|Working Group I: The Scientific Basis|
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Another complication comes from the way in which different chemical species are mixed in aerosols (e.g., Li and Okada, 1999). Radiative properties can change depending on whether different chemicals are in the same particles (internal mixtures) or different particles (external mixtures). Also, combining species may produce different aerosol size distributions than would be the case if the species were assumed to act independently. One example is the interaction of sulphate with sea salt or dust discussed in Section 22.214.171.124.
Fortunately, studies of the effects of mixing different refractive indices have yielded a fairly straightforward message: the type of mixing is usually significant only for absorbing material (Tang, 1996; Abdou et al., 1997; Fassi-Fihri et al., 1997). For non-absorbing aerosols, an average refractive index appropriate to the chemical composition at a given place and time is adequate. On the other hand, black carbon can absorb up to twice as much light when present as inclusions in scattering particles such as ammonium sulphate compared with separate particles (Ackerman and Toon, 1981; Horvath, 1993; Fuller et al., 1999). Models of present day aerosols often implicitly include this effect by using empirically determined light absorption coefficients but future efforts will need to explicitly consider how black carbon is mixed with other aerosols. Uncertainties in the way absorbing aerosols are mixed may introduce a range of a factor of two in the magnitude of forcing by black carbon (Haywood and Shine, 1995; Jacobson, 2000).
To assess uncertainties associated with the basic aerosol parameters, a compilation is given in Table 5.1, stratified by a crude geographic/aerosol type differentiation. The values for the size distribution parameters given in the table were derived from the references to the table. The mass scattering efficiency and upscatter fraction shown in the table are derived from Mie calculations for spherical particles using these size distributions and a constant index of refraction for the accumulation mode. The scattering efficiency dependence on relative humidity (RH) and the single-scattering albedo were derived from the literature review of measurements.
The uncertainties given in the table for the central values of number modal diameter and geometric standard deviation (Dg and sg) are based on the ranges of values surveyed in the literature, as are those for f(RH) and wo. Those for the derivative quantities asp and b, however, are based on Mie calculations using the upper and lower uncertainty limits for the central values of the size parameters, i.e., the propagation of errors is based on the functional relationships of Mie theory. The two calculations with coarse modes require some explanation.
While the accumulation mode is generally thought to dominate light scattering, recent studies – as discussed below – have suggested that sea salt and dust can play a large role under certain conditions. To include this possibility, a sea salt mode has been added to the Pacific marine accumulation mode. The salt mode extends well into the accumulation size range and is consistent with O’Dowd and Smith (1993). It is optically very important at wind speeds above 7 to 10 ms-1. For the case shown in the table, the sea salt mode accounts for about 50% of the local light scattering and could contribute over a third of the column optical depth, depending on assumptions about the scale height of the salt. Similarly, a soil dust coarse mode based on work by Whitby (1978) was added to the continental background accumulation mode. When present, this mode often dominates light scattering but, except for regions dominated by frequent dust outbreaks, is usually present over so small a vertical depth that its contribution to the column optical depth is generally slight. The importance of these coarse modes points to the importance of using size-resolved salt and dust fluxes such as those given in this report.
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