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More recent observations demonstrated that in remote oceanic and continental areas smaller concentrations can be measured than those listed in the table. Thus, according to Junge and Jaenicke (1971) over the northern Atlantic Ocean the concentration is 600 cm 3 on an average. Hogan et al. (1973) suggest that the lowest concentrations in the surface air over the North Atlantic can be measured between 20 -25 N and 30 -50 W. Over this area the mean value is only about 300 cm"3. In clean oceanic air of the Southern Hemisphere a smaller average concentration (~400cm"3) can be detected (A. Meszaros and Vissy, 1974) than the value reported by Junge and Jaenicke (1971). Moreover, according to information published by Japanese workers (Ohta and Ito, 1974), concentrations are generally between 150 200 cm"3 over the Pacific Ocean. Recent results obtained by Hogan and Bernard (1978) over Antarctica demonstrate that the concentration is less than 50 cm 3 in wintertime. During Antarctic summer the number of aerosol particles is around 100-200 cm 3 in agreement with the findings of Ohta and Ito (1974).

It is questionable whether the higher values reported for the Northern Atlantic Ocean are caused by anthropogenic pollutant emissions. Unfortunately, it is very difficult to answer to this important question, since we have no suitable homogeneous data sets to estimate possible secular concentration trends. However, some information is available concerning variations in the electrical mobility of the air over different oceans. These data show (Cobb and Wells, 1970) that electrical mobility of the surface air over the northern part of the Atlantic Ocean decreased by a factor of 2 during the last 70 years. In contrast this electrical parameter remained constant over the southern regions of the Pacific Ocean. Taking into account the relation between electrical mobility and aerosol particle concentration (Subsection 4.1.3) this finding suggests that the particle number concentration doubled during this century in remote areas of the Northern Hemisphere.

Fig. 24

■jpatial distribution of the concentration of Ihe Aitken particles at 3000 m over the European part of the U.S.S.R. (Selezneva. 1966). (By courtesy of Tellus)

Fig. 24

■jpatial distribution of the concentration of Ihe Aitken particles at 3000 m over the European part of the U.S.S.R. (Selezneva. 1966). (By courtesy of Tellus)

The vertical profile of aerosol particle concentration in the troposphere was widely studied by Soviet research workers. A great number of data obtained by aircraft flights is reviewed by Selezneva (1966). Her compilation indicates that on average the concentration drops to one-fourth of its ground level value at an altitude of 900 m on an average, and that above 3000-4000 m the particle number becomes constant. Selezneva speculates that this vertical distribution is mostly controlled\by atmospheric exchange and particle coagulation (flights were made generally under weather situations without precipitation). In Fig. 24 the average results at 3000 m altitude are plotted. One can see that concentrations increase with increasing continental source density. It can also be seen that the concentrations measured at this altitude agree well with those reported for remote oceanic surface air (see above). This means that, except the first few kilometers above the continents.

the troposphere is filled with aerosol particles of the same concentration. Disregarding sea salt particles of small number concentration, the composition of this aerosol, termed the tropospheric background aerosol (Junge, 1963), is also independent of place and time (see Section 4.4).

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