What causes high AQI values in Edmonton and Calgary?

In Edmonton and Calgary, air quality is rated Good at least 90 per cent of the time for any given year. The index will occasionally reach the Fair range (less than 10 per cent of the time). Air quality in both cities seldom reaches the Poor level (usually less than one per cent of the time), and the frequency of Very Poor index levels is very low.

Fair, Poor or Very Poor air quality conditions usually occur when there is a strong temperature inversion and light winds. This combination of weather conditions will often create a layer of cool, stagnant air near the ground. Air pollutants, mostly from automobiles, are trapped in this layer of stagnant air. In Edmonton these conditions usually occur with the approach of a warm front. In Calgary, strong temperature inversions are common before a Chinook arrives. Events such as forest fires can also lead to episodes of Fair, Poor and Very Poor air quality.

Fair and Poor air quality can also result from summer heat. In hot, sunny weather photochemical smog can form through complex chemical reactions involving oxides of nitrogen and volatile hydrocarbons. Photochemical smog has a light brown colour and can reduce visibility and affect breathing. Ground-level ozone is a photochemical smog component that is a major concern.

With the introduction of fine particulate matter (PM2.5) to the AQI in 2003, a small increase in the number of reported hours of Fair, Poor and Very Poor air quality is expected as the dust and smoke component of the old Index of the Quality of the Air (IQUA) did not adequately represent particulate matter concentrations. Recalculation of the AQI using PM2.5 data from the year 2002 from selected monitoring stations showed that the time periods associated with very poor air quality were during forest fire events and are therefore an accurate and credible description of the air quality at that time.