Chemical and Physical Processing of CCN and their Effects on Clouds
Cloud condensation nuclei (CCN) distributions are transformed by in-cloud processing. This can be chemical: aqueous oxidation; or physical: Brownian scavenging, collision and coalescence. Droplet evaporation leaves behind the cloud-processed CCN. Chemical processing increases CCN size (lower critical supersaturation; Sc) but does not change CCN concentration (NCCN) (Feingold and Kreidenweis, 2000). Physical processing leads to an increase in size and decrease of NCCN. These processes are especially important in stratus clouds that cover large areas and persist for long periods of time. Modified CCN in turn modify cloud droplet spectra.
Both chemical and physical processing were observed during the 2005 MArine Stratus/stratocumulus Experiment (MASE) field campaign. Figure 1 shows chemical concentrations associated with various CCN spectral modal ratings; 1-8, with 1 very bimodal and 8 strictly monomodal. Higher SO4 and NO3 with lower SO2 and O3 of the more bimodal spectra indicate chemical processing.
Figure 2 shows two nearby MASE CCN spectra. Increased NCCN at low Sc augment droplet activation (black data). Adiabatic cloud droplet growth model runs at various updrafts (W) show that the low Sc cloud processed mode creates higher cloud droplet concentrations (Nc) for low W characteristic of stratus clouds (Fig. 3a, black). Higher NCCN at low Sc (black data) also increases condensation competition and thus reduces cloud effective S (Seff) (Fig.3b). This increases W importance for determining Nc (Hudson and Noble, 2014). These high NCCN at low Sc and lower Seff reduce mean diameter (MD; Fig. 3c) and broaden droplet distributions (sigma; Fig. 3d). Increased Nc and decreased MD of chemical processing seems to augment the indirect aerosol effect (IAE) whereas inherently decreased Nc and increased MD of coalescence processing reduces IAE.
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Hudson, J. G., S. Noble, 2014. J. Atmos. Sci., 71, 312–331.