Fractionation of N₂O isotopomers in the stratosphere

Abstract

The vertical distribution of isotopomers of N 2 O ( 14 N 15 N 16 O, 15 N 14 N 16 O, and 14 N 14 N 18 O) in the lower and middle stratosphere was observed over Japan (39°N, 142°E) in 1999 using a balloon‐borne cryogenic sampler and ground‐based mass spectrometry. The abundance of the heavier isotopomers relative to 14 N 14 N 16 O increased with altitude, while the mixing ratio of N 2 O decreased due to photochemical depletion. Maximum fractionation was observed at the highest altitude, 34.5 km, where δ 15 N α air (isotopomer ratio of center nitrogen), δ 15 N β air (end nitrogen), and δ 18 O SMOW were 144.l‰, 42.4‰, and 119.0‰, respectively. The observed distribution is mostly accounted for by isotopic fractionation during consumption processes, which is in accordance with reported simulation experiments and theoretical prediction for photolytic fractionation of N 2 O isotopomers. However, the apparent fractionation factors in the lower (<∼24 km) and higher regions are different, which suggests that (1) the fractionation factor for stratospheric photolysis may depend on altitude or latitude, (2) transport and mixing processes in the stratosphere can affect the vertical profile, and (3) the relative contribution of photolysis and photo‐oxidation to total N 2 O sink is possibly dependent on altitude.