contribution of European emissions.
Chemical transformations convert primary directly emitted pollutants (such as sulphur dioxide or nitrogen oxides) into secondary pollutants (such as acids and particulate sulphates and nitrates). These can be formed through chemical oxidation (eg, by the very reactive hydroxyl molecule OH or by ozone and other oxidants). Photo-oxidants (eg, ozone and peroxyacetylnitrate or PAN) are formed by photochemical degradation of organic compounds in the presence of nitrogen oxides. Chemical transformation processes do not remove material from the atmosphere, but may convert it to substances with different deposition properties, and effects. For example, only a very small proportion of the gaseous nitrogen oxides (NOx) is removed by wet deposition. However, once NOx has been converted to nitric acid or the nitrate aerosol, wet deposition becomes one of the major removal processes.