Overview of an aircraft expedition into the Brazilian cerrado for the observation of atmospheric trace gases

Abstract

Tropospheric trace gases were measured from an aircraft platform. The flights were organized to sample air masses from the geographic area of central Brazil, where the vegetation, a savanna‐type environment with the local name of “cerrado”, is subject to burning every year, especially through August, September, and October. These measurements were made as a Brazilian local contribution to the international field campaign organized by NASA, the Transport and Atmospheric Chemistry Near the Equator‐Atlantic (TRACE A) mission, and the Southern African Fire Atmospheric Research Initiative (SAFARI). The major NASA TRACE A mission used the NASA DC 8 aircraft, with most flights over the South Atlantic Ocean region. In Brazil, missions using small aircraft measured ozone and carbon dioxide continuously, and carbon monoxide, nitrous oxide, and methane using grab sampling. In addition, ground‐based measurements were made continuously over most of the dry months of 1992, and ozonesondes were launched at three different sites. Geostationary Operational Environment Satellite‐East (GOES E) images and a special network of radio soundings provided meteorological information, and advanced very high resolution radiometer (AVHRR) images indicated the distribution of fire pixels in the region of interest. Most of the biomass burning in 1992 occurred in the state of Tocantins, with about 22% of all the burning in Brazil. The state of Mato Grosso was second, with 19% of all burning. The Brazilian aircraft was used mostly in these two states, near the cities of Porto Nacional and Cuiabá, for in situ sampling; 31 vertical profiles were made in air masses considered to be well mixed, that is, not in fresh plumes. Although the major interest was the dry season, sampling was also made during the previous wet season period in April 1992 for comparison; 10 vertical profiles were obtained using the same aircraft and measurement techniques. There is a clear difference between these two opposite seasonal periods, most evident in the O3 and CO data. Both Cuiabá and Porto Nacional show some 30–60 parts per billion by volume (ppbv) larger methane concentrations, for example, during the dry season, in comparison to the wet season, the difference at Cuiabá being larger. The methane data for the wet season show no significant differences between Cuiabá and Porto Nacional mixing ratios, which seems to exclude the existence of significant sources or sinks at these sites during this wet season. The ozone mixing ratios vary around 15 ± 5 ppbv in the wet season, and from a minimum of 35 to a maximum of 70 ± 10 ppbv, depending on height, in the dry season. The largest variability is seen in the carbon monoxide mixing ratios which vary from 90–100 ppbv in the wet season to maxima of 300 at 3.3 km and 600 ppbv at 1.2 km height in the dry season.