The radiocarbon concentrations in plant leaves from different altitudes at 3 sampling locations were measured with the new compact accelerator mass spectrometer (AMS) at Yamagata University to investigate air mixing on a global scale. The sampling locations are Yamagata in the mid-latitudes of the Northern Hemisphere (NH), Kenya in the equatorial region (EQ), and Chacaltaya in the Southern Hemisphere (SH). The 14C concentrations of the plant leaves ranged from 102 to 105 pMC. The 14C concentrations at high altitudes and mountain summits showed similar values of 104.2 ± 0.28, 104.3 ± 0.36, and 104.4 ± 0.23 pMC at the Yamagata, Kenya, and Chacaltaya sites, respectively. These results indicate that air from the free troposphere is well mixed on a global scale. The local Suess effect was calculated using the 14C concentrations of leaves at the land surface and mountain summits. The fractions were estimated as 1.25 ± 0.3% and 0.87 ± 0.44% at Yamagata and Nairobi, respectively. This estimation method is more advantageous than the conventional calculation. The life cycle of the leaves sampled is 1 or 2 yr, and hence the leaves allow us to study the 14C concentrations in the ambient atmosphere during a narrow and specific time period.