Temporal and latitudinal distributions of stratospheric N₂O isotopomers

Temporal and latitudinal variations of vertical profiles of N₂O isotopomers were observed in the stratosphere over Japan (39°N, 142°E), Sweden (68°N, 20°E), and Antarctica (69°N, 40°E) during the period between 1990 and 2001. Samples were collected with a balloon-borne cryogenic sampler and analyzed by mass spectrometry in the laboratory. Observed enrichment factors for heavier isotopomers (¹⁵N¹⁴N¹⁶O₂, ¹⁴N¹⁵N¹⁶O, and ¹⁴N¹⁴N¹⁸O) relative to ¹⁴N¹⁴N¹⁶O were nearly constant in the lower stratosphere (less than ∼22 km) but increased at higher altitudes (∼22-35 km) while showing seasonal and latitudinal differences. Enrichment factors during the photolysis and photo-oxidation of N₂O were also obtained in laboratory experiments and compared with those observed. We found that in the higher-altitude region (1) fractionation of the isotopomers is mainly determined by photolysis, but is also affected by physical processes, (2 subsidence of air masses in the winter polar vortex induces the intrusion of an upper stratospheric air mass depleted in N₂O, and (3) decay of the vortex in the local spring leads to rapid horizontal advection of midlatitude air masses. At lower altitudes, isotopomer ratios are determined by photolysis, photo-oxidation, and the mixing of air masses within the stratosphere or between the stratosphere and the troposphere. Secular trend of isotopomer profiles was not detectable over Japan during 11 years. Assuming that the lower stratospheric air over midlatitudes is exchanged with the troposphere, isotopomer ratios of the N₂O "back-flux" from the stratosphere were estimated. These values can be used in the isotopomeric mass balance model to constrain the global N₂O budget.