On the relative role of sea salt cloud condensation nuclei (CCN)

Contrasts between cloud condensation nuclei (CCN) spectral volatility (thermal fractionation) measurements in two aircraft field projects provide insight into the relative contribution of sea salt. During the much more cloudy Rain in Cumulus over the Ocean (RICO) project there was a high correlation coefficient (R) between refractory (non-volatile) CCN concentrations (NCCN) and horizontal wind speed (U), especially for low supersaturation (S) NCCN, whereas this R was significantly lower in the nearly cloud free Pacific Aerosol Sulfur Experiment (PASE) project. Volatile NCCN at all S were uncorrelated with U. Ambient particle concentrations over a broad range of large sizes displayed consistently high R with U in both projects that was similar to the R of refractory NCCN with U in RICO. The size range of this high R extended down to 0.2 μm in RICO but only down to 9 μm in PASE. In both projects particle concentrations smaller than these respective sizes were highly correlated with NCCN, at all S in PASE, but mainly with NCCN at high S in RICO. In each project NCCN at all S was uncorrelated with all ambient particle concentrations larger than these same respective sizes. NCCN at all S was also uncorrelated with U in both projects. The contrast in cloudiness between the two projects was responsible for many of the differences noted between the two projects. The results indicate that the effects of clouds on NCCN play a major role in the relative influence of sea salt on NCCN and ultimately on climate. Sea salt is a minor component of maritime CCN except at high wind speeds especially at low S.