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Comparing Australian cities with those overseas shows that, in general, air quality is relatively good in all the major capitals. Problems do, however, occur from time to time. Each capital suffers conditions which trap or circulate the air mass over urban areas during periods of high sunlight intensity or winter inversions. During these weather events, levels of smog or airborne particulates may approach or exceed air quality standards. Such events can be exacerbated by the occurrence of bushfires or hazard reduction burning in the vicinity of the airshed, or even at some distance from it.
An airshed is a body of air, bounded by meteorology and topography, in which a contaminant, once emitted, is contained. The capital cities, where the great majority of Australians live, are located on coastal strips with mountain ranges nearby. They experience high thermal contrasts, and on days which give rise to photochemical smog typically have offshore winds in the mornings, high sunlight flux, and on shore winds in the afternoons. This gives rise to circulation patterns in which morning, or previous evening, emissions of pollutants are converted to photochemical smog through the day, then circulated back over the cities, and typically concentrated into the regions well away from the central business district or the coast, such as Western Sydney or the southern parts of the Brisbane region. While each airshed is unique there are considerable commonalities between them, allowing a national approach to be undertaken on the issues, albeit with local characteristics.
Analysis of major airshed weather patterns by the Inquiry indicates that all capitals have between 20 and 40 "pollution conducive" days per year. Not all give rise to exceedence events. However the analysis demonstrates that, for these conducive circumstances, underlying day long concentrations of ozone and other smog components can be substantial even when exceedences do not occur. Unless there is a smelter or processing plant in or near the airshed, airborne lead is no longer a serious concern in Australian cities. Similarly, oxides of sulfur do not present difficulties in most urban airsheds.
Carbon monoxide emissions to the total airshed have been greatly reduced and are mainly a local issue associated with heavily trafficked corridors, local industry and winter wood burning in predominantly southern areas. The main areas of concern, now and for the future, relate to particulates, oxides of nitrogen and other smog precursors such as hydrocarbons. To summarise, while Australian urban ambient air quality is good in world terms, there is no room for complacency. Per capita, Australians are among the world's highest pollutant emitters although fortunately emissions per unit area are low by world standards due to the relatively low population density of our cities. Population trends over the next fifteen years and associated demand for services and travel necessitate that a cautious approach be taken in order to ensure that air quality is maintained.
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