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Noie 1 too! = .10.48 cm mechanism in clean tropospheric air. In a more recent study Ketseridis and Eichmann (1978) used an impactor to separate particles with radii smaller and larger than 1 fim. They report on the basis of their measurements carried out in Ireland, that the majority of organics are in the smaller size range where these materials comprise 25 % of the aerosol mass. It is very probable that organic particles of these sizes are formed by gas-to-particle conversion (see Subsection

In the upper troposphere the chemical composition of aerosol particles was widely studied by American research workers (Cadle, 1973). During aircraft flights, particles were collected on special filters, and the samples were subsequently analyzed by wet chemistry and neutron activation. The results obtained in the upper tropical troposphere near the Philippine Islands are tabulated in Table 22. These data show that the sulfate concentrations are much higher than those of silicon, chloride and sodium, showing that sea salt and mineral dust are unimportant at these altitudes. It is interesting that very small ammonium concentrations were found. This finding makes it likely that the majority of the sulfate consists of sulfuric acid droplets, which are less common near the Earth's surface (A. Mészáros and Vissy, 1974) owing to higher NH3 concentrations. Finally, it should be mentioned that Cadle argues that the relatively high nitrate concentrations are explained by the adsorption of HN03 vapour by the filter material15.

Table 23

Vertical profile of the mass concentration of particulate sulfur according to Gillette and BlifTord (1971)

Heighl Pacific Ocean Death Valley [km] Ogm '] [|<gm'J]

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