Air quality and main contaminants

The average person breathes about 12 cubic metres of air every day and dies if air supply stops for 5 minutes. The importance of air is obvious, and its monitoring and protection should be a top priority in any environmental program.

Traditionally, in preindustrial societies, human activities did not have significant effects on air quality. In some cases, overcultivation or overgrazing could increase the local aerosol content. However, these effects could be considered minor when compared with the changes introduced in the atmosphere by the industrial revolution. Since the beginning of this technological revolution, many billion tonnes of fossil and nonfossil fuels have been burned. Today, several billion gallons of petroleum and tonnes of coal and other fuels are burned worldwide. The effects on air quality are felt

almost everywhere. Changes are affecting practically all of its physical properties, including composition, concentration of particulates, temperature, and humidity.

Air and its principal contaminants

Normally, air at sea level contains about 78% nitrogen, 21% oxygen, and almost 1% argon. Carbon dioxide (CO2) is present in smaller quantities (0.03%). The biologically active components of the atmosphere are CO2 — the “raw material” for photosynthesis, which produces organic matter — and oxygen, which allows “burning” of this matter, forming CO2 and completing the cycle.

In addition to the natural components of air, others, which appear in variable concentrations, are related to human activities. Some of these substances are considered “contaminants” because they may be harmful to human health.

The main gaseous “contaminants” in air are sulphur dioxide, nitrogen oxides, carbon monoxide, chlorofluorocarbons, and ozone (Chovin and Roussel 1968).

Sulphur dioxide

Sulphur dioxide (SO2) is produced naturally as a derivative of biological hydrogen sulphide (H2S) and artificially by direct industrial emission (about 146 tonnes per year). It is highly soluble in water (10%); it combines with water to form sulphur trioxide (SO3) and sulphuric acid (H2SO4), and with nitrogen to form ammonium sulphate (NH4)2SO4, which eventually falls in precipitation. Rain falling downwind of sulphur dioxide emissions can be very acidic (acid rain), affecting natural ecosystems, such as lakes or some forest soils.

Nitrogen oxides

Nitrous oxide (N2O) is produced in small quantities by bacterial activity (about 600 million tonnes per year) and lightning. Artificial sources are important but, because this gas is not toxic, there is no need to alter them. Although nitric oxide (NO) is also produced naturally by bacterial activity (about 430 million tonnes per year), artificial sources are more important, particularly in combustion at temperatures above 1 300°C. It is removed from the air naturally by conversion to nitric acid (HNO3) and nitrate salts, and in precipitation. It is very toxic.

Nitrogen dioxide (NO2) is almost absent in nature. Its presence in the air is due to artificial processes, such as combustion at high temperatures (over 1 500°C). It is very toxic, and it is removed as nitric oxide. 

Hydrocarbons

These substances (methane, ethane, ethylene, toluene, benzene, terpene, etc.) come from various natural sources, such as bacterial activity and leaks from gas fields. The main artificial hydrocarbon is terpene, which leaks from petroleum fields and refineries. About 20% of the hydrocarbons found in the air are artificial; of this amount, 75% comes from vehicles and 25% from industrial activities.

Carbon monoxide

Carbon monoxide (CO) is produced mainly from methane (CH4) by chemical reaction with oxygen and hydroxyl ions (usually in swamps and other areas where methane is generated). Artificial sources relate to incomplete combustion processes (both in vehicle engines and industries). Carbon monoxide is very toxic to humans.

Carbon dioxide

This gas is produced by natural sources, such as cellular respiration, organic degradation,  volcanoes, natural fires, natural dissolution of carbonates, etc. Artificial sources include artificial fires, industrial combustion, and combustion in vehicle engines. It is removed by photosynthesis, chemical and organic precipitation of carbonates, and burial of organic matter.

In strict terms, CO2 cannot be considered a contaminant, but its excessive production by artificial sources without adequate removal can increase its concentration beyond natural limits, producing a “greenhouse effect” worldwide.

Chlorofluorocarbons

Chlorofluorocarbons (CFCs) are produced only artificially and are thought to reduce the density of stratospheric ozone molecules, increasing the effects of ultraviolet radiation. At ground level, the effect of CFCs is weak or nonexistent.

Ozone

Ozone is a natural component of the stratosphere. At ground level, however, its generation is related to various human activities, particularly automobile traffic.

Other contaminants

In addition to gaseous contaminants, many solid and liquid substances are often produced by industrial and engine emissions, traffic, or overcultivation in dry soils and enter the air as aerosols. 

Aerosols are especially important, not only because they worsen air quality, but also because they increase water vapor condensation. Because of this process cloud formation also increase and when they are widely extended they may have a cooling effect, compensating global warming tendencies.

Reproduced from Diversity, globalization and the ways of nature, Danilo Antón, IDRC books.