Global Warming Causes

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0.10 1.00 Diameter, nm

FIGURE 8.18 Typical remote continental aerosol number, surface, and volume distributions.

FIGURE 8.18 Typical remote continental aerosol number, surface, and volume distributions.

0.01

10.00

0.10 1.00 Diameter, fim

FIGURE 8.19 Typical free tropospheric aerosol number, surface, and volume distributions.

more particles in the accumulation mode relative to lower tropospheric spectra, suggesting precipitation scavenging and deposition of smaller and larger particles (Leaitch and Isaac 1991). The low temperature and low aerosol surface area make the upper troposphere suitable for new particle formation, and a nucleation mode is often present in the number distribution (Figure 8.19).

8.2.6 Polar Aerosols

Polar aerosols, found close to the surface in the Arctic and Antarctica, reflect their aged character; their concentrations are very low. Collections of data from aerosol measurements in the Arctic have been presented by a number of investigators (Rahn 1981; Shaw 1985; Heintzenberg 1989; Ottar 1989). The number distribution appears practically monodisperse (Ito and Iwai 1981) with a mean diameter of approximately 0.15 pm; two more modes at 0.75 and 8 pm (Shaw 1986; Jaenicke et al. 1992) (Figure 8.20) dominate the mass distribution.

During the winter and early spring (February to April) the Arctic aerosol has been found to be influenced significantly by anthropogenic sources, and the phenomenon is commonly referred to as "Arctic haze" (Barrie 1986). During this period the aerosol number concentration increases to over 200 cm-3. The nucleation mode mean diameter is at 0.05 pm and the accumulation mode at 0.2 pm (Covert and Heintzenberg 1993)

Diameter, jam

FIGURE 8.20 Typical polar aerosol number, surface, and volume distributions.

Diameter, jam

FIGURE 8.20 Typical polar aerosol number, surface, and volume distributions.

Arctic Haze

Arctic Haze

AJLU

0.10 Diameter, urn

1.00

FIGURE 8.21 Comparison of the aerosol distribution during Arctic haze with the typical polar distribution.

(Figure 8.21). Similar measurements have been reported by Heintzenberg (1980), Radke et al. (1984), and Shaw (1984).

The polar aerosol contains carbonaceous material from midlatitude pollution sources, sulfate, seasalt from the surrounding ocean, and mineral dust from arid regions of the corresponding hemisphere. Aerosol PM10 concentrations in the polar regions are less than 5 pg m-3 with sulfate representing roughly 40% of the mass.

8.2.7 Desert Aerosols

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.

Individual dust storms from the Sahara desert have been shown to transfer material from the northwest coast of Africa, across the Atlantic, to the east coast of the United States (Ott et al. 1991). For example, Prospero et al. (1987) suggested that enough Saharan dust is carried into the Miami area to significantly reduce visibility during the summer months. Similar dust transport occurs from the deserts of Asia across the Pacific Ocean

Diameter, (im

FIGURE 8.22 Typical desert aerosol number, surface, and volume distributions.

Diameter, (im

FIGURE 8.22 Typical desert aerosol number, surface, and volume distributions.

TABLE 8.4 Average Abundances of Major Elements in Soil and Crustal Rock

Elemental Abundance (ppm by mass)

TABLE 8.4 Average Abundances of Major Elements in Soil and Crustal Rock

Elemental Abundance (ppm by mass)

Element	Soil	Crustal Rock
Si	330,000	311,000
Al	71,300	77,400
Fe	38,000	34,300
Ca	13,700	25,700
Mg	6,300	33,000
Na	6,300	31,900
K	13,600	29,500
Ti	4,600	4,400
Mn	850	670
Cr	200	48
V	100	98
Co	8	12

Source: Warneck (1988).

Source: Warneck (1988).

TABLE 8.5 Properties of Atmospheric Aerosol Types

Type	Number (cm 3)	PM,(pg m~3)	PM10(ng nT3)
Urban (polluted)	105 - 4 x 106	30-150	100-300
Marine	100-400	14	10
Rural	2000-10,000	2.5-8	10-40
Remote continental	50-10,000	0.5-2.5	2-10

(Prospero 1995). While particles as large as 100 pm in diameter are found in the source regions, only particles smaller than 10 pm are transported over long distances, often farther than 5000 km.

The average number and volume concentration of the major aerosol types are summarized in Table 8.5.

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