Concentrations of dimethylsulfide (DMS) in the mixed layer depth (MLD) and in the atmospheric boundary layer (ABL) and carbon disulfide (CS2) and carbonyl sulfide (COS) in the ABL were measured in the northern North Pacific during summer (July--September 1997) and autumn-winter (November--December 1998). Seasonal and regional variability in sea surface DMS was roughly explained by a simple dilution model (DMS ¿ MLD = constant). Temporal and spatial comparison of COS and CS2 suggests that during autumn-winter, there would be strong perturbation by transport of terrestrial polluted air masses from the free troposphere (FT) into the ABL. The seasonal difference in the DMS budget in the ABL was assessed by the time series mass-balance model. This calculation showed that during autumn-winter, most DMS supplied from the ocean should have escaped from the ABL, while during summer, most DMS should be oxidized in the ABL. Our results suggest the possibility that the strong seasonality of oceanic and atmospheric DMS might be produced mainly by dilution effects due to strong mixing during winter.