Abstract
The belief that the isotope ratio of air helium is globally constant has meant that air helium is used in most laboratories as a helium isotopic standard. The well accepted air 3He/4He ratio of (1.39±0.01) × 10-6 is an average of the two isotopic determinations by Mamyrin et al. (1970) and Clarke et al. (1976). We measured absolute atmospheric 3He/4He ratio, taking into account the separation of helium from neon and possible occurrence of the helium isotope fractionation during the purification procedure. The 3He/4He ratio of (1.343±0.013)×10-6 observed at Ueno Park, Central Tokyo, Japan is apparently lower than the previous data. The discrepancy may be attributed to either experimental problems or natural phenomenon.
