Ii

i i 11 ml

—h-Wmil

i irunl

1 i ■ 1 1 ml I II

1 1 1 Mill

1 1 lU^

1 1 i Vuil -

-»-nil mil i i>

0.01

10.00

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.

Jaenicke Tropospheric Aerosols

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.

Was this article helpful?

0 0
How to Improve Your Memory

How to Improve Your Memory

Stop Forgetting and Start Remembering...Improve Your Memory In No Time! Don't waste your time and money on fancy tactics and overpriced

Get My Free Ebook


Post a comment