The influence of atmospheric
14CO
2 was evaluated on the determination of biogenic carbon ratios in industrial flue gases using accelerated mass spectrometry(AMS). Bioethanol, n-hexane, and their mixtures were combusted with a four-stroke engine, and
14CO
2 in exhaust gases was analyzed by AMS. The experimental biogenic carbon ratio determined by ASTM D6866 method was 1.2 times higher than the theoretical value of mixed fuel containing 3.18% biogenic carbons. In general, the influence of atmospheric
14CO
2 taken in combustion gases is neglected. It seems that the error cannot be neglected under international trading of emission allowances, where a large amount of carbons in the fuel were evaluated. The experimental value became to be the theoretical value by subtracting the amount of atmospheric
14C from that of the samples. As the contents of biofuel increased, the experimental biogenic carbon ratios reached the theoretical values and the influence of atmospheric
14CO
2 decreased. We recommend that the influence of atmospheric
14CO
2 should be corrected when fuel samples contain low amounts of
14C.
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