O₃, CO, Hydrocarbons and dimethyl sulfide over the Western Atlantic Ocean

The concentrations of O₃, CO, dimethyl sulfide (DMS) and light hydrocarbons (C₂-C₄) were measured from an instrumented aircraft during February-April 1985, near the U.S. East Coast and in the vicinity of Bermuda as part of the Western Atlantic Ocean Experiment (WATOX). Sampling flights were performed within the boundary layer (BL) and in the free troposphere (FT) at both locations. Photochemical generation of O₃ in polluted air parcels transported from the continent within the BL was identified as the probable source of excess O₃ (up to 50 ppbv above background). Convective lifting of boundary layer air carried pollutants into the free troposphere. The concentrations of HC compounds in air sampled near Bermuda had a significant inverse relation to air mass transport time from the continent. The BL concentrations of the more reactive HCs (ethylene, propane, propylene, normal- and isobutane) declined faster than the less reactive HCs (acetylene and ethane), and were found to be proportional to air mass transport time over the ocean. DMS was detected, with few exceptions, only within the BL at both sampling locations. The average concentrations in the BL samples collected near the U.S. East Coast and in the vicinity of Bermuda were 27 and 54 pptv. In all samples taken in the BL the DMS concentration decreased sharply as a function of altitude.