Spatial Distributions of Tropospheric Carbon Dioxide Over the Western North Pacific During Winter and Spring.

Atmospheric CO2 concentration was observed using a research aircraft (Gulfstream II) over the western North Pacific during the Pacific Exploration of Asian Continental Emission (PEACE) phase A (6 - 22 January 2002) and phase B (20 April - 16 May 2002). 13 and 12 flights were conducted during phase A and B, respectively, with its latitude range of 22 - 42 north latitude and maximum altitude of about 13 km. The primary objective of PEACE is to investigate chemical and transport processes of the continental outflow over the western North Pacific. Quantitative understanding of CO2 spatial and temporal distribution is important for understanding global carbon cycle, as its vertical distribution and seasonal change is scarcely revealed. The atmospheric air is taken and pressurized by a diaphragm pump and dried by a Nafion drier and a chemical desiccant column (Mg(ClO4)2). In situ measurement was carried out on board the aircraft using Li-Cor model 6262 non-dispersive inflared analyzer, and the CO2 concentration is determined against the high and low CO2-in-air standard gases based on the NIES (National Institute for Environmental Studies) standard scale. The response time is about 6 seconds, and the signal noise is in about +- 0.1 ppmv. The observed CO2 concentration is generally high in low altitude and low in high altitude. High CO2 concentration relative to the average CO2 distribution is sometimes observed during the flights. Its difference is about 8 ppmv at most. Trajectory analysis suggests that the observed air with high CO2 concentration is often affected by continental outflow. The averaged CO2 vertical distribution shows seasonal difference. The CO2 concentration decreases with altitude in winter at all latitude, however the CO2 concentration observed over 2.0 km at north of 25 north latitude in spring is almost constant. These differences are considered to be principally induced by phase delay of atmospheric CO2 change from the boundary layer to upper troposphere. Latitudinal difference of CO2 concentration at selected altitude band is also revealed during winter and spring. In winter, CO2 concentration generally increases with latitude at all altitude band, but in spring, highest CO2 concentration is observed between 25 - 40 north latitude, and the concentration is relatively low in high latitude.