The Distribution of Methyl Chloride, Dichloromethane, Trichloroethene, and Tetrachloroethene over the Atlantic

During the cruise ANT VIII/1 of the German R/V Polarstern in August/September 1989 the latitudinal distributions of the atmospheric concentrations of methylchloride, dichloromethane, trichloroethene, and tetrachloroethene were measured over the Atlantic between 45°N and 30°S by in situ gas chromatography. With the exception of trichloroethene they showed mixing ratios well above the lower limit of detection. The methylchloride distribution was uniform with average mixing ratios of 532 ± 8 and 550 ± 12 ppt in the northern and southern Hemispheres, respectively. Dichloromethane increased linearly between the Intertropical Convergence Zone and 45°N with average mixing ratios of 36 ± 6 ppt and was almost constant in the southern hemisphere with an average of 18 ± 1 ppt. Tetrachloroethene mixing ratios were between less than 1 and 10 ppt in the northern hemisphere and always below 3 ppt in the southern hemisphere. Similar to dichloromethane, tetrachloroethene was nearly constant in the southern hemisphere and increased linearly toward northern latitudes. This is compatible with the predominantly industrial origin of these compounds. Trichloroethene varied between 0.3 ppt and about 15 ppt in the northern hemisphere with an average of 3 ± 1 ppt and was generally lower than 1 ppt in the southern hemisphere with mixing ratios often near or below the detection limit of 0.1 ppt. For CH3Cl we estimate a global turnover of 3.5 × 1012 g/yr which is compatible with previous results. Using a simple model calculation our measurements imply a global turnover for CH2Cl2 and C2Cl4 of 0.9 × 1012 g/yr and 0.6 × 1012 g/yr, respectively.