Atmosphere Theme Report

Australia State of the Environment Report 2001 (Theme Report)
Lead Author: Dr Peter Manins, Environmental Consulting and Research Unit, CSIRO Atmospheric Research, Authors
Published by CSIRO on behalf of the Department of the Environment and Heritage, 2001
ISBN 0 643 06746 9

Urban Air Quality (continued)

Indicators of the condition of air quality (continued)

Ozone in urban areas [A Indicator 3.2]

  • Implications
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    The smog that occurs in Australian cities is photochemical in origin. This surface ozone is not related to the stratospheric ozone dealt with in Stratospheric ozone. Concentrations of ozone provide an estimate of photochemical smog. Since the 1980s there has been a steady decline in the maximum value of hourly ozone concentrations in the biggest cities of Australia (Figure 92). This has led to a common perception that there is a decline in the number of occurrences of smog episodes. Figure 93 casts some doubt on this interpretation: there is no downward trend in the maximum value of ozone when averaged over four hours. For the smaller cities (i.e. other than Sydney and Melbourne) it appears that maximum ozone concentrations are steady or have increased slightly from 1980 or 1990.

    Figure 92: Maximum one-hour average ozone concentrations in selected cities.

     Maximum one-hour average ozone concentrations in selected cities

    Source: Commonwealth, state and territory environment authorities

    Figure 93: Maximum four-hour ozone concentrations in selected cities.

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    Source: Commonwealth, state and territory environment authorities

    The situation is more complex than previously thought (Figures 94 to 96). The steady decline, from 1982 to 1990, in the number of days that exceed the NEPM Standard in Sydney and Melbourne contributed to the view that smog events were on a sustained decline. The situation reversed in the early 1990s, and there has been an increase in the number of days that exceed the NEPM standards.

    Figure 94: Number of days per year with maximum hourly ozone greater than the NEPM standard of 0.10 ppm for selected cities.
    Negative values indicate zero exceedences. Zero values indicate no data.

     Number of days per year with maximum hourly ozone greater than the NEPM standard of 0.10 ppm for selected cities

    Source: Commonwealth, state and territory environment authorities; NEPC (2001)

    Figure 95: Number of days per year with maximum hourly ozone greater than 0.08 ppm for selected cities.
    Negative values indicate zero exceedences. Zero values indicate no data.

     Number of days per year with maximum hourly ozone greater than 0.08 ppm for selected cities

    Source: Commonwealth, state and territory environment authorities

    Figure 96: Number of days per year with maximum four-hourly ozone greater than NEPM Standard of 0.08 ppm for selected cities.
    Negative values indicate zero exceedences. Zero values indicate no data.

     Number of days per year with maximum four-hourly ozone greater than NEPM Standard of 0.08 ppm for selected cities

    Source: Commonwealth, state and territory environment authorities

    Ozone has a marked seasonal distribution. Highest ozone concentrations are observed in summer (when solar radiation is strongest). Figure 97 shows the monthly variation in daily maximum one-hour ozone concentration in south-east Queensland during 1997 and 1998.

    Figure 97: Monthly variation in daily maximum one-hour ozone concentration in south-east Queensland during 1997 and 1998.
    The line above the box reaches to the highest readings. The box marks the 90th and 50th percentiles. The line below the box extends to the 10th percentile.

     Monthly variation in daily maximum one-hour ozone concentration in south-east Queensland during 1997 and 1998

    Source: Data from Queensland EPA

    Implications

    Unlike carbon dioxide, ozone is not emitted directly from motor vehicles. The direct emissions of oxides of nitrogen and hydrocarbons react with sunlight to eventually form ozone. From 1976, when they were first introduced, until 1998, the ADR set a limit in the range of 1.9 to 1.93 g/km for NOx. In 1998, this was reduced to 0.63 g/km under ADR37/01. Hydrocarbon emissions have been more stringently controlled. In 1976, ADR27A set a hydrocarbon emission limit of 2.1 g/km, which was successively reduced to 1.75 g/km in 1981, to 0.93 g/km in 1986 and to 0.26 g/km in 1998.

    The emission limits set in 1986 were not sufficiently stringent to enable compliance with the NEPM that was introduced in 1998 (Figures 92 and 93). Hopefully, the stricter emission limits imposed in 1998 by ADR37/01 will overcome this.