Air quality

Fine particulate matter

Fine particulate matter (PM_2.5) is emitted directly to the air and can also be formed in the air through the interactions of other pollutants, such as nitrogen oxides, sulphur oxides, ammonia and volatile organic compounds. The largest human-caused emission sources of PM_2.5 are open sources, mainly dust from unpaved roads, construction operations and agriculture (crop production). Home firewood burning is the largest non-open source contributor to PM_2.5 emissions. PM_2.5 is also emitted in wildfire smoke. PM_2.5 is one of the major components of smog and one of the most widespread outdoor pollutants. Exposure to PM_2.5 can lead to the onset or development of adverse respiratory and cardiovascular effects, such as asthma attacks, chronic bronchitis, heart attacks as well as lung cancer.^{Footnote 5} Fine particulate matter can also damage vegetation and structures, contribute to acidification and eutrophication^{Footnote 6} of ecosystems, contribute to haze and reduce visibility.

National annual average fine particulate matter concentrations

Key results

• From 2009 to 2023,
  • no significant trend was detected in the national average PM_2.5 concentrations
  • the national average PM_2.5 concentration exceeded the 2020 standard of 8.8 micrograms per cubic metre (µg/m³) only in 2023
• In 2023, the national average PM_2.5 concentration was the highest over the 15-year record, consistent with the unprecedented wildfire season

National average fine particulate matter concentrations, Canada, 2009 to 2023

How is this indicator calculated

Note: n/a = not applicable. The national average PM_2.5 concentration indicator is based on the annual average of the daily 24-hour average concentrations recorded at 161 monitoring stations across Canada. The comparison to the Canadian Ambient Air Quality Standard is provided for illustrative purposes only. A statistically significant trend is reported when the Mann-Kendall test indicates the presence of a trend at the 95 percent confidence level. For more information, consult the Methods section.
Source: Environment and Climate Change Canada (2025) National Air Pollution Surveillance (NAPS) Program - Open Government Portal.

From 2009 to 2023, no trend was observed in the national average PM_2.5 concentration. The highest concentration was recorded in 2023 (9.5 micrograms per cubic metre), which was the only year in the 15-year reporting period when the concentration was above the 2020 standard. The 2023 national average PM_2.5 concentration was 62 percent larger than 2009. This was consistent with the 2023 wildfire season, which had the largest area burned by wildfires since 1970.

Year-to-year variation in average PM_2.5 concentrations are related not only to changes in the quantity of emissions of PM_2.5 and its precursors, but also to wildfire conditions and annual variations in weather conditions that influence the formation, dispersion and transport of PM_2.5, including transboundary movement of PM_2.5 from the United States.

The variations observed in average PM_2.5 concentrations were also affected by the progressive introduction of monitoring equipment, from the mid 2000s to 2013, that uses newer measurement technologies. These new instruments measure an additional (semi-volatile) portion of the PM_2.5 mass not captured by the older instruments. This should be considered when comparing measurements from newer monitors with those from years in which older instruments were used.

Regional annual average fine particulate matter concentrations

Key results

• From 2009 to 2023,
  • an increasing trend was detected for annual average PM_2.5 concentrations in British Columbia
  • decreasing trends were detected in the Atlantic Canada and southern Quebec regions
  • no trends were detected in other regions
• In 2023, the annual average PM_2.5 concentrations exceeded the 2020 standard of 8.8 micrograms per cubic metre in the northern territories and the Prairies and northern Ontario regions

Regional annual average fine particulate matter concentrations, Canada, 2009 to 2023

How is this indicator calculated

Note: n/a = not applicable. The regional annual average PM_2.5 concentration indicator is based on the annual average of the daily 24-hour average concentrations recorded at 16 monitoring stations in the Atlantic Canada region, 35 in the southern Quebec region, 37 in the southern Ontario region, 35 in the Prairies and northern Ontario region, 35 in British Columbia and 3 in northern territories region. The comparison to the Canadian Ambient Air Quality Standard is provided for illustrative purposes only. A statistically significant trend is reported when the Mann-Kendall test indicates the presence of a trend at the 95 percent confidence level. For more information, consult the Methods section.
Source: Environment and Climate Change Canada (2025) National Air Pollution Surveillance (NAPS) Program - Open Government Portal.

In 2023, the regions of the Prairies and northern Ontario and the northern territories exceeded the 2020 standard of 8.8 micrograms per cubic metre, with concentrations of 16.1 micrograms per cubic metre and 15.7 micrograms per cubic metre, respectively. The southern Ontario, Atlantic Canada and British Columbia regions reported regional average concentrations of 8.8 micrograms per cubic metre, 5.4 micrograms per cubic metre and 7.0 micrograms per cubic metre, respectively.

Average fine particulate matter concentrations at monitoring stations

The National Air Pollution Surveillance program measures air pollutant concentrations at monitoring stations across Canada. The Canadian Environmental Sustainability Indicators program provides access to this information through an interactive map. The map allows users to explore average PM_2.5 concentrations at specific monitoring stations.

Key results

In 2023, annual average PM_2.5 concentrations were recorded at 201 monitoring stations across Canada.

• 92 stations recorded annual average concentrations above the 2020 standard of 8.8 micrograms per cubic metre. These stations were in Alberta (38), Ontario (20), Quebec (15), British Columbia (12), Saskatchewan (6) and the Northwest Territories (1)
• 2 stations recorded annual average concentrations below 4.0 micrograms per cubic metre, both located in British Columbia
• no monitoring station data were available for Manitoba or Nunavut

Average fine particulate matter concentrations by monitoring station, Canada, 2023

Navigate data using the interactive map

How this indicator was calculated

Source: Environment and Climate Change Canada (2025) National Air Pollution Surveillance (NAPS) Program - Open Government Portal.

National annual average peak fine particulate matter concentrations

Key results

• From 2009 to 2023,
  • no trend was detected in the national average peak PM_2.5 concentration
  • national average peak concentrations exceeded the 2020 standard of 27 micrograms per cubic metre in 2018 and 2023
• In 2023, the national average peak PM_2.5 concentration was the highest over the 15-year record, largely due to the unprecedented wildfire season

National average peak fine particulate matter concentrations, Canada, 2009 to 2023

How is this indicator calculated

Note: n/a = not applicable. The national average peak PM_2.5 concentration indicator is based on the annual 98th percentile of the daily 24-hour average concentrations recorded at 161 monitoring stations across Canada. The comparison to the Canadian Ambient Air Quality Standard is provided for illustrative purposes only. A statistically significant trend is reported when the Mann-Kendall test indicates the presence of a trend at the 95 percent confidence level. For more information, consult the Methods section.
Source: Environment and Climate Change Canada (2025) National Air Pollution Surveillance (NAPS) Program - Open Government Portal.

In 2023, the national average peak PM_2.5 concentration was 45.1 micrograms per cubic metre, the highest in the last 15 years and 167 percent and 141 percent larger than 2009 and 2022 respectively. It also exceeded the 2020 standard of 27 micrograms per cubic metre. This record-high concentration was the result of the wildfire affecting all regions in Canada that year. These wildfires saw the largest area burned since 1970.

From 2009 to 2023, no trend was detected in the national average peak PM_2.5 concentrations. Over that period, concentrations remained relatively stable except for the years 2017, 2018 and 2023. The higher concentrations observed for those years can be attributed primarily to wildfire activity.

Changes in peak PM_2.5 concentrations are linked not only to changes in the quantity of emissions, but also to annual variations in wildfire activity and in weather conditions that influence the formation, dispersion, and transport of PM_2.5 including transboundary movement of PM_2.5 from the United States.

The PM_2.5 concentrations were also influenced by the progressive introduction of monitoring equipment that uses newer measurement technologies. From 2000 to 2013, new PM_2.5 monitoring equipment was progressively introduced across Canada to replace older monitoring equipment. These new instruments measure an additional (semi-volatile) portion of the PM_2.5 mass not captured by the older instruments. This should be considered when comparing measurements from newer monitors with those from years in which older instruments were used.

Regional annual average peak fine particulate matter concentrations

Key results

• From 2009 to 2023,
  • a decreasing trend was detected in annual average peak PM_2.5 concentrations in the Atlantic Canada region
  • no trend was detected for the other regions

• In 2023,
  • annual average peak PM_2.5 concentrations exceeded the 2020 standard of 27 micrograms per cubic metre in all regions, except Atlantic Canada.
  • the southern Quebec and southern Ontario regions recorded concentrations exceeding the standard for the first time in last 15 years

Regional annual average peak fine particulate matter concentrations, Canada, 2009 to 2023

How is this indicator calculated

Note: n/a = not applicable. The regional average peak PM_2.5 concentration indicator is based on the annual 98th percentile of the daily 24-hour average concentrations recorded at 16 monitoring stations in the Atlantic Canada region, 35 in the southern Quebec region, 37 in the southern Ontario region, 35 in the Prairies and northern Ontario region, 35 in British Columbia and 3 in the northern territories region. The comparison to the Canadian Ambient Air Quality Standard is provided for illustrative purposes only. A statistically significant trend is reported when the Mann-Kendall test indicates the presence of a trend at the 95 percent confidence level. For more information, consult the Methods section.
Source: Environment and Climate Change Canada (2025) National Air Pollution Surveillance (NAPS) Program - Open Government Portal.

