An experiment to determine atmospheric CO concentrations of tropical South Atlantic air samples

New observations of atmospheric carbon monoxide, CO, are described, from tropical South Atlantic air samples. A new observational site, Maxaranguape, was set up in a clean remote environment right next to the ocean on the north‐east coast of Brazil, to obtain CO mixing ratios and auxiliary data (meteorological parameters, ozone (O 3 ), carbon dioxide (CO 2 ) and methane (CH 4 )) during three sequential seasonal cycles. The seasonal variations of temperature, humidity and precipitation are shown for the new site. Chromatographic separation followed by mercury oxide detection is used to measure CO. The seasonality of the CO data was clearly established. Minima are seen during April, May and June showing wet‐period averages of 56.1 parts per billion by volume (ppbv), with standard deviation 8.7 ppbv; during dry‐period months, August to November, the average was 77.7±16.5 ppbv. For comparison, CO concentrations were also measured over continental areas in Brazil. Much larger values have been found in moderate ‘burning’ regions, such as the south of the state of Mato Grosso and the north‐western part of the state of Parana, where 200 ppbv in the dry season has been observed. Since normally the air masses have travelled for several days over the ocean, the air masses over the site present low chemical activity. Daily variations of CO 2 are very small, of the order of a few percent relative to the diurnal mean. Only on rare occasions, when the wind direction changes, is the sampled air contaminated from flowing over the inhabited shoreline to the south, and then CO 2 varies inversely with O 3 . The monthly mean CH 4 data does not show a clear seasonal variation, possibly because the amplitude of the CH 4 variation is only of the order of 1%, which is close to the precision of the measuring instrument. Copyright © 2003 Royal Meteorological Society