2.4 Photochemical smog

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Priority Rating: 3

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Description

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Photochemical smog is considered to be a problem associated with modern industrialised cities. It is characterised by the reaction of ozone (O₃), oxides of nitrogen (NO_x) and reactive organic compounds (also known as volatile organic compounds) in sunlight and at high temperatures. A mixture of these chemicals forms a layer of visible, brown or white haze in the sky (Government of Western Australia, 1998). Perth is most susceptible to smog, especially from late spring through to early autumn, when sunlight is abundant, temperatures are hot, and oscillating wind patterns mean that the city's air is recirculated. In contrast, visible haze at other times of the year is likely to be caused by particulates (see 'Particulates').

Changes in precursor pollutants can influence the formation of photochemical smog. Sources of the precursor pollutants include motor vehicles (for both oxides of nitrogen and reactive organic compounds), combustion processes (oxides of nitrogen and ozone), and refining, petrochemical and solvent based industries (reactive organic compounds). The concentration of ozone in a polluted atmosphere is typically used as an indicator of the total amount of photochemical smog, as it usually makes up about 85% of the total photochemical smog concentration. Photochemical smog can also be transported over large distances, with weather patterns principally influencing its dispersal. The absence of vertical air mixing, as occurs with temperature inversion layers close to Earth, also prevents its dispersion. Ozone as a pollutant in the troposphere (close to the surface of the Earth) should not be confused with the hole in the ozone layer (see 'Stratospheric ozone depletion').

Photochemical smog can cause a number of human health issues, the most common being respiratory problems. It also contains peroxyacetyl nitrate, a compound that causes the eyes to water profusely. Hospitalization rates often rise during incidents of photochemical smog, and mortality of people aged 65 and older from cardio-vascular problems increases with exposure to ozone. The health of native vegetation and crop productivity may also be detrimentally affected.

Objectives

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• To protect health, amenity and the environment by ensuring that the levels of photochemical smog in the air meet the relevant standards.

Condition

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There are standards in place for photochemical smog via the National Environment Protection (Ambient Air Quality) Measure (NEPM) to ensure community health is not compromised. They include:

Photochemical oxidants (as ozone):

Averaging period: One hour; Maximum concentration: 0.10 parts per million; Goal by 2008 (maximum exceedences): one day per year;

Averaging period: Four hours; Maximum concentration: 0.08 parts per million; Goal by 2008 (maximum exceedences): one day per year 

Nitrogen dioxide:

Averaging period: one hour; Maximum concentration: 0.12 parts per million; Goal by 2008 (maximum exceedences): 1 day per year

Averaging period: one year; Maximum concentration: 0.03 parts per million;Goal by 2008 (maximum exceedences): none

It is worth noting that standards for health are measured as nitrogen dioxide, whereas environmental standards emissions are measured as total oxides of nitrogen.

Indicator A13: Ambient ozone concentration compared with National Environment Protection (Ambient Air Quality) Measure standard and goal.

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Perth's air quality is of a high standard compared to other Australian and international cities, but photochemical smog is regularly experienced over the city in summer. When compared to the NEPM standard, there has been a trend of decreasing ozone concentrations in the maximum range of one hour averages at Caversham (Figures A4.1) and Rockingham (Figure A4.2). Ozone concentrations exceeding the standard are very infrequent at South Lake (Figure A4.3). Caversham is an eastern inland Perth suburb, Rockingham is a southern coastal suburb, and South Lake is a southern inland suburb.

Figure A4.1: Ambient yearly range of ozone levels (one hour average) at Caversham air quality monitoring station, compared with the Australian standard for exposure.

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Data source: Department of Environment and Conservation.

Figure A4.2: Ambient yearly range of ozone levels (one hour average) at Rockingham air quality monitoring station, compared with the Australian standard for exposure.

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Data source: Department of Environment and Conservation.

Figure A4.3: Ambient yearly range of ozone levels (one hour average) at South Lake air quality monitoring station, compared with the Australian standard for exposure.

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Data source: Department of Environment and Conservation.

Indicator A14: Number of exceedences of the ozone National Environment Protection (Ambient Air Quality) Measure.

