Cj5

tor1

100 rH 10

Fig. 35

Size distribution and concentration (N) of sea salt particles from different geographical regions. (/): according to A. Mészàros and Vissy (1974) (by courtesy of J. of Aerosol Science); (2). according to Metnieks (19S8) (by courtesy of School of Cosmic Physics); (J): according to E. Mészàros (1964) (by courtesy of J. de Recherches Atmosphériques)

The quantity of sea salt particles, identified as NaCl, was found to be particularly important over the Indian Ocean. This is explained by stormy weather conditions during the sampling period. Curve 1 in Fig. 35 gives the size distribution of sea salt particles measured over the Indian Ocean (the total number concentration, N, is also plotted). To represent the ad vection of sea salt particles over the continents, two other spectra are also shown. Curve 2 was measured by Metnieks (1958) in Ireland while distribution 3 was observed in the surface air in Hungary (E. Meszaros, 1964). These latter investigators used gelatin layers sensitized with silver nitrate to identify chloride particles. Figure 35 makes clear that the sea salt concentration is very small in a continental environment. Furthermore, the maximum of the distribution is shifted in the direction of larger particles. However, even over the ocean very few sea salt particles have a radius smaller than 0.1 ¿¿m (3.2 cm ~3 in our case) as compared to the total concentration of Aitken particles (Subsection 4.3.1). Finally, over the Indian Ocean practically all giant particles consisted of sea salt14.

It follows from Table 21 that the great majority of aerosol particles consist either of ammonium sulfate or from a mixture of ammonium sulfate and sea salt. Thus, if we disregard sea salt particles we can conclude that the particulate matter in tropospheric background air consists mainly of sulfur-containing species. This finding was more recently confirmed by Butor (1976) who made samplings over the North Atlantic and identified particles by electron microscopy. Moreover, the investigation of Meszaros and Vissy also shows that 70% of the number of ammonium sulfate particles are in the range of 0.03 iS r ^ 0.1 ¿¿m in agreement with the results of continental observations (see Table 20). It should be noted here that

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