International comparison of urban air quality indicators: data sources and methods, chapter 5

5. Caveats and limitations

Caution must be taken when comparing air quality among urban areas. Differences in monitoring equipment, in the placement of stations and in the number of stations can influence the comparability of the ambient concentrations provided from different urban areas and for different years. In Canada, since 2007, older fine particulate matter monitoring instruments have been gradually replaced by newer models that measure a portion (semi-volatile) of the fine particular matter mass that may not have been fully captured by the older monitors. This technology change may partly explain the increases in the fine particulate matter concentrations observed in some Canadian urban areas.

Furthermore, concentrations measured at monitoring stations can be influenced by many highly complex determinants such as climate, station elevation and geography, local emissions and transboundary pollution. Depending on these factors ambient air concentrations may vary substantially from one year to the next.

All data used for these comparisons have been published and revised by their respective institutions. However, no guarantee is given as to the accuracy of the data. Data collected through the various agencies were taken as they were publicly available at the time of production of these indicators.