In 2023, annual average peak PM_2.5 concentrations were larger than in 2022 for all regions: by 24 percent in British Columbia, 25 percent in Atlantic Canada, 95 percent in southern Quebec, 108 percent in southern Ontario, 263 percent in the northern territories and 361 percent in the Prairies and northern Ontario. The substantial increase in concentrations is largely driven by the record 2023 wildfire season.

Except for Atlantic Canada and British Columbia, the regional average peak PM_2.5 concentrations in 2023 were the highest in the 15-year period.

Peak fine particulate matter concentrations at monitoring stations

The National Air Pollution Surveillance program measures air pollutant concentrations at monitoring stations across Canada. The Canadian Environmental Sustainability Indicators program provides access to this information through an interactive map. The map allows users to explore peak PM_2.5 concentrations at specific monitoring stations.

Key results

In 2023, annual average peak PM_2.5 concentrations were recorded at 204 monitoring stations across Canada. The highest peak PM_2.5 concentrations were generally recorded at monitoring stations in central Canada.

• 120 stations recorded annual average peak concentrations above the 2020 standard 27.0 micrograms per cubic metre, ranging from 27 micrograms per cubic metre to 212 micrograms per cubic metre. The majority of these stations were located in Alberta (40), Ontario (37), Quebec (19) and British Columbia (15)
• no stations recorded annual average peak concentrations below 10.0 micrograms per cubic metre
• no monitoring station data were available for Manitoba or Nunavut

Peak fine particulate matter concentrations by monitoring station, Canada, 2023

Navigate data using the interactive map

How this indicator was calculated

Source: Environment and Climate Change Canada (2025) National Air Pollution Surveillance (NAPS) Program - Open Government Portal.

Ground-level ozone

Ozone (O₃) is a gas that, when present in the upper atmosphere (10 to 50 kilometres above the earth's surface), protects plant, animal and human health from the sun's harmful ultraviolet radiation. In the lower atmosphere and at ground level, O₃ is a secondary pollutant formed through reactions between precursor gases such as nitrogen oxides and volatile organic compounds in the presence of sunlight. Exposure to O₃ is harmful to human health and can cause throat irritation, coughing, shortness of breath and aggravation of existing conditions such as asthma. Over time, exposure to O₃ may lead to development of asthma, reduced lung function and other lung conditions.^{Footnote 5}   Ground-level O₃ can impact vegetation, decrease the productivity of some crops, and may contribute to forest decline. It can also damage synthetic materials and textiles, cause cracks in rubber, accelerate fading of dyes and speed deterioration of some paints and coatings. Ground-level O₃ is a major component of smog, along with fine particulate matter.

National annual average ground-level ozone concentrations

Key results

• From 2009 to 2023,
  • no trend was detected in the national average O₃ concentrations
  • national average concentrations showed low variability ;
• In 2023, the national average O₃ concentration showed the highest level since 2009

National average ozone concentrations, Canada, 2009 to 2023

How this indicator was calculated

Note: n/a = not applicable. The national average O₃ concentration indicator is based on the annual average of the daily maximum 8-hour average concentrations recorded at 169 monitoring stations across Canada. No comparison with CAAQS is shown as there is no comparable O₃ standard. A statistically significant trend is reported when the Mann-Kendall test indicates the presence of a trend at the 95 precent confidence level. For more information, consult the Methods section.
Source: Environment and Climate Change Canada (2025) National Air Pollution Surveillance (NAPS) Program - Open Government Portal and the Canadian Air and Precipitation Monitoring Network (CAPMoN).

In 2023, the national average O3 concentration was 34.2 parts per billion, slightly higher than the average concentration observed in previous years (2009-2022), which ranged from 31.9 parts per billion to 34.0 parts per billion.

Regional annual average ground-level ozone concentrations

Key results

• From 2009 to 2023,
  • increasing trends were detected in annual average O₃ concentrations for the Atlantic Canada and southern Quebec regions
  • no trends were detected for the other regions
• In 2023, the southern Ontario region showed the highest regional average O₃ concentration

Regional average ozone concentrations, Canada, 2009 to 2023

How this indicator was calculated

Note: n/a = not applicable. The regional annual average O₃ concentration indicator is based on the annual average of the daily maximum 8-hour average concentrations recorded at 18 monitoring stations in the Atlantic Canada region, 39 in the southern Quebec region, 40 in the southern Ontario region, 34 in the Prairies and northern Ontario region, and 34 in British Columbia and 4 in the northern territories region. No comparison with CAAQS is shown as there is no comparable O₃ standard. A statistically significant trend is reported when the Mann-Kendall test indicates the presence of a trend at the 95 percent confidence level. For more information, consult the Methods section.
Source: Environment and Climate Change Canada (2025) National Air Pollution Surveillance (NAPS) Program - Open Government Portal and the  Canadian Air and Precipitation Monitoring Network (CAPMoN).

In 2023, the southern Ontario region had a regional average O₃ concentration of 37.1 parts per billion, followed by the Prairies and northern Ontario region with 36.6 parts per billion, the southern Quebec with 34.6 parts per billionand the Atlantic Canada regions with 33.8 parts per billion. The average concentrations in the northern territories and British Columbia regions were lower, at 29.3 parts per billion and 28.9 parts per billion, respectively.

Average ground-level ozone concentrations at monitoring stations

The National Air Pollution Surveillance program measures air pollutant concentrations at monitoring stations across Canada. The Canadian Environmental Sustainability Indicators program provides access to this information through an interactive map. The map allows users to explore average O₃ concentrations at specific monitoring stations.

Key results

In 2023, annual average O₃ concentrations were recorded at 214 monitoring stations across Canada. Of these:

• 10 stations had an annual average concentration above 40 parts per billion, ranging from 41 parts per billion to 44 parts per billion. One (1) station was in Newfoundland and Labrador, and the 9 other stations were in Alberta
• 7 stations had an annual average concentrations below 25 parts per billion. Of these stations, 5 were in British Columbia and 1 station each in Manitoba and the Northwest Territories

Average ozone concentrations by monitoring station, Canada, 2023

Navigate data using the interactive map

How this indicator was calculated

Source: Environment and Climate Change Canada (2025) National Air Pollution Surveillance (NAPS) Program - Open Government Portal and the Canadian Air and Precipitation Monitoring Network (CAPMoN).

National annual average peak ground-level ozone concentrations

Key results

From 2009 to 2023,

• no trend was detected in the national average peak O₃ concentrations
• national average peak concentrations remained below the 2020 standard of 62 parts per billion

National average peak ozone concentrations, Canada, 2009 to 2023

How this indicator was calculated

Note: n/a = not applicable. The national average peak O₃ concentration indicator is based on the annual 4th-highest of the daily maximum 8-hour average concentrations recorded at 169 monitoring stations across Canada. The comparison to the Canadian Ambient Air Quality Standard is provided for illustrative purposes only. A statistically significant trend is reported when the Mann-Kendall test indicates the presence of a trend at the 95 percent confidence level. For more information, consult the Methods section.
Source: Environment and Climate Change Canada (2025) National Air Pollution Surveillance (NAPS) Program - Open Government Portal and the Canadian Air and Precipitation Monitoring Network (CAPMoN).

In 2023, the national average peak O₃ concentration was 59.0 parts per billion.

From 2009 to 2023, no trend was observed; concentrations showed low variability and were below the 2020 standard of 62 parts per billion.

Regional annual average peak ground-level ozone concentrations

Key results

• From 2009 to 2023, a decreasing trend was detected for regional average peak O₃ concentrations only in the northern territories region
• In 2023, regional average peak concentrations in the southern Ontario and Prairies and northern Ontario regions exceeded the 2020 standard of 62 parts per billion

Regional average peak ozone concentrations, Canada, 2009 to 2023

How this indicator was calculated

Note: n/a = not applicable. The regional annual average peak O₃ indicator is based on the annual 4th-highest of the daily maximum 8-hour average concentrations recorded at 18 monitoring stations in the Atlantic Canada region, 39 in the southern Quebec region, 40 in the southern Ontario region, 34 in the Prairies and northern Ontario region, 34 in British Columbia and 4 in the northern territories region. The comparison to the Canadian Ambient Air Quality Standard is provided for illustrative purposes only. A statistically significant trend is reported when the Mann-Kendall test indicates the presence of a trend at the 95 percent confidence level. For more information, consult the Methods section.
Source: Environment and Climate Change Canada (2025) National Air Pollution Surveillance (NAPS) Program - Open Government Portal and the Canadian Air and Precipitation Monitoring Network (CAPMoN).

In 2023, the southern Ontario region recorded the highest regional average peak O₃ concentration at 66.2 parts per billion while the northern territories region recorded the lowest at 45.3 parts per billion.

Peak ground-level ozone concentrations at monitoring stations

The National Air Pollution Surveillance program measures air pollutant concentrations at monitoring stations across Canada. The Canadian Environmental Sustainability Indicators program provides access to this information through an interactive map. The map allows users to explore peak O₃ concentrations at specific monitoring stations.

