Air pollutants come from a range of different sources such as road traffic, industrial processes, combustion of oil and wood, construction machinery and shipping. In addition to these anthropogenic sources, air pollutants are also emitted through natural processes such as volcanic eruptions and forest fires. In larger urban areas and cities like Stockholm and Uppsala, the main source of air pollutant emissions is from road traffic.
Depending upon the nature of an emissions source, it is possible to define it either as a point source (e.g. industrial emissions from a chimney), line source (e.g. road) or an area source (e.g. agriculture). The source type, along with its height above ground and the temperature of emitted pollutants, are important to pollutant dispersion. Exhaust gases with high temperature, emitted from a high chimney, will disperse further and affect a larger area than, for example, emissions from a petrol station which is at ground level. As air pollutants disperse they also become diluted, resulting in lower concentrations.