Annual regional air quality index statistics for 2011

In 2011, the AQI was calculated for 24 weather regions, with the island of Montréal subdivided into four subregions.

The percentage of days when the AQI was deemed “good” or “acceptable” ranged from 77.9% on the island of Montréal (some 282 days out of 365) to 100% in the region of Montmagny.

Air quality was deemed “poor*” most often in Montréal (22.1% of the time, or 80 days out of 365, an increase of 6% compared to 2010). Note that the further a region is from Montréal, in any direction, the better the air quality becomes.

Overall, air quality was “good”, “acceptable” or “poor” in the following proportions, respectively: 54%, 42% and 4% of the time. Compared to 2010, “acceptable” AQI ratings increased slightly, by 2%, to the detriment of “good” AQI ratings.

It is important to note that since 2010, a new generation of instruments is used across the network to measure 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.