Abstract
Abstract. As a basic understanding of the dynamics of the atmospheric circulation, it has been believed that gravitational separation of atmospheric components is observable only in the atmosphere above the turbopause. However, we found, from our high-precision measurements of not only the isotopic ratios of N2, O2 and Ar but also the mole fraction of Ar, that gravitational separation occurs even in the stratosphere below the turbopause; their observed vertical profiles are in good agreement with those expected theoretically from molecular mass differences. The O2/N2 ratio observed in the middle stratosphere, corrected for gravitational separation, showed the same mean air age as estimated from the CO2 mole fraction. Simulations with a 2-dimensional model of the middle atmosphere indicated that a relationship between gravitational separation and the age of air in the stratosphere would be significantly affected if the Brewer–Dobson circulation was enhanced due to global warming. Therefore, gravitational separation is usable as a new indicator of changes in the atmospheric circulation in the stratosphere.
Highlights
Our air sampler consists of 12 stainless-steel sample containers, a liquid helium dewar, a receiver, a transmitter, a control unit and batteries; all these components are housed in a water- and pressureproof aluminum chamber (Honda, 1990; Honda et al, 1996)
To detect gravitational separation in the stratosphere, the stratospheric air samples collected over Japan were analyzed for δ(15 N) of N2, δ(18 O) of O2, δ(O2 /N2 ), δ(Ar/N2 )
Of N2 were (2.1 ± 0.2), (11.9 ± 1.4) and (4.2 ± 0.6) per meg per meg−1, respectively. These ratios are consistent with the corresponding values of 2, 12 and 4 per meg per meg−1 expected from gravitational separation, but are clearly different from (1.55 ± 0.02), (16.2 ± 0.1) and (2.75 ± 0.05) per meg per meg−1 determined experimentally for a possible thermal diffusion effect at the air intake of the cryogenic air sampler
Summary
The collection of stratospheric air has been carried out using a balloon-borne cryogenic air sampler over Sanriku (39◦ N, 142◦ E) and Taiki (43◦ N, 143◦ E), Japan, since 1985 (e.g., Nakazawa et al, 1995; Aoki et al, 2003). Our air sampler consists of 12 stainless-steel sample containers, a liquid helium dewar, a receiver, a transmitter, a control unit and batteries; all these components are housed in a water- and pressureproof aluminum chamber (Honda, 1990; Honda et al, 1996). The other end of the motor-driven valve is connected to a sample intake, located 3.5 m below the bottom of the aluminum chamber, through a manifold and a stainless-steel bellows tube with an inner diameter of 15 mm reinforced with mesh. Air samples were collected in the containers at assigned altitudes by opening and closing the motor-driven valves using a telecommand system. The flow rates of sample air at the respective altitudes were 50–100 L min−1 at ambient pressures, and typical amounts of air samples collected were about 25 L at standard temperature (0 ◦ C) and pressure (1013.25 hPa)
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