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)

Haze in urban areas [A Indicator 3.8]

  • Implications
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    Haze in urban areas generally denotes the presence of air pollution, but there may be several different causes for the haze. Photochemical smog leads to urban haze, but in Australian cities, smoke from wildfires or from poorly planned hazard reduction burns can also lead to haze. Haze is generally measured in terms of visibility: the inability to see objects further away than 20 km is evidence that there is haze. This is called 'local visual distance'.

    There was a decline in haze occurrences (Figure 110) during the late 1980s, but the occasional years of extreme bushfires will lead to episodic spikes in the number of hazy days.

    Figure 110: Number of days per year when one-hour visibility was less than 20 km.
    Negative values indicate zero exceedences. Zero values indicate no data.

     Number of days per year when one-hour visibility was less than 20 km.<br />
Negative values indicate zero exceedences. Zero values indicate no data

    Source: State environmental authorities

    Implications

    Haze is an episodic event. On most occasions, visibility is substantially greater than 20 km in all Australian cities. Nevertheless, even if visibility reductions occur only 10% of the time, this will equate to 36 exceedences per year.

    During the middle years of the 1990s (Figure 110), visibility exceedences were occurring in most Australian cities about 10% of the time, arresting, and in some cases reversing, a downward trend since the 1980s. A similar observation has already been made in relation to ozone (Figure 94) and it seems likely that the air pollution conditions are being influenced by major climatic occurrences, most probably resulting from El Nio. In such years the prevailing subsidence conditions and the higher incidence of bushfires cause greater occurrences of haze and other air pollution events.