Air Quality Index (AQI)

Air Quality Index (AQI) Calculation Method

Each hour, an air quality index is calculated on the basis of the following five pollutants: ozone, fine particulate matter, sulfur dioxide, nitrogen dioxide and carbon monoxide.

For each of the pollutants measured at an air monitoring station, a sub-index is calculated first. The sub-index is calculated by dividing the concentration of a pollutant monitored by its corresponding reference value and multiplying the result by 50. A pollutant’s reference value is the concentration at which air quality is considered "poor". This value is determined on the basis of criteria to protect human health. The reference values are as follows:

Pollutant

Type of Measurement

Reference Value

Ozone (O3)

Hourly average

82 ppb

Particulate matter (PM2.5)

Average over 3 hours

35 µg/m3

Sulfur dioxide (SO2)

Maximum over 4 minutes1

200 ppb

Nitrogen dioxide (NO2)

Hourly average

213 ppb

Carbon monoxide (CO)

Hourly average

30 ppm

1Maximum  4 minutes average concentration over a given hour.

The results of the highest sub-index are then used as the air quality index for that station. Not all pollutants have to be monitored at one station to calculate the AQI. The following is an example of a calculation where ozone, particulate matter and sulfur dioxide are measured.

Example of the calculation

Sub-index O3 = (90 ppb / 82 ppb) X 50 = 55

Sub-index PM = (51 µg/m3 / 35 µg/m3) X 50 = 73

Sub-index SO2 = (49 ppb / 200 ppb) X 50 = 12

The air quality index is the highest of the sub-indices: AQI = 73

The AQI for a region or city is based on the highest of the air quality indices measured at representative stations in the area.


Representative spatial scales

Regional

Stations used for calculating regional AQI are spatially representative to within 100-150 km. Regions displayed on the map are meteorological regions. They correspond to regions used for broadcasting weather prediction and air quality information. On this scale, pollutants taken into account for high AQI values are fine particulates or ozone during summer; during winter, only fine particulates are used.

Local (sector)

Stations used to calculate local AQI are spatially representative to a maximum of 1 km. These zones, designated “urban sectors,” are shown on the map as circles. At this level, atmospheric pollutants like sulphur dioxide and fine particulates may significantly influence AQI values due to industrial, transport or residential wood heating local emission sources. Consequently, these areas often have unique profiles. Some emission sources such as residential wood heating bear witness to human polluting activities that are likely to influence a great number of places in Québec. The types of influence that characterize any given area vary with local sources.

Caution
The number of areas defined using the AQI is relatively small. Moreover, all pollutants are not measured. Consequently, the air quality picture as presented here is only partial in nature.

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