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 (S_eff) of nearby clouds (Hoppel et al. 1986). When particle composition was assumed these Hoppel minima diameters converted to critical S, S_c by using particle hygroscopicity (kappa). This was based on the principle that lower S_c particles produced the cloud droplets. Cloud processing then proceeded only on the droplets grown on the lower S_c 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 S_c whereas unactivated CCN did not change size or S_c. This resulting size gap occurred at the maximal S_c from which the activated CCN had grown to cloud droplets. The size at this “Hoppel minimum” then represented S_eff 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 S_eff . In MASE and ICE-T there were also simultaneous DMA measurements to compare with CCN spectra by transposing size to S_c 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 S_c 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 S_eff, kappa, cloud droplet concentrations, mean diameter, spectral width and albedo. These changes and lower S_eff from Hoppel minima than S_eff 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. 2^nd column is the number of comparisons, 3^rd column is the number of comparisons that provided Hoppel minima, S_eff, 4^th column is mean hygroscopicity, kappa, of the processed modes (lower S_c), 5^th column is mean kappa of the corresponding unprocessed modes (higher S_c), 6^th column is the mean mode rating (1 to 8; 1 very bimodal; 8 strictly monomodal), 7^th column is mean S_eff from Hoppel minima, 8^th column is S_eff by matching below cloud CCN spectra with mean cloud droplet concentration, 9^th column is CCN concentration at 1% S.    

Hoppel et al., 1986:  Geophys. Res. Lett., 13 (1), 125-128.