Note: Values in last column give relative quantity of sulfate ions associated to ammonium. Concentrations are expressed in /jg m ~1

Note: Values in last column give relative quantity of sulfate ions associated to ammonium. Concentrations are expressed in /jg m ~1

Martell (1966) concluded that large stratospheric sulfate particles are due to the coagulation of Aitken-size sulfate particles transported from the troposphere. This assumption is based among other things on the fact (at the time of the publication of Martell's paper this was only a hypothesis) that Aitken particles in the upper troposphere are composed of sulfates (see the last section). Ten years later Podzimek (1976) also reported evidence based on his particle counts that stratospheric Aitken particles are not produced in the lower stratosphere, but arise in the troposphere and are mixed upwards. He also states, however, that this process is negligible during strong volcanic eruptions which eject many gaseous and particulate materials to higher altitudes.

We can conclude that the origin of stratospheric particles is not well established. It is proposed, however, that volcanic activity is at least one of the most important factors governing the aerosol cycle in the stratosphere. It is obvious that much work remains to be done in measuring not only particulate, but also gaseous sulfur compounds in the stratosphere (see Georgii, 1978).

The size distribution of aerosol particles was presented in Subsection 4.3.2. We have not discussed, however, the relation between the size distribution and the relative humidity of the air, since for such a discussion the knowledge of the chemical composition is necessary. The aim of this section is to summarize the problem with the presentation of some results of measurements. The interested reader is referred for further details to Hanel (1976).

The size distribution of aerosol particles varies as a function of the relative humidity because of the presence of water-soluble materials in the particulate matter. To understand the principle of this phenomenon let us consider a single water-soluble particle consisting of a given substance. The particle, with radius r0, is in a dry state, that is, in an environment containing no water vapour. If the relative humidity of the particle's environment is increased the radius initially remains the same, disregarding the adsorption process which is of little importance (see Fig. 37). At a relative humidity determined by the nature of the substance (and also somewhat by the particle size) the radius of the particle suddenly changes to a larger value. This phenomenon is due to the fact that the particle has changed to a solution

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