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Air Quality Index (AQI)

Air Quality Index (AQI) Calculation Method

An air quality index is calculated hourly using the following five contaminants: ozone, particulate matter, sulfur dioxide, nitrogen dioxide and carbon monoxide.

For each of the contaminants measured at an air monitoring station, a sub-index is calculated first. The sub-index is calculated by dividing the concentration of a contaminant monitored by its corresponding reference value and multiplying the result by 50. A contaminant'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:

Contaminant

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 contaminants 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 sector is based on the highest of the air quality indices measured at representative stations in the area.


Representative spatial scales

The air quality index is shown on the basis of two representative spatial scales.

Regional

On this scale, the calculation of AQI is essentially based on readings at regional stations outside urban areas, far from local sources of air emission contaminants and where concentrations measured will be representative of the entire region. The regions shown on the map are used by Environment and Climate Change Canada for meteorological and air quality forecasting (Info-Smog). On this scale, particulate matter and ozone levels are used to calculate AQI.

Sector

At the sector level, AQI calculation is essentially based on readings at a single urban station. The defined sectors are shown as circles on the map. In some cases, atmospheric contaminants such as sulfur dioxide and particulate matter may significantly impact AQI values due to specific local sources like industry, transport, residential woodstove heating, etc., giving each sector a unique profile. This means that spatial representativeness can vary greatly from one station to the next.

Caution
The number of sectors defined for the AQI is relatively small. Moreover, all contaminants are not measured at every station. Consequently, the air quality picture as presented here is only partial in nature.

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