Effect of cloud processing on the CCN supersaturation spectrum

5. Effect of cloud processing on the CCN supersaturation spectrum

The previous examples illustrate that most aerosolrCCN, on evaporation after exiting clouds, are significantly larger than the aerosolrCCN entering the cloud. This mass enhancement has been attributed to the uptake of soluble precursor gas phase species into cloud droplets where they are subsequently oxidised to less volatile aerosol mass. The effect of increasing the mass of a CCN by addition of soluble material is to allow it to be activated at a lower cloud supersaturation since the critical activation of a CCN is Ž . dependent on the number of soluble ions Hudson and Clarke, 1992 . Therefore, after chemical processing in clouds, the CCN supersaturation spectrum will be modified, leading to a higher concentration of CCN at a lower supersaturation. This effect is illustrated in Fig. 6 where normalised CCN supersaturation spectra under cloud-free conditions are compared with that under cloudy conditions. The CCN Ž supersaturation spectra are constructed using Kohler curve equations Pruppacher and Ž . Fig. 6. Top Changes in the CCN supersaturation distribution for typical cloud-free and cloudy environmental y3 Ž . conditions. Number concentration is normalised to N s100 cm . Bottom Changes in the CCN supersatura- tion distribution between aerosol observed below cumulus cloud base, around cumulus cloud edge, and under stratocumulus cloud base on 24r06r1992. . Klett, 1978 and by assuming that the observed aerosol is composed of ammonium sulphate. Although this is not strictly correct as volatility analysis illustrated the presence of sulphuric acid and soot carbon in the accumulation mode size range, the assumption that all the aerosol is ammonium sulphate serves to illustrate the relative change in the CCN supersaturation spectra. Fig. 6a compares the CCN spectra derived from the CBL, CFBL and CFFT aerosol distributions shown in Fig. 1. The CFBL and CFFT spectra exhibit very close similarities as shown in Fig. 1. By comparison, the CBL spectrum, at supersaturation less than 0.1, exhibits approximately twice as many CCN as that calculated for both the CFFT and CFBL spectra. These results suggest that clouds Ž . will form more readily i.e. a lower supersaturation is required on aerosol which has already been cycled through clouds. The changes in CCN spectra for 24r06r1992, shown in Fig. 6b also exhibit a similar pattern for CCN observed in the vicinity of clouds compared with those observed in the general background.

6. Conclusions