Sources of air pollutants
Sources of air pollution can be of two major categories: anthropogenic and natural sources.
Natural sources - Many pollutants are formed and emitted through natural processes:
- An erupting volcano emits particulate matter as well as gases such as hydrogen sulphide, sulphur dioxide, and methane.
- Forest and prairie fires produce great quantities of pollutants in the form of smoke, nitrogen oxides, CO, unburned hydrocarbons, and ash.
- Dust storms are a common source of particulate matter in many parts of the world.
- Oceans produce aerosols in the form of salt particles.
- Plants and trees are a major source of hydrocarbons (e.g. terpenes) on the planet, and the blue haze that is so familiar over forested mountain areas is mainly from atmospheric reactions with volatile organics produced by the trees. Plants also produce pollen and spores, which cause respiratory problems and allergic reactions.
- Other natural sources of air pollutants include the emissions of methane and other gases due to digested food by animals (e.g. cattle).
Anthropogenic sources - These substances come primarily from three sources:
(1) Combustion derived from burn fossil fuel for power and heating, or exhaust emissions from transportation vehicles (e.g. motor vehicles, marine vessels, aircraft) that use gasoline or diesel fuels;
(2) industrial processes;
(3) mining and drilling;
(4) pesticide application.
The main pollutants from combustion are smoke, fly ash, sulphur, and nitrogen oxides, as well as CO2 and CO. Combustion of oil and coal - both of which contain significant amounts of sulphur - yields large quantities of sulphur oxides. One effect of the production of sulphur oxides is the formation of acidic deposition, including acid rain (explained below). Nitrogen oxides are formed by thermal oxidation of atmospheric nitrogen at high temperatures. Therefore, almost any combustion process will produce nitrogen oxides. Carbon monoxide is a product of the incomplete combustion of fuel. It is known that the more efficient the combustion, the higher is the ratio of CO2 to CO.
Transportation sources, particularly automobiles, are a main source of air pollution and include smoke, lead particles from tetraethyl lead additives, hydrocarbons, nitrogen oxides, CO, and more recently, the platinum group metals, which are used in automobile catalytic converters. Additionally, fine and inhalable particles produced by automobiles that use essentially diesel fuel are one of the main problems in cities and have been responsible for many deaths. Since the mid-1960s, there has been significant progress in reducing exhaust emissions, mainly with the use of no-lead or low-lead gasoline as well as the use of oxygenated fuels (e.g. fuels containing ethanol or methyl t-butyl ether (MTBE)). Over the past 10 years there have been advances also with the introduction of particulate filters.
Industries may emit different pollutants relating to their manufacturing processes: acids (acetic, nitric, sulphuric, and phosphoric); gases (ammonia and chlorine); solvents and resins; and metals (lead, zinc, cadmium, and copper).
Some pollutants entering the atmosphere also come from solid waste disposal (e.g. deposition of waste in landfills which generates methane), agricultural burning and military equipments (e.g. toxic gases, nuclear weapons, germ warfare and rocketry).
Indoor pollutants - Generally, the term indoor air pollution refers to home and nonfactory public buildings such as office buildings and hospitals, and results in a contamination frequently referred to as "sick building syndrome." This type of pollution can come from tobacco smoking, heating and cooking, pesticides, dust emitted from fertilizer, radon, gases, and most commonly, microbes such as fungi (moulds) and bacteria that grow in the structure or the cooling and heating systems due to excessive moisture.
Although indoor air pollution has increased in developed nations due to tighter building construction and the use of building materials that may give off gaseous chemicals, this type of air pollution is a particular problem in developing countries. Wood, animal dung, crop residues, and other forms of biomass are used extensively for heating and cooking, often in poorly ventilated rooms. This leads to high exposures of air pollutants (e.g. polycyclic aromatic hydrocarbons, CO) particularly in women and children.
For additional information on anthropogenic and natural sources of air pollution see the following link:
