Fractionation of N2O isotopomers in the stratosphere

The vertical distribution of isotopomers of N2O ((NNO)-N-14-N-15-O-16, (NNO)-N-15-N-14-O-16, and (NNO)-N-14-N-14-O-18) in the lower and middle stratosphere was observed over Japan (39 degreesN, 142 degreesE) in 1999 using a balloon-borne cryogenic sampler and ground-based mass spectrometry. The abundance of the heavier isotopomers relative to (NNO)-N-14-N-14-O-16 increased with altitude, while the mixing ratio of N2O decreased due to photochemical depletion. Maximum fractionation was observed at the highest altitude, 34.5 km, where delta 15N(air)(alpha) (isotopomer ratio of center nitrogen), delta 15N(air)(beta) (end nitrogen), and delta O-18(SMOW) were 144.1 parts per thousand, 42.4 parts per thousand, and 119.0 parts per thousand, 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 N2O isotopomers. However, the apparent fractionation factors in the lower (< <similar to>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 N2O sink is possibly dependent on altitude.