Abstract
Abstract. Unique measurements of vertical size-resolved aerosol particle concentrations, trace gas concentrations and meteorological data were obtained during the Arctic Summer Cloud Ocean Study (ASCOS, www.ascos.se), an International Polar Year project aimed at establishing the processes responsible for formation and evolution of low-level clouds over the high Arctic summer pack ice. The experiment was conducted from on board the Swedish icebreaker Oden, and provided both ship- and helicopter-based measurements. This study focuses on the vertical helicopter profiles and onboard measurements obtained during a three-week period when Oden was anchored to a drifting ice floe, and sheds light on the characteristics of Arctic aerosol particles and their distribution throughout the lower atmosphere. Distinct differences in aerosol particle characteristics within defined atmospheric layers are identified. Within the lowermost couple hundred metres, transport from the marginal ice zone (MIZ), condensational growth and cloud processing develop the aerosol population. During two of the four representative periods defined in this study, such influence is shown. At altitudes above about 1 km, long-range transport occurs frequently. However, only infrequently does large-scale subsidence descend such air masses to become entrained into the mixed layer in the high Arctic, and therefore long-range transport plumes are unlikely to directly influence low-level stratiform cloud formation. Nonetheless, such plumes can influence the radiative balance of the planetary boundary layer (PBL) by influencing formation and evolution of higher clouds, as well as through precipitation transport of particles downwards. New particle formation was occasionally observed, particularly in the near-surface layer. We hypothesize that the origin of these ultrafine particles could be in biological processes, both primary and secondary, within the open leads between the pack ice and/or along the MIZ. In general, local sources, in combination with upstream boundary-layer transport of precursor gases from the MIZ, are considered to constitute the origin of cloud condensation nuclei (CCN) particles and thus be of importance for the formation of interior Arctic low-level clouds during summer, and subsequently, through cloud influences, for the melting and freezing of sea ice.
Highlights
The Earth has experienced a statistically significant warming over the past century, mainly as a result of the anthropogenic greenhouse effect (IPCC, 2007)
Two other campaigns have conducted similar measurements of the vertical stratification of aerosol, and gas phase dimethyl sulphide (DMS), over the high Arctic north of 80◦: the Arctic Ocean Experiment 1996 (AOE-96) (Bigg et al, 2001; Leck et al, 2001) and the Arctic Ocean Experiment 2001 (AOE2001) (Lundén et al, 2010), with the latter including acetonitrile measurements.The present study focuses on 38 of the vertical helicopter profiles, obtained during a threeweek period while the Swedish icebreaker Oden was anchored to a drifting ice floe within the high Arctic pack ice area
The following corrections were applied to the instrumentation data: first, adjustments were made to the Ultrafine Condensation Particle Counter (UCPC) data in order to account for changes in the amount of air pumped by the instrument with altitude
Summary
The Earth has experienced a statistically significant warming over the past century, mainly as a result of the anthropogenic greenhouse effect (IPCC, 2007). Due to the usually low concentrations of aerosol particles over the inner Arctic pack ice area in summer, natural surface particle sources have been emphasized as being much more important than transport from continental sources (Leck and Persson, 1996b; Leck and Bigg, 1999, 2005a, 2010; Bigg and Leck, 2001b; Kerminen and Leck, 2001; Leck et al, 2002; Chang et al, 2011) One such local source over the central Arctic Ocean is dimethyl sulphide (DMS), which, via photochemical oxidation and subsequent condensation on pre-existing particles, forms methanesulfonate (CH3SO−3 ) and non-sea-salt sulphate (nss SO24−) (Heintzenberg and Leck, 1994; Leck and Persson, 1996a, b; Quinn et al, 2007). In order to better understand low-level stratiform cloud formation over the pack ice in summer, emphasis should be placed on vertical profiling of the atmosphere Such an approach provides valuable information on the importance of the respective aerosol sources as CCN precursors, and on the mechanisms via which aerosol particles are transported vertically and entrained within lowlevel stratiform clouds. The vertical stratification of aerosol particles and its dependence, or lack thereof, on local sources, aerosol processing, transport pathways and atmospheric mixing are presented
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