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 sulphur dioxide and nitrogen oxides also undergo chemical transformation as they are dispersed in the atmosphere, forming sulphuric acid and nitric acid respectively, which may be deposited downwind as acid rain.