Water vapor isotopic composition of a stratospheric air intrusion: Measurements from the Chajnantor Plateau, Chile

Measurements of water vapor isotopic composition in stratospheric air intrusions can be used to constrain the dilution of the intrusion as it mixes into the middle troposphere. The intrusion studied here occurred on 17 and 18 August 2012 with measurements obtained at an altitude of 5 km in the Chilean Andes at the Atacama Large Millimeter Array astronomical observatory on the Chajnantor Plateau. Surface ozone concentrations rose 16 ppb in 6 h and were associated with a potential vorticity intrusion on the 330 K isentropic surface. A simulated stratospheric ozone tracer reaching Chajnantor also supports the interpretation of a stratospheric intrusion. Beginning around 18:00 UTC on 17 August, the mixing ratio dropped from 3000 ppmv to 430 ppmv as the water vapor δ D values dropped from −153‰ to −438‰ over 13 h while the δ 18 O values dropped from −20‰ to −63‰. The average mixing ratio, δ D, and δ 18 O values during August 2012 were measured to be 1500 ppmv, −250‰, and −34‰, respectively. The minimum water vapor concentration during the intrusion was in the driest 5% of measurements made during that month, while the minimum δ D and δ 18 O values were within the lowest 0.5% of measurements made during that month. Simple two‐component models of mixing between stratospheric or upper tropospheric air with boundary layer air fail to reproduce observations, but a three‐component mixing model, in which the stratospheric intrusion mixes with an upper tropospheric background air mass, as it mixes with boundary layer air on Chajnantor, matches the observations.