Vertical and meridional distributions of the atmospheric CO₂ mixing ratio between northern midlatitudes and southern subtropics

The atmospheric CO 2 mixing ratio was measured using a continuous measurement system onboard a Gulfstream‐II aircraft between the northern midlatitudes and the southern subtropics during the Biomass Burning and Lightning Experiment Phase A (BIBLE A) campaign in September–October 1998. The vertical distribution of CO 2 over tropical regions was almost constant from the surface to an altitude of 13 km. CO 2 enhancements from biomass burning and oceanic release were observed in the tropical boundary layer. Measurements in the upper troposphere indicate interhemispheric exchange was effectively suppressed between 2°N–7°N. Interhemispheric transport of air in the upper troposphere was suppressed effectively in this region. The CO 2 mixing ratios in the Northern and Southern Hemispheres were almost constant, with an average value of about 365 parts per million (ppm) and 366 ppm, respectively. The correlation between the CO 2 and NO y mixing ratios observed north of 7°N was apparently different from that obtained south of 2°N. This fact strongly supports the result that the north‐south boundary in the upper troposphere during BIBLE A was located around 2°N–7°N as the boundary is not necessary a permanent feature.