We make good use of an understanding of the processes of condensation and precipitation when probing into manmade changes in the atmospheric environment. These are inadvertent changes, largely the result of urbanization and the growth of industrial processes, which have, enormously increased the rate of combustion of hydrocarbon (fossil) fuels in the past half-century or so. Now we turn to consider the kinds of foreign matter, or pollutants, injected by man into the lower atmosphere, and the effects of pollution upon air quality and urban climate.
We recognize two classes of atmospheric pollutants. First, there are solid and liquid particles, which are designated collectively as particulate matter. Dusts found in smoke of combustion, as well as droplets naturally occurring as cloud and fog, fall into the category of particles. Second, there are compounds in the gaseous state, included under the general term chemical pollutants in the sense that they are not normally present in measurable quantities in clean air remote from densely populated, industrialized regions.
One group of chemical pollutants of industrial and urban areas is of primary origin, that is, produced directly from a source on the ground. Gases included in this group are carbon monoxide (CO), sulphur dioxide (SO2), oxides of nitrogen (NO, NO2, NO3), and hydrocarbon compounds. However, these chemical pollutants cannot be treated separately from particulate matter, since they are often combined within a single suspended particle. Certain dusts are said to be hydroscopic because they have an affinity for water and easily take on a covering water film(ordinary table salt is an example). The water film in turn absorbs the chemical pollutants. Hygroscopic particles of salt, derived from the sea surface, are normally present in great numbers in the atmosphere.
When particles and chemical pollutants are present in considerable density over an urban area, the resultant mixture is known as smog. Almost everyone living in middle latitudes is familiar with smog through its irritating effects upon the eyes and respiratory system and its ability to obscure distant objects. When concentrations of suspended matter are less dense, obscuring visibility of very distant objects, but not other wise objectionable, the atmospheric condition is referred to as haze. Atmospheric haze builds up quite naturally in stagnant air masses as a result of the infusion of various surface materials. Haze is normally present whenever air reaches high relative humidity because water films grow on hygroscopic nuclei. Nuclei of natural atmospheric haze particles consist of mineral dusts from the soil, crystals of salt blown from the sea surface, hydrocarbon compounds (pollens and terpenes) exuded by plant foliage, and smoke from forest and grass fires. Dusts from volcanoes may, on occasion, add to atmospheric haze.
It is evident at this point that what we are calling atmospheric pollutants are of both natural and man-made origin, and that man's activities can supplement the quantities of natural pollutants present.
Not all man-made pollution comes from the cities. Isolated industrial activities can produce pollutants far from urban areas. Particularly important are smelters and manufacturing plants in small towns and rural areas. Sulphide ores (metals in combination with sulphur compounds) are processed by heating in smelters close to the mine. Here, sulphur compounds are sent into the air in enormous concentrations from smokestacks. Fallout over the surroundings area is destructive to vegetation.
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