Aircraft Observation of CO₂, CO₂ O₃ and H₂ over the North Pacific during the PACE-7 Campaign

Aircraft observation under the Pacific Atmospheric Chemistry Experiment (PACE) program was performed from February 13 to 21, 2000 to examine in detail the distributions of CO₂ in the free troposphere between 5 and 11 km. Continuous measurements of CO₂ mixing ratios were made using an on-board measuring system over the northern North Pacific between Nagoya, Japan and Anchorage, Alaska, and the western North Pacific between Nagoya and Saipan. Other trace gases, such as CO and O₃, were also observed using continuous measuring systems at the same time. CO₂ over the northern Pacific (35°N and higher) showed highly variable mixing ratios, ranging from 374 ppm in the upper troposphere to 366 ppm in the lowermost stratosphere. This highly variable distribution of CO₂ was quite similar to that of CO, but the relationship between CO₂ and O₃ showed a strong negative correlation. These results indicated that the exchange process between the stratosphere and the troposphere significantly influences the large CO₂variation. On the other hand, the CO₂ over the western North Pacific to the south of Japan showed no significant variation in the upper troposphere at 11 km but a relatively larger variability at 5 km. The CO₂ enhancement at lower altitudes coincided with the CO elevation due to the intrusion of a polluted air mass. Trajectory analysis indicated that the Asian continental outflow perturbed the CO₂ distributions over the western Pacific. Very low mixing ratios of O₃ of less than 20 ppb were distributed in the latitude band of 15–30°N at 11 km, reflecting the effects of transport from the equatorial region.