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 stratos
phere was observed over Japan (39 degreesN, 142 degreesE) in 1999 using a b
alloon-borne cryogenic sampler and ground-based mass spectrometry. The abun
dance 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 photochemica
l depletion. Maximum fractionation was observed at the highest altitude, 34
.5 km, where delta 15N(air)(alpha) (isotopomer ratio of center nitrogen), d
elta 15N(air)(beta) (end nitrogen), and delta O-18(SMOW) were 144.1 parts p
er thousand, 42.4 parts per thousand, and 119.0 parts per thousand, respect
ively. The observed distribution is mostly accounted for by isotopic fracti
onation during consumption processes, which is in accordance with reported
simulation experiments and theoretical prediction for photolytic fractionat
ion of N2O isotopomers. However, the apparent fractionation factors in the
lower (< <similar to>24 km) and higher regions are different, which suggest
s that (1) the fractionation factor for stratospheric photolysis may depend
on altitude or latitude, (2) transport and mixing processes in the stratos
phere can affect the vertical profile, and (3) the relative contribution of
photolysis and photo-oxidation to total N2O sink is possibly dependent on
altitude.