Itinerary and summary On 20 April 2011 Polarstern started its Atlantic transfer from Cape Town to Bremerhaven as last leg of Antarctic cruise ANT-XXVII/4. The cruise was utilized for continuous investigations of atmospheric and marine properties as well as for energy and material fluxes between ocean and atmosphere. The cruise ended in Bremerhaven on 20 May 2011 after a short stop in Las Palmas. The following projects were carried out: 3.1 Autonomous measurement platforms for energy and material exchange between ocean and atmosphere (OCEANET - Atmosphere & Ocean) In order to provide a solid basis for the observational monitoring of energy and material exchange between ocean and atmosphere it is planned to develop an autonomous observation system for operational use onboard cargo- and research vessels. The project is based on a network of expertise between IFM-GEOMAR (CO2-/O2-fluxes, photosynthetic status, energy budget, remote sensing), IfT (lidar measurements), the GKSS research center (ferry box, remote sensing of marine biology with ENVISAT/ MERIS) and AWI-Bremerhaven (CO2-system, marine infrastructure of Polarstern). 3.2 Chemical and physical characterization of marine aerosols on board Polarstern Exchange of gases and aerosol particles between ocean and atmosphere is not well understood currently, although it has received considerably and intensively attention. New measurements were carried out here using a LIDAR system to understand the formation mechanism of secondary fraction in marine aerosols particles and to characterize the optical properties of marine aerosols. 3.3 Aerosols measurements using MICROTOPS In addition to the LIDAR aerosol measurements, optical thickness densities of aerosols were measured by using the MICOROTOPS. 3.4 Rate measurement of cosmic particles in dependence on latitude and weather conditions Continuous rate measurements of cosmic particles allow estimating their dependency on latitude due to the earth’s magnetic field. Simultaneous cloud observations allow investigating whether the concentration of cosmic particles influences cloudiness or vice versa.