Survey of the distribution of trace substances in pure and polluted atmospheres.

The main reactive air pollutants are SO2, H2S, sulphate compounds, NH3, NO, N02 and other oxidants and hydrocarbons. On the basis of a survey on the production and mean distribution of these compounds in pure and polluted atmospheres several chemical reactions are considered. The meteorological parameters, sun radiation, water vapour and the liquid water content of clouds modify these chemical reactions in the atmosphere. The distribution of trace substances in the atmosphere is a very complex subject. In a survey trace substances may be subdivided according to their importance in air chemistry, hygiene, and meteorological aspects in the troposphere. HYDROCARBONS IN THE ATMOSPHERE With chemical reactions in the atmosphere, great emphasis has been laid on hydrocarbons, which are emitted mainly by automobiles (evaporation of fuel, blow by and exhaust). Early analyses of these contaminants are referred to by Magill and Littman in l953. The composition of exhaust gases depends on the state of the automobile and on driving conditions (see for example43' 47). In the literature, hydrocarbon concentrations are referred to as carbon-, methane-, hexane,propaneor ethene-equivalents. The reactivity of hydrocarbons (HC) was studied by Altshuller1, who defined the reactivity according to product yields and troublesome effects (production of ozone, PAN, formaldehyde, aerosols and the effects of eye irritation and plant damage). On this basis, C5and heavier paraffins, olefiris and most aromatics are reactive. These conclusions are drawn from bulk analyses of HC. Stephens53 analysed outdoor air before and after irradiation (UV) for single HC-components. From these measurements Figure. 1 shows the concentration of a number of light hydrocarbons (full line). The dashed line represents the percentage of the HC found after 24 hours irradiation. This percentage is also an indication of reactivity. There seems to be a difference according to whether single HC-components are studied or a mixture of different HC and nitrogen oxides34 as is found in outdoor air. N-butane was present (Figure 1) in the sample to the extent of 120 ppb and this was found after irradiation to be reduced to 65 per cent = 78 ppb, i.e. 42 ppb had been destroyed. This unreactive n-butane from ambient air loses more