Key results

In 2023, annual average peak O₃ concentrations were recorded at 214 monitoring stations across Canada:

• 85 stations had an annual average peak concentration above the 2020 standard of 62 parts per billion, mostly located in Ontario (34), Alberta (32) and Quebec (12)
• 12 stations recorded an annual average peak concentration below 45 parts per billion. Of these stations, 8 were in British Columbia and 1 station each in Northwest Territories, Manitoba, New Brunswick and Newfoundland and Labrador

Average peak ozone concentrations by monitoring station, Canada, 2023

Navigate data using the interactive map

How this indicator was calculated

Source: Environment and Climate Change Canada (2025) National Air Pollution Surveillance (NAPS) Program - Open Government Portal and the Canadian Air and Precipitation Monitoring Network (CAPMoN).

Nitrogen dioxide

Nitrogen dioxide (NO₂) belongs to a group of substances called nitrogen oxides (NO_X). Nitrogen oxides are emitted into the atmosphere from high-temperature combustion processes such as those found in vehicle engines, power plants and industrial processes. The main sources of nitrogen oxide emissions in Canada are industry, (particularly oil and gas) and transportation, mainly off-road vehicles and mobile equipment. The majority of emitted NO_X is nitrogen monoxide (NO); however, once in the atmosphere, NO reacts quickly with volatile organic compounds and ozone to form NO₂. Exposure to NO₂ can result in adverse health effects, such as lung irritation, decreased lung function, and increased susceptibility to allergens for people with asthma. Long-term exposure to NO₂ may lead to the development of allergies and asthma.^{Footnote 5} NO₂ also has adverse environmental impacts. It contributes to the formation of O₃ and PM_2.5 as well as acid deposition (acid rain) and eutrophication^{Footnote 6} of aquatic ecosystems.

National annual average nitrogen dioxide concentrations

Key results

From 2009 to 2023,

• a decreasing trend was detected in the national average NO₂ concentrations
• the national average concentrations remained below the 2020 standard of 17.0 parts per billion

National average nitrogen dioxide concentrations, Canada, 2009 to 2023

How this indicator was calculated

Note: n/a = not applicable. The national average NO₂ concentration indicator is based on the annual average of the hourly concentrations recorded at 128 monitoring stations across Canada. The comparison to the Canadian Ambient Air Quality Standard is provided for illustrative purposes only. A statistically significant trend is reported when the Mann-Kendall test indicates the presence of a trend at the 95 percent confidence level. For more information, consult the Methods section.
Source:Environment and Climate Change Canada (2025) National Air Pollution Surveillance (NAPS) Program - Open Government Portal.

In 2023, the national average NO₂ concentration was 6.1 parts per billion, which was the second lowest concentration since 2009.

From 2009 to 2023, the national average NO₂ concentration did not exceed the 2020 standard of 17.0 parts per billion, and a decreasing trend was detected. The national average concentration decreased by 34.5 pourcent (3.2 parts per billion) between 2009 and 2023. This trend is mainly attributable to 2 factors:

• lower emissions from vehicles and engines following the adoption of new technologies and clean fuel for vehicles and the introduction of progressively more stringent emission regulations by the federal government
• lower emissions from fossil-fuel-fired (for example, coal-fired) power-generating utilities due to improved emission control technologies and the closures of some coal-fired power plants

Regional annual average nitrogen dioxide concentrations

Key results

• From 2009 to 2023, decreasing trends were detected for all regions
• Since 2009, regional average NO₂ concentrations remained below the 2020 standard of 17.0 parts per billion in all regions

Regional average nitrogen dioxide concentrations, Canada, 2009 to 2023

How this indicator was calculated

Note: n/a = not applicable. The regional annual average NO₂ concentration indicator is based on the annual average of the hourly concentrations recorded at 12 monitoring stations in the Atlantic Canada region, 16 in the southern Quebec region, 32 in the southern Ontario region, 34 in the Prairies and northern Ontario region and 32 in British Columbia. There were not enough stations to report results for the northern territories region. The comparison to the Canadian Ambient Air Quality Standard is provided for illustrative purposes only. A statistically significant trend is reported when the Mann-Kendall test indicates the presence of a trend at the 95 percent confidence level. For more information, consult the Methods section.
Source: Environment and Climate Change Canada (2025) National Air Pollution Surveillance (NAPS) Program - Open Government Portal.

In 2023, British Columbia had the highest regional average NO₂ concentration, at 7.3 parts per billion. The Prairies and northern Ontario, southern Quebec and southern Ontario regions followed with concentrations of 6.4 parts per billion, 6.3 parts per billion and 5.9 parts per billion, respectively. The lowest regional average concentration was recorded in the Atlantic Canada region, at 2.3 parts per billion.

Average nitrogen dioxide concentrations at monitoring stations

The National Air Pollution Surveillance program measures air pollutant concentrations at monitoring stations across Canada. The Canadian Environmental Sustainability Indicators program provides access to this information through an interactive map. The map allows users to explore average NO₂ concentrations at specific monitoring stations.

Key results

In 2023, average NO₂ concentrations were recorded at 171 monitoring stations across Canada:

• no stations had annual average concentrations above 17.0 parts per billion
• 51 stations had annual average concentrations below 4.0 parts per billion; notably in Alberta (13) and Ontario (9)
• no monitoring station data were available for Nunavut and the Northwest Territories

Average nitrogen dioxide concentrations by monitoring station, Canada, 2023

Navigate data using the interactive map

How this indicator was calculated

Source: Environment and Climate Change Canada (2025) National Air Pollution Surveillance (NAPS) Program - Open Government Portal.

National annual average peak nitrogen dioxide concentrations

Key results

From 2009 to 2023,

• a decreasing trend was detected in the national average peak NO₂ concentrations
• national average peak concentrations remained below the 2020 standard of 60 parts per billion

National average peak nitrogen dioxide concentrations, Canada, 2009 to 2023

How this indicator was calculated

Note: n/a = not applicable. The national average peak NO₂ concentration indicator is based on the annual 98th percentile of the daily maximum 1-hour average concentrations recorded at 128 monitoring stations across Canada. The comparison to the Canadian Ambient Air Quality Standard is provided for illustrative purposes only. A statistically significant trend is reported when the Mann-Kendall test indicates the presence of a trend at the 95 percent confidence level. For more information, consult the Methods section.
Source: Environment and Climate Change Canada (2025) National Air Pollution Surveillance (NAPS) Program - Open Government Portal.

In 2023, the national average peak NO₂ concentration was 31.9 parts per billion, the lowest concentration recorded in the last 15 years.

From 2009 to 2023, the national average peak NO₂ concentration was below the 2020 standard of 60 parts per billion with a downward trend detected. This trend is mainly attributable to 2 factors:

• lower emissions from vehicles and engines following the adoption of new regulations and clean fuel for vehicles and the introduction of progressively more stringent emission regulations by the federal government
• lower emissions from fossil-fuel-fired (for example, coal-fired) power-generating utilities from improved emission control technologies and the closures of some coal-fired power plants

Regional annual average peak nitrogen dioxide concentrations

Key results

• From 2009 to 2023, decreasing trends were detected for all regions
• Since 2009, regional average peak NO₂ concentrations have remained below the 2020 standard of 60 parts per billion in all regions

Regional average peak nitrogen dioxide concentrations, Canada, 2009 to 2023

How this indicator was calculated

Note: n/a = not applicable. The regional average peak NO₂ concentration indicator is based on the annual 98th percentile of the daily maximum 1-hour average concentrations recorded at 12 monitoring stations in the Atlantic Canada region, 16 in the southern Quebec region, 32 in the southern Ontario region, 34 in the Prairies and northern Ontario region and 32 in British Columbia. There were not enough stations to report results for the northern territories region. The comparison to the Canadian Ambient Air Quality Standard is provided for illustrative purposes only. A statistically significant trend is reported when the Mann-Kendall test indicates the presence of a trend at the 95 percent confidence level. For more information, consult the Methods section.
Source: Environment and Climate Change Canada (2025) National Air Pollution Surveillance (NAPS) Program - Open Government Portal.

In 2023, 3 of the 5 regions recorded the lowest regional average peak NO₂ concentrations of the last 15 years: the southern Quebec region, the southern Ontario region and British Columbia. The Prairies and northern Ontario and the southern Quebec regions had the highest regional average peak NO₂ concentrations, at 35.4 parts per billion and 35.2 parts per billion, respectively, followed by the southern Ontario region with a concentration of 32.5 parts per billion and British Columbia with 29.9 parts per billion. The Atlantic Canada region had the lowest regional average peak concentration, with 22.4 parts per billion.

Peak nitrogen dioxide concentrations at monitoring stations

The National Air Pollution Surveillance program measures air pollutant concentrations at monitoring stations across Canada. The Canadian Environmental Sustainability Indicators program provides access to this information through an interactive map. The map allows users to explore peak NO₂ concentrations at specific monitoring stations.

