Point-source and diffuse-source pollution
As previously noted, air quality in Australia’s urban areas is strongly influenced by short-term meteorology (including extreme events), seasonal conditions, local topography and distance from the sea. The size of the urban centre and the presence of major industrial facilities also play a role in shaping the varying levels of air quality experienced in an urban airshed.
Historically, the most common image of air pollution has been a highly visible plume of unknown content being emitted from a power station or industrial plant. However, both state and national pollutant inventories show that, although industrial point sources still dominate some emission types (notably sulfur dioxide), with the exception of major industrial centres (such as Mount Isa and Port Pirie), diffuse or area sources tend to be the main factors affecting air quality at an airshed scale. These sources include motor vehicles, domestic and commercial solvents, service stations and domestic lawn mowers. The generalisation remains true whether the focus is on the key criteria pollutants (ozone and its precursors and particles) or on the main hazardous air pollutants (air toxics such as benzene, toluene and xylene) (Figure 3.31).139,157
It is the diffuse sources (motor vehicles, domestic solid fuel heating, bushfires and various types of planned burning, and dust from roads and agricultural activities) that are the most challenging for government policy makers, regulators and program managers working to improve air quality.
Among diffuse sources of air pollution, motor vehicles are the most pervasive and have the largest impact on urban air quality and human health. In our capital cities, they are the dominant source of NOx (a generic term for nitric oxide and nitrogen dioxide) and VOCs—the precursors of photochemical smog. Although the combined emissions from industry, electricity generation and wood heating are a larger source of PM10 than motor vehicles, because of their ubiquitous presence in our cities, motor vehicles tend to be a more important source of human exposure. Furthermore, discharges from major industrial and power-generation facilities are elevated and thus have less influence at ground level than corresponding ground-level emissions.
In addition, very fine particles (<1 micrometre) form a major part of vehicle particulate emissions. It is these, together with particles in the range 1 micrometre to less than 2.5 micrometres, that are the focus of increasing concern in relation to cardiovascular and respiratory disease, with which they are strongly correlated.158 The Australian Bureau of Transport and Regional Economics estimates that, in 2000, motor vehicle pollution was responsible for 900–4500 cases of respiratory and cardiovascular disease and bronchitis, and as many as 2000 premature deaths.
Within an airshed at a neighbourhood level, as state regulators and local government officials know only too well, a broad range of small-scale industrial and commercial activities have the potential to impact on local amenity and health, most often through emissions of odour, dust and noise. Such widespread diffuse-source problems are often historical in nature (the result of residential areas having developed in close proximity to incompatible land uses) and are particularly difficult to resolve.
An important diffuse source of particulate pollution in cool–temperate parts of Australia is domestic wood heaters and open fires. In autumn and winter, in cities such as Melbourne, Hobart, Canberra and Launceston, and in many smaller centres in Tasmania, Victoria and inland New South Wales, smoke from domestic wood heaters is the major source of particulate pollution. In inland centres such as Canberra, cold nights and clear skies frequently occur in autumn and winter, creating temperature inversions. These trap wood smoke near ground level, leading to particle levels above both the NEPM 24-hour PM10 standard and the PM2.5 advisory level.6 In centres such as Launceston, local valley topography can increase the frequency and strength of such inversions, leading to incidents of significant particulate pollution.
The term ‘planned burning’ encompasses a broad range of activities associated with forestry, public land management and agriculture. Depending on their location and scale, the smoke generated by such activities has the potential to impact on health and amenity, affecting areas such as tourism, viticulture and outdoor events if the burns are not well planned and executed. Recent work by the Environment Protection Division of the Tasmanian Department of Primary Industries, Parks, Water and Environment indicates that planned burns are a significantly more important diffuse source of particulate pollution than estimated by the National Pollutant Inventory.159 However, although the potentially adverse impacts of planned burns need to be recognised and managed, they should be considered in the context of potential benefits, such as a reduction in the risk of wildfires.
The term ‘planned burning’ could also be applied to burning carried out as a traditional management practice by Indigenous land custodians in tropical savanna grasslands in northern Australia. These low-impact burns have been employed by Aboriginal people for many thousands of years.160 Because they take place in remote areas away from population centres, these traditional practices do not raise concerns over impacts on health or amenity, such as are often associated with planned burning in the southern parts of the country.