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Chapter 2

Ozone in Ontario

Ground-level ozone is a gas formed when nitrogen oxides and volatile organic compounds react in the presence of sunlight. Ground-level ozone is a primary component of smog and is different from the ozone in the layer high above the earth that protects us from the sun's harmful ultra-violet (UV) rays. The formation and transport of ozone are strongly dependent on meteorological conditions. Changing weather patterns contribute to short-term and year-to-year differences in ozone concentrations. In Ontario, elevated concentrations of ground-level ozone are generally recorded on hot, sunny days from May to September between noon and early evening.

Characteristics, sources and effects

Ozone is a colourless, odourless gas at ambient concentrations, and is a major component of smog.

Ground-level ozone is not emitted directly into the atmosphere, however, it results from chemical reactions between volatile organic compounds (VOCs) and nitrogen oxides (NO_X) in the presence of heat and sunlight. Figure 2.1 shows estimates by sector of Ontario's VOC emissions caused by human activity. Transportation sectors account for approximately 30 per cent of VOC emissions. However, owing top the large forested area in Northern Ontario, biogenic VOC emissions in Ontario are significant - approximately three times those from sources caused by human activity. The sources of NO_X are discussed in the nitrogen dioxide section in Chapter 4.

Ozone irritates the respiratory tract and eyes. Exposure to high levels of ozone results in chest tightness, coughing and wheezing. Children active outdoors during the summer when ozone levels are at their highest are most at risk of experiencing such effects. Other groups at risk include individuals with pre-existing respiratory disorders such as asthma and chronic obstructive lung disease. Ground-level ozone has been linked to increased hospital admissions and premature deaths. Ozone also causes agricultural crop loss each year in Ontario and noticeable leaf damage in many crops, garden plants and trees.

Figure 2.1: Ontario VOC Emissions by Sector

(Emissions From Human Activity, 2000 Estimates)

Monitoring results for 2000

Ground-level ozone was monitored at 38 ambient locations during 2000, however, only 36 sites (26 urban and 10 rural) had sufficient data to be used in the analysis presented here. Once again the highest annual mean (33.1 ppb) was measured at Long Point, a rural site on the northern shore of Lake Erie, while the lowest annual mean (16.9 ppb) was measured at Hamilton West. Generally, ozone is lower in urban areas because it is removed by reaction with nitric oxide emitted locally by vehicles.

Among urban sites in 2000, London recorded the highest one-hour concentration (110 ppb), Kitchener recorded the greatest number of instances (25 hours) of ozone above the one-hour AAQC of 80 ppb, and Peterborough recorded the highest annual urban mean (28.1 ppb).

At rural sites, Grand Bend on the eastern shore of Lake Huron measured the highest one-hour concentration (128 ppb) and Long Point recorded the most number of instances (105 hours) above Ontario's one-hour AAQC.

Ground-level ozone continues to exceed its provincial criterion across the province. In 2000, Ontario's one-hour ozone AAQC was exceeded at 32 of 36 monitoring stations on at least one occasion. All sites in Ontario except Ottawa, Cornwall, Thunder Bay and North Bay recorded at least one hour of elevated ozone (above 80 ppb) in 2000. People with heart and lung problems are at higher risk when ozone levels exceed 80 ppb. Sensitive people may experience trouble breathing and their health may be affected if they engage in vigorous exercise.

Figure 2.2 shows the number of ozone exceedance days (a day with at least one hour above the 80 ppb AAQC) at sites across Ontario for 2000. Long Point recorded the most exceedance days (20) or about five per cent of the days in 2000. This is significantly lower than the 38 and 42 exceedance days recorded at Long Point in 1998 and 1999 respectively and, is mostly attributed to the cool, wet summer of 2000. Among the next eight ozone sites recording the highest number of exceedance days in 2000, all are located in southwestern Ontario and six of the eight are monitoring in rural areas.

Figure 2.2: Number of Ozone Exceedance Days at Sites Across Ontario (2000)

Note: Rural sites are italicized; an ozone exceedance day has at least 1 hour > 80 ppb.

In general, ozone levels in southern Ontario decrease from southwest to northeast. As previously mentioned, more than 50 per cent of provincial ozone levels during widespread smog episodes are due to long range transport of ozone and its precursors from neighbouring U.S. States. This U.S. contribution is expected to be much higher (as much as 90 per cent) in Ontario cities and towns on the northern shores of Lake Erie, the eastern shores of Lake Huron and in the extreme southwest near the U.S. border.

Trends

Figure 2.3 shows the distribution of the province-wide ozone exceedance days (at least one hour > 80 ppb) and the number of hot days (those days with maximum air temperatures greater than 30°C) for the period 1991 to 2000. The number of hot days and the number of ozone exceedance days show a similar trend over the 10-year period. Just as the highest number of exceedance days in 1999 is attributed to the weather (highest number of hot days), the lowest numbers in 2000 reflect conditions less favourable to the formation and transport of ozone from the U.S.

Figure 2.3: Trend of Ozone Exceedance Days and 'Hot' Days (1991 - 2000)

Note: 22 ozone sites operated over 10 years; an ozone exceedance day has at least one hour > 80 ppb.

Number ot 'Hot' Days is based on observations from 8 sites where maximum daily air temperatures > 30 degrees C.

Province- wide VOC emissions show a decreasing trend for the period 1991 to 1996, after which they have remained fairly constant to 2000 (Figure 2.4). New vehicle emission standards in the early 1990s and the shift in the consumption of residential fuels from oil and wood to natural gas probably contributed to this trend. Emissions from forest fires and natural sources are not included in the trend; these emissions may be as high as three times the emissions from human activity.

Figure 2.4: Trend of VOC Emission Estimates (1991 - 2000)

The introduction of lower gasoline volatility beginning in the summer of 1989 has resulted in a 3.1 per cent decrease in VOC emissions during 2000. As well, the implementation of Ontario's Drive Clean vehicle inspection program has contributed to an additional reduction in VOC levels. Overall, there has been a 17 per cent reduction in VOC emissions over the period 1991 to 2000.

The trend in composite one-hour maximum ozone concentrations is shown for 1991 to 2000, in Figure 2.5. For the 10-year period, the composite mean of the maximum one-hour concentrations range from 98 to 117 ppb. The composite rural hourly maximum ozone concentration is on average about 10 per cent higher than the corresponding composite urban one-hour maximum.

Figure 2.5: Trend of Ozone Concentrations (1991 - 2000)

Note: Annual composite maximum one-hour ozone based on 18 ozone sites operated over 10 years.