Primary marine aerosol-cloud interactions off the coast of California

Primary marine aerosol (PMA)-cloud interactions off the coast of California were investigated using observations of marine aerosol, cloud condensation nuclei (CCN), and stratocumulus clouds during the Eastern Pacific Emitted Aerosol Cloud Experiment (E-PEACE) and the Stratocumulus Observations of Los-Angeles Emissions Derived Aerosol-Droplets (SOLEDAD) studies. Based on recently reported measurements of PMA size distributions, a constrained lognormal-mode-fitting procedure was devised to isolate PMA number size distributions from total aerosol size distributions and applied to E-PEACE measurements. During the 12 day E-PEACE cruise on the R/V Point Sur, PMA typically contributed less than 15% of total particle concentrations. PMA number concentrations averaged 12 cm(-3) during a relatively calmer period (average wind speed 12m/s(1)) lasting 8 days, and 71cm(-3) during a period of higher wind speeds (average 16m/s(1)) lasting 5 days. On average, PMA contributed less than 10% of total CCN at supersaturations up to 0.9% during the calmer period; however, during the higher wind speed period, PMA comprised 5-63% of CCN (average 16-28%) at supersaturations less than 0.3%. Sea salt was measured directly in the dried residuals of cloud droplets during the SOLEDAD study. The mass fractions of sea salt in the residuals averaged 12 to 24% during three cloud events. Comparing the marine stratocumulus clouds sampled in the two campaigns, measured peak supersaturations were 0.20.04% during E-PEACE and 0.05-0.1% during SOLEDAD. The available measurements show that cloud droplet number concentrations increased with >100 nm particles in E-PEACE but decreased in the three SOLEDAD cloud events.