Observations of atmospheric methane and carbon monoxide in Brazil: SCAR-B mission

Methane (CH4) concentrations have been measured in the lower atmosphere in Brazil on board a Brazilian research aircraft. Air samples were obtained at different heights by grab sampling, and taken for analysis by flame ioniza tion detector (FID) gas chromatography at INPE (the Brazilian Institute for Space Research). The sampling region is the cerrado area of central Brazil , around the cities of Brasilia, Cuiaba, Alta Floresta, and Porto Nacional. Instead of concentrating on plume characteristics, an effort was made to d escribe the average mixed atmosphere, keeping a distance from direct burnin g sources. The measurements of atmospheric CH4 during the dry season have s hown larger mixing ratios compared to the wet season. This finding has cons istently been observed for different dry seasons, but especially for the Sm oke, Clouds, and Radiation: Brazil (SCAR-B) field mission. For comparison, measurements near the ground in a region not directly affected by biomass b urning (Natal, Brazil) show concentrations that are 39 parts per billion by volume (ppbv) less than the biomass burning results, which is 4 times the size of the seasonal amplitude at oceanic stations within about 10 degrees of the equator, thus making the biomass burning values comparable to those observed at northern hemisphere midlatitudes. The vertical distribution for these regions seems to be well mixed, showing differences of about 40 ppbv between the different sampling areas of the SCAR-B experiment. The CH4 mix ing ratio was in general well correlated with the CO mixing ratio.