Atmospheric and dissolved carbonyl sulfide (COS) concentrations were measured on 473 samples during three cruises into the northeast Atlantic Ocean. The cruises took place in April/May 1992, January 1994, and September 1994, covering three seasons. In January 1994, persistent undersaturation of COS in seawater with respect to the atmosphere was observed. This is the first data set to show a strong and persistent under-saturation with the mean saturation ratio (SR) being 46% and the standard deviation 13%. In April 1992, the seawater was slightly supersaturated, with a SR of 126¿58%. Only in September 1994, strong supersaturation of 214¿86% was observed. The measured air concentrations were relatively uniform, averaging 410¿67 pptv in January 1994, 466¿42 pptv in April 1992, and 396¿18 pptv in September 1994.

Sea-to-air fluxes of COS were estimated using three different exchange models. We obtained moderate to low COS emissions in September (19 to 33 nmol m-2 d-1) and April/May (5 to 10 nmol m-2 d-1), in contrast to a significant flux from the atmosphere into the ocean in January (-76 to -31 nmol m-2 d-1). The strong seasonal variation of COS emissions with the possibility of reversed fluxes into the ocean during winter must be considered in future oceanic source estimates. The possible effect of an open ocean winter sink on global marine emissions of COS could be a reduction by some 10--15%. ¿ American Geophysical Union 1995.