Measurements of CH4 and CO during ship cruises in the South Atlantic

Shipboard air grab samplings were made in November 2000, and in March 2001, to study the distribution of the atmospheric CH4 and CO in the South Atlantic region. The observations were performed in two campaigns during the cruise between Rio de Janeiro (22°54′S, 43°10′W) and the Brazilian Comandante Ferraz Antarctic Station (62°05′S, 58°24′W), and the cruise back to Rio de Janeiro. The sampling was performed using two canisters filled in a sequential way for each sampling day. Most of the average mixing ratios of CH4 for November 2000 were slightly lower than the expected values for this region, with an average of 1707.3±8.4 ppbv, without a clear latitudinal tendency. The mixing ratios for CO presented averages very near to the expected values for that region, with an average of 52.7±6.5 ppbv, showing a latitudinal variation as observed by other studies. For March 2001, the CH4 average mixing ratios showed a somewhat lower latitudinal spread than that observed in November 2000, with two sampling points showing lower mixing ratios in comparison with the other. However, the average of 1688.8±7.3 ppbv is very near that observed at the same latitude by other studies. The CO average mixing ratios presented a very low dispersion, with an average of 41.1±7.6 ppbv, with a very clear latitudinal variation. Both trace gases showed a seasonal difference between the two periods, which was very clear in the ratio CH4/CO. To determine the origin of sampled air masses, back trajectories for 48 h were calculated for isentropic surfaces of 1.5 km altitude or lower. With the exception of some trajectories between the latitudes of 30°S and 34°S, which were originated in the continent, most of them had origin from west direction, beginning in the Pacific Ocean.