Nitrogen dioxide (NO2) arises directly and indirectly from combustion processes. Concentrations are generally highest close to their emission sources, primarily road transport followed by the power generation industry and other industrial and commercial sector sources.
The nature of the combustion process and fuel used will determine the presence of other pollutants, such as particulate matter, polycyclic aromatic hydrocarbons, benzene, carbon monoxide and sulphur dioxide. It is not possible to define the proportions that these pollutants may be present in at any particular location at any one time. Proportions will vary with time, the distance from sources, meteorology, and chemistry depending on the type of combustion process and emissions released.