The term secondary air pollutants is generally applied to pollutants which are not emitted directly into the air but are formed in the atmosphere by chemical reactions between other pollutants and atmospheric gases. The three pollutants discussed here are ozone, nitrogen dioxide and secondary particles. Limit values for each of these pollutants are included in proposed EU 'Daughter Directives' on air quality and objectives have been included in the UK National Air Quality Strategy (DoE et al, 7997, DETR et al, 1999) (secondary particles contribute to the regulated pollutant PM10). Secondary air pollutants are of particular concern and interest because the response of ambient concentrations to emissions control measures can often be non-linear and more complex than for primary pollutants, such as sulphur dioxide, carbon monoxide or benzene. The origins, conditions leading to episodes of high concentrations and historical and likely future trends are described for each pollutant.
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