An Assessment of the Role of Natural and Anthropogenic CCN on Droplet Size Distributions in Marie Stratus in the North Atlantic

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This pilot field experiment will determine the effects of variations in natural and anthropogenic cloud condensation nuclei (CCN) on marine stratus cloud droplet size distributions in the Canary Islands. The hypothesis to be tested is that increases in CCN numbers, caused by transport of anthropogenic and natural aerosols from Europe and Africa, contrasted to the natural marine situation, will cause a systematic shift to smaller droplet sizes. Although previous work has demonstrated that anthropogenic aerosols have affected cloud microphysics, in case studies, such effects have not been shown to be significant climatologically on a global scale, nor have specific sources of CCN been identified in a remote region. To justify the assertion that the interaction between pollution aerosols and clouds is altering climate, ie., to demonstrate the Twomey Effect, it is necessary to show that a significant portion of the observed variance of the cloud droplet size distribution under background conditions is caused by pollution-derived CCN. This experiment will be conducted at Tenerife in the Canary Islands, where wide variations in natural and anthropogenic CCN are expected, where an existing AEROCE sampling station is located, in the region where the ACE-2 (Aerosol Characterization Experiment) is planned. Marine boundary-layer stratus clouds will be sampled near could base at a ground-based, elevated site. The droplet number and size distribution will be monitored continuously using a PMS- CSASP-100-HV probe (2 to 95 um dia.), and discrete droplet samples will be collected for detailed chemical analysis. The cloud-water chemistry will be used as a surrogate for the chemical composition of the pre-cloud CCN. The relative effects of anthropogenic and natural sources on variations in droplet size distributions will be identified using receptor modeling. Aerosol chemistry from the AEROCE Izania Observatory (2600m) will be compared with the cloud chemistry of m arine stratus.