Statistical profile: poor air quality and smog

Ozone and fine particles in suspension are two components of smog often linked to the poor quality of the air we breathe. Until quite recently, the best mesurable indicator used in determining the number of days of smog or poor quality of the air was the presence of ozone. As the instrumentation available to measure fine particles has improved considerably during the last few years, it is now possible to use the presence of fine particles matter as indicator of poor air quality. However, since the simultaneous detection of these two components of smog only really started in 2004 in most of Québec's regions, we cannot track the progression of the problem prior to then by taking both indicators into account at the same time.

Poor air quality days

On a regional basis, a poor air quality day is one in which the reading from at least one measuring station is characterized as 'poor' for an hour or more. This tag is assigned once the average concentration of particulates during three hours is greater than 35 micrograms per cubic metre (µg/m³) or when the average concentration of ozone during one hour is more than 82 parts per billion (ppb). This indicator encompasses days of thick smog occurring over a region, and days when poor air quality is observed locally during a short period.

Number of poor air quality days* due to fine particulate matter and ozone
by administrative region, 2004-2009

*Based on the occurrence as measured by at least one station in the region of (fine particles)concentration greater than 35 µg/m³ during three hours or of a concentration of ozone over 82 ppb for one hour.

n/a = not available

Smog days and episodes

A "smog day" is one during which atmospheric pollutants combine with weather conditions to give rise during several hours to elevated concentrations of fine particles or of ozone over an extended area. Three criteria are used in determining smog days:

1. Intensity: concentrations of fine particles (averaged over 3 hours) or of ozone (hourly average) must be greater than 35 µg/m³ and 82 ppb, respectively
2. Duration: elevated concentrations must be observed during at least three hours
3. Surface area: the concentrations must be representative observations of the administrative region.

A smoggy episode refers to a day or a group of days during which smog is present continuously.

Number of smog days and episodes due to fine particulate matter and ozone by administrative region, 2004-2009

n/a = not available

The number of days and smog episodes varies greatly by region (see above table). In 2009, 2 of the 14 areas covered by the monitoring network recorded no smog days at all. In other areas, smog day frequency varied between one for the Laurentides, Abitibi-Témiscamingue, Estrie and Chaudière-Appalaches regions, to 19 episodes (35 days in all) for the Montréal region. When 2009 data is compared to data from previous years, it is clear that the average number of episodes and smog days (respectively 5.1 episodes and 8.5 smog days) is on the rise since 2008, yet is lower than 2004 and 2005 averages. The increase is mainly due to new fine particle measuring equipment (see above box). The new devices are more sensitive and can measure higher concentrations of pollutants, but this should not be interpreted as deterioration in air quality. Moreover, the presence or absence of weather conditions that favour the dispersion of pollutants into the atmosphere influences air quality. Average episode length remains stable at 1.7 days.

During the period covered (2004–2009), the longest smoggy episode (9 days) occurred between January 31, 2005 and February 8th of that year, when a stagnant air mass hovered over a great part of the St. Lawrence lowlands. The highest particulate concentration of that period was measured at 117 µg/m3 in Montréal. Elsewhere in southern Québec, fine particles concentrations ran from 52 µg/m3 in Estrie to 102 µg/m3 in the Québec City area. It is worth noting that the greatest fine particles concentrations observed from 2004 through 2009 were of natural origin; readings of 125 to 360 µg/m3 were taken from June 3-5, 2005 in the Saguenay–Lac-Saint-Jean region where forest fires were raging.

During the 2004–2009 period, ozone was a very slight contributing factor in smog formation, accounting for a single smog day in the Outaouais region in 2009.