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The Arctic is one of the most vulnerable regions affected by climate change.Extensive measurement data are needed to understand the atmosphericprocesses governing this vulnerability. Among these, data describing cloudformation potential are of particular interest, since the indirect effect ofaerosols on the climate system is still poorly understood. In this paper wepresent, for the first time, size-resolved cloud condensation nuclei (CCN)data obtained in the Arctic. The measurements were conducted during twoperiods in the summer of 2008: one in June and one in August, at theZeppelin research station (78°54´ N, 11°53´ E) inSvalbard. Trajectory analysis indicates that during the measurement periodin June 2008, air masses predominantly originated from the Arctic, whereasthe measurements from August 2008 were influenced by mid-latitude air masses.CCN supersaturation (SS) spectra obtained on the 27 June, beforesize-resolved measurements were begun, and spectra from the 21 and 24 August, conducted before and after the measurement period, revealedsimilarities between the 2 months. From the ratio between CCNconcentration and the total particle number concentration (CN) as a functionof dry particle diameter (Dp) at a SS of 0.4 %, the activationdiameter (D50), corresponding to CCN / CN = 0.50, was estimated.D50 was found to be 60 and 67 nm for the examined periods in June andAugust 2008, respectively. Corresponding D50 hygroscopicity parameter(κ) values were estimated to be 0.4 and 0.3 for June and August 2008, respectively. These values can be compared to hygroscopicity valuesestimated from bulk chemical composition, where κ was calculated tobe 0.5 for both June and August 2008. While the agreement between the 2months is reasonable, the difference in κ between the differentmethods indicates a size dependence in the particle composition, which islikely explained by a higher fraction of inorganics in the bulk aerosolsamples

Size-resolved cloud condensation nuclei concentration measurements in the Arctic: two case studies from the summer of 2008