Age and gravitational separation of the stratospheric air over Indonesia

The gravitational separation of major atmospheric components, in addition to the age of air, would provide additional useful information about stratospheric circulation. However, observations of the age of air and gravitational separation are still geographically sparse, especially in the tropics. In order to address this issue, air samples were collected over Biak, Indonesia in February 2015 using four large plastic balloons, each loaded with two compact cryogenic samplers. With a vertical resolution of better than 2km, air samples from seven different altitudes were analyzed for CO₂ and SF₆ mole fractions, δ¹⁵N of N₂, δ¹⁸O of O₂, and δ(Ar•N2) to examine the vertically dependent age and gravitational separation of air in the tropical tropopause layer (TTL) and the equatorial stratosphere. By comparing their measured mole fractions with aircraft observations in the upper tropical troposphere, we have found that CO₂ and SF₆ ages increase gradually with increasing altitude from the TTL to 22km, and then rapidly from there up to 29km. The CO₂ and SF₆ ages agree well with each other in the TTL and in the lower stratosphere, but show a significant difference above 24km. The average values of δ¹⁵N of N2, δ¹⁸O of O₂, and δ(Ar•N2) all show a small but distinct upward decrease due to the gravitational separation effect. Simulations with a two-dimensional atmospheric transport model indicate that the gravitational separation effect decreases as tropical upwelling is enhanced. From the model calculations with enhanced eddy mixing, it is also found that the upward increase in air age is magnified by horizontal mixing. These model simulations also show that the gravitational separation effect remains relatively constant in the lower stratosphere. The results of this study strongly suggest that the gravitational separation, combined with the age of air, can be used to diagnose air transport processes in the stratosphere.