The atmospheric lifetime of dust depends on particle size; large particles are quickly removed from the atmosphere by gravitational settling, while sub-micron sized particles can have atmospheric lifetimes of several weeks. A number of models of dust mobilisation and transport have been developed for regional to global scales (Marticorena et al., 1997; Miller and Tegen, 1998; Tegen and Miller, 1998).

To estimate the radiative effects of dust aerosol, information is required about particle size, refractive index, and whether the minerals are mixed externally or as aggregates (Tegen et al., 1996; Schulz et al., 1998; Sokolik and Toon, 1999; Jacobson, 2001). Typical volume median diameters of dust particles are of the order of 2 to 4 mm. Refractive indices measured on Saharan dust have often been used to estimate the global dust radiative forcing (Tegen et al., 1996). Since this dust has a single scattering albedo significantly below one, the resulting forcing is small due to partial cancellation of solar and thermal forcing, as well as cancellation of positive and negative forcing over different geographic regions (Tegen and Lacis, 1996). However, different refractive indices of dust from different regions as well as regional differences in surface albedo lead to a large uncertainty in the resulting top-of-atmosphere dust forcing (Sokolik and Toon, 1996; Claquin et al., 1998, 1999).

5.2.2.2 Sea salt

Sea salt aerosols are generated by various physical processes, especially the bursting of entrained air bubbles during whitecap formation (Blanchard, 1983; Monahan et al., 1986), resulting in a strong dependence on wind speed. This aerosol may be the dominant contributor to both light scattering and cloud nuclei in those regions of the marine atmosphere where wind speeds are high and/or other aerosol sources are weak (O’Dowd et al., 1997; Murphy et al., 1998a; Quinn et al., 1998). Sea salt particles are very efficient CCN, and therefore characterisation of their surface production is of major importance for aerosol indirect effects. For example, Feingold et al. (1999a) showed that in concentrations of 1 particle per litre, giant salt particles are able to modify strato-cumulus drizzle production and cloud albedo significantly.

Sea salt particles cover a wide size range (about 0.05 to 10 mm diameter), and have a correspondingly wide range of atmospheric lifetimes. Thus, as for dust, it is necessary to analyse their emissions and atmospheric distribution in a size-resolved model. A semi-empirical formulation was used by Gong et al. (1998) to relate the size-segregated surface emission rates of sea salt aerosols to the wind field and produce global monthly sea salt fluxes for eight size intervals between 0.06 and 16 mm dry diameter (Figure 5.2g and Table 5.3). For the present-day climate, the total sea salt flux from ocean to atmosphere is estimated to be 3,300 Tg/yr, within the range of previous estimates (1,000 to 3,000 Tg/yr, Erickson and Duce, 1988; 5,900 Tg/yr, Tegen et al., 1997).

5.2.2.3 Industrial dust, primary anthropogenic aerosols

Transportation, coal combustion, cement manufacturing, metallurgy, and waste incineration are among the industrial and technical activities that produce primary aerosol particles. Recent estimates for the current emission of these aerosols range from about 100 Tg/yr (Andreae, 1995) to about 200 Tg/yr (Wolf and Hidy, 1997). These aerosol sources are responsible for the most conspicuous impact of anthropogenic aerosols on environmental quality, and have been widely monitored and regulated. As a result, the emission of industrial dust aerosols has been reduced significantly, particularly in developed countries. Considering the source strength and the fact that much industrial dust is present in a size fraction that is not optically very active (>1 mm diameter), it is probably not of climatic importance at present. On the other hand, growing industrialisation without stringent emission controls, especially in Asia, may lead to increases in this source to values above 300 Tg/yr by 2040 (Wolf and Hidy, 1997).

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