Cloud Processing of CCN Spectra

Abstract:

Cloud processing often makes bimodal aerosol spectra from which size at minimal concentration infers cloud effective supersaturation (S_eff) (Hoppel et al. 1986). Particle hygroscopicity (κ) converts this Hoppel minimum to critical S, S_c. Only lower S_c particles that produce cloud droplets are physically (coalescence) or chemically (gas-to-particle conversion) processed, which increases soluble content so that upon evaporation, these CCN have even lower S_c whereas the unactivated CCN do not change size or S_c. This results in the size gap at S_eff.

DRI CCN spectrometers have revealed bimodality in 6 projects for which S_eff can be obtained without κ. However in 2 projects, MASE and ICE-T, simultaneous DMA measurements also provided κ by transposing DMA sizes to S_c; the κ that makes the DMA spectra agree with simultaneous CCN spectra (Fig). There was DMA-CCN agreement for 227 MASE and 50 ICE-T measurements. Since unlike Fig. a mean κ of the processed modes was greater than mean κ of the unprocessed modes, chemical processing was indicated; since most κ were lower than ammonium sulfate κ (0.61) chemical processing should move processed κ closer to 0.61. Chemical processing was also indicated in MASE by greater sulfate and nitrate concentrations for bimodal spectra and greater sulfur dioxide and ozone concentrations for monomodal spectra. MASE above cloud measurements showed higher κ and less bimodality than below cloud measurements, this is consistent with the higher above cloud N_CCN, that κ is lower in pollution and for these less cloud interacted samples. Interspersed bimodal and monomodal CCN spectra under the ubiquitous MASE stratus suggested less than well-mixed boundary layers. Somewhat surprisingly there was more bimodality for the cumulus ICE-T clouds than the MASE stratus. ICE-T indicated more physical than chemical cloud processing.

Cloud-processing of CCN spectra is as important as CCN sources; it alters S_eff, cloud droplet concentrations, mean diameter, spectral width and albedo. These changes and the lower Hoppel minima S_eff than S_eff from spectral comparisons with droplet concentrations; i.e., probably due to unprocessed small droplets, could be additional or counter indirect aerosol effects.