Research: Seasonality of CO2 under the monsoon meteorological regime —

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

Atmospheric CO₂ 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 CO₂ 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 CO₂ 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 CO₂ measurement data taken over Asia Pacific. From ground-based global atmospheric monitoring networks, it is now well known that atmospheric CO₂ 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 CO₂. The CONTRAIL data revealed clear seasonality of CO₂ over Asia that varies with latitude, longitude and altitude. In particular, we observed distinct depletion of CO₂ concentration over South Asia to Southeast Asia in August to September. The low CO₂ 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 CO₂ 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 CO₂ uptakes in South Asia and of dynamical development and decay of the Asian summer monsoon anticyclone have remarkable impact in distributing atmospheric CO₂ over Asia and to the Pacific.

The CONTRAIL commercial airliner measurements provide high-frequency CO₂ data over regions under-sampled by the current world atmospheric monitoring networks. The CONTRAIL datasets will complement measurements at other observational platforms (ground stations and satellites) and will be of increasing help to better understand carbon cycles and atmospheric transport of trace gases important in climate change and air pollution.

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Journal Reference:

1. Taku Umezawa, Hidekazu Matsueda, Yousuke Sawa, Yosuke Niwa, Toshinobu Machida, Lingxi Zhou. Seasonal evaluation of tropospheric CO₂ over the Asia-Pacific region observed by the CONTRAIL commercial airliner measurements. Atmospheric Chemistry and Physics, 2018; 18 (20): 14851 DOI: 10.5194/acp-18-14851-2018