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Trends in Ozone and its Precursors
TRENDS IN OZONE AND ITS PRECURSORS
This section shows how the annual national and regional average levels of ozone and its precursors (NO, NO2, and VOC) have varied over time. It also provides an estimation of the trends in these annual averages. The annual average for a given year is simply the average of all the considered station-specific values for that year (for both regional and national averages). These averages are based on data from monitoring stations with a 75% data capture over the period considered. The ozone averages are based on data from monitoring stations in both urban and non- urban areas, while for NOx and VOC, only stations located in urban areas were considered (primarily because most measurements of NOx and VOC occur in urban areas).
The values of the regional and national annual averages vary from year to year as the ambient levels react to changing conditions such as emissions and meteorology. Despite this variation, there may have been an overall upward or downward tendency in these values. This tendency is typically qualified by the slope (the rate of change) of a linear line fitted through the actual values, and it is this slope and the direction of change in values (increasing or decreasing) that is referred to as the trend.
In this report, the trend is estimated by applying the Sen’s Non-parametric Estimator of Slope. The Sen method was also used to test if a trend was statistically different from zero at the 95% confidence level. A non-statistically significant trend means that random variations alone may have been responsible for the trend and, as such, there is likely no systematic trend in levels. For this report, only the direction of the trend is indicated and trends that are not statistically significant are reported as No trends.
Trends in Ozone
Figure 7 shows how the annual national and regional average ozone levels in the form of the CWS varied over the 15-year period from 1991 to 2005. Except for New Brunswick, national and regional average ozone levels have remained more or less unchanged (i.e. the trends were not statistically different from zero) over the 15-year period. Levels in New Brunswick experienced a decreasing trend, although this is largely attributable to the substantial drop in levels at the beginning of the period.
Figure 7 also shows how the average ozone levels compare to the numerical value of the CWS. This is shown only as a relative indication of the magnitude of the measured levels, and not as an indication of achievement of the CWS. The national average ozone levels were either just above or just below the CWS over most of the 15-year period. In the four western provinces, the regional averages have been consistently below the CWS, with the highest levels found in Alberta. The regional average has been above the CWS every year in Ontario, and in all but two years in Quebec.
The "no change” observed in ozone levels over the 15-year period would suggest a corresponding "no improvement” in population health risk associated with ambient ozone levels in the form of the CWS.
In New Brunswick, the average was above the CWS at the beginning of the period, and has remained just below it ever since. For Nova Scotia, only one non-urban monitoring station (located in Kejimkujik National Park) satisfied the data completeness criteria. The ozone levels at this station have been mostly above the CWS. For Newfoundland and Labrador, the only station considered, located in St. John’s, and has had ozone levels consistently below the CWS. Overall, levels in the eastern regions have experienced a downward tendency over the last three years of the reporting period.
Figure 7: Trends in ozone levels in the form of the ozone CWS, 1991-2005
Notes: Indicated levels are consecutive 3-year averages. The ozone CWS is shown only as an indication of how the levels compare, on average, to the Standard and not as an indication of achievement of the CWS over the years. The direction of the trend in levels (decreasing or increasing) is indicated only if the obtained value of the trend is statistically different from zero at the 95% confidence level. Otherwise it is indicated as "No trend”. Data generated by Environment Canada from measurements collected through NAPS. Large urban (LU) stations are located in communities with populations over 100,000; Small urban (SU) stations are located in communities with populations of 100,000 or less; Non-urban (NU) stations are located in areas where the land use is predominantly rural.
Trends in NOx
Figure 8 shows how the annual warm season (April to September) national and regional average of the 1-hour NO and NO2 levels varied over the 15-year period 1991 to 2005. April to September is the period in Canada where the peak short-term (1- to 8-hour averages) ozone levels are typically the highest. The ambient NO and NO2 levels discussed here are based only on monitoring stations located in urban communities and are presented only for regions with sufficient data.
The ambient NO and NO2 levels both decreased substantially, nationally and regionally, with statistically significant downward trends. Nationally, NO levels in 2005 were about 50% lower than in 1991, and NO2 about 30% lower. Similar reductions are also seen in each considered region. Of interest to note in Figure 8 is the fact that reductions in NO were almost double those of NO2.
Since only urban monitoring stations were considered, the measured ambient NO and NO2 levels at these stations is largely a reflection of locally generated emissions of NOx. For most Canadian urban areas the largest sources of NOx emissions come from on-road vehicles. As such, the observed reductions in ambient NO appears to be consistent with the NOx reductions from on-road vehicles of about 40% between the 1990 emissions and the projected emissions for 2005 (see Section 6.2.2).
Figure 8: Trends in 1-hour NO2 and NO levels, April to September, 1991-2005
Notes: Downward trends are observed in all regions for both NO2 and NO and these trends are all statistically significant at the 95% confidence level. The indicated percentage changes are the percentage difference in levels between the end year and the beginning year. In brackets are the number of Large Urban stations considered. Data were generated by Environment Canada from measurements collected through NAPS.
Trends in VOC
Ambient VOC levels are not measured every hour like NO and NO2. Rather, they are measured over a 24-hour period, with the measurements taken every three or six days. This section shows how the annual warm season (April to September) national and regional average of these levels varied over the period 1993 to 2005 (the longest period of available data for a larger number of monitoring stations). The VOC levels discussed here are based only on monitoring stations located in urban communities and are presented only for regions with sufficient data.
As seen in Figure 9, ambient VOC levels decreased both nationally and regionally by about 50%. The decreasing trend was (statistically) significant nationally and in all regions except at the New Brunswick monitoring station. The decrease in ambient VOC appears to be consistent with the VOC emission reductions from on-road vehicles of about 50% between the 1990 emissions (676 kilotonnes, kt), and the projected emissions in 2005 (274 kt).
Figure 9: Trends in 24-hour VOC levels, April to September, 1993-2005
Notes: Downward trends are observed in all regions and the trends are all statistically significant (95% confidence level), except for New Brunswick where the trend is not statistically significant. The indicated percentage changes are the percentage difference in levels between the end year and the beginning year. In brackets are the number of Large Urban station considered. Data were generated by Environment Canada from measurements collected through NAPS.
Discussion on Trends
As noted above, national average ozone levels in the form of the CWS (which considers the 4the highest ozone levels) remained unchanged over the 15-year period form 1991 to 2005. During this time period, ambient levels of ozone precursors decreased substantially. The decrease in ambient NO levels at many urban locations, with a resulting decrease in ozone scavenging (as discussed in section 2), is the most apparent reason why the 4th highest ozone levels resulting from the yearly change in meteorological conditions. These not only have substantial impacts on the higher ozone levels, but they also mask the long-term trends in ozone levels associated with changes in emissions of NOx and VOC.
Ozone levels measured at a given location depend not only on the emissions of the precursors in the community, but also on a number of other factors such as the prevailing meteorological conditions, the chemical processes, the direction of the wind and the associated possible transport of ozone and its precursors into the community from upwind source regions, and the long-range transport of these pollutants. As such, comparison between the local ozone levels and the local ambient levels of its precursors alone is not sufficient to account for trends in ozone levels.
Modelling and observational analysis continue to support the view that reductions in both VOC and NOx will benefit urban areas while NOx reductions may be more effective in lowering widespread ozone concentrations, benefiting rural areas.
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