Potential source regions and processes of the aerosol in the summer Arctic

Abstract

Abstract. Sub-micrometer particle size distributions measured during four summer cruises of the Swedish icebreaker Oden 1991, 1996, 2001, and 2008 were combined with concurrent gas data, back trajectories and daily maps of pack ice cover in order to investigate source areas and aerosol formation processes of the boundary layer aerosol in the central Arctic. With a clustering algorithm potential aerosol source areas were explored. Clustering of particle size distributions together with back-trajectories delineated five potential source regions and three different aerosol types that covered most of the Arctic basin. Clustering the open water information along the trajectories clearly separated marine from pack ice aerosol. Clustering the two subpopulations above and below the 50% value of average open water during the course of the trajectories yielded further details about marine and pack ice aerosol sources. In the pack ice subpopulation three potential source areas over the pack ice with very low percentages of open water from which aged aerosol with only one major mode around 40 nm derived. A significant difference between newly formed and aged aerosol over the pack ice became clear when comparing the course of open water under the trajectories for these two aerosol types. In both subpopulations the air had spent ten days over pack ice with less than 50% open water while traveling over ever more contiguous ice. Trajectories connected with high concentrations of newly formed small particles, however, experienced more open water during the last four days before arrival in heavy ice conditions at Oden. Thus we hypothesize that both, long travel times over the more contiguous ice, combined with more open water conditions during the last days before air mass arrival were an essential factor controlling the formation of ultrafine particles over the central Arctic pack ice. A comparison the Oden data with summer size distribution data from Alert, Nunavut and Mt. Zeppelin, Spitsbergen confirmed the Oden findings with respect to particle sources over the central Arctic.