Remote continental aerosol

As Table 7-16 shows, the relative abundances of the major elements in the aerosol do not differ greatly from those in bulk soil, crustal rock, or average shale—that is, the elements are neither greatly enriched nor seriously depleted. A good match with any of the three reference materials is not obtained, however. The differences must be significant, since they are greater than conceivable analytical errors. Consider silicon as an example. Tables 7-13 and 7-16 indicate an average Si/Al ratio of 2.7, which is lower than that for either bulk soil or crustal rock and is more similar to that in shales. Fly ash exhibits a particularly low Si/Al ratio. It is possible that the low aerosol value in heavily industrialized Tees-side (Table 7-13) is due to a mixture of natural and combustion aerosols, but this explanation cannot be extended to the remote continental aerosol. A more likely explanation for the silicon deficiency is the size distribution of the Si/Al ratio in soil particles. The very coarse quartz particles, which are rich in silicon, are not readily mobilized. Since only the fine fraction of soil particles contributes to aerosol formation, the Si/Al ratio in the aerosol will be determined by that of silts and clays (see Table 7-7 for definitions). Common clay... 

Table 7-lb. Average Absolute and Relative Abundances of Major Elements in Crustal Rock, Soil, and Shale-, Relative Abundances of Elements in Fly Ash from Coal and Fuel-Oil Combustion and Relative Abundances of Major Elements in the Remote Continental Aerosol, with Enrichment Factors (Aerosol) EF= (X)/(AI)aeroso,/(X)/(AI)crusta, rock ... 

Element Elemental abundances (ppmw) Relative composition (X)/(A1) Remote continental aerosol ... 

Typical remote continental aerosol number, surface, and volume distributions. 

Desert aerosol, of course present over deserts, actually extends considerably over adjacent regions such as oceans (Jaenicke and Schutz 1978 d Almeida and Schutz 1983 Li et al. 1996). The shape of its size distribution is similar to that of remote continental aerosol but depends strongly on the wind velocity. Its number distribution tends to exhibit three overlapping modes at diameters of 0.01 pm or less, 0.05 pm, and 10 pm, respectively (Jaenicke 1993) (Figure 8.22). An average composition of soils and crustal material is shown in Table 8.4. The soil composition is similar to that of the crustal rock, with the exception of the soluble elements such as Ca, Mg, and Na, which have lower relative concentrations in the soil. 

Primary particles (e.g., dust, pollens, plant waxes) and secondary oxidation products are the main components of remote continental aerosol (Deepak and Gali, 1991). Aerosol number concentrations average around 2000 to 10,000 cm and PM lo concentrations are... 

FIGURE 7.26 Representative vertical distribution of aerosol number concentration (Jaenicke, 1993). A range of concentrations is shown for marine and remote continental aerosols. 

Particulate matters are classified in different types, depending on their origins. While marine aerosols are formed of sea salt particles, remote continental aerosols are of primary particles (Uke dust, pollens and plant waxes), as well as secondary oxidation products. Moreover, desert aerosols, which resemble remote continental aerosols in their shape and size, are found over deserts and adjacent regions and strongly depend on the wind velocity. Urban aerosols are then considered a... 

Remote continental aerosols that occur near the earth s surface but have experienced little or no exposure to the results of human activity. 

According to a review by J. Heintzenberg [Tellus, 41B 149 (1989)], in North / taica and Europe, uiban fine aerosols typieally contain 28 % sulfate, 31 % oiganics, 9 % BC, 8 % ammonium, 6 % nitrate, and 18 % other material (mean mass = 32 tg/m ) suburban aerosols contain 37 % sulfate, 24 % tn-ganies, 5 % BC, 11 % ammonium, 4 % nitrate, and 19 % other material (mean mass = 15 pg/m ) and remote continental aerosols contain 22 % sulfate, 11 % organics, 3 % BC, 7 % ammonium, 3 % nitrate, and 56 % other material (mean mass = 4.8 pg/m ). Additional data on different aero-sol types, consistent with this review, are presented by J.H. Seiirfeld and S.N. Pandis [Atmospheric Chemistry and Physics (Wiley New York, 1998)]. 

Table VI summarizes aerosol mass concentrations and composition in different regions of the troposphere. It is interesting to note that average total fine particle mass (that associated with particles of diameter less than about 2 /im) in non-urban continental, i.e., regional, aerosols is only a factor of two lower than urban values. This reflects the relatively long residence time of particles (recall the estimate of a lifetime of fine particles by dry deposition of 10 days). Correspondingly, the average composition of non-urban continental and urban aerosols is roughly the same. The average mass concentration of remote aerosols is a factor of three lower than that of non-urban continental aerosols. The elemental carbon component, a direct indicator of anthropogenic combustion sources, drops to 0.3% in the remote aerosols, but sulfate is still a major compo-...

Fig. 7-1. Left Idealized particle size distributions for the rural continental and the maritime aerosols. The distribution of sea-salt particles that contribute to the maritime aerosol is shown separately. The transition from the rural to the urban aerosol is indicated. Right Determination of remote tropospheric aerosol size distribution by a combination of instrumental techniques. [ Single-stage and free-wing impactors, O—O set of five double-stage impactors singleparticle optical scattering analyzer these data were obtained at the observatory lzana, Tenerife,...

The third section of Table 7-16 shows the relative composition of two continental aerosols to provide a direct comparison with the data in the second part of the table. The first aerosol given represents an average over samples taken at several remote sites in Europe, North America, and Africa (Rahn, 1975b). The second aerosol is the 1974 average obtained by Moyers et al. (1977) at two sites in rural Arizona. The (X)/(A1) ratios for the elements Si, Fe, Na, K, and Ca in the aerosols treated in Table 7-13 are shown at the bottom of that table. To further illustrate the behavior of important elements in the atmospheric aerosol, we show in Fig. 7-23 a number of scatter diagrams. The combination of these data will be discussed below. 

Fig. 7-23. Scatter diagrams for iron, titanium, magnesium, calcium, vanadium, and chromium in the atmospheric aerosol versus aluminum as reference element. Concentrations are given in units of ng/ m3. Symbols indicate the type of sampling site O marine, A marine-influenced, 9 remote continental, Antarctic. The range of X/Al ratios in crustal rock is shown by the solid lines and X/Al ratios in soil by the dashed lines. [Adapted from Rahn (1975b).]...

Patterson, E. M., C. S. Kiang, A. C. Delany, A. F. Wartburg, A. C. D. Leslie, and B. J. Huebert (1980). Global measurements of aerosols in remote continental and marine regions concentrations, size distributions and optical properties. J. Geophys. Res. 85, 7361-7376. 

In the absence of significant transport of continental aerosols, particles over the remote oceans are largely of marine origin (Savoie and Prospero 1989). Marine atmospheric particle concentrations are normally in the range of 100-300 cm-3. Their size distribution is usually characterized by three modes (Figure 8.15) the Aitken (Dp < 0.1 pm) the... 

The essence of the potential effect of aerosols on cloud reflectively is embodied in Figure 24.17 which shows cloud albedo as a function of cloud droplet number concentration at various cloud thicknesses at a constant liquid water content of L — 0.3 g m 3. At constant liquid water content and constant cloud thickness, cloud albedo increases with increasing CDNC. For a cloud 50 m thick with this liquid water content, an increase of CDNC from 100 to 1000 cm-3, corresponding to going from remote marine to continental conditions, leads to almost a doubling of cloud albedo. 

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