We present climatological carbon dioxide (CO 2 ) distributions over the Asia-Pacific region obtained from the CONTRAIL (Comprehensive Observation Network for Trace gases by Airliner) measurements. The high-frequency in-flight CO 2 measurements over 10 years reveal a clear seasonal variation of CO 2 in the upper troposphere (UT), with a maximum occurring in April–May and a minimum in August–September. The CO 2 mole fraction in the UT north of 40° N is low and highly variable in June–August due to the arrival of air parcels with seasonally low CO 2 caused by the summertime biospheric uptake in boreal Eurasia. For August–September in particular, the UT CO 2 is noticeably low within the Asian summer monsoon anticyclone associated with the convective transport of strong biospheric CO 2 uptake signal over South Asia. During September as the anticyclone decays, a spreading of this low CO 2 area in the UT is observed in the vertical profiles of CO 2 over the Pacific Rim of the continental East Asia. Simulation results identify the influence of anthropogenic and biospheric CO 2 fluxes in the seasonal evolution of the spatial CO 2 distribution over the Asia-Pacific region. It is found, for example, that a substantial contribution to the UT CO 2 over the northwestern Pacific comes from the continental East Asian emissions in the spring, but switches to South Asian and/or Southeast Asian air masses affected dominantly by the biospheric CO 2 uptake in the summer monsoon season. The CONTRAIL CO 2 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.