Seasonal and Secular Variations of Atmospheric 14CO2 Over the Western Pacific Since 1994

Abstract

Air sample collections over the western Pacific have continued since 1992 as a part of Center for Global Environmental Research, National Institute for Environmental Studies (CGER-NIES) global environmental monitoring program. The air samples collected on the Japan-Australia transect made it possible to trace the seasonal and secular 14CO2 variations, as well as an increasing trend of greenhouse gases over the western Pacific. A subset of CO2 samples from latitudes of 10-15 degrees N and 23-28 degrees S were chosen for accelerator mass spectrometry (AMS) 14C analysis using a NIES-TERRA AMS with a 0.3-0.4% precision. These 14CO2 records in maritime air show seasonal variations superimposed on normal exponential decreasing trends with a time constant of about 16 yr. The Delta-14C values in the Northern Hemisphere are lower those in the Southern Hemisphere by 3-4 per mil during 1994-2002. The Northern Hemisphere record shows relatively high seasonality (2.3 +/1.5 per mil) as compared with the Southern Hemisphere (1.3 +/1.2 per mil). The maximum values of seasonal cycles appear in late autumn and early winter in the Northern and Southern Hemispheres, respectively. Oscillations of 1-10 yr over the western Pacific are found to correlate possibly with the El Nino/Southern Oscillation (ENSO) events.