Fig. 38

Variation of geometric mean radius (r50„ ) of sulfate particles as a function of relative humiditv (crosses). Solid line represents the theoretical growth of an ammonium sulfate particle with r = 0.14 urn (E. Meszaros, 1970). (By courtesy of Tellus)

4.5.2 Results of measurements

Variations of the size distribution of aerosol particles as a function of relative humidity can be observed in situ in the air. Changes in particle size distributions are detected by optical counters (Laktionov and Bogomolov, 1972), by nephelometers (Charlson et ai, 1969) or by diffusion channels (Sinclair et al, 1974).

Another possibility is to capture particles, e.g. on impactor slides, and to study them after drying at different humidities under a microscope. Different relative humidities are generally created by different saturated salt solutions (isopiestic method). An investigation of this kind was carried out by A. Meszaros (1971) who

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