Observed and modeled seasonal variation of 13 C, 18 O, and 14 C of atmospheric CO at Happo, a remote site in Japan, and a comparison with other records

Measurements of the 13 C/ 12 C and 18 O/ 16 O ratios and 14 C of atmospheric CO were carried out for 2 years (1997 and 1998) at Happo (36°41′N, 137°48′E, 1840 m above sea level) in Japan. This is the first measurement of time series for isotopic compositions of CO at a remote site in Asia. The seasonal cycle of the 18 O/ 16 O ratio shows a maximum of 10‰ (Vienna SMOW) in February and a minimum of 2‰ in July. This minimum value is heavier than the values reported for high latitude in the Northern Hemisphere, and this indicates that CO from fossil fuel combustion, which has a large 18 O/ 16 O ratio, affects midlatitude in the Northern Hemisphere considerably. On the contrary, the 13 C/ 12 C ratio shows a clear seasonal variation with little scatter; maximum −24.5‰ (Vienna Peedee belemnite) in April and minimum −28.5‰ in July‐August. The seasonal variation at Happo has different values and phases compared to those in the Southern Hemisphere and in the northern high latitude. 14 CO concentration at Happo is similar to that at high latitude in the Northern Hemisphere. A simple box model calculation is presented for these seasonal variations, and the model reproduces the observed seasonal cycles of CO concentration, 18 O/ 16 O ratio, and 14 CO concentration within the limitations of the simplified model, but not 13 C/ 12 C ratio. To reproduce the spring maximum of CO concentration, an enhanced CO production from biomass burning or other CO sources in spring are inferred in east Asia.