G. McFiggans et al. Aerosol effects on warm cloud activation 2599 300 200 100 draft=0. 1 draft=0. 175 m s 0 01002003004000100200300400 Sub-cloud aerosol (cm Fig. 5. Cloud droplet concentration as a function of sub-cloud aerosol where the sub-cloud aerosol comprises an extemal mix of sulphate nd sea-salt CCN The simulations results are shown in Fig. 5 for updraughts sation of these properties(e. g. Whitby, 1978; Van Dingenen of 0. I ms- and 0. 175 ms. For low sulphate CCN concen- et al., 2004) trations, the addition of sea-salt CCN increases the number of Figure 6 shows representative average distributions in a activated droplets significantly while for high sulphate con- variety of locations. Most particles greater than 200 nm di- centrations, the number of activated droplets decreases sig- ameter with moderate amounts of soluble material will acti- nificantly. For the higher updraught, the point at which the vate under reasonable supersaturations. Assuming that those result of the addition of sea-salt nuclei switches from an in- particles greater than 200 nm in Fig. 6 are moderately sol- crease to a decrease in droplet concentration reduces for the uble, it can be seen that the sizes critical to determining the gher updraught and the impact of the reduction in droplet droplet number in an aerosol population fall in the range with concentration increases for increasing updraught significant contributions from both Aitken and accumulation The main processes driving this phenomenon are(1)sea- mode particles(around 100 nm diameter). It is therefore nec- salt CCN are typically larger than sulphate CCN; (2)for a essary to capture the features of the aerosol size distribu- en size, sea-salt is more active as a CCn than sulphate, tion in both modes in order to realistically describe cloud ()although the relative concentration of larger sea-salt CCn activation behaviour, (Martinsson et al., 1999). The follow is significantly lower than sulphate CCN, they contribute to ing sections investigate further properties of real atmospheric a significant reduction in the peak supersaturation reached in aerosol and the potential impacts of these properties on cloud cloud and thus inhibit the activation of sulphate nuclei. This activation example demonstrates that for even simple two-component It should be noted that, the critical size range for cloud aerosol systems the dynamic competition is quite complex activation of about 50 to 150 nm is not accessible to most and non-linear and that the effect of increasing the availabil- optical sizing instruments, but may be probed by mobility ity of ccn does not necessarily lead to an increase in cloud instruments. This significantly reduces the amount of data droplet concentration. Similar non-linearities are evident in available at cloud altitudes because mobility analyses can be the effects of composition on droplet activation and caution challenging on aircraft due to the time required to scan a size should be exercised in translating a composition change to distribution an equivalent change in drop number concentration(Ervens et al., 2005). The results of such responses are strongly de- 3.1.3 Relative importance of size distribution, composition pendent on water vapour supply (i.e. updraught)and conden sation rates(dependent on size distribution and composition) The activation of seasalt and sulphate in marine stratiform Feingold (2003) performed a sensitivity anal cloud described in this section is a particularly simple case in aspects of the relative importance of aerosol size and compo- terms of both composition and the limited range of updraught sition, in so far as both properties affect activation, using a velocity. Ambient aerosol size distributions are highly vari- cloud parcel model. Input aerosol size distributions(parame- able from location to location. The reader is referred to a terised as lognormal functions described by Na, rg, g), and range of review articles for a broad and detailed characteri- prescribed updraught velocities, w, were varied over a large www.atmos-chem-phys.net/6/2593/2006/ Atmos. Chem. Phys., 6, 2593-2649, 2006G. McFiggans et al.: Aerosol effects on warm cloud activation 2599 Fig. 5. Cloud droplet concentration as a function of sub-cloud aerosol where the sub-cloud aerosol comprises an external mix of sulphate and sea-salt CCN. The simulations results are shown in Fig. 5 for updraughts of 0.1 ms−1 and 0.175 ms−1 . For low sulphate CCN concen￾trations, the addition of sea-salt CCN increases the number of activated droplets significantly while for high sulphate con￾centrations, the number of activated droplets decreases sig￾nificantly. For the higher updraught, the point at which the result of the addition of sea-salt nuclei switches from an in￾crease to a decrease in droplet concentration reduces for the higher updraught and the impact of the reduction in droplet concentration increases for increasing updraught. The main processes driving this phenomenon are (1) sea￾salt CCN are typically larger than sulphate CCN; (2) for a given size, sea-salt is more active as a CCN than sulphate; (3) although the relative concentration of larger sea-salt CCN is significantly lower than sulphate CCN, they contribute to a significant reduction in the peak supersaturation reached in cloud and thus inhibit the activation of sulphate nuclei. This example demonstrates that for even simple two-component aerosol systems the dynamic competition is quite complex and non-linear and that the effect of increasing the availabil￾ity of CCN does not necessarily lead to an increase in cloud droplet concentration. Similar non-linearities are evident in the effects of composition on droplet activation and caution should be exercised in translating a composition change to an equivalent change in drop number concentration (Ervens et al., 2005). The results of such responses are strongly de￾pendent on water vapour supply (i.e. updraught) and conden￾sation rates (dependent on size distribution and composition). The activation of seasalt and sulphate in marine stratiform cloud described in this section is a particularly simple case in terms of both composition and the limited range of updraught velocity. Ambient aerosol size distributions are highly vari￾able from location to location. The reader is referred to a range of review articles for a broad and detailed characteri￾sation of these properties (e.g. Whitby, 1978; Van Dingenen et al., 2004). Figure 6 shows representative average distributions in a variety of locations. Most particles greater than 200 nm di￾ameter with moderate amounts of soluble material will acti￾vate under reasonable supersaturations. Assuming that those particles greater than 200 nm in Fig. 6 are moderately sol￾uble, it can be seen that the sizes critical to determining the droplet number in an aerosol population fall in the range with significant contributions from both Aitken and accumulation mode particles (around 100 nm diameter). It is therefore nec￾essary to capture the features of the aerosol size distribu￾tion in both modes in order to realistically describe cloud activation behaviour, (Martinsson et al., 1999). The follow￾ing sections investigate further properties of real atmospheric aerosol and the potential impacts of these properties on cloud activation. It should be noted that, the critical size range for cloud activation of about 50 to 150 nm is not accessible to most optical sizing instruments, but may be probed by mobility instruments. This significantly reduces the amount of data available at cloud altitudes because mobility analyses can be challenging on aircraft due to the time required to scan a size distribution. 3.1.3 Relative importance of size distribution, composition and updraught Feingold (2003) performed a sensitivity analysis comparing aspects of the relative importance of aerosol size and compo￾sition, in so far as both properties affect activation, using a cloud parcel model. Input aerosol size distributions (parame￾terised as lognormal functions described by Na, rg, σg), and prescribed updraught velocities, w, were varied over a large www.atmos-chem-phys.net/6/2593/2006/ Atmos. Chem. Phys., 6, 2593–2649, 2006