ASCOS was an interdisciplinary expedition to the central Arctic on the icebreaker Oden from 1 August to 9 September 2008. The expedition focused on processes relating to cloud formation in the summer Arctic and included observations of marine biology and chemistry, atmospheric chemistry, physical oceanography, aerosol chemistry and physics, and meteorology. This dataset contains 30 min-averaged data measured with an eddy covariance system at the open lead site from August 17 to September 1, 2008. The sonic anemometer was mounted on a tripod approximately 300 m from the open lead main site. A condensation particle counter was used to measure the total particle number concentration (diameter > 10 nm).