Cloud Processing making Bimodal CCN spectra
Clear air aerosol spectra often display modes, attributed to cloud processing. For bimodal spectra sizes at minimal concentrations infer effective supersaturations (Seff) of nearby clouds (Hoppel et al. 1986). When particle composition was assumed these Hoppel minima diameters converted to critical S, Sc by using particle hygroscopicity (kappa). This was based on the principle that lower Sc particles produced the cloud droplets. Cloud processing then proceeded only on the droplets grown on the lower Sc CCN. These physical (coalescence among droplets and Brownian capture of interstitial particles) and chemical (gas-to-particle conversion of sulfate or nitrate) processes then increased the soluble content of cloud droplets so that when evaporated, as typical, emerging dry particles had even lower Sc whereas unactivated CCN did not change size or Sc. This resulting size gap occurred at the maximal Sc from which the activated CCN had grown to cloud droplets. The size at this “Hoppel minimum” then represented Seff of the processing clouds.
Detailed spectra from the DRI CCN spectrometers has revealed bimodality in RICO (wintertime Caribbean cumuli), MASE (polluted California stratus), PASE (central Pacific), ICE-L (wintertime Colorado and Wyoming), POST (California stratus) and ICE-T (summertime Caribbean cumuli). Since CCN measurements are in terms of S, particle composition (i.e., kappa) is not required to estimate Seff . In MASE and ICE-T there were also simultaneous DMA measurements to compare with CCN spectra by transposing size to Sc by using kappa. The kappa that made the DMA spectra agree with simultaneous CCN spectra (Fig) thus revealed kappa. Such DMA-CCN agreement was consistent for 227 MASE measurements and 50 ICE-T measurements. For some spectra kappa differed for the two modes (Fig. a). An opposite average kappa difference pattern in MASE indicated chemical processing. Since most kappa were lower than ammonium sulfate kappa (0.61) chemical processing should push kappa closer to 0.61 for the cloud-processed lower Sc mode. In MASE greater sulfate and nitrate concentrations for the more bimodal spectra (lower modal rating) and greater sulfur dioxide and ozone concentrations for more monomodal spectra (higher modal rating) also indicated chemistry. MASE above cloud measurements showed higher kappa and less bimodality, which is consistent with the higher CCN concentrations above, that kappa is lower in pollution and for the less cloud interacted samples above than below cloud. That bimodality was not universal under the ubiquitous MASE stratus suggests not so well-mixed boundary layers. Somewhat surprisingly there was more bimodality for the cumulus ICE-T clouds than the MASE stratus. ICE-T did not indicate chemical processing.
Cloud-processing of CCN spectra is as important as CCN sources; it alters Seff, kappa, cloud droplet concentrations, mean diameter, spectral width and albedo. These changes and lower Seff from Hoppel minima than Seff from CCN spectral comparisons with droplet concentrations; i.e., probably due to unprocessed small droplets, could be additional or counter indirect aerosol effects.
Table 1. MASE data is divided by the stratus; below and above. 2nd column is the number of comparisons, 3rd column is the number of comparisons that provided Hoppel minima, Seff, 4th column is mean hygroscopicity, kappa, of the processed modes (lower Sc), 5th column is mean kappa of the corresponding unprocessed modes (higher Sc), 6th column is the mean mode rating (1 to 8; 1 very bimodal; 8 strictly monomodal), 7th column is mean Seff from Hoppel minima, 8th column is Seff by matching below cloud CCN spectra with mean cloud droplet concentration, 9th column is CCN concentration at 1% S.
Hoppel et al., 1986: Geophys. Res. Lett., 13 (1), 125-128.