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There were 13 exceedences of the ozone NEPM standard for metropolitan Perth from January 1998 to December 2006 and none were recorded in other monitored areas. The ambient air quality protection goal of no more than one exceedence per site per year would have allowed four exceedences over this period. While of concern, this is not high compared to other locations in Australia. For example, the Sydney, Newcastle and the Hunter Valley region of NSW had 392 exceedences of the standard at air quality monitoring sites between July 1998 and June 2006 (NSW Environment Protection Authority 1998-2006). All exceedences in Perth have occurred in January and February, due to the high summer temperatures required for smog to form. The number of exceedences of the standard for ozone has generally decreased over time, although some sites had more in 2004 than in previous years.

Ozone is also measured at Quinns Rock, Rolling Green and Swanbourne sites, where no exceedences of the ambient air quality standard have been recorded. The behaviour of ozone in the airshed contributes to hotspots in concentration, which can move according to meteorological conditions, and consequently these hotspots are not necessarily picked up by existing monitoring stations.

Indicator A15: Background concentration of ozone in the Perth airshed.

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Even though the total number of ozone exceedences of the NEPM standard is not high (Figures A4.1, A4.2 and A4.3), a concerning trend is the increase in background ozone concentrations. A rise in background concentration means that Perth airshed is gradually moving closer to the NEPM standard, and less of a daily increase is required to exceed the standard (Figure A4.4). For example, the number of days showing moderate levels of ozone (0.03 ppm to 0.07 ppm) increased between 1992 and 2002 at Caversham - but still remains below the NEPM standard. A levelling out was seen between 2003 and 2005, but may have started rising again in 2006 (Figure A4.4).

Figure A4.4: Number of days at Caversham air quality monitoring station where the daily maximum one-hour averaged ozone concentration was lower or higher (but below the National Environment (Ambient Air Quality) Protection Measure exceedence limit), 1992-2006.

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Data source: Department of Environment and Conservation. Analysis: Department of Environment and Conservation. Note: An ozone concentration of 0 ppm to 0.03 ppm is considered low. A concentration between 0.03 ppm to 0.07 ppm is higher, but still below the NEPM exceedence limit.

Indicator A16: Number of exceedences of the nitrogen dioxide National Environment Protection (Ambient Air Quality) Measure.

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Nitrogen dioxide levels in Perth are generally low and below the NEPM standard at all monitoring sites. Ambient nitrogen dioxide levels for most suburban sites are low (i.e. Caversham, Hope Valley, North Rockingham, Quinns Rock, Rolling Green, South Lake and Swanbourne, Duncraig) and have remained stable over time (for example, see Figure A4.5). In contrast, nitrogen dioxide levels at the Queens Building monitoring site in Perth's central business district shows some exceedences of the NEPM standard, but a general decreasing trend is observed (Figure A4.6).

Figure A4.5: Ambient yearly range of nitrogen dioxide levels at Duncraig air quality monitoring station, compared with the Australian standard for exposure.

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Data source: Department of Environment and Conservation.

Figure A4.6: Ambient yearly range of nitrogen dioxide levels at Queens Building (Perth central business district) air quality monitoring station, compared with the Australian standard for exposure.

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Data source: Department of Environment and Conservation.

Indicator A17: Background concentration of nitrogen dioxide.

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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 in 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 which are 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.

Pressures

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Indicator A18: Emissions of precursors to photochemical smog.

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Oxides of nitrogen and reactive organic compounds are major precursors for the formation of photochemical smog.

Oxides of nitrogen are released from motor vehicles, and other fuel combustion and point sources. Motor vehicles and industrial emissions contribute 42% and 37%, respectively, to the oxides of nitrogen load in the Perth airshed (Department of Environmental Protection, 2002a).

Estimated oxides of nitrogen emissions in the Perth airshed have increased between 1992-93 and 1998-99 (Figure A4.7; Department of Environmental Protection, 2002b). Increases were recorded for all sources, except biogenic and natural sources which remained stable. Increases occurred in the number of cars on the road, the number of kilometres being travelled, and a greater number and/or volume of industrial and commercial emissions. The significant increase in area-based emissions results from off-road vehicles (i.e. trains, aircraft) and ships, and a change in methodology to address underestimates in earlier figures (Department of Environmental Protection, 2002b).

