International comparison of urban air quality indicators: data sources and methods, chapter 3

3. Data

3.1 Data source

The ambient levels of air pollutant data used for the international comparison were obtained from many different databases.


United States

  • The United States Environmental Protection Agency's AirData Download Data Files from the AirData website, which provides access to monitored air quality data from the Air Quality System Data Mart.



Hong Kong

The guidelines for the air pollutants used in the international comparison were obtained from the World Health Organization's document entitled Air Quality Guidelines Global Update 2005. Particulate matter, ozone, nitrogen dioxide and sulfur dioxide.

3.2 Spatial coverage

Ambient air levels in urban areas in Canada, the United States, Europe, Australia, and China are compared in these indicators. For the purpose of these indicators, an urban area is a city, group of cities (metropolitan area) or region with a population greater than one million. More specifically, an urban area corresponds to the census metropolitan area for Canadian cities. For the United States, the metropolitan statistical areas were used. For the European countries different urban areas corresponding to the areas covered by the available monitoring stations were used. Three different urban areas were considered: urban area, large urban zone and core city (or greater city). For Australian cities, the greater capital city statistical area was used. The Hong Kong Special Administrative region was used for Hong Kong.

3.3 Temporal coverage

The indicators were calculated with concentrations measured from 2009 to 2014. All data are measured hourly and provided as hourly, daily or annual values depending on the web tools available from each country.

3.4 Data completeness

For any given ozone or fine particulate matter monitoring station used in the calculation of pollutant concentrations for a particular urban area, measurements covering at least 75% of the year were required.

A sulphur dioxide or nitrogen dioxide monitoring station was used in the calculation of concentrations for an urban area only if the data available meets the following conditions:

  • A valid month requires that at least 50% of the hours have valid measurements;
  • A valid quarter (three months) requires data for at least two valid months; and
  • A station is included only if 50% of hours in year are valid and has four valid quarters.

3.5 Data timeliness

The gap between a given year and publication of air pollutant concentration data for that year varies considerably among urban areas. For some urban areas, annual data is publicly available within a few months of a reported year while for others, data validation takes longer. The year 2014 was selected as the latest year of data to accommodate the variation in data timelines.

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