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Abstract:
Within the framework of the RACEPAC (Radiation-Aerosol-Cloud Experiment in the Arctic Circle) project, the Arctic aerosol, arriving at a ground based station in Tuktoyaktuk (Mackenzie River delta area, Canada), was characterized during a period of 3 weeks in May 2014. The observations of basic meteorological parameters and particle number size distributions (PNSDs) were indicative for the rapid transition from Arctic spring to summer that took place during the measurement period. Two distinct types of air masses were found. One type were typical Arctic haze air masses, termed as spring-type air masses, characterized by a mono-modal PNSD with a pronounced accumulation mode at sizes above 100 nm. These air masses were observed during a period when back trajectories indicate an air mass origin in the north east of Canada. The other air mass type is characterized by a bi-modal PNSD with a clear minimum around 90 nm, and with an Aitken mode consisting of freshly formed aerosol particles. Back trajectories indicate that these air masses, termed as summer-type air masses, originated from the northern Pacific. Generally total particle number concentrations (NCN) ranged from 20 to 500 cm−3, while cloud condensation nuclei (CCN) number concentrations were found to cover a range between less than 10 up to 250 cm−3 for a supersaturation (SS) between 0.1 and 0.7 %. The hygroscopicity parameter κ of the CCN was determined to be 0.23 on average and variations in kappa were largely attributed to measurement uncertainties.
Furthermore, simultaneous PNSD measurements at the ground station and on the Polar 6 research aircraft were performed. We found a good agreement of ground based PNSDs with those measured between 200 and 1200 m. During two of the four overflights, particle number concentrations at 3000 m were found to be up to twenty times higher than those measured below 2000 m, and for one of these two flights, PNSDs measured above 2000 m showed a different shape than those measured at lower altitudes. This is indicative for long range transport from lower latitudes into the Arctic that can advect aerosol from different regions in different heights.
