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; S_c) but does not change CCN concentration (N_CCN) (Feingold and Kreidenweis, 2000).  Physical processing leads to an increase in size and decrease of N_CCN.  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 N_CCN at low S_c augment droplet activation (black data).  Adiabatic cloud droplet growth model runs at various updrafts (W) show that the low S_c cloud processed mode creates higher cloud droplet concentrations (N_c) for low W characteristic of stratus clouds (Fig. 3a, black).  Higher N_CCN at low S_c (black data) also increases condensation competition and thus reduces cloud effective S (S_eff) (Fig.3b).  This increases W importance for determining N_c (Hudson and Noble, 2014).  These high N_CCN at low S_c and lower S_eff reduce mean diameter (MD; Fig. 3c) and broaden droplet distributions (sigma; Fig. 3d).  Increased N_c and decreased MD of chemical processing seems to augment the indirect aerosol effect (IAE) whereas inherently decreased N_c and increased MD of coalescence processing reduces IAE.

Feingold, G., and S. Kreidenweis, 2000. J. Geophys. Res., 105(D19), 24351-24361.

Hudson, J. G., S. Noble, 2014. J. Atmos. Sci., 71, 312–331.