Key results

In 2023, peak NO₂ concentrations were recorded at 171 monitoring stations across Canada. Of these:

• 1 station located in Alberta recorded an annual peak concentration above the 2020 standard of 60.0 parts per billion (70.3 parts per billion)
• 76 stations had annual peak concentrations below 30.0 parts per billion; most of them were in British Columbia (23), Alberta (16) and Ontario (12)
• no monitoring station data were available for Nunavut and the Northwest Territories

Peak nitrogen dioxide concentrations by monitoring station, Canada, 2023

Navigate data using the interactive map

How this indicator was calculated

Source: Environment and Climate Change Canada (2025) National Air Pollution Surveillance (NAPS) Program - Open Government Portal.

Sulphur dioxide

Sulphur dioxide (SO₂) is emitted when a fuel or raw material containing sulphur is burned or used in industrial processes such as metal ore smelting. The main sources of sulphur dioxide emissions in Canada are the oil and gas industry, combustion of fossil fuels for electricity generation, and processes in the non-ferrous smelting and refining industry. Sulphur dioxide emissions contribute to acid deposition and are a major precursor to fine particulate matter. Exposure to high concentrations of SO₂ can adversely affect the respiratory systems of humans and animals. Sulfur dioxide (SO₂) exposure can irritate the lungs, reduce lung function, and increase susceptibility to allergens in people with asthma. It can also damage vegetation and contributes to the deterioration of building materials such as paint or concrete.

National annual average sulphur dioxide concentrations

Key results

• From 2009 to 2023,
  • a decreasing trend was detected in the national average SO₂ concentrations
  • national average concentrations remained below the 2020 standard of 5.0 parts per billion
• In 2023, the national average SO₂ concentration was the lowest in the last 15 years

National average sulphur dioxide concentrations, Canada, 2009 to 2023

How this indicator was calculated

Note: n/a = not applicable. The national average SO₂ concentration indicator is based on the annual average of the hourly concentrations recorded at 88 monitoring stations across Canada. The comparison to the Canadian Ambient Air Quality Standard is provided for illustrative purposes only. A statistically significant trend is reported when the Mann-Kendall test indicates the presence of a trend at the 95 percent  confidence level. For more information, consult the Methods section.
Source: Environment and Climate Change Canada (2025) National Air Pollution Surveillance (NAPS) Program - Open Government Portal.

In 2023, the national average SO₂ concentration was 0.7 parts per billion, which was the lowest in 15 years and 4.2 percent lower than the 2022 level.

From 2009 to 2023, no exceedance of the 2020 standard was recorded in the national concentrations. A decreasing trend was also detected over this period. National concentrations decreased by 51.2 percent (0.7 parts per billion) between 2009 and 2023. This trend is mainly attributable to reductions in sulphur oxide emissions in Canada resulting from technological upgrades and closures of non-ferrous metal smelters (including aluminium smelters) and pulp and paper facilities, the phase-out of coal-fired electricity generation and transition to natural gas-fired electricity generation, better emission control technologies within the oil and gas sector, and the implementation of federal regulations related to sulphur content in fuels.

Regional annual average sulphur dioxide concentrations

Key results

• From 2009 to 2023, decreasing trends were detected for all regions
• Since 2009, regional average SO₂ concentrations remained below the 2020 standard of 5.0 parts per billion in all regions

Regional average sulphur dioxide concentrations, Canada, 2009 to 2023

How this indicator was calculated

Note: n/a = not applicable. The regional annual average SO₂ concentration indicator is based on the annual average of the hourly concentrations recorded at 10 monitoring stations in the Atlantic Canada region, 9 in the southern Quebec region, 10 in the southern Ontario region, 31 in the Prairies and northern Ontario region, 26 in British Columbia. There were not enough stations to report results for the northern territories region. The comparison to the Canadian Ambient Air Quality Standard is provided for illustrative purposes only. A statistically significant trend is reported when the Mann-Kendall test indicates the presence of a trend at the 95 percent confidence level. For more information, consult the Methods section.
Source: Environment and Climate Change Canada (2025) National Air Pollution Surveillance (NAPS) Program - Open Government Portal.

Regional average SO₂ concentrations should not be compared between regions because concentrations are highly dependent on the locations of the monitoring stations within the region. Concentrations can also be affected by year-to-year changes in sampling, for example if certain stations are offline in a given year.^{Footnote 7}

Average sulphur dioxide concentrations at monitoring stations

The National Air Pollution Surveillance program measures air pollutant concentrations at monitoring stations across Canada. The Canadian Environmental Sustainability Indicators program provides access to this information through an interactive map. The map allows users to explore annual average SO₂ concentrations at specific monitoring stations.

Key results

In 2023, average SO₂ concentrations were recorded at 121 monitoring stations across Canada. Of these:

• 1 station in Quebec recorded an annual average concentration above the 2020 standard of 5.0 parts per billion, at 5.7 parts per billion
• 67 stations had annual average concentrations below 0.5 parts per billion. Most of these stations were in British Columbia (23) and Alberta (20)
• no monitoring station data were available for Nunavut or Yukon

Average sulphur dioxide concentrations by monitoring station, Canada, 2023

Navigate data using the interactive map

How this indicator was calculated

Source: Environment and Climate Change Canada (2025) National Air Pollution Surveillance (NAPS) Program - Open Government Portal.

National annual average peak sulphur dioxide concentrations

Key results

From 2009 to 2023,

• a decreasing trend was detected in the national average peak SO₂ concentrations
• national average peak concentration remained below the 2020 standard of 70 parts per billion for all years

National average peak sulphur dioxide concentrations, Canada, 2009 to 2023

How this indicator was calculated

Note: n/a = not applicable. The national average peak SO₂ concentration indicator is based on the annual 99th percentile of the daily maximum 1-hour average concentrations recorded at 88 monitoring stations across Canada. The comparison to the Canadian Ambient Air Quality Standard is provided for illustrative purposes only. A statistically significant trend is reported when the Mann-Kendall test indicates the presence of a trend at the 95 percent confidence level. For more information, consult the Methods section.
Source: Environment and Climate Change Canada (2025) National Air Pollution Surveillance (NAPS) Program - Open Government Portal.

In 2023, the national average peak SO₂ concentration was 22.5 parts per billion, which is the fourth lowest peak concentration recorded over the past 15 years.

From 2009 to 2023, a decreasing trend was detected, with national concentrations falling by 38.1 percent (13.9 parts per billion). This trend is mainly attributable to reductions in sulphur oxide emissions in Canada and the United States due to technological upgrades and closures of non-ferrous metal smelters, the phase-out of coal-fired electricity, better emission control technologies within the oil and gas sector and the implementation of federal regulations related to sulphur content in fuels.

Regional annual average peak sulphur dioxide concentrations

Key results

From 2009 to 2023,

• decreasing trends were detected in average peak SO₂ concentrations for all regions except the Prairies and northern Ontario
• regional average peak SO₂ concentrations remained below the 2020 standard of 70 parts per billion in all regions

Regional average peak sulphur dioxide concentrations, Canada, 2009 to 2023

How this indicator was calculated

Note: n/a = not applicable. The regional average peak SO₂ concentration indicator is based on the annual 99th percentile of the daily maximum 1-hour average concentrations recorded at 10 monitoring stations in the Atlantic Canada region, 9 in the southern Quebec region, 10 in the southern Ontario region, 31 in the Prairies and northern Ontario region and 26 in British Columbia. There were not enough stations to report results for the northern territories region. The comparison to the Canadian Ambient Air Quality Standard is provided for illustrative purposes only. A statistically significant trend is reported when the Mann-Kendall test indicates the presence of a trend at the 95 percent confidence level. For more information, consult the Methods section.
Source: Environment and Climate Change Canada (2025) National Air Pollution Surveillance (NAPS) Program - Open Government Portal.

Regional peak SO₂ concentrations should not be compared between regions because concentrations are highly dependent on the locations of the monitoring stations within the region. Concentrations can also be affected by year-to-year changes in sampling, for example if certain stations are offline in a given year.^{Footnote 7} 

Peak sulphur dioxide concentrations at monitoring stations

The National Air Pollution Surveillance program measures air pollutant concentrations at monitoring stations across Canada. The Canadian Environmental Sustainability Indicators program provides access to this information through an interactive map. The map allows users to explore peak SO₂ concentrations at specific monitoring stations.

Key results

In 2023, peak SO₂ concentrations were recorded at 122 monitoring stations across Canada:

• 7 stations recorded annual average peak concentrations above the 2020 standard of 70 parts per billion, ranging from 71.6 parts per billion to 198.7 parts per billion. Of these stations, 1 was in Alberta and 2 were located each in Quebec, Ontario and British Columbia
• 34 stations had annual average peak concentrations below 5 parts per billion. Of these, most were in British Columbia (15) and in the Prairies (11)
• no monitoring station data were available for Nunavut or Yukon

Peak sulphur dioxide concentrations by monitoring station, Canada, 2023

Navigate data using the interactive map

How this indicator was calculated

Source: Environment and Climate Change Canada (2025) National Air Pollution Surveillance (NAPS) Program - Open Government Portal.

