Seasonal evaluation of tropospheric CO ₂ over the Asia-Pacific region observed by the CONTRAIL commercial airliner measurements

Measurement of atmospheric carbon dioxide (inline-formulaCO₂) is indispensable for top-down estimation of surface inline-formulaCO₂ sources/sinks by an atmospheric transport model. Despite the growing importance of Asia in the global carbon budget, the region has only been sparsely monitored for atmospheric inline-formulaCO₂ and our understanding of atmospheric inline-formulaCO₂ variations in the region (and thereby that of the regional carbon budget) is still limited. In this study, we present climatological inline-formulaCO₂ distributions over the Asia-Pacific region obtained from the CONTRAIL (Comprehensive Observation Network for TRace gases by AIrLiner) measurements. The high-frequency in-flight inline-formulaCO₂ measurements over 10 years reveal a clear seasonal variation in inline-formulaCO₂ in the upper troposphere (UT), with a maximum occurring in April–May and a minimum in August–September. The inline-formulaCO₂ mole fraction in the UT north of 40inline-formula^∘ N is low and highly variable in June–August due to the arrival of air parcels with seasonally low inline-formulaCO₂ caused by the summertime biospheric uptake in boreal Eurasia. For August–September in particular, the UT inline-formulaCO₂ is noticeably low within the Asian summer monsoon anticyclone associated with the convective transport of strong biospheric inline-formulaCO₂ uptake signal over South Asia. During September as the anticyclone decays, a spreading of this low-inline-formulaCO₂ area in the UT is observed in the vertical profiles of inline-formulaCO₂ over the Pacific Rim of continental East Asia. Simulation results identify the influence of anthropogenic and biospheric inline-formulaCO₂ fluxes in the seasonal evolution of the spatial inline-formulaCO₂ distribution over the Asia-Pacific region. It is inferred that a substantial contribution to the UT inline-formulaCO₂ over the northwestern Pacific comes from continental East Asian emissions in spring; but in the summer monsoon season, the prominent air mass origin switches to South Asia and/or Southeast Asia with a distinct imprint of the biospheric inline-formulaCO₂ uptake. The CONTRAIL inline-formulaCO₂ data provide useful constraints to model estimates of surface fluxes and to the evaluation of the satellite observations, in particular for the Asia-Pacific region.