[H-TT35_1PO1] Developments and applications of AMS techniques for earth and human environmental research

[HTT35-P12] Observations of atmospheric radiocarbon in carbon dioxide at Hateruma Island and Cape Ochi-ishi, Japan

*Yukio TERAO¹, Hitoshi MUKAI¹ (1.Center for Global Environmental Research, National Institute for Environmental Studies)

Atmospheric radiocarbon in carbon dioxide (¹⁴CO₂) is a powerful tracer for understanding of carbon cycles, e.g. oceanic and biospheric CO₂ exchanges and CO₂ emissions from fossil fuel combustion. Observation sites for radiocarbon concentrations, ∆¹⁴C, are not many enough to evaluate the global and regional carbon flux. We present an analysis of trends, interannual variability (IAV) and seasonal cycle of ¹⁴CO₂ in background air from July 2004 to December 2012 at two NIES/CGER monitoring stations; Hateruma Island (HAT; latitude 24.06N, longitude 123.81E) and Cape Ochi-ishi (COI; latitude 43.16N, longitude 145.50E). The air samples were collected in 2 L Pyrex glass flasks. The sampling frequency was monthly. CO₂ was extracted from the whole air at NIES and CO2 samples were converted to graphite and analyzed ratios of ¹⁴C/¹²C by accelerator mass spectrometry (AMS, National Electrostatics Corp., 1.5SDH) at Paleo Labo Co., Ltd., Japan. Analytical precision in ∆¹⁴C determined from statistical uncertainty (number of ¹⁴C counts) was ±1.7-2.0 ‰ for most samples. The repeatability of measurements using modern reference air was ±1.9 ‰ . A decreasing trend in ∆¹⁴C was -5‰ yr^-1 in average but large IAV was observed at both stations: large decreases in 2007-2008 and in 2010-2011 (-8 to -9 ‰ yr^-1) and almost zero decrease in 2009. We also observed clear seasonal cycle of ∆¹⁴C. The peak-to-peak amplitudes in the seasonal cycle determined from the smooth curve fits were 7‰ at both stations and the maximum of ∆¹⁴C appeared in July and the minimum in January at HAT, and the maximum in September and the minimum in May at COI. The differences in phase of ∆¹⁴C seasonal cycle between HAT and COI suggested that the atmospheric ∆¹⁴C at COI was influenced by CO₂ emitted from terrestrial biosphere.