M. H. Strong

Hydrogen isotope correction for laser instrument measurement bias at low water vapor concentration using conventional isotope analyses: application to measurements from Mauna Loa Observatory, Hawaii

The hydrogen and oxygen isotope ratios of water vapor can be measured with commercially available laser spectroscopy analyzers in real time. Operation of the laser systems in relatively dry air is difficult because measurements are non-linear as a function of humidity at low water concentrations. Here we use field-based sampling coupled with traditional mass spectrometry techniques for assessing linearity and calibrating laser spectroscopy systems at low water vapor concentrations. Air samples are collected in an evacuated 2 L glass flask and the water is separated from the non-condensable gases cryogenically. Approximately 2 µL of water are reduced to H(2) gas and measured on an isotope ratio mass spectrometer. In a field experiment at the Mauna Loa Observatory (MLO), we ran Picarro and Los Gatos Research (LGR) laser analyzers for a period of 25 days in addition to periodic sample collection in evacuated flasks. When the two laser systems are corrected to the flask data, they are strongly coincident over the entire 25 days. The δ(2)H values were found to change by over 200‰ over 2.5 min as the boundary layer elevation changed relative to MLO. The δ(2)H values ranged from -106 to -332‰, and the δ(18)O values (uncorrected) ranged from -12 to -50‰. Raw data from laser analyzers in environments with low water vapor concentrations can be normalized to the international V-SMOW scale by calibration to the flask data measured conventionally. Bias correction is especially critical for the accurate determination of deuterium excess in dry air.

Measurements of water vapor D/H ratios from Mauna Kea, Hawaii, and implications for subtropical humidity dynamics

[1] Water vapor D/H ratios were measured from samples collected on Mauna Kea, Hawaii, in July 2006, and provide new constraints on the processes that control subtropical humidity. D/H ratios ranged from -88%o at sea level to -321‰ on the summit of Mauna Kea, with sharply decreased D/H ratios above the trade inversion. A simple Rayleigh distillation model underpredicts the observed clear-sky D/H ratios by as much as 160%o at the summit. A model that accounts for large-scale condensation, fractionation, mixing, and transport of water vapor, but ignores more detailed microphysical processes, is able to reproduce the first-order characteristics of the clear-sky free troposphere relative humidity and D/H ratios. These results are consistent with remote sensing studies of subtropical D/H ratios and suggest that subtropical clear-sky water vapor isotopologues may be relatively insensitive to microphysical processes.