Volatile organic compounds^{Footnote 8} 

Volatile organic compounds (VOCs) are carbon-containing gases and vapours that are found in many common products such as gasoline and solvents. They are emitted from the oil and gas industry, solvent usage, and transportation (for example, exhaust emissions). Long-term exposure to some VOCs can cause cancer and other serious health problems. Short-term exposure to high levels of VOCs can result in fatigue, nausea, dizziness, headaches, breathing problems and irritation of the eyes, nose and throat. VOCs contribute to the formation of fine particulate matter and ground-level ozone, which are the main components of smog.

National annual average volatile organic compound concentrations

Key results

• From 2009 to 2023, a decreasing trend was detected in the national annual average VOC concentrations
• In 2023, the national average concentration was at its lowest level in the last 15 years

National average volatile organic compound concentrations, Canada, 2009 to 2023

How this indicator was calculated

Note: n/a = not applicable. The national annual average VOC concentration indicator is based on the annual average of the daily time-integrated concentrations (24-hour for urban stations and 4-hour for rural stations) recorded at 29 monitoring stations across Canada. VOC sampling in 2020 was limited and no station met the data completeness criteria for that year. During 2011, 2021 and 2022, VOC sampling was paused at several stations. For these years, the national average concentration is likely biased high compared to other years, as described in the Caveats and limitations section. No comparison with CAAQS is shown as there is no comparable VOC standard. A statistically significant trend is reported when the Mann-Kendall test indicates the presence of a trend at the 95 percent confidence level. For more information, consult the Methods section.
Source: Environment and Climate Change Canada (2025) National Air Pollution Surveillance (NAPS) Program - Open Government Portal.

In 2023, the national average VOC concentration was 62.0 parts per billion Carbon (parts per billion carbon), 10.6 percent (7.3 parts per billion carbon) lower than in 2022.

From 2009 to 2023, a decreasing trend was detected. This is consistent with the reduction in VOC emissions from cars and trucks, which is attributable to the introduction of new technologies, cleaner fuels and more stringent emissions standards and from the implementation of federal regulations related to the production and use of paints, solvents and cleaners.

Regional annual average volatile organic compound concentrations

Key results

• From 2009 to 2023, decreasing trends were detected in average VOC concentrations for the southern Quebec region and British Columbia
• In 2023, the regional average VOC concentration was 1 of the 3 lowest recorded over the last 15 years in all regions except British Columbia

Regional average volatile organic compound concentrations, Canada, 2009 to 2023

How this indicator was calculated

Note: n/a = not applicable. The regional annual average VOC concentration indicator is based on the annual average of the daily time-integrated concentrations (24-hour for urban stations and 4-hour for rural stations) recorded at 4 monitoring stations in the Atlantic Canada region, 5 in the southern Quebec region, 9 in the southern Ontario region, 4 in the Prairies and northern Ontario region and 7 in British Columbia. There were not enough stations to report results for the northern territories region. VOC sampling in 2020 was limited and no station met the data completeness criteria for that year. During 2011, 2021 and 2022, VOC sampling was paused at several stations. For these years, the regional average concentration is likely biased high compared to other years, as described in the Caveats and limitations section. No comparison with CAAQS is shown as there is no comparable VOC standard. A statistically significant trend is reported when the Mann-Kendall test indicates the presence of a trend at the 95 percent confidence level. For more information, consult the Methods section.
Source: Environment and Climate Change Canada (2025) National Air Pollution Surveillance (NAPS) Program - Open Government Portal.

Regional average VOC concentrations should not be compared between regions because concentrations are highly dependent on the locations of the monitoring stations within the region. Concentrations can also be affected by year-to-year changes in sampling, for example if certain stations are offline in a given year.^{Footnote 7}

Average volatile organic compounds concentrations at monitoring stations

The National Air Pollution Surveillance program measures air pollutant concentrations at monitoring stations across Canada. The Canadian Environmental Sustainability Indicators program provides access to this information through an interactive map. The map allows users to explore average VOC concentrations at specific monitoring stations.

Key results

In 2023, annual average VOC concentrations were recorded at 36 monitoring stations across Canada:

• 4 stations recorded an annual average concentration above 100 parts per billion carbon, ranging from 116.4 parts per billion carbon to 338.8 parts per billion carbon. One (1) station was in each of New Brunswick and Alberta, and 2 stations were in British Columbia
• 3 stations had an annual average concentration below 20.0 parts per billion carbon. Two (2) were in Ontario and 1 was in British Columbia
• no monitoring data were available for Prince Edward Island, Yukon, Northwest Territories and Nunavut

Average volatile organic compounds concentrations by monitoring station, Canada, 2023

Navigate data using the interactive map

How this indicator was calculated

Source: Environment and Climate Change Canada (2025) National Air Pollution Surveillance (NAPS) Program - Open Government Portal.

Air quality monitoring stations are located across the country but are more concentrated in urban areas and in Canada’s south. The indicators for PM_2.5, O₃, SO₂, NO₂ and VOCs are provided nationally and by region. The regions used for these indicators are listed and shown in the following table and map.

Regions used for the regional Air quality indicators

Ambient levels of PM_2.5, O₃, SO₂, NO₂ and VOCs measured by monitoring station are also shown in the Canadian Environmental Sustainability Indicators interactive indicator maps.

Data quality assurance and quality control for the National Air Pollutant Surveillance program

Monitoring agencies contributing to the National Air Pollution Surveillance program all strive to adhere to established quality assurance and quality control standards, which are developed by Environment and Climate Change Canada in consultation with the provincial, territorial, and regional governments participating in the program.

Ensuring data quality involves identifying the appropriate data quality objectives and methodologies that can be used to meet these objectives. The key data quality objectives for the National Air Pollution Surveillance program are:

• Representativeness: the degree to which data measurements represent a pollutant concentration of interest
• Comparability: the measure of confidence with which one data set or method can be compared to another at other participating National Air Pollutant Surveillance program sites across Canada
• Accuracy: the assessment of the overall agreement of a measurement with a known value (Table 2). Such assessment can include analysis of agreement among repeated measurements (precision) and measures of positive or negative systematic errors (bias)
• Completeness: the assessment as to whether enough information is being collected to ensure confidence in conclusions or decisions made based on data

Routine assessments of network operations provide assurance that the monitoring systems and data processing procedures produce an acceptable level of data quality to meet National Air Pollution Surveillance guidelines and to identify areas where improvements may be required. Three (3) main streams of audits and assessment are used in the National Air Pollution Surveillance network:

• Performance and systems audits: conducted externally either by an ECCC auditor or by another agency separate from the monitoring agency. These audits are performed using independently verified reference standards and provide an unbiased quantitative assessment to defend the quality of the data
• Interagency measurement program involves analysis by the monitoring agency of an unknown sample concentration provided by ECCC. These tests help verify instrument accuracy and help determine data comparability across sites
• Data quality assessments: involve the statistical analysis of environmental data to determine if collected and reported data meet network and data quality objectives

Additional audits and assessments are performed by ECCC's air quality laboratories in Ottawa for the analysis of integrated VOC samples. Consult the National Air Pollution Surveillance Program: Ambient Air Monitoring and Quality Assurance/Quality Control Guidelines (PDF; 2.8 MB) for more information.

Note: Units: µg/m³ = micrograms per cubic metre, ppb = parts per billion, ppbC = parts per billion carbon.

Average indicators are used to capture prolonged or repeated exposures over longer periods or chronic exposure while peak indicators are used to capture immediate or acute short-term exposures.

Canadian Ambient Air Quality Standards

In October 2012, federal, provincial and territorial ministers of the environment, with the exception of Quebec,^{Footnote 12} agreed to begin implementing the Air Quality Management System. This system provides a collaborative, cross-Canada framework for action for reducing air pollution to further protect human health and the environment, including through continuous improvement of air quality. Under the system, the Canadian Ambient Air Quality Standards (CAAQS, the standards) are drivers for air quality improvements across the country. The CAAQS are health- and environment-based air quality objectives for pollutant concentrations in outdoor air. Underpinning by management levels requiring increasingly more stringent action the closer the concentration is to the level of the standard,^{Footnote 13} the CAAQS act as drivers for air quality improvements. The standards are not "pollute-up-to levels", and the Air Quality Management System encourages governments to take action to continuously improve air quality, considering that some pollutants can affect human health even at concentrations below the standards.

Under the Canadian Environmental Protection Act, 1999, the 2020 CAAQS were established in the first instance for:

• PM_2.5 and O₃ in May 2013
• SO₂ in October 2017
• NO₂ in December 2017

The standards have been updated, with 2020 standards in place for all 4 pollutants, and 2025 standards in place for SO₂, NO₂, and O₃. PM_2.5 standards are in place for 2030. The 2020 Canadian Ambient Air Quality Standards^{Footnote 14} are presented in the table below. Calculation of the Air quality indicators mostly follows the same data-handling conventions as those used in calculating the concentrations to use for comparison to the standards. Formal comparison to the standards to determine if concentrations exceed a standard can only be done using ambient concentrations as measured at individual monitoring stations and not using national or regional average concentrations. As such, comparisons of the indicator values (such as the national and regional average concentrations) to the standards are provided for illustrative purposes only and not for assessing whether the standards are achieved. Indicator values that are below a standard do not imply that concentrations at individual monitoring stations are also below the standard. Furthermore, the indicators are not adjusted for exceptional events (such as wildfires) or for pollution from transboundary flows.

