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 = 5080 m, pressure ≈ 550 hPa), has a mean deuterium excess (d‐excess = δD − 8*δ18O) of 46‰ ± 5‰ and frequently exceeds 100‰ at low water vapor mixing ratios (q ≤ 500 ppmv). 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. Key Points Measured d‐excess is controlled by in‐cloud processes and mixingRayleigh fractionation at RH = 105 to 130% largely controls observed d‐excessWater vapor undergoes mixing en route to Chajnantor