[1] Temporal and latitudinal variations of vertical profiles of N 2 O isotopomers were observed in the stratosphere over Japan (39°N, 142°E), Sweden (68°N, 20°E) O 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 N 2 O 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 N 2 O, 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 N 2 O ''back-flux'' from the stratosphere were estimated. These values can be used in the isotopomeric mass balance model to constrain the global N 2 O budget.

Abstract. The vertical distribution of isotoporners ofN20 (14NISNI60, 15NI4NI60, and 14NI4NI80) in the lower and middle stratosphere was observed over Japan (39øN, 142øE) in 1999 using a balloon-borne cryogenic sampler and ground-based mass spectrometry. The abundance of the heavier isotoporners relative to 14NI4NI60 increased with altitude, while the mixing ratio of N20 decreased due to photochemical depletion. Maximum fractionation was observed at the highest altitude, 34.5 km, where (515NC•ai r (isotoporner ratio of center nitrogen), (515N13ai r (end nitrogen), and (•I8OsMow were 144.1%o, 42.4%0, and 119.0%o, 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 N20 isotopolners. However, the apparent fi'actionation factors in the lower (< •24 kin) 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 aft•ct the vertical profile, and (3) the relative contribution of photolysis and photo-oxidation to total N20 sink is possibly dependent on altitude.

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We carried out reactions of methyl-substituted cyclohexenes and α-pinene with ozone in air and elucidated the mechanisms of formation of the minor products (peroxides and formic acid). Peroxyacetic acid was formed only from the cyclohexenes with a methyl group on the double bond, whereas formic acid was produced in higher yields from the cyclohexenes without a methyl group on the double bond. These differences in product yields allowed us to elucidate the mechanism of formation of the products.