Data collection and validation

Data obtained from National Air Pollution Surveillance monitoring stations are converted to a format compatible with the Canada-wide Air Quality Database. All data in the Canada-wide Air Quality Database have a comparable level of quality because jurisdictions adhere to established quality assurance and quality control procedures as outlined in the National Air Pollution Surveillance Monitoring and Quality Assurance/Quality Control Guidelines (PDF; 2.8 MB). These procedures include site and sampling system design, use of monitoring methods that meet defined minimum performance specifications, operation, maintenance and calibrations and data validation techniques. National Air Pollution Surveillance monitoring organizations are responsible for submitting quality-assured data, as per the specifications in the Guidelines, to the Canada-wide Air Quality Database. Data submitted to the National Air Pollution Surveillance database are in the hour-ending format (that is, minute data collected between 01:01 and 02:00 are averaged and reported as the 02:00 hour).

Data completeness criteria

The following criteria are used to determine which stations have sufficient hourly and daily measurements in each year to be considered valid for inclusion in the indicators.

Fine particulate matter (PM_2.5)

For the annual average PM_2.5 indicator:

• a daily 24-hour average concentration was considered valid if at least 75 percent (18 hours) of the 1 hour concentrations were available on a given day
• an annual average concentration was considered valid if at least 75 percent of the daily average concentrations were available for the year and at least 60 percent of the daily average concentrations were available in each quarter^{Footnote 15}of a calendar year

For the peak (98th percentile) 24-hour PM_2.5 indicator:

• a daily 24-hour average concentration was considered valid if at least 75 percent (18 hours) of the 1 hour concentrations were available on a given day
• a 98th percentile of the daily average concentration was considered valid if at least 75 percent of the daily average concentrations were available for the year and at least 60 percent of the daily average concentrations were available in each quarter of a calendar year
• a station was also included in the 98^th percentile if the daily average concentration exceeded the 24 hour standard of 27 micrograms per cubic metre (µg/m³), and had at least 75 percent of the daily average concentrations available for the year

Ground-level ozone (O₃)

For the annual average O₃ indicator:

• rolling (or moving) 8-hour average concentrations were calculated for each hour of the day from the 1-hour average concentrations, resulting in up to 24 8-hour average concentrations per day. The 8-hour average concentrations are reported to the end hour.
• to be valid a rolling 8-hour average concentration must have at least 6 1-hour average concentrations
• a daily maximum 8-hour average concentration was considered valid if at least 75 percent (18) of the 8 hour rolling average concentrations were available in the day
• the annual maximum 8-hour average concentration was considered valid if at least 75 percent of all daily maximum 8-hour average concentrations were available for the period from April 1 to September 30

For the peak (4th-highest) 8-hour O₃ indicator:

• rolling (or moving) 8-hour average concentrations were calculated for each hour of the day from the 1-hour average concentrations, resulting in up to 24 8-hour average concentrations per day. The 8-hour average concentrations are reported to the end hour
• to be valid a rolling 8-hour average concentration must have at least 6 1-hour average concentrations
• a daily maximum 8-hour average concentration was considered valid if at least 75 percent (18) of the 8­­-hour rolling average concentrations were available in the day or if the daily maximum 8-hour average concentration exceeded the 8-hour standard of 62 ppb
• the annual 4th-highest daily maximum 8-hour average concentration was considered valid if there were at least 75 percent of all daily maximum 8-hour average concentrations in the period from April 1 to September 30
• a station was also included if the annual 4th-highest daily maximum 8-hour average concentration exceeded the 8-hour standard of 62 parts per billion (ppb), even if the above data completeness criteria were not satisfied

Nitrogen dioxide (NO₂)

For the annual average NO₂ indicator:

• an annual average concentration was considered valid if at least 75 percentof all the 1-hour average concentrations were available for the year and at least 60 percent were available in each quarter
• a station was also included if the annual average concentration exceeded the annual standard of 17.0 ppb, and at least 50 percent of the NO₂ 1-hour values are available in each calendar quarter

For the peak (98th percentile) 1-hour NO₂ indicator:

• the daily maximum 1-hour average concentration was considered valid if at least 75 percent (18) of the hourly concentrations were available on a given day or if the daily maximum 1-hour average concentration exceeded the 1-hour standard of 60 ppb
• the 98th percentile of the daily maximum 1-hour average concentrations was considered valid if at least 75 percent of the daily maximum 1-hour average concentrations for the year were available and at least 60 percent in each quarter were available
• a station was also included if it exceeded the 1-hour standard of 60 ppb, even if the above data completeness criteria were not satisfied

Sulphur dioxide (SO₂)

For the annual average SO₂ indicator:

• an annual average concentration was considered valid if at least 75 percent of all the 1-hour average concentrations were available for the year and at least 60 percent were available in each quarter
• a station was also included if the annual average concentration exceeded the annual standard of 5.0 ppb, and at least 50 percent of the SO₂ 1-hour values are available in each calendar quarter

For the peak (99th percentile) 1-hour SO₂ indicator:

• the daily maximum 1-hour average concentration was considered valid if at least 75 percent (18 hours) of the hourly concentrations were available on a given day or if the daily maximum 1-hour average concentration exceeded the 1-hour standard of 70 ppb
• the annual 99th percentile of the daily maximum 1-hour average concentrations was considered valid if at least 75 percent of all the daily maximum 1-hour average concentrations for the year were available and at least 60 percent in each quarter were available
• a station was also included if it exceeded the 1-hour standard of 70 ppb, even if the above data completeness criteria were not satisfied

Volatile organic compounds (VOCs)

There are fewer data available for VOCs and therefore the data completeness criteria for this indicator are different. At urban monitoring stations, VOC samples are usually collected over a 24-hour period once every 6 days; conversely at rural stations, samples are collected over a 4-hour sampling period (12:00 to 16:00) once every 3 days.^{Footnote 16}

For the annual average VOC indicator:

• a daily total VOC concentration was considered valid if the sample was collected over a consecutive period of 24 hours (± 1 hour) at an urban station or a consecutive 4 hours (± 0.5 hours) at a rural station and if valid concentration measurements were available for ethane, ethylene, acetylene, and at least one of benzene, ethylbenzene, toluene, m and p-xylene, and o‑xylene.
• a quarter (3 months) was considered valid if had at least 5 valid daily total VOC concentrations
• a station was only included if there were 3 valid quarters in the year

After the data completeness criteria have been met, the pollutant concentrations are calculated for the selected stations.

Pollutant-specific calculations

Fine particulate matter

Fine particulate matter concentrations are expressed in micrograms per cubic metre (µg/m³). The PM_2.5 average and peak (98th percentile) 24-hour indicators are based on the 24-hour daily average concentrations (daily average) for the whole year. The daily average value for PM_2.5 is measured from midnight to midnight.

For a given station, the average indicator is calculated by summing all valid daily averages and dividing by the number of valid days. The peak (98th percentile) 24-hour indicator is obtained by determining the 98th percentile value of all 24-hour daily values for a given year. The 98th percentile value corresponds to the concentration for which 98 percent of all the daily 24-hour values are less than or equal to it and 2 percent are greater than or equal to it. For example, the 98th percentile value of 25 micrograms per cubic metre at a given station means that 98 percent of all daily 24-hour average concentrations are less than or equal to 25 micrograms per cubic metre and only 2 percent are greater than or equal to 25 micrograms per cubic metre. In a year with a complete dataset, the 98th percentile corresponds to the 8th highest value. The following table provides the rank of the 98th percentile value based on the number of available daily measurements.^{Footnote 17} 

The regional and national indicators (average and peak [98th percentile] 24-hour) for PM_2.5 are calculated by averaging the station-level annual average and station-level annual peak values for all stations that met the completeness criteria within either the region or Canada as a whole.

Ground-level ozone

Ozone concentrations are expressed in parts per billion (ppb). There are 24 consecutive 8 hour average concentrations (8 hour rolls) that can possibly be calculated for each day. The highest value of the 24 8‑hour average concentrations per day is the daily maximum. An illustration of the calculation running 8‑hour average concentrations and the selection of the daily maximum is provided in the figure below.

Calculation of the ground-level ozone daily maximum 8-hour average concentration

For each station, the average O₃ indicator is calculated by taking the average of the daily maximum 8 hour (ending) averages for the period from January 1 to December 31. The regional and national averages for O₃ are obtained by averaging the station-level annual averages for selected stations within the region or Canada as a whole.

For each station, the peak (4th-highest) 8-hour O₃ indicator is based on the 4th-highest of the daily maximum 8-hour average concentrations measured over a given year. All of the daily maximum 8-hour average concentrations are ordered in an array from highest to lowest, with equal values repeated as often as they occur. Each value is assigned a rank. For a given year, the 4th-highest ranking value in the array is identified as the annual peak (4th-highest) 8-hour O₃ concentration for that station.

