Concurrent measurement of aerosols, cloud condensation nuclei (CCN) and clouddroplet activation were carried out as a part of the third Pallas CloudExperiment (PaCE-3) which took place at a ground based site located onnorthern Finland during the autumn of 2009. In this study, we investigaterelationships between the aerosol properties, CCN and size resolved clouddroplet activation. During the investigated cloudy periods, the inferrednumber of cloud droplets (CDNC) varied typically between 50 and150 cm and displayed a linear correlation both with the number ofparticles having sizes over 100 nm and with the CCN concentrations at0.4% supersaturation. Furthermore, the diameter corresponding to the50% activation fraction, , was generally in the range of 80 to 120nm. The measured CCN concentrations were compared with predictions of anumerical model which used the measured size distribution and size resolvedhygroscopicity as input. Assuming that the droplet surface tension is equalto that of water, the measured and predicted CCN concentrations weregenerally within 30%. We also simulated size dependent cloud dropletactivation with a previously developed air parcel model. By forcing the modelto reproduce the experimental values of CDNC, adiabatic estimates for theupdraft velocity and the maximum supersaturation reached in the clouds werederived. Performed sensitivity studies showed further that the observedvariability in CDNC was driven mainly by changes in the particle sizedistribution while the variations in the updraft velocity and hygroscopicitycontributed to a lesser extent. The results of the study corroborateconclusions of previous studies according to which the number of clouddroplets formed in clean air masses close to the Arctic is determined mainlyby the number of available CCN.
展开▼
