Air Quality Indicator

Detailed information for 2003

The air quality indicator reflects the potential for long term exposure of Canadians to ground-level ozone, a key component of smog and one of the most common and harmful air pollutants to which people are exposed. The air quality indicator is a population weighted estimate of average concentrations of ground-level ozone observed by monitoring stations across Canada. Statistics Canada calculates the indicator based on monitoring data provided by Environment Canada's National Air Pollution Surveillance (NAPS) network.

Data release - December 14, 2005 (in " Canadian Environmental Sustainability Indicators ", online catalogue # 16-251-XWE.)

• Description
• Data sources and methodology
• Data accuracy

Description

The national air quality indicator focuses on human exposure to ground-level ozone, a key component of smog. Human exposure to ground-level ozone is of concern because there are no established thresholds below which this pollutant poses zero risk to human health.

Ambient air quality data are provided by Environment Canada. Further details on the monitoring methods are available from the following website: http://www.etc-cte.ec.gc.ca/NAPS/.

The data from each monitoring station used in the indicator was population weighted to provide a more accurate estimation of human exposure to ground-level ozone. The methodology chosen uses a population-weighted mean, which involves estimating the population within a certain distance (in this case, 40 km) of each station and then using that estimate to derive the relative weight that each station is given in the analysis of averages and trends.

The air quality indicator is presented nationally and regionally as scatter plots of weighted ambient mean concentration per year.

The indicator is based on the methodology proposed by the National Roundtable on the Environment and the Economy (NRTEE) in the report State of the Debate: Environment and Sustainable Development Indicators for Canada. This was synthesized from a number of sources including recommendations from a cluster group of experts on atmospheric issues and proceedings from two national conferences.

A warm season mean for ozone is calculated for each individual monitoring site. These sites are weighted according to the share of population within 40km of the site related to the total population within 40km of all sites chosen for the reference year (based on the data completeness criteria outlined above). These weighted observations are summed to produce a mean pollutant concentration for each year of the time series. Higher values indicate higher ambient levels of air pollution.

Annual fluctuations in meteorology can have a considerable impact on air quality since ambient levels of ozone are a function of temperature, sunlight, and local emissions of precursors. To ensure a consistent monitoring basis, monitoring data were taken from warm season observations only (April to September).

Statistical activity

The goal of the Canadian Environmental Sustainability Indicators (CESI) is to provide Canadians with more regular and reliable information on the state of their environment and how it is linked with human activities. Environment Canada, Statistics Canada and Health Canada are working together to develop and communicate these indicators. Reflecting the joint responsibility for environmental management in Canada, this effort has benefited from the co-operation and input of the provinces and territories.

A description of each indicator is available through the following link:

• Additional documentation

Subjects

• Environment
• Environmental quality

Data sources and methodology

Target population

The universe for this survey is the network of air quality monitoring stations in Canada. The target population includes those sites with sufficiently representative data. The site selection criteria are from Environment Canada, and require for each station that:
- Each eight-hour period must have data for six or more hours.
- Each day must have data for at least 18 of the 24 eight-hour periods.
- Each season must have values for at least 75% of days. For the smog season (April 1 -- September 30), 138 of the 183 days are required.
- To be included in the time series, stations must have yearly seasonal values for 75% or more of the years.
- Stations missing more than two consecutive years at the start or end of the period are excluded to avoid using data from stations commissioned or decommissioned during the period.

Instrument design

This methodology does not apply.

Sampling

It is recognized that the current collection of monitoring stations was not designed to be geographically representative of Canada and all population exposure to air pollution; rather, sites are situated to respond to specific federal, provincial or regional needs. Monitoring sites included in this analysis are almost all located in populated areas and other areas for which it is suspected that air quality is affected by surrounding land uses, such as specific industrial sites. Even so, sites do not comprehensively cover all geographic areas with potential air quality issues or problems across Canada.

The data used to calculate the air quality indicator were derived from samples collected at sites across the country during a fourteen year period. Data were combined to calculate a single index value for each year.

Minimum daily sample criteria for the calculation of 8-hour averages for ozone are six hourly readings for each eight hour period. Daily 8-hour maxima are only reported if there is a minimum of 18 of the possible 24 eight-hour averages for the day. Each station must have values for at least 75% of the days of the smog season. And, stations are only included in the 14 year trend analysis if they have 11 or more years of data, and if they have less than three consecutive years missing at the start and end of the time series.

As a result of applying the criteria for data completeness, 79 stations had sufficient data for the national trend analysis (1990-2003), 74 were used for regional trend analysis and 154 were used for the 2003 snapshot.

Data sources

Data are extracted from administrative files and derived from other Statistics Canada surveys and/or other sources.

Air quality data used in the calculation of the air quality indicator were obtained from existing air quality monitoring programs across the country. Detail on the specifics of the National Air Pollution Surveillance system can be found at the following website: http://www.etc-cte.ec.gc.ca/NAPS/.

Population data for the weighting of the indicator are based on the Census of Population and additional intercensal estimates from Demography Division at Statistics Canada.

Further details on the population estimates are available at the following Internet address: http://www22.statcan.ca/ccr_r000_e.htm.

Error detection

Each monitoring site follows standardized methods for sample collection in the field to ensure reliability of measurements. Data are subsequently analyzed by Environment Canada's Meteorological Service to correct for errors.

Imputation

This methodology does not apply.

Quality evaluation

Information on quality evaluation of the air quality monitoring data is available from the following website: http://www.etc-cte.ec.gc.ca/NAPS/.

Information in the quality evaluation of Census of Population data is available at the following Internet address: http://www22.statcan.ca/ccr_r000_e.htm.

Disclosure control

Statistics Canada is prohibited by law from releasing any information it collects that could identify any person, business, or organization, unless consent has been given by the respondent or as permitted by the Statistics Act. Various confidentiality rules are applied to all data that are released or published to prevent the publication or disclosure of any information deemed confidential. If necessary, data are suppressed to prevent direct or residual disclosure of identifiable data.

In order to prevent any data disclosure, confidentiality analysis is done using the Statistics Canada Generalized Disclosure Control System (G-Confid). G-Confid is used for primary suppression (direct disclosure) as well as for secondary suppression (residual disclosure). Direct disclosure occurs when the value in a tabulation cell is composed of or dominated by few enterprises while residual disclosure occurs when confidential information can be derived indirectly by piecing together information from different sources or data series.

Revisions and seasonal adjustment

Across the country, air quality varies seasonally and annually due to fluctuations in weather such as the timing and amount of sunlight, heat, and wind. The air quality indicator records only observations during the warm season to ensure consistency in the monitoring data since certain types of monitoring equipment are affected by low temperature extremes.

Data accuracy

The accuracy of the population weighted mean is dependent primarily on the accuracy of the population and air quality monitoring data used to generate the indicator. An additional factor is the degree to which the 40km zones used for population estimation defines the population affected by the pollution observed at each monitoring site. Information about the monitoring data can be found from the following website : http://www.etc-cte.ec.gc.ca/NAPS/.

Information about the population data are available at the following Internet address: http://www22.statcan.ca/ccr_r000_e.htm.

The use of 40km zones around the monitoring stations was based on expert opinion. Future indicator work will address the issue of population representation in more detail.