[43] The trimodal distribution exhi

[43] The trimodal distribution exhibited larger droplets due to the activation of the 0.90 μm mode. This large mode is a source of GCCN, which could be important for initiating precipitation processes [Yin et al., 2000]. Although collision and coalescence were not included in the simulation, strong kinetic effects may enhance the precipitation process, because the driving force for droplet growth (i.e., the difference between the saturation ratio at the droplet surface and that of the surrounding air) is greater for larger droplets. Hence, larger droplets, which are necessary to initiate collision and coalescence, could effectively draw water vapor away from the smaller droplets. The competition for water vapor by GCCN will likely be enhanced at higher aerosol concentrations when kinetic limitations are more pronounced. A strong kinetic effect, however, could also decrease the precipitation efficiency because the overall droplet population becomes smaller due to evaporative and deactivation mechanisms shown by Nenes et al. [2001b]. This question should be addressed in future studies.
7.2. Dry Season CCN Spectra

[44] In spite of the similarity of the modeled dry season CCN spectra to the measured results between 0.15% and 1.5% Sv (Figure 2b), variations in number distribution and chemical composition can yield climatically significant differences in cloud properties. The nonconverging behavior of the effective cloud radii for the dry season simulations, shown in Figure 10b, illustrates the differences in activation and growth of the cloud droplets. Relative differences in cloud effective radius between simulations can exceed 10%, which could yield differences in indirect forcing as high as 2 W m−2 [Rotstayn, 1999]. These results suggest that changes in cloud albedo are more influenced by chemical and physical properties in cases of high aerosol concentrations, such as those found during the biomass-burning season.

[45] The importance of chemical composition and the influence of kinetic effects for the dry season CCN spectra are highlighted by DDST and DEST (Table 1 and Figure 10b), where the slower initial rate of droplet activation allows a few particles to grow into cloud droplets before kinetic limitations become significant at larger droplet concentrations. The enhanced CCN activity of larger particles (i.e., smaller critical supersaturation) (Figure 2b) due to surface tension effects of DDST appears to effectively compete for water vapor as reff continues to increase relative to the mean droplet size. The surface tension effect essentially serves the same role as GCCN, and the kinetic limitations at high droplet concentrations appear to amplify this effect.

[46] Although the CCN spectra of DA and DEST are nearly identical, even at smaller supersaturations than measured, Figure 10b illustrates significant differences in the activation and growth of the droplets. The difference in cloud effective radii for the two simulations is ca. 4.5%, which is of climatic significance [Ghan et al., 2001], and could be related to surface tension effects. Surface tension effects not only reduce the droplet's critical supersaturation, but also influence the shape of the Köhler curve, which can alter the growth behavior of a droplet. Therefore, knowledge of the CCN spectra alone may not be adequate in fully assessing the climatic effects of aerosol.