Hydrogen isotope measurement corrections for low water vapor concentrations: Mauna Loa Observatory, Hawaii

We present a comparison of laser spectroscopy measurements and traditional mass spectrometry techniques for measuring the hydrogen isotope composition of atmospheric water vapor. Laboratory experiments indicate that the Picarro gas analyzer has a negative relationship between inverse concentration and D/H ratio, so flask measurements were used to calibrate the laser spectroscopy systems at low water vapor concentrations and assess linearity. Field-based measurements made at the Mauna Loa Observatory (MLO) in Hawaii included data collection from both Picarro and Los Gatos Research laser analyzers and periodic sample collection in evacuated flasks for a period of 25 days. Air samples were collected in evacuated 2 L glass flasks and water vapor was separated from the non-condensable gases cryogenically. The remaining water was reduced to H2 gas and measured on an isotope ratio mass spectrometer. When the two laser systems are corrected to the flask data, they are strongly coincident over the entire 25 day sample period. Corrected δD values ranged from -106‰ to -332‰. The δD values of atmospheric water vapor changed by 200‰ within 2.5 minutes as the boundary layer elevation changed relative to MLO. This study demonstrates that field campaigns using laser analyzers in environments with low water vapor concentrations can be corrected to the international V-SMOW scale by calibration to the flask data measured conventionally. Bias correction is especially critical for accurate determination of deuterium excess in dry air.