The hypothesis

1. Possible feedback on Arctic cloud formation Can the Arctic biosphere affect the melting of ice? Measurement domain Michael Tjernström and Caroline Leck Department of Meteorology, Stockholm University SE-106 91 Stockholm, Sweden Corresponding author: michaelt@misu.su.se http://www.fysik.lu.se/eriksw/aoe2001/aoe2001.htm Meteorology The lowest troposphere - the atmospheric boundary layer (ABL) - during Arctic summer is different compared to most other locations on Earth. Near-surface characteristics are con-trolled by freezing and melting of snow and ice at the surface, as illustrated by the peaks in the probability of near-surface temperature. The relative humidity is always high and the lowest atmosphere is often cloudy. The lowest cloud base is most often found below 100 m and frequent fogs occur. The ABL is shallow, ~ 100 - 200 m, but its static stability is often small and it remains well mixed by local turbulence. Characteristics aloft are governed by long range transport. The air aloft is often substantially warmer and moister than the ABL air, as can be seen in time-height cross-sections from soundings. This results in strong capp-ing inversions where absolute humidity often increase with height through the inversion. The resulting vertical structure facilitates mixing between the surface and a few hundred meters, and thus transport of aerosols formed near the surface. Exchange between the free troposphere, with long-range transported aerosol precursors, and the surface is inhi-bited. Turbulent flux of moisture both from below and above maintains a very moist ABL. Aerosol production In-situ observations from the central Arctic basin show a far larger local aerosol production than expected, and temporal variations in aerosol concentration also by far exceeds those expected in this pristine and horizontally homogeneous environment. Our measurements indicate that the surface microlayer of the open water in the Arctic plays an important role in maintaining the population of particles smaller than about 50nm diameter, and that their presence does not rely on any source in the free troposphere. Their sub-sequent acquisition of sulfuric acid provides a much more direct and faster path to CCN status than having to grow all the way from nucleated particles. This new picture on the evolution of the Arctic aerosol greatly expands the possibility of a climate feedback effect, since there is a much greater involvement of biological processes than with DMS alone. If climate change means a reduction of the ice cover, this could lead to changes in the local aerosol production that causes a feedback through the effects on the radiative properties of the low clouds. Soundings were launched every six hour during research stations ice drift ~100nm ~100 nm A radio-controlled boat used in collecting the surface microlayer on a hydrophilic Teflon drum, at 89°N. ~1000 nm ~1000nm Aggregated compact balls 106 ml-1 The hypothesis In the Arctic, stratocumulus almost always warms the surface and in summer the cloud cover is persistently large, mostly by low clouds. Changes in cloud microphy-sics then becomes important for the surface energy balance. Arctic stratocumulus have a relatively low albedo, presumably due to relatively low concentration of Cloud Condensation Nuclei (CCN) in the Arctic. More abundant CCN could change the drop size distribution and thus cause a change in the cloud albedo. The question is: “Could an enhanced greenhouse warming cause changes in local CCN production and thereby alter the radiation balance at the surface, through the effect of the CCN concentration on the cloud drop-size distribution?” To understand this, we must first know how CCN are naturally produced in the Arc-tic. Previous understanding involves large sources of CCN precursors at the margi-nal ice zone. CCN then form as the air is advected in over the pack ice. We have found evidence of local biogenic aerosol production in the central Arctic basin. An anthropogenic warming, leading to an increased open water fraction, would impact on the biological activity and could therefore also change the local CCN production. 3 x bulk water 108 -1014 ml-1 a group of ”virus like” particles coated with acid Bacterium ”virus like” bacterium Pentagonal/hexagonal habits ? organic film with inclusion sea-salt chain aggregates The ice camp seen from Oden’s bridge, with the sodar and tethersonde site in the foreground and the meteorological mast in the background Sea-salt, organic film, H2SO4, bacterium sulfuric acid NH4-H2SO4-MSA The Arctic Ocean Experiment 2001 (AOE-2001) The discussion in this poster builds on results from three experiments to the central Arctic basin, on the Swedish icebreaker Oden. The latest (AOE-2001) was a two-month atmospheric experiment, including a three-week ice drift from 89 N to 88 N. The measurements include marine biology, atmospheric chemistry, aerosols and meteorology.