Observations of CCN Concentrations and Concentration Variations Related to Clouds and Cloud Scavenging

Maritime CCN concentrations have traditionally been defined as number per cubic centimeter < 200. However, extensive aircraft measurements in the remote mid Pacific boundary layer during the PASE project (Aug.-Sep. 2007) consistently displayed CCN concentrations in excess of 300 per cubic centimeter. By contrast measurements in the Caribbean during the RICO project (Dec.-Jan., 2004-05) showed concentrations during 17 flights that were consistent with most previous maritime measurements, < 200 per cubic centimeter. The main difference between these projects was clouds, because RICO was a small cumulus cloud study whereas the location and season for PASE was chosen to minimize clouds. Boundary layer concentrations were quite consistent throughout each of the 8-hour flights during both of these projects. In fact the measured concentration consistency was comparable to the statistical variability of the instruments. The concentration variability among the RICO flights was a factor of four (Hudson and Mishra 2007) whereas concentrations varied by only a few percent among the PASE flights. Two summertime status cloud projects off the central California coast displayed stark contrasts in CCN concentrations and variability. Like PASE and RICO the 2005 MASE project showed consistent boundary layer concentrations throughout most of each of 11 four hour flights. Concentration variability among the MASE flights was intermediate of the PASE and RICO projects, but the MASE concentrations were always higher than PASE or RICO and thus not maritime; i.e., polluted. By contrast the 2008 POST stratus cloud project displayed much more day-to-day and within flight boundary layer CCN concentration variability. During POST concentrations ranged up to those of MASE (polluted) but were often lower; i.e., down into the traditional maritime range, often < 100 per cubic centimeter. The polluted MASE cloud deck was widespread with little horizontal or vertical variability during or among the flights. By contrast the horizontal and vertical variability of the POST clouds both within and among the 15 flights was much greater and comparable to that of the CCN. Even though PASE was remote (2 degrees N latitude directly south of Honolulu) the CCN concentrations were consistently higher than traditional maritime because of the near absence of cloud scavenging. Lower concentrations were found near the rare PASE clouds. RICO CCN concentrations were within the traditional maritime range because of cloud scavenging. MASE CCN concentrations were so consistent because the high CCN concentrations made the clouds colloidally stable and reduced coalescence. This tended to maintain the clouds and minimize CCN scavenging. The lower and more variable POST CCN and cloud droplet concentrations provided greater cloud scavenging, which reduced CCN concentrations and cloud extent in spite of anthropogenic influence. Coupled with the fact that these stratus clouds are low and thus within the smaller stratus boundary layer, this led to the greatest within and among flight CCN variability. Since the RICO cumulus clouds were small and ephemeral compared to the stratus and within a deeper boundary layer this resulted in a slower scavenging process, which resulted in only the day-to-day variability that was observed. Hudson, J. G., and S. Mishra (2007): Geophys. Res. Lett., 34, L16804, doi:10.1029/2007GL030044.