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Air pollution occurs when the air contains gases, dust, fumes or odour in harmful amounts. Aerosols are a subset of air pollution that refers to the tiny particles suspended everywhere in our atmosphere. These particles can be both solid and liquid, and are collectively referred to as atmospheric aerosol particles. Natural processes, such as volcanic eruptions and human, industrial and agricultural activities ,create aerosols. Light-coloured aerosol particles can reflect incoming heat energy from the sun in cloud-free air and dark aerosol particles can absorb it. Thus, aerosols can actually modify how much energy clouds reflect and change atmospheric circulation patterns. In short, aerosols can modify our climate. The question is whether aerosols, due to their reflective properties, provide a probable long-term curative alternative to the current eminent problem of global warming. The short answer to the question is no. However, with progress in science it might become probable to use aerosol solutions than what technology promises the society right now. But before we take a stand on the question, let’s give the question a deeper look.
The aerosol particles grow by absorbing water vapour and other gases. If the relative humidity in a region is high (80 per cent or more), tiny water drops can form on some of the particles. A subset of these, called cloud condensation nuclei, then grow into cloud drops that eventually fall as rain or snow. In and near major cities, where the concentration of aerosol particles is high, the concentration of cloud droplets is also very high. The increased number of little drops causes the reflectivity of clouds to increase. Seen from above, clouds near polluted areas are often brighter than those above cleaner regions of the earth. Water droplets and ice particles are basically white, so they reflect solar radiation. In contrast, the condensed water also traps and emits long wave radiation producing heat. Thus, clouds can have either cooling or warming effects on a local area, depending on whether the reflecting or trapping effects are high. Due to many unknowns relating to aerosol particles, the magnitude of long-term aerosol impacts on climate remains among the most uncertain factors in climate projections. For example, sulfate aerosol particles in the stratosphere from the Pinatubo volcanic eruption in 1991 produced measurable cooling for two years over much of the globe.
At present, human-caused aerosol air pollution has a relatively minor impact on the climate. Since aerosol particles of human origin both reflect and absorb solar energy as the solar beam travels down through the atmosphere, these particles can diminish the energy that arrives on earth as heat. Scientists estimate that particles produced by human activities have led to a net loss of solar energy at the ground by as much as 8 per cent in densely populated areas over the past few decades. This effect, referred to as solar dimming, may have masked some of the late 20th century global warming due to greenhouse gases.
However, human activities that result in production of both reflecting and absorbing aerosol particles have also been curtailed by legislation and use of modern technology in many places all over the world. Thus, the pea soup fogs that so bedeviled London in Sherlock Holmes’ day, for example, were caused by particles produced by incomplete combustion of coal. These fogs are now a thing of the past, thanks to mandatory scrubbers and other advanced combustion techniques in the UK. Similarly, clean air regulation in the US has also decreased particle concentrations considerably. Even today, though haze clouds seen over major cities give a startling proof of the effects of human-induced particles in the US, while atmospheric soot production is still very high in many parts of Asia, including countries like India and China.
Several geoengineering strategies for reducing global warming propose using atmospheric aerosol particles to reflect the sun’s energy away from earth. As aerosol particles do not stay in the atmosphere for very long accumulated heat-trapping gases will overpower any temporary cooling due to short-lived aerosol particles. However, as global warming is such a serious threat, some scientists and engineers have explored the idea of harnessing the reflective power of some aerosol particles to temporarily fight global warming, while renewable fuel energy sources are being fully developed. The idea is to artificially increase the concentrations of the white aerosol particles above the surface of the ocean and in the lower stratosphere in order to reflect more of the sun’s energy away from earth. The field of geoengineering, though still in its infancy, has the potential to buy us some time in the attempt to maintain relatively slow warming rates.
However, experimentation with our very complex climate system by dramatically increasing reflecting aerosols carries with it the potential for large unintended and potentially dangerous side-effects on ecosystems, agriculture, including the human health. In particular, health consequences associated with further increases in particulate pollution are a grave concern.
Although aerosol technology does not seem to hold a strong promise to combat global warming at present, it has led to much research and there’s still hope for mankind.