The regional and national average peak O₃ indicators are obtained by averaging all 4th-highest values from all stations that met the completeness criteria within either the region or Canada as a whole.

Nitrogen dioxide

Nitrogen dioxide concentrations are expressed in parts per billion (ppb). The NO₂ average indicator is based on the annual average of all 1-hour concentrations while the peak (98th percentile) 1-hour indicator is based on the annual 98th percentile of the daily maximum 1-hour average concentrations. The daily maximum 1-hour average value for NO₂ is measured from midnight to midnight.

For a given station, the average indicator is calculated by summing all valid 1-hour averages and dividing by the number of total hours. The peak (98th percentile) 1-hour indicator is obtained by determining the 98th percentile value of all daily maximum 1-hour average for a given year. The 98th percentile value corresponds to the concentration for which 98 percent of all the daily maximum values are less than or equal to it and 2 percent is greater than or equal to it. For example, the 98th percentile value of 25 ppb at a given station means that 98 percent of all daily maximum 1-hour average concentrations are less than or equal to 25 ppb and only 2 percent are greater than or equal to 25 ppb.

The national and regional indicators (average and peak [98th percentile] 1-hour) for NO₂ are calculated by averaging the station-level annual average and station-level annual peak values for all stations that met the completeness criteria within either the region or Canada as a whole.

Sulphur dioxide

Sulphur dioxide concentrations are expressed in parts per billion (ppb). The SO₂ average indicator is based on the annual average of the 1-hour concentrations, while the peak (99th percentile) 1-hour indicator is based on the annual 99th percentile of the daily maximum 1-hour average concentrations. The daily maximum 1-hour average value for SO₂ is measured from midnight to midnight.

For a given station, the average indicator is calculated by summing all valid 1-hour averages and dividing by the number of total hours. The peak (99th percentile) 1-hour indicator is obtained by determining the 99th percentile value of all daily maximum 1-hour concentrations for a given year. The 99th percentile value corresponds to the concentration for which 99 percent of all the daily maximum 1-hour concentrations are less than or equal to and 1 percent are greater than or equal to it. For example, the 99th percentile value of 65 ppb at a given station means that 99 percent of all daily maximum 1-hour average concentrations are less than or equal to 65 ppb and only 1 percent are greater than or equal to 65 ppb. In a year with a complete dataset, the 99th percentile corresponds to the 4th highest value. The following table provides the rank of the 99th percentile value based on the number of available daily measurements.

The national and regional indicators (average and peak [99th percentile] 1-hour) for SO₂ are calculated by averaging the station-level annual average and station-level annual peak values for all stations that met the completeness criteria within the region or throughout Canada.

Volatile organic compounds

Volatile organic compounds are reported as a daily sum of individual compounds. The number of compounds included in the reported sum may slightly vary subject to the analytical validity of the individual compound concentrations. Urban VOC station indicators are calculated from the average of daily total VOC concentrations (24-hour time-integrated concentrations) while rural VOC station indicators are calculated from the average of daily 4-hour total VOC concentrations (time-integrated samples collected from 12:00 to 16:00). The daily 24-hour average concentrations are based on measurements taken from midnight to midnight. As presented in the Data completeness criteria, a daily average concentration was only considered valid if measurements for certain compounds were available. For a station, the average indicator is calculated by taking the average of the daily total concentrations for a given year. 

The national and regional indicators for VOCs are obtained by averaging the station-level annual averages from all stations that met the completeness criteria within the region and throughout Canada. 

While the concentration unit for individual VOCs is usually expressed as micrograms per cubic metre (µg/m³), parts per billion carbon (ppbC) are used in this indicator to assess the quantity of mixed VOC species. 

Station selection criteria for inclusion in national and regional indicators (time‑series)

Station-level indicators were calculated for the years 2009 to 2023 for all air pollutants. Each station was then assessed for its suitability (sufficient data, no large gaps at the beginning or end) for inclusion in the national and regional time series. The specific criteria are as follows:

• for the national and regional time series, a station is included if it satisfies the data completeness criteria for at least 11 of the 15 years. Due to a significant gap in VOC data between 2020 and 2022, a station is required to satisfy the data completeness for at least 10 years to be considered in the VOC concentration indicator
• stations are included if data are available for at least 1 of 3 years at the beginning and at the end of the time series, this measure avoids the use of data from stations that were commissioned or decommissioned at the beginning or end of the time series.

In addition to the time series selection criteria, a minimum of 3 monitoring stations is required to calculate the indicator for a region.

Station selection results

The following table indicates the number of monitoring stations that satisfied the selection criteria (data completeness and time series)over the period from 2009 to 2023 and were thus included in the time series for the national and regional Air quality indicators. Further details are available in a list of selected stations.

Note: The sum of the regional stations may not match the national station numbers because a minimum of 3 monitoring stations is required to calculate the indicator for a region. Where there were not enough stations in the northern territories region, results from stations located in this region were only included in the national totals.

Local (station-level) indicators for O₃, PM_2.5, NO₂, and SO₂ are also presented in the Canadian Environmental Sustainability Indicators interactive maps. Stations that meet the data completeness criteria for the year 2023 may not meet the data completeness criteria for the 15-year national and regional indicators. Likewise, stations that meet the 15-year national and regional indicator data completeness criteria, may not meet the data completeness criteria for 2023 alone. As such, stations displayed on the map satisfy annual data completeness criteria for the year 2023, but this does not imply that data from these stations were used to calculate 15-year national or regional indicators.

Imputation

Stations that do not have enough measurements to meet the 15-year time series criteria are excluded from the national and regional indicators. However, when one monitoring station closes and a comparable monitoring station opens nearby, the data from the 2 stations may be combined to meet the 15-year criteria. When this is done, the 2 stations are counted as one.

Monitoring equipment

Fine particulate matter monitoring equipment

Six (6) types of monitoring equipment were used to monitor ambient PM_2.5 concentrations under the categories of^{Footnote 18} :

PM_2.5 Pre-Federal Equivalency Method (FEM) and Non-Federal Equivalency Method Instruments:

• Non-FEM: Rupprecht & Patashnick tapered element oscillating microbalance (TEOM) monitor or TEOM® Series 1400/1400a with sample equilibrium system (SES) monitor

OR

PM_2.5 Designated Federal Equivalency Method (FEM) Instruments:

• FEM: Thermo Scientific TEOM 1400a with the Series 8500C Filter Dynamics Measurement System (FDMS) monitor (prior to mid-2009 this method was designated as a pre-FEM)
• FEM: Met One BAM-1020 Beta Attenuation Mass monitor
• FEM: Thermo Scientific 5030 or 5030i SHARP (Synchronized Hybrid Ambient Realtime Particulate) monitor (introduced as an FEM in 2010)
• FEM: GRIMM Environmental Dust Monitor model EDM 180 (introduced as an FEM in 2011)
• FEM: Teledyne Advanced Pollution Instrumentation Model T640 PM mass monitor with or without network data alignment enabled^{Footnote 19}  (introduced as an FEM in 2016)

The Thermo Scientific TEOM 1400a with 8500C FDMS, Met One BAM-1020, Thermo Scientific SHARP, GRIMM 180and Teledyne T640monitors have been approved by the United States Environmental Protection Agency as Class III Federal Equivalent Methods and have been deployed across the National Air Pollution Surveillance network replacing non-FEM tapered element oscillating microbalance instruments, which in some circumstances may under report the PM_2.5 mass concentrations relative to the National Air Pollution Surveillance PM_2.5 Reference Method. Since 2005, the tapered element oscillating microbalance monitors have gradually been replaced by the federal equivalent methods monitors. The FEM monitors measure a portion (semi-volatile) of the PM_2.5 mass not captured by the older instruments. Because of these measurement differences between the new and the old monitoring equipment, concentrations measured with the new monitors may not be directly comparable with the measurements from years in which older instruments were used.

Ground-level ozone monitoring equipment

Ozone measurements are made using ultraviolet photometry. Sample air passes through a beam of light from an ultraviolet lamp, which is absorbed by O₃. The amount of ultraviolet light absorbed is proportional to the amount of O₃ in the sample.

Nitrogen dioxide monitoring equipment

Nitrogen dioxide is calculated by subtraction following the measurement of total of nitrogen oxides (NO_X) and nitrogen monoxide (NO). Nitrogen monoxide (NO) concentrations are determined photometrically by measuring the light intensity from the chemiluminescent reaction of NO mixed with excess O₃. The chemiluminescence method detects only NO, therefore, NO₂ must first be converted to NO for measurement purposes. Sample flow is either directed through a converter to reduce NO₂ to NO, or it bypasses the converter to allow detection of only NO. The sample stream with reduced NO₂ is a measurement of NO plus NO₂, which is expressed as NO_X (that is, NO_X = NO₂ + NO). The difference between NO_X and NO detection is taken as the NO₂ concentration (that is, NO₂ = NO_X - NO).

Sulphur dioxide monitoring equipment

Sulphur dioxide measurements are made using pulse-fluorescence ultraviolet adsorption instruments. This technology is based on the principle that SO₂ molecules absorb ultraviolet light at one wavelength and emit ultraviolet light at a different wavelength. The intensity of the emitted light is proportional to the number of SO₂ molecules in the sample gas.

