Annual regional air quality index statistics for 2010

In 2010 the AQI was calculated for 24 weather regions, with the island of Montréal being subdivided into four subregions.

The percentage of days when the AQI was deemed “good” or “acceptable” ranged from 83.6% on the island of Montréal (some 305 days out of 365) to 100% in Lac-Saint-Jean.

Air quality was deemed “poor*” most often in Montréal (16.4% of the time, or 60 days out of 365, somewhat less than in 2009). In terms of hours, the AQI was “poor” 4.2% of the time (369 out of 8760 valid hours).

Compared to the previous year and looking at all regions together, the total percentage of days when air quality was “poor*” fell by 10%, while the percentage of days when the AQI was “good” fell by 4%. At the same time, the number of days when air quality was “acceptable” increased by 8%. From 2009 to 2010, the greatest improvement in air quality was in Montréal. Despite these variations, the regional AQI statistics for 2010 are similar to those observed since 2004. Note that the further a region is from Montréal, in any direction, the better the air quality becomes.

Air quality in 2010 was significantly affected by the large forest fires in Haute-Mauricie (from late May to late June). The negative effects were detected in every region where the AQI is calculated. For example, on May 31, 2010 hourly concentrations of fine particulate matter (PM2.5) reached 239 µg/m³ at the Ditton station (05810) in Estrie, more than 350 kilometres from where the fires were raging!

2010 was the first full year in which the new generation of instruments was used throughout the network for detecting fine particulate matter (PM_2.5). These devices measure fine particulate matter (the semi-volatile part) that could not be measured with older instruments, which results in higher fine particulate matter readings.

Since fine particulate matter is the pollutant that is most often responsible for high AQI values, one must be cautious when comparing data from 2010 and later with data from previous years. Therefore, an increase in the frequency of “poor” or “acceptable” AQI after 2009 does not necessarily mean there is air quality deterioration.

* Air quality is deemed “poor” when at least one station in the region reports a concentration of fine particulate matter greater than 35 µg/m3 (over a three-hour period), or an hourly concentration of ozone greater than 82 ppb.