Age and gravitational separation of the stratospheric air over Indonesia

Abstract. Stratospheric 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 2 km, 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 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 observation in the upper tropical troposphere, we have found that CO2 and SF6 age increases gradually with increasing altitude from the TTL to 22 km, and then rapidly from there up to 25 km. The CO2 and SF6 age agrees well with each other in the TTL and in the lower stratosphere, but shows a significant difference above 24 km. The average values of δ15N of N2, δ18O of O2, and δ(Ar/N2) all show 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.