Air indicators – Nitrogen dioxide

About the indicator

Nitrogen dioxide, or NO₂, is an air pollutant that can negatively affect human health and the environment. Exposure to NO₂ can lead to increased risk of respiratory problems. NO₂ also adversely affects ecosystems when it is deposited onto the ground, for example, through acidic deposition, commonly known as acid rain.

This indicator reports on the concentration of NO₂ from 2000 to 2023 and provides information on the variation across Alberta and how concentrations have changed over time. This indicator also compares NO₂ concentrations with Alberta’s Ambient Air Quality Objectives (AAAQOs), which set thresholds for air pollutants to protect human and ecosystem health. 

Nitrogen dioxide facts

• Nitric oxide (NO) and NO₂, collectively referred to as NO_x, are gases produced during high-temperature combustion in air. Most NO_x is emitted into the atmosphere as NO but is quickly converted to NO₂.
• Major sources of NO₂ are the emissions caused by the burning of fossil fuels used in vehicles, home heating and industrial processes. NO₂ also occurs naturally in the environment due to forest fires, lightning or emissions from the soil.
• NO₂ contributes to the development of other air pollutants, such as ozone, nitric acid and particulate matter. Near large cities, it contributes to smog which is visible as the brownish haze in the lower atmosphere.
• NO₂ is part of the Air Quality Health Index (AQHI) that reports on health risks associated with local air quality across Alberta in real-time.

Methods

For information on how the results in this indicator were calculated and for references, see: Condition of the Environment Report – Air Component

Summary of key results

Last updated: February 2025

• NO₂ concentrations have been decreasing throughout much of the province over the past 2 decades due to improved emissions control technology at sources, including personal vehicles and industry.
• Since 2005, NO₂ concentrations have consistently been below the annual AAAQO across the province. Historically, exceedances of the one-hour AAAQO have occurred infrequently.
• NO₂ concentrations are higher in urban areas, with Alberta’s highest concentrations in Calgary and Edmonton.

Variation across Alberta

• Alberta’s cities see higher concentrations of NO₂ when compared to smaller communities, with Calgary and Edmonton showing the highest concentrations of NO₂ in the province (Figures 1a and 1b).
• Higher NO₂ concentrations in cities occur due to a higher number of emission sources including vehicles. Industrial sources that emit NO₂ are also often located in or near urban areas.
• NO₂ concentrations are also elevated near major industrial sources, such as in the mineable oil sands region and the industrial heartland northeast of Edmonton.

Changes over time

• Between 2000 and 2023, Alberta saw a statistically significant decrease in annual average and peak concentrations of NO₂ when averaged across all monitoring stations (Figures 2a and 2b, respectively). This general trend is consistent with trends across Canada as improved technologies have reduced NO₂ concentrations from various emissions sources.
• Most major population centres, including Edmonton and Calgary, saw a statistically significant decrease in NO₂ annual average and peak concentrations between 2000 and 2023. Grande Prairie and Fort McMurray saw steady annual average and peak concentrations over this period, potentially due to increased industrial operations and population growth that may have offset improved emission technologies in these regions. Peak concentrations in Medicine Hat also remained steady over this period.
• Examining the trends over more recent years and for all stations across the province provides additional information on the changes over time for the indicator. Between 2014 and 2023, there was a statistically significant increasing trend in monthly average NO₂ concentrations at some monitoring stations (Fort Chipewyan, Fort McKay-Bertha Ganter and Fort McKay South in northeastern Alberta, Tamarack and St. Lina near Cold Lake, Beaverlodge near Grande Prairie, and Range Road 220, located northeast of Edmonton). However, NO₂ concentrations at these stations remained low compared to stations located in larger cities. For more information on spatial variability in NO₂ concentrations, see Figures 1a and 1b.

Comparison to provincial objectives

• Alberta's Ambient Air Quality Objectives provide thresholds for over 30 airborne compounds to protect human and ecosystem health. The AAAQOs are used in the design of industrial facilities and to establish emissions requirements. The Alberta government tracks AAAQO exceedances and assesses the need for management action.
• Since 2005, NO₂ concentrations have consistently been below the annual AAAQO for all stations across the province (Figure 2a). The annual objective for NO₂ is 24 parts per billion (ppb) based on the protection of ecosystems.
• In 2023, there were no exceedances of the one-hour AAAQO for NO₂. Historically, exceedances of the one-hour AAAQO for NO₂ have occurred infrequently – 0 to 3 times per year – in various locations across Alberta. The one-hour AAAQO for NO₂ is 159 ppb based on the protection of human health.

Air quality reporting and resources

• Alberta reports annually on the status of air quality for existing regional air quality management frameworks (AQMFs) as well as the Canadian Ambient Air Quality Standards (CAAQS).
  • NO₂ is one of the pollutants reported on through the South Saskatchewan Region (SSR) AQMF and Lower Athabasca Region (LAR) AQMF as well as the CAAQS.
  • The most recent assessment results are available in Alberta’s Air Zone Reports. 
• Published reports and scientific papers on air quality in Alberta are available via the Air Indicators landing page under ‘Related Publications’.
• Alberta also provides air quality data and information to the public through the near real-time AQHI website and provides access to quality assured data via Alberta’s Air Data Warehouse.

Focused study

Meteorology influences the dispersion, transformation, and deposition of pollutants in the environment. Meteorological conditions such as surface-based temperature inversions, where the temperature increases with height, create a stable lower atmosphere that can trap pollutants near the ground.

A recent study by the Government of Alberta (Brown et al, 2025) investigated the impact of surface-based temperature inversions on NO₂ concentrations at Alberta community monitoring stations from 2015 to 2019. A better understanding of how temperature inversions relate to pollutant concentrations provides a more complete picture of the processes affecting air quality in Alberta.

Key findings include:

• Elevated (≥ 32 ppb) daily maximum NO₂ concentrations were more likely during the morning commuter period and in the evenings under cool, stable atmospheric conditions (Figure 3). This illustrates the influence both local emissions and meteorology have on elevated NO₂ concentrations.
• Elevated daily maximum NO₂ concentrations were most frequently observed in the winter months (Figure 3) when atmospheric inversions are more frequent and long lasting.
• The presence of inversions tended to increase NO₂ concentrations during times when emissions would have caused only moderate levels in their absence.
• While inversions may not always directly cause elevated NO₂ concentrations, they may contribute to the frequency of observed elevated concentrations.