Primary pollutants emitted directly into the atmosphere can be a major cause of poor air quality.
Globally, quantities of nitrogen oxides produced naturally (by bacterial and volcanic action and lightning) far outweigh man-made emissions. Man-made emissions are mainly due to fossil fuel combustion from both stationary sources, for example power generation (24%), and mobile sources, such as transport (49%). Other atmospheric contributions come from non-combustion processes, for example nitric acid manufacture, welding processes and the use of explosives.
The most important sources of sulfur dioxide are fossil fuel combustion, smelting, manufacture of sulfuric acid, conversion of wood pulp to paper, incineration of refuse and production of elemental sulfur. Coal burning is the single largest man-made source of sulfur dioxide accounting for about 50% of annual global emissions, with oil burning accounting for a further 25-30%.
When fuel burns efficiently, the major byproducts are carbon dioxide and water. In the presence of a limited supply of oxygen, however, incomplete combustion of fuel releases carbon monoxide. Idling and slow-moving road vehicles commonly emit Carbon Monoxide. Up to three-quarters of all Carbon Monoxide emissions are from road transport. Smaller contributions come from processes involving the combustion of organic matter, for example in power stations and waste incineration.
Particulate matter is emitted from a wide range of sources, the most significant primary sources being road transport (25%), non-combustion processes (24%), industrial combustion plants and processes (17%), commercial and residential combustion (16%) and public power generation (15%). Natural sources are less important; these include volcanoes and dust storms. Particulate matter can also be formed by the transformation of gaseous emissions such as oxides of sulfur and nitrogen, and VOCs.
Hydrocarbons are emitted from petrol evaporation and incomplete combustion, leakage of natural gas from distribution systems and evaporation of solvents, used in paints or industrial degreasing processes.
Air Pollution Chemistry
Some air pollutants that are released into the atmosphere by man-made activities pose environmental and health risks directly. These primary pollutants include carbon monoxide, particulate matter, nitrogen oxides and lead, emitted from exhausts of road vehicles. Additional impacts, however, result from the conversion of primary pollutants by a complex series of chemical reactions in the atmosphere, to secondary pollutants, many of which are potentially more harmful than their precursors. Since much of the pollutant chemistry is driven by the presence of sunlight, the secondary products are commonly referred to as photochemical pollutants.
A well-known secondary photochemical pollutant is ozone (O3). Its formation results from the sunlight-initiated oxidation (reaction with oxygen) of volatile organic compounds (VOCs) such as benzene in the presence of nitrogen oxides (NOx), mostly nitric oxide (NO) and nitrogen dioxide (NO2). Once formed, ozone is scavenged by NO, and in the absence of other competing reactions, a "photostationary state" is formed where concentrations of NO, NO2 and O3 are all inter-related. In rural areas away from major sources of NO, such as urban road transport, ozone scavenging by NO is lower, and consequently ozone concentrations in the atmosphere are higher.
The primary pollutants sulfur dioxide and nitrogen oxides undergo chemical transformation as they are dispersed in the atmosphere, forming sulfuric acid and nitric acid respectively, which may be deposited downwind as acid rain.