Box 4B Atmospheric constituents and their residence times

The chemical composition of the atmosphere has been evolving since the Earth was formed; it has now reached an equilibrium which sustains life. The 'natural' atmosphere is made up almost entirely of nitrogen, oxygen, argon and water vapour (Table 4.1). Air pollutants are very minor constituents of the atmosphere, but nevertheless can disturb the equilibrium, and thus affect human health, ecosystems and climatic conditions on Earth.

Atmospheric constituents are removed from the atmosphere through deposition and chemical reactions. Deposition can occur in both wet ­ with rain or snow ­ and dry forms. Wet deposition is an efficient but episodic cleansing mechanism of the atmosphere for both gases and particles. Particles can be trapped in raindrops or snowflakes during the process of condensation or by the impact of falling drops. Particles are also deposited by impaction, sedimentation (for large particles), or filtration. Many gases are taken up by vegetation through stomata (pores in leaves), or absorbed on wet surfaces. Dry deposition is a continuous process depending on the properties of the surface and of the depositing species.

The atmospheric residence times of the gases in the atmosphere vary widely (Table 4.1): from less than one hour (eg, reactive volatile organic compounds such as cyclopentadiene), to several decades (for chlorofluorocarbons or CFCs). Winds can easily transport pollutants over continental-scale distances even if their residence times are as short as 1 or 2 days, and the pollutants will generally be confined to the lowest 1 to 2 km of the troposphere. Thus, air pollution levels in Europe are dominated by the 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.