Surface measurements of upper tropospheric water vapor isotopic composition on the Chajnantor Plateau, Chile

Simultaneous, real-time measurements of atmospheric water vapor mixing ratio and isotopic composition (delta D and delta O-18) were obtained using cavity ringdown spectroscopy on the arid Chajnantor Plateau in the subtropical Chilean Andes (elevation 5080 m or 550 hPa; latitude 23 degrees S) during July and August 2010. The measurements show surface water vapor mixing ratio as low as 215 ppmv, delta D values as low as -540 parts per thousand, and delta O-18 values as low as -68 parts per thousand, which are the lowest atmospheric water vapor delta values reported from Earth's surface. The results are consistent with previous measurements from the base of the tropical tropopause layer (TTL) and suggest large-scale subsidence of air masses from the upper troposphere to the Earth's surface. The range of measurements is consistent with condensation under conditions of ice supersaturation and mixing with moister air from the lower troposphere that has been processed through shallow convection. Diagnostics using reanalysis data show that the extreme aridity of the Chajnantor Plateau is controlled by condensation in the upper tropical troposphere. Citation: Galewsky, J., C. Rella, Z. Sharp, K. Samuels, and D. Ward (2011), Surface measurements of upper tropospheric water vapor isotopic composition on the Chajnantor Plateau, Chile, Geophys. Res. Lett., 38, L17803, doi:10.1029/2011GL048557.