What is clean air?
At locations that are remote from pollutant sources, air concentrations reach „background? levels. „Background level? concentrations refer to measurements done far from pollution sources. Due to the mixing and dispersion processes in the atmosphere, background levels may represent concentrations that are low but significantly different from the unpolluted air of pre-industrial periods.
The concentration of pollutants in urban air is one to three orders of magnitude (10x to 1000x) greater than levels in „background? or unpolluted air. For example, Cape Point „background? concentrations for ozone (average 20-25ppb) and CO (average +-55ppb) may be compared with values within the City of Cape Town of up to 100ppb for ozone and 10ppm for CO. In some cities, peak ozone values of more than 200 ppb are not uncommon. (ppb: parts per billion; ppm: parts per million).
The common air pollutants
. *PM10: Particulate matter with aerodynamic diameter<10µm
**PM2.5: Particulate matter with aerodynamic diameter<2.5µm
Primary and secondary air pollutants
Primary pollutants: SO2, CO2, CO, NO, NO2, Particulate Matter (PM) and VOCs are primary pollutants - they are released directly into the atmosphere. Vehicle emissions and emissions from other combustion sources may be significant primary sources of fine fraction (PM2.5) particulate emissions. The combustion of the hydrocarbons - fuel oil, diesel and petrol - produces elemental carbon as a primary particulate. Windblown dust contributes to the coarse fraction of PM10 as a primary pollutant.
Secondary pollutants are formed in the atmosphere through chemical reactions and physical processes. For example, SO2 and NO2 react with ammonia or other alkaline species, atmospheric oxygen and water vapour to form sulphates (ammonium bisulphate and /or sulphuric acid) and nitrates (ammonium nitrate, peroxyacetylnitrate (PAN) and/ or nitric acid). The nuclei that form when these substances condense may grow through the physical processes of deposition and agglomeration.