Primary pollutants emitted directly to the atmosphere, including nitrogen oxides, sulphur dioxide, carbon monoxide, particulate matter and volatile organic compounds (VOCs), 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 sulphur dioxide are fossil fuel combustion, smelting, manufacture of sulphuric acid, conversion of wood pulp to paper, incineration of refuse and production of elemental sulphur. Coal burning is the single largest man-made source of sulphur dioxide accounting for about 50% of annual global emissions, with oil burning accounting for a further 25-30%.
When fuel is burnt 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. Carbon monoxide is commonly emitted from idling and slow-moving road vehicles. 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 sulphur 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.