Deuterium excess in subtropical free troposphere water vapor: Continuous measurements from the Chajnantor Plateau, northern Chile

Water vapor measured continuously by cavity ring-down spectroscopy from July 2012 to March 2013 on the hyperarid Chajnantor Plateau, northern Chile (elevation=5080m, pressure approximate to 550hPa), has a mean deuterium excess (d-excess=D-8*O-18) of 465 and frequently exceeds 100 at low water vapor mixing ratios (q500ppmv). These measurements provide empirical support for theoretical predictions of free troposphere d-excess. The d-excess measured at this site can be understood in terms of supersaturation with respect to ice at relative humidities between 100% and 130%, followed by mixing with moist midtropospheric or lower tropospheric air en route to the plateau. The d-excess measured at Chajnantor is consistent with predictions for d-excess in the upper troposphere from isotope-enabled general circulation models and with high vapor saturation over ice in cloud-resolving and microphysical models.