Nonurban Continental Aerosol

Nonurban continental aerosol is often acidic as a result of anthropogenic sulfate or nitrate. Typical aerosol number concentrations of nonurban continental aerosol are in the range of 103/cm3, with mass concentrations around 30 pg/m3 (Anderson et al., 1993). The aerosol mass distribution usually exhibits a trimodal structure similar to that of urban aerosol.

Figure 2 shows approximate atmospheric aerosol number concentration N and volume concentration V as a function of altitude z. Because of the large variations in number and volume concentrations in the lower troposphere (remote continental, rural continental, urban, marine, polar), the vertical profiles fan out at low altitudes

10"

10"

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1 1

1

— Low Altitudes

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-

~C—

—Tropopause- — — —

Volume in desert dust storms

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Figure 2 Atmospheric aerosol number N and volume V concentrations as a function of altitude z. Adapted from Jaenicke (1988).

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ra 3

1

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Urban'

Desert

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• Rural

-

Remote

Background

Continental

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Figure 3 Typical aerosol number concentrations in accumulation and nuclei modes for six classes of global aerosols. Clement and Ford (1997), based on number concentrations reported by Jaenicke (1993). Reprinted from Journal of Aerosol Science, Vol. 28, No. 1, C. F. Clement and I. J. Ford, Properties and modelling of global aerosols, 5743-4, Copyright 1997, with permission from Elsevier Science.

Nuclei Mode

Figure 3 Typical aerosol number concentrations in accumulation and nuclei modes for six classes of global aerosols. Clement and Ford (1997), based on number concentrations reported by Jaenicke (1993). Reprinted from Journal of Aerosol Science, Vol. 28, No. 1, C. F. Clement and I. J. Ford, Properties and modelling of global aerosols, 5743-4, Copyright 1997, with permission from Elsevier Science.

(dashed lines). Aerosol number concentration decreases continuously with increasing altitude, whereas volume concentration decreases up to the tropopause (about 10 km) and then increases in the stratospheric aerosol layer, reaching maximum between 15 and 20 km altitude.

Atmospheric aerosol number concentrations can be naturally divided into three groups: nuclei mode particles with diameters < 0.01 |im, accumulation mode particles with diameters 0.01 to 1 |im, and coarse particles with diameters exceeding 1 |im. Figure 3 shows the total number concentrations of the nuclei and accumulation modes for urban, rural, desert, remote continental, marine, and background aerosols. (There is no obvious nucleation mode for the polar aerosol.) Number concentrations of the two modes are seen to be equal over a range of 3 orders of magnitude in different parts of the atmosphere.

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