Airborne observations of the tropospheric CO 2 distribution and its controlling factors over the South Pacific Basin

Highly precise measurements of CO 2 mixing ratios were recorded aboard both the NASA DC‐8 and P3‐B aircraft during the Pacific Exploratory Mission‐Tropics conducted in August‐October 1996. Data were obtained at altitudes ranging from 0.1 to 12 km over a large portion of the South Pacific Basin representing the most geographically extensive CO 2 data set recorded in this region. These data along with CO 2 surface measurements from the National Oceanic and Atmospheric Administration/Climate Monitoring and Diagnostics Laboratory (NOAA/CMDL) and the National Institute of Water and Atmospheric Research (NIWA) were examined to establish vertical and meridional gradients. The CO 2 spatial distribution in the southern hemisphere appeared to be largely determined by interhemispheric transport as air masses with depleted CO 2 levels characteristic of northern hemispheric air were frequently observed south of the Intertropical Convergence Zone. However, regional processes also played a role in modulating background concentrations. Comparisons of CO 2 with other trace gases indicated that CO 2 values were influenced by continental sources. Large scale plumes from biomass burning activities produced enhanced CO 2 mixing ratios within the lower to midtroposphere over portions of the remote Pacific. An apparent CO 2 source was observed in the NOAA/CMDL surface data between 15°N and 15°S and in the lower altitude flight data between 8°N and 8.5°S with a zone of intensity from 6.5°N to 1°S. Inferred from these data is the presence of a Southern Ocean sink from south of 15°S having two distinct zones seasonally out of phase with one another.