Volatile organic compound monitoring equipment

A combined gas chromatography-flame ionization detector system is used for quantification of VOCs containing 2 carbons, while a combined gas chromatography-mass selective detector system operating in selected ion monitoring mode is used for quantification of VOCs containing 3 to 12 carbons. Approximately 120 VOCs (including a number of biogenic species such as isoprene and pinenes) are targeted for quantification in the samples, but not all VOCs are detectable in each sample. The total concentration of VOCs in parts per billion carbon is calculated from the total mass of 77 of these species when detectable in the sample. Air samples are collected in either 6-litre or 3.2-litre stainless steel canisters. The canisters are then shipped to the Environment and Climate Change Canada analysis laboratory in Ottawa.

Statistical analysis

Non-parametric statistical tests were carried out on temporal concentration data to detect the presence of a linear trend and, if present, to determine the orientation (positive or negative) and magnitude of the rate of change (slope). The standard Mann-Kendall trend test was used to detect trend presence and orientation, while the Sen's pairwise slope method was used to estimate the slope. Both tests were applied to the national and regional data for PM_2.5, O₃, NO₂, SO₂ and VOCs.

The Mann-Kendall trend test considers the full time series of concentration data when assessing the presence of a trend, which is why sometimes it concludes that no significant trend is present despite a large concentration increasing during the final year of the timeseries.

Percentile bounds

A percentile is a statistical measure used to indicate the value below which a percentage of the data falls. For example, the 10th percentile is the value below which 10 percent of the data may be found. Likewise, the 90th percentile is the value below which 90 percent of the data may be found.

A percentile range is the difference between 2 determined percentiles. The 10th to 90th percentile range is the most common and is referred to as the 10th to 90th percentile bounds in the Air quality indicators. If sufficient data values are available, the bounds capture 80 percent of the data. When few data values are available, the calculated percentile range may vary greatly from one year to the next or may not be visible for a given year. This can be observed in the results for the northern territories region or for some regions in the regional VOC indicator.

Revisions to station selections

Monitoring stations are selected based on the 15-year time series criteria for the calculation of the Air quality indicators. As this is a rolling 15-year time period, the number of stations selected may vary from 1 iteration of the indicators to the next and may change the historical trends. Caution should be exercised when comparing different iterations of the Air quality indicators.

Year-to-year bias due to data gaps

The group of monitoring stations that are used in the calculation of the national and regional indicators may vary from year-to-year if some stations do not meet the minimum data requirements for a given year. This introduces bias in the annual values. For example, if a station in an especially polluted area has no station-level value for 2023, then the national indicator value and the relevant regional values for 2023 will be biased low relative to the other years in the 15-year period. This is because the values for all other years are pulled upward by the high concentrations recorded at the station in the polluted area, while the value for 2023 is not influenced by the station in the polluted area. The bias caused by missing a station-level value from a single station is usually negligible, due to the large number of stations that factor into the calculation of the national and regional indicators. However, if a value is missing from a station that typically records extremely high concentrations, if values are missing from many stations, or if the number of stations factoring into the calculation of the national or regional indicators is small, then the bias may be large. In cases with larger biases, year-to-year fluctuations in the indicators do not necessarily represent real changes in air quality.

Regional indicators for the northern territories are especially susceptible to year-to-year bias, due to the small number of stations that contribute to the calculation of these indicators. The indicators for SO₂ and VOCs are also highly susceptible to year-to-year bias, due to the moderate number of stations that contribute to these indicators and the high local variability of SO₂ and VOC concentrations. For the SO₂ and VOC indicators, the bias due to data gaps was estimated for each year by comparing the actual annual concentration to the expected annual concentration if data gaps were not present. The expected annual concentration was calculated by filling in gaps in the station-level concentrations with estimated values, based on a Sen’s slope fit to each station’s concentrations from other years. Cases where the bias was estimated to be large (≥ +15 percent or ≤ –15 percent) are noted in the text below.

For the regional average SO₂ indicator, the following annual concentrations are likely biased relative to other annual concentrations:

• the 2017 average SO₂ concentration for Atlantic Canada is estimated to be biased low by 20 percent due to missing data from a station that typically records high average SO₂ concentrations
• the 2022 and 2023 average SO₂ concentrations for southern Quebec are estimated to be biased high by 27 percent and 46 percent, respectively, due to missing data from multiple stations that typically record low average SO₂ concentrations
• the 2023 average SO₂ concentration for southern Ontario is estimated to be biased high by 23 percent due to missing data from multiple stations that typically record low average SO₂ concentrations

For the regional peak SO₂ indicator, the following annual concentrations are likely biased relative to other annual peak concentrations:

• the 2009 peak SO₂ concentration for Atlantic Canada is estimated to be biased high by 21 percent due to missing data from multiple stations that typically record low peak SO₂ concentrations
• the 2022 and 2023 peak SO₂ concentrations for southern Quebec are estimated to be biased high by 23 percent and 39 percent, respectively, due to missing data from multiple stations that typically record low peak SO₂ concentrations
• the 2023 peak SO₂ concentration for southern Ontario is estimated to be biased high by 22 percent due to missing data from multiple stations that typically record low peak SO₂ concentrations

For the national average VOC indicator, the following concentration biases are estimated: +19 percent in 2011, +25 percent in 2021 and +17 percent in 2022.

For the regional average VOC indicator, the following annual concentrations are likely biased relative to other annual concentrations:

• the 2022 regional average VOC concentrations are estimated to be biased high by 22 percent for Atlantic Canada, by 45 percent for southern Quebec, by 34 percent for southern Ontario, and by 45 percent for the Prairies and northern Ontario region
• the 2023 regional average VOC concentrations are estimated to be biased high by 18 percent for Atlantic Canada, by 25 percent for southern Quebec, by 31 percent for southern Ontario, and by 47 percent for the Prairies and northern Ontario region
• the 2022 average VOC concentration for British Columbia is estimated to be biased low by 20 percent, since a station that typically records very high average VOC concentrations was also paused, outweighing the effect of VOC monitoring stations in less polluted areas being paused
• the 2018 average VOC concentration for Atlantic Canada is estimated to be biased low by 42 percent due to missing data from a station that typically records very high average VOC concentrations

Regional biases in SO₂ indicators due to sampling locations

Regional average and peak SO₂ concentrations are highly dependent on the locations of SO₂ monitoring stations within the region. Stations that are close to a major stationary emission source, such as a smelter, tend to measure much higher SO₂ concentrations than other stations. The proportion of SO₂ monitoring stations located near major stationary emission sources differs greatly by region. For example, 50 percent of the stations contributing to the southern Ontario average and peak SO₂ indicators are located near a major emissions source, while only 10 percent of the stations contributing to the Prairies and northern Ontario average and peak SO₂ indicators are located near a major emissions source. For this reason, regional differences in the average and peak SO₂ concentrations are mainly due to differences in station placement, rather than differences in SO₂ concentrations experienced by the general population of the regions. Therefore, the average and peak SO₂ concentrations presented in these indicators should not be compared between regions.

Regional biases in VOC indicators due to sampling locations

Regional average VOC concentrations are highly dependent on the locations of the VOC monitoring stations within the region. Stations that are close to a major stationary emission source, such as an oil and gas extraction facility, tend to measure much higher concentrations than other stations. The proportion of VOC monitoring stations located near major stationary emission sources differs greatly by region. For example, 57 percent of the stations contributing to the British Columbia average VOC indicator are located near a major emissions source, while only 20 percent of the stations contributing to the southern Quebec average VOC indicator are located near a major emissions source. For this reason, regional differences in the average VOC concentration are mainly due to differences in station placement, rather than differences in VOC concentrations experienced by the general population of the regions. Therefore, the average VOC concentrations presented in this indicator should not be compared between regions.

Effect of new fine particulate matter measurement technologies

Since 2005, the Rupprecht & Potashnick tapered element oscillating microbalance (TEOM) monitors used in the National Air Pollution Surveillance program have gradually been replaced by newer monitoring technologies (federal equivalency method-approved instruments). Many studies conducted in Canada, the United States and other countries have found that the TEOM monitors under-report concentrations compared with the newer monitors, especially when the air contains a large proportion of semi-volatile particulate matter. This may be the case during cooler seasons when the air contains a greater proportion of ammonium nitrate and semi-volatile organic compounds.

Some of the year-to-year variations in the PM_2.5 air quality indicator may be due, in part, to the introduction of the newer monitoring technologies across the National Air Pollution Surveillance Network rather than to changes in actual ambient concentrations only. As such, trends in PM_2.5 concentrations may not be a true reflection of the changes that have occurred over the time period concerned.

To address a consistent positive bias observed in the Teledyne T640 instruments, a mass concentration alignment factor was applied to T640 instruments across Canada. This alignment factor is intended to improve the consistency of PM_2.5 measurement with those obtained from the NAPS Federal Reference Method (FRM). ECCC, along with provincial, territorial and jurisdictional partners of NAPS have agreed to implement this alignment factor for the T640 instruments.^{Footnote 19}