Methane in the tropical South Atlantic: Sources and distribution during the late dry season

Methane (CH 4 ) mixing ratios in the South Atlantic basin were sampled from a DC‐8 aircraft during the TRACE A expedition over the months of September and October 1992. This high‐precision (±0.1%), high‐resolution data set (independent measurements every 5 s) includes a total of 67,335 observations and ranges from 1586.6 to 2152.8 parts per billion by volume (ppbv). The observed values were influenced by emissions from biomass burning and local urban/industrial areas, stratosphere‐troposphere exchange, active convective mixing, and possible long‐range transport of pollution. Average mixing ratios increased from the atmospheric mixed layer (0–2 km) to the free troposphere (2–6 km) and were greatest at altitudes above 6 km. A longitudinal trend observed between 5°N and 40°S latitude suggests that CH 4 inputs were greater on the South American side of the basin. A latitudinal trend in values between the near‐surface and 11 km exhibits the expected gradient from north to south, but differs from the latitudinal gradient observed in surface level clean air at this time. Higher concentrations at altitude, at least at this time of year, indicate that ground‐based atmospheric monitoring sites may underestimate pollution inputs to the region.