Figure A4.7: Change over time in estimated amount and source of emissions of oxides of nitrogen in the Perth airshed.

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Data source: Department of Environmental Protection (2002b).

Reactive organic compounds were examined in the Perth Airshed Inventory Update 1998-1999 (Department of Environmental Protection, 2002b). Total estimated emissions reduced between 1992-93 and 1998-99, and the major source changed from motor vehicles to area-based sources (Figure A4.8). Reduced motor vehicle emissions were likely to be a result of better emission controls (associated with engine design improvements) and a significant decrease in industrial emissions (primarily the result of improvement work undertaken at the BP Kwinana refinery).

Figure A4.8: Change over time in estimated amount and source of reactive organic compound emissions in the Perth airshed.

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Data source: Department of Environmental Protection (2002b).

Work on a third inventory of emissions into Perth's airshed is underway, however details are not yet available.

Current responses

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National Environment Protection Measures: were established in 1998 by the National Environment Protection Council to set uniform standards for ambient air quality. Amongst other pollutants, standards have been defined for ozone and nitrogen dioxide as precursors to photochemical smog. Recent reviews of the standards by the Environmental Protection and Heritage Council has shown that health effects related to ozone exposure occur in cities with low atmospheric levels of ozone as well as in cities with high levels, such as Los Angeles (Environment Protection and Heritage Council, 2005a). There is no evidence from epidemiological studies of a threshold level for adverse health effects; if there is a threshold it is below background ozone levels (Environment Protection and Heritage Council, 2005b). There is 100-fold variability in response to ozone across the Australian population. Around 10% of people are particularly sensitive, but are not necessarily asthmatic or prone to respiratory illness. Lowering the standard from 0.10 ppm to 0.08 ppm (as has been suggested by some groups) would not necessarily protect against long-term exposure effects but the full review has not yet been completed.

TravelSmart: is a community based program that encourages people to use alternatives to travelling in their private car. TravelSmart forms part of the Metropolitan Transport Strategy and aims to reduce car-as-driver trips of Perth residents by 35% over the next 30 years.

Conversion of vehicles to gas: Vehicle conversion to gas has a lot of potential to reduce emissions and pollution to urban airsheds. Since 2000, the Department of Planning and Infrastructure has provided a subsidy for more than 11 000 vehicle owners wishing to convert petrol cars to liquid petroleum gas or to buy factory made gas-run vehicles (Department for Planning and Infrastructure, 2005). In 2006, the subsidy was expanded by the Australian Government.

Urban planning: Although the Network City plan for Perth and the Peel region focuses on managing growth of the urban area, making fuller use of urban land, nurturing the environment and encouraging the use of public transport, it does have the opportunity to plan for reduced car use and consequently improve air quality (Western Australian Planning Commission & Department for Planning and Infrastructure, 2004). However, with present urban growth rates, traffic concentration and commuting distances in private vehicles continues to be of concern.

Environmental Protection Authority Guidance Statement no. 15: The EPA has developed a guidance statement for emissions of NO_x for the installation of new gas turbines (Environmental Protection Authority, 2000). This is intended to promote installation of gas turbines that achieve Australian Environment Council and National Health and Medical Research Council guidelines (which were the predecessors of NEPM standards).

Implications

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Photochemical smog can cause a number of population health problems. Ozone and oxides of nitrogen are especially harmful for senior citizens, children, and people with heart and lung conditions such as emphysema, bronchitis, and asthma. They can irritate breathing passages (nose and throat), irritate the eyes, cause a choking sensation and shortness of breath, wheezing and coughing, and may decrease the lung's working capacity. It is known to interfere with the body's ability to fight infection, increasing susceptibility to illness. Hospital admissions and respiratory deaths increase during severe smog episodes. Native vegetation, horticultural and agricultural crops can show reduced growth and visible injury with prolonged ozone exposure, even at low concentration. Concentrations of ozone and reactive organic compounds are likely to increase into the future with increasing population, rapid economic growth, and increasing dependence on vehicles.

Suggested responses

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2.15 Implement the Perth Air Quality Management Plan. Although the plan was released in 2000 and much progress made, many parts have yet to be implemented.

2.16 Develop and implement an air quality management plan for photochemical smog in susceptible regional areas, using the Perth Air Quality Management Plan as a model.