\i»if. Values in parentheses give the number of observations. Samples were collected in l%7

15 The results of Spicer and Schumacher (1977) suggest that alkaline sites on many filters collect gaseous nitric acid and cause it to be reported as particulate nitrate.

The vertical profile of different elements in particulate matter was investigated by Gillette and Blifford (1971) who used cascade impactors and filters to collect particles. The sulfur profiles obtained over the Pacific Ocean near California and over the American continent (Death Valley) are represented by the data given in Table 23. By re-calculating their concentrations in sulfate we can say that in the upper troposphere the sulfate level is 0.1-0.2 ngm~3 which agrees reasonably well with figures in Table 22 as well as with the results of near surface measurements of Winkler (1975) and Flyger and Heidam (1978) carried out in remote areas. The X-ray fluorescence analyses of Gillette and Blifford (1971) also showed that 80-90 % of the sulfate (sulfur) found occurs in the Aitken size range.

On the basis of the foregoing discussion it is concluded that tropospheric background particles consist mainly of sulfur compounds. Sulfate particles contain hydrogen or ammonium ions as a function of the ammonia gas available. These mostly Aitken size particles are (externally or internally) mixed with some organic material at least near the Earth's surface.16 More research is needed to know in more detail the chemistry of this aerosol.

443 Stratospheric particles

At the end of the fifties a program was started in the U.S.A. to study stratospheric aerosol particles. The results of the program were analyzed by Junge and his associates (see Junge, 1963). The most important achievement of this study was the discovery of an aerosol layer between 15-20 km consisting mostly of large particles (see Subsection 4.3.3).

To investigate their chemical composition, stratospheric particles were collected by aircraft- and balloon-borne impactors. Elements with atomic numbers of 12-30 were identified in the samples by electron microprobe and X-ray fluorescence techniques. Table 24 summarizes the results obtained (Junge, 1963). In this table the sulfur is given as sulfate since further wet chemical analyses showed that the sulfur occurred as sulfate particles. In can be seen from the data tabulated that 89 % of the mass of the components identified is sulfate. Data also indicate that the quantity of NH4 is sufficient only to neutralize about one third of the sulfate ions. Further flights carried out over a wide range of latitudes (60° S-70° N) demonstrated that this sulfate layer can be observed everywhere in the stratosphere.

About ten years later a new stratospheric aerosol program was performed. In this case, particles were collected between 17 and 28 km by absolute filters having a collection efficiency of virtually 100 % in all size ranges. Table 25 gives the results of a sampling day when particles were collected at an altitude of 18 km. It the last column of the table the percentage of sulfate ions possibly neutralized by NH4 is also presented. It can be seen that the concentrations measured by this more recent

16 According to Jaenicke (1978) in the Aitken size range 25 % of particles consists of organic materials under tropospheric background conditions.

program exceed by at least one order of magnitude the concentration found in the previous program (see Table 24). Furthermore, Lazrus and his co-workers (1971) found high sulfate mass concentrations even when the optical measurements designed to detect large particles did not indicate the presence of an aerosol layer.

Table 24

Chemical composition of stratospheric aerosol particles (Junge, 1963)

Table 24

Chemical composition of stratospheric aerosol particles (Junge, 1963)


Number of observations

Concentration [10-'jigm"1]

Relative composition


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