Age of air and gravitational separation in the stratosphere over Indonesia

Stratospheric air collections were carried out at Biak, Indonesia in February 2015, by using the compact cryogenic air sampler. Eight sets of air sampler were launched from the experiment field in LAPAN observatory (001°10'32" S, 136° 06'02" E) using 4 large plastic balloons. Air samples were analyzed for concentrations of CO2, CH4, N2O, and SF6. In this study, we estimated the mean age of air in the tropical stratosphere over Biak, and compared them with the results obtained from the previous experiments at Japan. CO2-age and SF6-age were estimated by comparing the data records in the tropical troposphere. As a result, the mean age of air was estimated to be about 3.1±0.3 years at 29 km altitude. This value was significantly lower than those obtained from the satellite SF6 measurements. The collected air samples were also analyzed for d15N of N2, d18O of O2, d(Ar/N2), and d40Ar by using a mass spectrometer. Each d value was normalized to the values corresponding to a mass difference of 1 amu. Average values of normalized d showed small but significant decrease with increasing altitude, probably due to gravitational separation. A 2-dimensional model was used to evaluate the basic structures of age and gravitational separation in the stratosphere. The model-simulated age and gravitational separation were significantly underestimated compared with observed values. Therefore, the meridional mass stream function was adjusted so that the age in the equatorial stratosphere became close to the results observed over Indonesia. However, the model-calculated gravitational separation became overestimated. This discrepancy suggests that the relative contributions of mean circulation and mixing process are different between age and gravitational separation. The contribution of horizontal mixing was also examined by calculating with enhanced eddy mixing condition. As a result, age increased further with increasing altitude, due to additional aging by horizontal mixing. However, gravitational separation didn't show a significant increase, especially in the lower stratosphere. These results suggest that gravitational separation is sensitive to mean circulation change, but not to horizontal mixing change.