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Photochemical Smog

Historically, the term smog referred to a mixture of smoke and fog, hence the name smog. The industrial revolution has been the central cause for the increase in pollutants in the atmosphere over the last three centuries. Before 1950, the majority of this pollution was created from the burning of coal for energy generation, space heating, cooking, and transportation. Under the right conditions, the smoke and sulfur dioxide produced from the burning of coal can combine with fog to create industrial smog.

In high concentrations, industrial smog can be extremely toxic to humans and other living organisms. London is orld famous for its episodes of industrial smog. The most famous London smog event occurred in December, 1952 when five days of calm foggy weather created a toxic atmosphere that claimed about 4000 human lives. Today, the use of other fossil fuels, nuclear power, and hydroelectricity instead of coal has greatly reduced the occurrence of industrial smog.

However, the burning of fossil fuels like gasoline can create another atmospheric pollution problem known as photochemical smog. Photochemical smog is a condition that develops when primary pollutants (oxides of nitrogen and volatile organic compounds created from ossil fuel combustion) interact under the influence of sunlight to produce a mixture of hundreds of different and hazardous chemicals known as secondary pollutants. Development of photochemical smog is typically associated with specific climatic conditions and centers of high population density.

Cities like Los Angeles, New York, Sydney, and Vancouver frequently suffer episodes of photochemical smog. One way in which the production of photochemical smog is initiated is through the photochemical reaction of nitrogen dioxide (NO2) to form ozone. There are any sources of photochemical smog, including vehicle engines (the number one cause of photochemical smog), industrial emissions, and area sources (the loss of vapors from small areas such as a local service station, surface coatings and thinners, and natural gas leakage).

Vehicle engines, which are extremely numerous in all parts of the world, do not completely burn the petroleum they use as fuel. This produces nitrogen dioxide which is released through the vehicle exhaust along with a high concentration of hydrocarbons. The absorption of solar radiation by the nitrogen dioxide results n the formation of ozone (O3). Ozone reacts with many different hydrocarbons to produce a brownish-yellow gaseous cloud which may contain numerous chemical compounds, the combination of which, we call photochemical smog.

Both types of smog can greatly reduce visibility. Even more importantly, they pose a serious threat to our health. They form as a result of extremely high concentrations of pollutants that are trapped near the surface by a temperature inversion. Many of the components which make up these smogs are not only respiratory irritants, but are also known carcinogens.

The Perth Photochemical Smog Study, a joint effort of Western Power Corporation and the Department of Environmental Protection (DEP), was undertaken to determine, for the first time, the extent to which photochemical smog had become a problem in Perth.

Measurements of photochemical smog in Perth’s air began in 1989, at a single site in the suburb of Caversham, 15 kilometers north-east of the city center. Despite the common perception that Perth is a windy city and therefore not prone to air pollution, the first summer of measurements revealed that the city was sometimes subjected to smog levels which approached or exceeded the guidelines recommended by the National Health and Medical Research Council of Australia (NHMRC).

In 1991 the State Energy Commission of Western Australia (SECWA, now Western Power Corporation) sought to extend the capacity of the gas turbine power station it operated at Pinjar, some 40 kilometers north of the Perth central business district. In view of the Caversham data, the Environmental Protection Authority expressed concern that increasing the NOx emissions at Pinjar could contribute to Perth’s emerging photochemical smog problem which, at that stage, was poorly defined.

A consequent condition on the development at Pinjar was that SECWA undertake a study of the formation and distribution of photochemical smog in Perth, a particular outcome of which would be to determine the effect of the Pinjar power station’s emissions on smog in the region.

Given the DEP’s concerns and responsibility in relation to urban air quality, the Perth Photochemical Smog Study (PPSS) was developed as a jointly operated and managed project, funded by SECWA and with DEP contributing facilities and scientific expertise.

The primary objective of the Perth Photochemical Smog Study was to measure, for the first time, the magnitude and distribution of photochemical smog concentrations experienced in the Perth region and to assess these against Australian and international standards, with consideration given to health and other environmental effects.

The study’s monitoring and data analysis program was very successful in defining the distribution of Perth’s smog. The Perth region experiences photochemical smog during the warmer months of each year. On average, during the three year period July 1992 to June 1995, there have been 10 days per year on which the peak hourly ozone concentration exceeded 80 parts per billion (ppb) somewhere over the Perth region.

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