International comparison of urban air quality indicators: data sources and methods, chapter 4

4. Methods

Air pollutant concentrations were obtained from the air quality monitoring networks and agencies listed in section 3.1. For ozone, the annual average of daily maximum 8-hour concentrations was used. For fine particulate matter,the annual average of 24-hour concentrations was used. For sulphur dioxide and nitrogen dioxide, the annual average of all hourly concentrations were used. When the ambient concentrations were not provided in the required metrics, hourly measurements were used to calculate the indicators for each station.

Annual average ambient levels from all stations found in an urban area were averaged to obtain the reported value. This average was an arithmetic average and not weighted by the population covered by each station.

The ambient concentrations for the fine particulate matter indicator are measured in micrograms per cubic metre, while those for the ozone, sulphur dioxide and nitrogen dioxide indicators are measured in parts per billion. When necessary, the ambient concentrations reported in micrograms per cubic metre by some jurisdictions were converted to parts per billion using a temperature of 25°C and a pressure of 101.325 kilopascals.

The World Health Organization's guidelines for fine particulate matter and nitrogen oxide were used within the charts solely for indicative purposes and to show how Canadian urban areas fare compared to the guidelines. The World Health Organization's guidelines for ozone and sulphur dioxide were not used because the definitions of the indicators and the guidelines were different.

4.1 Urban areas selection

The urban areas for these indicators were selected on the basis of the following criteria.

• The urban area had to have a population of at least one million.
• Availability of the air quality data after the station selection criteria (see section 3.4) were applied.