State of the Environment Report 2007
Oxides of nitrogen is classed as an emerging issue due to its potential to cause significant impacts on human health and the atmosphere, however it is currently well below NEPM standards in areas where it is monitored. Subsequent State of the Environment reports may include it as a full issue. Some data is included in 'Photochemical smog' as it is a significant precursor for this issue.
'Oxides of nitrogen' (NOx) is a broad term used to include nitric oxide (NO), nitrogen dioxide (NO2), nitrous oxide (N2O), or mixtures of these compounds. They are all precursors of photochemical smog (see 'Photochemical smog'). Nitric oxide is a colourless and odourless gas that oxidises in the atmosphere to form nitrogen dioxide, an odorous, brown, acidic and highly corrosive gas that can affect human health and the wider environment. Nitrogen dioxide is a lung irritant and may lower immunity to respiratory infections. Exposure to high levels of nitrogen dioxide causes severe lung injury and has been demonstrated to increase the effects of exposure to pollutants such as ozone, sulfur dioxide and inhalable particulates (National Environment Protection Council, 1998). At high concentrations, nitrogen dioxide can reduce plant growth and cause visible injury, including damage to foliage that can inhibit crop yield. It can also fade and discolour fabrics, react with surfaces and furnishings and reduce visibility (Environmental Protection Agency & Queensland Parks and Wildlife, 2003). Nitrous oxide is a colourless non-flammable gas with a pleasant, slightly sweet odour. It has an anaesthetic effect (commonly known as laughing gas), but is also a potent greenhouse gas.
Oxides of nitrogen are emitted by internal combustion engines, industrial and commercial boilers in power generation and from industrial combustion processes, with motor vehicle emissions being the major source in urban areas. Natural sources of nitrogen oxides are from lightning and oxidation of ammonia, but these constitute a very small proportion of overall atmospheric concentrations. Indoor sources of nitrogen dioxide include unflued gas stoves and heaters and other combustion devices.
Standards are in place for nitrogen dioxide via the National Environment Protection (Ambient Air Quality) Measure (NEPM) to ensure community health is not compromised. They include:
Averaging period: one hour; Maximum concentration: 0.12 parts per million;Goal by 2008 (maximum exceedences): one day per year
Averaging period: one year; Maximum concentration: 0.03 parts per million; Goal by 2008 (maximum exceedences): none
Standards for health are measured as nitrogen dioxide, whereas standards for environmental emissions are measured as total oxides of nitrogen.
Nitrogen dioxide and other oxides of nitrogen are generally low in WA and below the NEPM standards. The Air Quality in Perth: 1992-2002 study (Department of Environmental Protection, 2003) found there had been no improvement in ambient nitrogen dioxide concentrations over the study period. Slight trends to increasing daily maximum concentrations (based on one-hour averages) were recorded at some monitoring sites. Emissions from area-based sources (e.g. commercial shipping and off-road vehicles), motor vehicles and industry have significantly increased since 1992. Increased emissions from motor vehicles are a result of increase in the number of vehicles and age of the vehicle fleet (average of 11 years) in Perth (Australian Bureau of Statistics, 1999, cited in Department of Environmental Protection, 2003).
Monitoring for nitrogen oxides has also been conducted at Dampier and Karratha for the Pilbara Air Quality Study (Department of Environment, 2004a), Port Hedland (BHP Billiton, 2005) and Wagerup in the South West (Alcoa Australia, 2005). Past monitoring has shown low nitrogen dioxide levels in these regions, below the NEPM standard. However, monitoring is no longer conducted at the Pilbara study sites, and data from industry sites is not readily available, so it is not possible to be certain the standard has been met in these areas in recent years.
Although design changes to vehicles and higher fuel standards have been implemented in WA to reduce nitrogen oxides, it is likely that population growth will result in more vehicles and therefore more nitrogen oxide pollution in Perth and regional centres. Changes to industrial processes have incorporated low nitrogen oxide emission burners in new installations, and the technologies are now well accepted by industry. Generally, greater reliance on natural gas fired power generation may have the adverse effect of increasing emissions of nitrogen oxides, but this has to be balanced against other environmental benefits.
