of a combination of increased traffic during the re-opening period and different inter-annual variability between the pre-COVID and re-opening periods.

Changes in PM2.5 concentrations across the province

  • 2020 stay-at-home period: Average change percentage of PM2.5 concentrations was 5%, ranging from -5% at North Bay to 17% at Thunder Bay.
  • 2020 re-opening period: Average change percentage of PM2.5 concentrations was 9%, ranging from -1% at North Bay to 19% at Hamilton West.
  • The change in PM2.5 concentrations varied among stations across the province reflecting the influence of local and regional PM2.5 sources (e.g., forest fires) and its secondary formation in air.

Changes in O3 concentrations across the province

  • 2020 stay-at-home period: Average change percentage of O3 concentrations was -2%, ranging from -4% at Windsor Downtown and Thunder Bay to 1% at a few stations.
  • 2020 re-opening period: Average change percentage of O3 concentrations was 6%, ranging from -8% at North Bay to 13% at Hamilton West.
  • The changes in O3 concentrations varied among stations across the province. The slight reduction during the stay-at-home period is attributable to decreased precursor emissions (i.e., volatile organic compounds and nitrogen oxides). The increase during the re-opening period reflects recovered transportation and industrial related activities. Higher O3 levels are also attributable to reduced titration effects due to the reduction in nitric oxide emissions (e.g., transportation emissions).

Additional information on specific air contaminants is presented in the Appendix: Baseline concentrations of black carbon and changes in 2020; Baseline concentrations of nitrogen dioxide and changes in 2020; Baseline concentrations of fine particulate matter and changes in 2020; and Baseline concentrations of ozone and changes in 2020.