Measurements of atmospheric CO2 and CH4 using a commercial airliner from 1993 to 1994

A new automatic flask sampling system for the Boeing 747 commercial airliner was developed to observe CO2 and CH4 mixing ratios in the upper atmosphere at altitudes of 9-13 km. It was confirmed by a test flight that sample air collected using our system was useful for precise measurements of the trace gases in the upper atmosphere. Monthly air sampling was performed over the western Pacific between Narita in Japan and Cairns in Australia during 1993-1994. Measurements of both CO2 and CH4 in the Northern Hemisphere showed a clear seasonal cycle that was largely influenced by the seasonal variation in the lower troposphere. A significant decrease of mixing ratio during the winter season was observed in the CH4 variation, suggesting the intrusion of lower stratospheric air into the upper troposphere. The seasonal variation of both the gases gradually decayed toward the equator, but a different seasonal cycle appeared in the Southern Hemisphere. This change indicated the significance of meridional transport of both gases through the upper troposphere into the Southern Hemisphere. The mixing ratio level of both gases showed a recent increase in the upper troposphere.