The Canadian Environmental Sustainability Indicators: On population-weighted ground-level ozone
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Soheil Rastan and Joe St. Lawrence, Environment Accounts and Statistics Division
The Canadian Environmental Sustainability Indicators (CESI) are a series of indicators published by the federal government so that Canadians better understand the linkage between the economy, the environment and human health. One such indicator is the ground-level ozone exposure indicator. This indicator reflects the trend in population-weighted ozone levels.
This study extends the trend analysis and builds upon the CESI indicator. It presents two additional population-weighted ground-level ozone concentration trends from 1990 to 2005: a trend based on a lower end or 25th percentile of the concentration data and a trend based on an upper end or 75th percentile of the concentration data (see text box for additional information).
From 1990 to 2005 the estimated increase, based on the lower-end of the annual concentration data, was statistically more significant than the estimated increase based on the middle range of the annual concentration data. On the other hand, trend analysis on the upper-end of the concentration data did not reveal any statistically significant increase or decrease.
This study is based on data from Canadian Environmental Sustainability Indicators (CESI), 2007. Information on the data sources and methods underlying the ground-level ozone exposure indicator can be found in Canadian Environmental Sustainability Indicators: Air Quality Indicators: Data Sources and Methods, Catalogue no. 16-254-X.
Over 250 air quality monitoring stations are located across Canada. Most stations collecting ground-level ozone data are organized under the National Air Pollution Surveillance (NAPS) program. The CESI ozone indicator is an estimate of the trend in population exposure to ground-level ozone. A population-weighted methodology is used to take into consideration the number of people living within 40-km of monitoring stations. Ozone concentration data in larger populated areas are given a higher weight than those in less populated areas to adjust for the differences in populated versus less populated areas.
The 1990 to 2005 ozone indicator is based on population-weighted average concentrations collated from 76 monitoring stations satisfying the CESI inclusion criteria, from April to September when ozone concentrations are relatively higher than in other months of the year. The trend is expressed in terms of an annualized rate of change, both as parts-per-billion per year (ppb/year) and as a percentage change per year, with associated confidence intervals.
25th, 50th and 75th percentiles
The 25th and the 75th percentile concentrations delineate the data into the lowest quarter, highest quarter, and the middle half of observations during the 180-day warm period, April to September.
The lower end of the spectrum represents days with ozone concentrations that are approximately below 30 ppb. These days have concentrations that are similar to background levels and are referred to in this study as the "good days" of the warm period in terms of ground-level ozone concentrations.
The higher end of the spectrum represents days with ozone concentrations that are approximately above 40 ppb. These days have concentrations that are fairly above those of background levels and are referred to in this study as the "bad days" of the warm period in terms of ground-level ozone concentrations.
The mid range of the spectrum holds both the median point (the 50th percentile) and the mean point. These represent ground-level ozone concentration in an average warm period day. The mid range of the spectrum represents average days with ozone concentrations at around 35 ppb.
The trend analysis conducted in this study follows the same non-parametric linear regression test used in the 2007 CESI report. The term "significant" in this study refers to statistical significance. Reported trends have probability (p) values and confidence intervals (CI). Synthesis and analysis of spatio-temporal and environmental data to examine cause-effect relationships are beyond the scope of this analysis.
Ground-level ozone is an air pollutant. It is a highly reactive and unstable compound that reacts with almost anything it comes into contact with— mainly other pollutants present in the ambient air including nitrogen oxides (NOx) and volatile organic compounds (VOC). Together these air pollutants result in the formation of smog, especially during the warmer months of the year.
Exposure to ground-level ozone is higher during warm periods than during cold periods of the year. Some of the health risks associated with exposure to ground-level ozone range from minor to severe respiratory problems.
Human activities affect the formation of ground-level ozone; however, ozone is also present in the natural environment at concentrations known as background levels.
In general, the annual average ozone concentration at Canadian background stations is between 25 and 35 parts-per-billion (ppb), a range similar to background sites in the United States and around the world.1 Such background levels in ozone concentration represent almost half of the Canada-Wide Standard (CWS) threshold limit for ground-level ozone.2
During the warm months of the year from April to September, daily ground-level ozone concentrations vary from as little as 10 ppb to over 100 ppb, depending on temperature, sunlight, wind pattern, NOx concentrations, and the spatial proximity of the monitoring stations to sources of ozone-forming and depleting pollutants.
In terms of accumulated exposure, days with ozone levels similar to or higher than the 75th percentile3 carry a higher degree of risk if compared with days that have ozone levels similar to or lower than the 25th percentile.4
Conducting a trend analysis on the upper end of the ozone concentration spectrum, the 75th percentile, revealed no statistically significant increase or decrease.5
Trend analysis on the mid range, both the mean and the median, of the ozone concentration spectrum, days with ozone concentrations at around 35 ppb, suggested a statistically significant upward trend; an increase of 0.3 ppb/year.6 This rate is equivalent to an annual average percentage increase of 0.8%.7
However, a trend analysis on the lower end of the ozone concentration spectrum, the 25th percentile, suggested a statistically more significant increase of 0.4 ppb/year.8 This rate is equivalent to an annual average percentage increase of 1.5%.9 Chart 1 presents the results of the trend analyses on the three concentration spectrums.
The results of this hypothesis-generating study indicate that as far as the national estimate of population-weighted ground-level ozone concentration is concerned, the good days are getting worse and the bad days remain the same.
The extent to which rising temperature, on the one hand, and falling NO emissions, on the other hand, has influenced these increasing trends remains to be examined.
Analyzing the influence of such parameters is beyond the scope of this study. Future work could further evaluate the role of some of these factors in influencing the magnitude of our cumulative exposure to ground-level ozone during the warm months of the year.
- Ian G. McKendry, 2006, Background Concentrations of PM2.5 and Ozone in British Columbia, Canada (PDF), Geography / Atmospheric Science, Prepared for the British Columbia Ministry of the Environment, (accessed April 28, 2008).
- The Canada-Wide Standard for ground-level ozone is 65 ppb, 8-hour averaging time, achievement to be based on the 4th-highest annual ambient measurement, averaged over 3 consecutive years, by 2010. See Canadian Council of Ministers of the Environment, 2008, Particulate Matter and Ground-level Ozone, (accessed April 14, 2008).
- The 75th percentile of the data represents days with ozone concentrations approximately above 40 ppb.
- The 25th percentile of the data represents days with ozone concentration approximately below 30 ppb.
- p = 0.260; 90% CI: -0.1 to 0.5 ppb/year.
- p = 0.054; 90% CI: 0.1 to 0.5 ppb/year.
- 90% CI: 0.1% to 1.7%.
- p = 0.001; 90% CI: 0.2 to 0.5 ppb/year.
- 90% CI: 0.7% to 1.8%.
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