3.3 The effects of atmospheric aerosols
While many aspects of sulphur pollution are detrimental to the surface environment, another of its features may be counteracting global warming. There is growing evidence that global warming is at least partially offset by minute particles of dust (aerosols), including those of various sulphates, in the atmosphere. The effect is caused by emissions from power stations in Europe, North America and Asia, dust storms in the Sahara, burning tropical forests, iron and steel manufacture, and, importantly, by the tremendous growth in air transport.
Aerosols cool the atmosphere in two ways. They reflect and scatter solar radiation, slightly reducing the amount that reaches the surface. Sulphate aerosols also act as nuclei for condensation of water vapour, thereby encouraging the formation of clouds. Large numbers of aerosols produce small water droplets, of which reflective clouds are made. Clouds shade the ground during hot summer days. At night and in winter they warm the surface layers of the atmosphere by absorbing long-wavelength radiation emitted from the Earth's surface. Sulphur dioxide that enters the stratosphere also forms aerosol droplets of sulphuric acid that further reduce incoming solar radiation — but its source is explosive volcanism that is powerful enough to inject material to altitudes greater than 10 km.
In the USA, average daytime cloud cover has risen from a little less than 50% between 1900 to 1940 to above 58% since 1960. Whereas the average daytime temperature of much of the global landmass has risen since 1950, the areas with high SO2 emissions have cooled. Average maximum daytime temperatures between June and November over land in the Northern Hemisphere fell by about 0.41 °C between the 1950s and 1990s. This is opposite to the trend shown by Figure 6.12b for average annual surface temperature over the whole of the Northern Hemisphere, and has been ascribed to the effects of atmospheric aerosols released by pollution.
This increased direct return of solar energy to space has been dubbed 'global dimming' due to a decrease in solar radiation reaching parts of the Earth's surface, a trend observed until the early 1990s. Thereafter, growing controls on emissions from European and North American power stations have led to a 'brightening-up' of the skies — an increase in insolation. Ironically, reduced particulate pollution is likely to exacerbate the current global warming trend, since it had in part been offset by global dimming.
An important additional consequence of the combustion of fossil fuels is the effect the products of such burning have on air quality. In December 1952, a cold fog (smog) that contained high levels of smoke and sulphur dioxide hung over London for almost a week. It was one of the worst in a series of 'pea-souper' fogs that descended on London at that time, and was directly responsible for about 4000 deaths through bronchial infections and heart attacks. A public outcry led to smoke controls and wider use of smokeless solid fuel. At the time, doctors blamed high smoke levels in the fog, but more recent research suggests that the formation of highly acidic particles may have been important. The 1952 London smog had a pH of 1.6 — more acidic than lemon juice.
The advent of North Sea gas and the siting of power stations outside centres of population dramatically reduced the incidence of smoke-laden fogs. However, they have been replaced by a different form of air pollution — NOx and smoke particles from vehicle emissions. A thick haze that built up from London traffic fumes during four windless days in December 1991 directly caused the deaths of 160 people. Two pollutants were exceptionally concentrated in the London air: NO2 levels reached 423 parts per billion, the highest level ever recorded in the UK, and black smoke particles reached 228 µ g m-3.
Particularly on sunny summer days, the main respiratory irritant in 'modern' polluted air is ozone (O3), only naturally abundant in the stratosphere. This ground-level ozone is formed from photochemical reactions involving both NOx and traces of hydrocarbons in the air. Whereas the London smogs of the 1950s were the result mainly of domestic coal combustion, the NOx that forms one of the main ozone precursors comes mainly from car exhausts and power station flues. While the consequences of ground-level ozone can have severe consequences for human health, the implications of this pollution can be even further reaching — natural ecosystems suffer and agricultural production falls. Already, rice yields in polluted parts of Asia are thought to be 10% lower than they would have been under 'ozone-clean' conditions, while soy bean crops are even more susceptible, with a projected 30% drop in yield by 2020.