Comparison of Measured and Predicted Cloud Condensation Nuclei in the Free Troposphere over the Eastern Pacific Ocean during DYCOMS-II

Hiroshi Takagi, MS candidate, University of Wyoming

Atmospheric aerosols that nucleate cloud droplets are known as cloud condensation nuclei or CCN. CCN modulate cloud droplet concentration and size and thus cloud albedo and precipitation. These connections are most evident within marine stratocumulus cloud systems; however budgets for marine boundary layer CCN are poorly constrained. Two confounding factors are 1) the transport of free-tropospheric CCN into the marine boundary layer, and 2) the prediction of CCN from aerosol size spectra. This work focuses on both issues by analyzing airborne measurements of CCN and aerosol obtained during 30-minute flight segments conducted 100 to 400 m above marine stratocumulus cloud during DYCOMS-II. Flight-to-flight variability of both CCN and aerosol is dependent on back trajectories. Larger concentrations were documented for three study days associated with back trajectories that passed over British Columbia, Washington, Oregon or California before reaching the DYCOMS-II study area (~200 km west of San Diego, California). In contrast, two study days associated with oceanic back trajectories which did not pass over North America exhibited substantially lower concentrations. In a closure study, CCN concentrations measured at an effective supersaturation of 0.8% were compared to concentrations predicted using aerosol size spectra. Closure was achieved for the two study days uninfluenced by pollution originating from North America. This result could be used to either initialize or validate aerosol models which treat the coupling of free troposphere and marine boundary layer. However, the measured-to-predicted CCN ratios were lower than unity for the cases associated with back trajectories which passed over North America and for one of these cases the measured-to-predicted CCN ratio remained smaller than unity after accounting for possible biases in the measurements and calculations. Plausible reasons for this disparity are explored.