The best known of the secondary pollutants are certain gases that are synthesized by photochemical reactions in the lower atmosphere. The primary emitted chemicals in these reactions are hydrocarbons and gaseous oxides of nitrogen such as nitric oxide and nitrogen dioxide. These emitted chemicals participate in a complex of ultraviolet-driven photochemical reactions on sunny days to synthesize some important secondary pollutants, most notably ozone, peroxy acetyl nitrate, hydrogen peroxide, and aldehydes. These secondary compounds, especially ozone, are the harmful ingredients of oxidizing or photochemical smogs that cause damages to people and vegetation exposed to this type of pollution.
Most ozone is found in the upper atmosphere, where it acts to screen out much of the harmful radiation from the sun. Upper-level ozone is an important part of the earth's life support sytem. Lower-level ozone is created when sunlight hits hydrocarbons and nitrogen oxides released into the lower atmosphere by industrial and natural processes. Ozone is well known as an irritant to human respiratory systems, as a strong oxidant that causes materials to age rapidly and degrade in strength, and as a toxic chemical to plants. In terms of causing damage to agricultural and wild plants, ozone is the most damaging air pollutant in North America. Low-level ozone also acts as a greenhouse gas, restricting the escape of heat from the earth's surface and thus contributing to the global warming process.
Scientists estimate that the amount of low-level ozone currently in the earth's atmosphere is 100-200 times higher than it was only 100 years ago. The formation of low-level ozone can be slowed by reducing emissions of human-created hydrocarbons and nitrogen oxides into the atmosphere. Reducing hydrocarbon emissions by using catalytic converters on vehicles and generally reducing automobile travel time helps, as does the use of filtering devices to scrub industrial air emissions. However, many filters and converters fail to remove nitrogen oxides from emissions, and human-produced nitrogen oxides can combine with naturally produced hydrocarbons just as easily as with human-produced hydrocarbons. Planting tress and plants doesn't help, but the development and installation of converters and filters for removing both hydrocarbons and nitrogen oxides can.
Secondary pollutants can also be formed in other ways. For example, when soils and surface waters become acidified through atmospheric depositions or other processes, naturally occurring aluminum in soil or sediment minerals becomes more soluble and therefore, becomes more available for uptake by organisms. The soluble, ionic forms of aluminum are the most important toxic factor to plants growing in acidic soils and to fish in acidic waters. In this context, aluminum can be considered to be a secondary pollutant because it is made biologically available as a consequence of acidification.
A few pesticides generate toxic chemicals when they are chemically transformed in the environment, and this phenomenon can also be considered to represent a type of secondary pollution. For example, dithiocarbamate is a fungicide used in the cultivation of potatoes. Ethylene thiourea is an important metabolite of this chemical, formed when the original fungicide is broken down by microorganisms in soil. Ethylene thiourea is relatively stable in soils and also somewhat mobile so that it can leach into ground water. Ethylene thiourea has been demonstrated to be carcinogenic in mammals, and it therefore represents an important type of toxicity that was not characteristic of the original fungicide.
See also Smog.