The National Institute for Environmental Studies (NIES), Japan, and Meteorological Research Institute (MRI), Japan have been conducting an atmospheric measurement project called CONTRAIL (Comprehensive Observation Network for Trace gases by Airliner). This is the world's first program that measures atmospheric CO 2 concentrations continuously on board passenger aircraft. Japan Airlines' (JAL) aircraft regularly carry a Continuous CO 2 Measuring Equipment (CME), which measures atmospheric CO 2 concentrations continuously from take-off to touch down along the flight track. Since the project started in 2005, over 7 millions of CO 2 data from over 12 thousands of flights have been collected worldwide, which enabled us to explore detailed spatiotemporal variations of atmospheric CO 2 over Asia Pacific, the region that has been only sparsely monitored for CO 2 concentration.

The study was published in Atmospheric Chemistry and Physics on October 17, 2018.

Atmospheric CO 2 concentration is increasing and actions for climate change need accurate knowledge of the global carbon cycle. Asia has become increasingly important in the global carbon budget by rapidly growing economy. There are still large uncertainties in the estimates of CO 2 emissions from human-related sectors and emissions and uptakes by natural vegetation in Asia. Atmospheric measurements have been useful information since abundance and gradient of CO 2 in the air are essentially determined by spatiotemporal distributions and intensities of emissions and uptakes and by atmospheric transport. It has been however a challenge to establish a well-facilitated monitoring network across Asia. Commercial airliner is one practical solution, since it provides regular and long-term opportunities of flying measurements over different countries worldwide.

This study analysed 10 years of the CONTRAIL CO 2 measurement data taken over Asia Pacific. From ground-based global atmospheric monitoring networks, it is now well known that atmospheric CO 2 undergoes clear seasonal variation with summertime depletion and springtime elevation (in the Northern Hemisphere) due to biospheric activity (photosynthesis and respiration), and thus northern biosphere is well recognized as a driver of seasonal cycles of atmospheric CO 2 . The CONTRAIL data revealed clear seasonality of CO 2 over Asia that varies with latitude, longitude and altitude. In particular, we observed distinct depletion of CO 2 concentration over South Asia to Southeast Asia in August to September. The low CO 2 area was found to be confined within the Asian summer monsoon anticyclone -- persistent anticyclonic circulation in upper layers of the atmosphere (above 10 km altitude) associated with the seasonally varying monsoon regime -- and to be imprinted by strong CO 2 uptake by vegetation in South Asia. The Asian summer monsoon meteorology efficiently conveys signals from South Asian ground vegetation upward and it propagates eventually out to the Pacific after being trapped within the anticyclone. Seasonal evolutions of CO 2 uptakes in South Asia and of dynamical development and decay of the Asian summer monsoon anticyclone have remarkable impact in distributing atmospheric CO 2 over Asia and to the Pacific.