As well as particulate pollution produced by urban and industrial activities involving coal and coke combustion, there is the associated generation of pollutant gases. Before the Clean Air Act in the UK, it was estimated that domestic fires produced 80 to 90 per cent of London's smoke. However, these were responsible for only 30 per cent of the sulphur dioxide released into the atmosphere - the remainder being contributed by electricity power-stations (41 per cent) and factories (29 per cent). After the early 1960s, improved technology, the phasing out of coal fires and anti-pollution regula tions brought about a striking decline in sulphur dioxide pollution in many European and North American cities (Figure 12.21). Nevertheless, the effect of the regulations was not always clear. The decrease in London's atmospheric pollution was not apparent until eight years after the introduction of the 1956 Clean Air Act, whereas in New York City the observed decrease began in the same year (1964) - prior to the air pollution control regulations there.
Urban complexes in many parts of the world are significantly affected by pollution resulting from the combustion of gasoline and diesel fuel by vehicles and aircraft, as well as from petrochemical industries. Los Angeles, lying in a topographically constricted basin and often subject to temperature inversions, is the prime example of such pollution, although this affects all modern cities. Even with controls, 7 per cent of the gasoline from private cars is emitted in an unburned or poorly oxidized form, another 3.5 per cent as photochemical smog and 33 to 40 per cent as carbon monoxide. Smog involves at least four main components: carbon soot, particulate organic matter (POM), sulphate (SO42-) and peracyl nitrates (Pans). Half of the
Source: From Brimblecombe (1986).
aerosol mass is typically POM and sulphate. However, there are important regional differences. For example, the sulphur content of fuels used in California is lower than in the eastern United States and Europe, and NO2 emissions greatly exceed those of SO2 in California. The production of the Los Angeles smog, which, unlike traditional city smogs, occurs characteristically during the daytime in summer and autumn, is the result of a very complex chain of chemical reactions termed the disruptedphotolytic cycle (Figure 12.22). Ultraviolet radiation dissociates natural NO2 into NO and O. Monatomic oxygen (O) may then combine with natural oxygen (O2) to produce ozone (O3). The ozone in turn reacts with the artificial NO to produce NO2 (which goes back into the photochemical cycle forming a dangerous positive feedback loop) and oxygen. The hydrocarbons produced by the combustion of petrol combine with oxygen atoms to produce the hydrocarbon free radical HcO*, and these react with the products of the O3-NO reaction to generate oxygen and photochemical smog. This smog exhibits well-developed annual and diurnal cycles in the Los Angeles basin (see Figures 12.19C and D). Annual levels of photochemical smog pollution in Los Angeles (from averages of the daily highest hourly figures) are greatest in late summer and autumn, when clear skies, light winds and temperature inversions combine with high amounts of solar radiation. The diurnal variations in individual components of the disrupted photolytic cycle indicate complex reactions. For
example, an early morning concentration of NO2 occurs due to the buildup of traffic and there is a peak of O3 when incoming radiation receipts are high. The effect of smog is not only to modify the radiation budget of cities but also to produce a human health hazard.
Evolving state and city regulations in the United States have given rise to considerable differences in the type and intensity of urban pollution. For example, Denver, Colorado, situated in a basin at 1500-m altitude, regularly had a winter 'brown cloud' of smog and high summer ozone levels in the 1970s and 1980s. By the beginning of this century, substantial improvements had been achieved through the mandatory use of gasoline additives in winter, restrictions on wood burning, and scrubbers installed on power plants.
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Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.