| Summary: | The isotopic composition of water vapor is widely used in atmospheric science as a tracer for the hydrological cycle, because of the preferential condensation of the heavier isotopologues. However, isotopic fractionation factors between water vapor and ice have not previously been directly measured at temperatures and in conditions characteristic of most of the atmosphere. Measurements have been made at comparatively warm temperatures > 235 K, and therefore do not sample conditions under which cirrus (ice) clouds form. Climate models that incorporate water isotopes use values extrapolated from measurements made at warmer temperatures (Merlivat and Nief, 1967). Furthermore, ice growth in the atmosphere often occurs in supersaturated conditions where effective isotopic fractionation should be suppressed from the equilibrium case, but the commonly-assumed diffusive model of Jouzel and Merlivat, 1984 has never been quantitatively validated by laboratory measurements. More recent theoretical studies have suggested that additional surface attachment kinetic effects could significantly alter the diffusive model (Nelson, 2011). Finally, recent in-situ observations in the upper troposphere and lower stratosphere have led to the suggestion that at very cold temperatures, ice growth is also significantly suppressed by surface effects: measurements within cirrus clouds < 205 K showed anomalously high supersaturation. Although various mechanisms to explain this high supersaturation in the presence of ice crystals have been proposed, a definitive explanation has not yet been identified.
I show results from the IsoCloud campaigns at the AIDA Aerosol and Cloud Chamber, which investigated isotopic fractionation and ice growth in atmospherically relevant conditions by simulating cirrus clouds at temperatures and pressures characteristic of the upper troposphere. The IsoCloud campaigns involved a series of adiabatic expansion experiments at temperatures between 190 and 233 K, during which multiple in-situ and extractive instruments measured water vapor, total water, ice particle number density, and water vapor isotopic composition. This analysis uses isotopic composition measurements from the Chi-WIS, a new mid-IR tunable diode laser absorption spectrometer developed specifically for the IsoCloud campaigns to measure the evolving isotopic ratio of HDO to H2O in the vapor phase. I describe its design and performance in AIDA, and use its measurements to determine the degree of preferential condensation of HDO over H2O over this temperature range.
Results show an equilibrium isotopic fractionation factor lower (1-3%) than the extrapolated relationship of Merlivat and Nief in 1967, and far below that of Ellehoj et al., 2013. I also use ice growth experiments at a variety of supersaturations to provide direct tests of kinetic isotope effects resulting from vapor deposition in supersaturated conditions. I show that isotopic fractionation is reduced in supersaturated conditions consistently with the model of Jouzel and Merlivat, 1984. The IsoCloud experiments, with small ice crystals (radii <10 um), should be sensitive to surface kinetic effects. I show these effects are constrained to be small. Finally, I show that the ice growth deposition coefficient for cirrus conditions shows no temperature dependence, i.e. I find no suggestion of a suppression of ice growth at cold temperatures.
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