Transport model diagnosis of the mean age of air derived from stratospheric samples in the tropics

Abstract. Stratospheric profiles of the mean age of air estimated from cryogenic air samples acquired during the CUBE/Biak field campaign over Indonesia are investigated with the aid of an atmospheric chemistry transport model nudged to ERA-Interim meteorological fields. Application of the boundary impulse response (BIR) method and Lagrangian backward trajectories to the transport field simulated by a single model prove useful in interpreting the observational results, which include discrepancies between CO₂- and SF₆-derived mean ages. This may be because the BIR method takes unresolved diffusive processes into account while the Lagrangian method distinguishes the pathways the air parcels have taken before reaching the sample site. The capability to estimate the vertical profiles of the clock tracer concentrations and the water vapor “tape recorder” is another advantage of the Lagrangian method, confirming the reality of the trajectory calculations. The profile of CO₂-mean age is reproduced reasonably well by trajectory-derived mean age, while BIR-derived mean age is much greater than CO₂ age at 28 and 29 km, possibly due to high diffusivity in the transport model. On the other hand, SF₆ age is reproducible only in the lower stratosphere, but far exceeds the trajectory-derived mean age above 25 km. As air parcels of mesospheric origin are missing in the Lagrangian age estimation, this discrepancy, together with the fact that the observed SF₆ concentrations are much lower than the trajectory-derived values in this height region, is consistent with the idea that the stratospheric air samples are mixed with SF₆-depleted mesospheric air, leading to overestimation of the mean age.