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

Measurements of the ¹³C/¹²C and ¹⁸O/¹⁶O ratios and ¹⁴C 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 ¹⁸O/¹⁶O 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 ¹⁸O/¹⁶O ratio, affects midlatitude in the Northern Hemisphere considerably. On the contrary, the ¹³C/¹²C 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. ¹⁴CO 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, ¹⁸O/¹⁶O ratio, and ¹⁴CO concentration within the limitations of the simplified model, but not ¹³C/¹²C 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.