Isotope Fractionation of Water during Evaporation without Condensation

Abstract

The microscopic events engendering liquid water evaporation have received much attention over the last century, but remain incompletely understood. We present measurements of isotope fractionation occurring during free molecular evaporation from liquid microjets and show that the isotope ratios of evaporating molecules exhibit dramatic differences from equilibrium vapor values, strong variations with the solution deuterium mole fraction, and a clear temperature dependence. These results indicate the existence of an energetic barrier to evaporation and that the evaporation coefficient of water is less than unity. These new insights into water evaporation promise to advance our understanding of the processes that control the formation and lifetime of clouds in the atmosphere.

Determination of the evaporation coefficient of D<sub>2</sub>O

“…The apparent lack of a temperature dependence to γ e observed in this study and that of Smith et al (Smith et al, 2006) may appear to be in contrast to an earlier study by Cappa et al (Cappa et al, 2005) wherein a stronger temperature dependence to γ e was suggested. As noted in that work, and discussed in a later publication (Cappa et al, 2007), the prediction involved several assumptions and high uncertainty.…”

“…In a previous work (Cappa et al, 2005) we discussed a possible source of discrepancy between our experiments and those of Li et al (2001), claiming that our formulations of γ e and α m are different, with γ e ∼(1−α m ). We now recognize that this prior analysis was incorrect and resulted from equating two different rates in formulations of the evaporation and mass accommodation processes which are not equivalent.…”

“…This velocity is calculated from the liquid flowrate and the orifice size. As described previously, the orifice size is determined by measuring the liquid jet diameter immediately after the nozzle using Mie scattering with the VOAG turned off (Cappa et al, 2005). The initial temperature of the droplets was determined by collecting the Raman spectrum of the droplet train in ambient air, where evaporative cooling is minimal (Smith et al, 2006).…”

“…The liquid jets and droplets also provide a renewing surface, minimizing contamination issues. Measurements of isotopic ratios in evaporation between 264 and 295 K showed that γ e <1 and that it varied with the H/D ratio in the liquid (Cappa et al, 2005). Using Raman thermometry we derived a precise value of γ e from the temperature change associated evaporation of pure H 2 O, yielding a value of 0.62±0.09 over a temperature range of 245-295 K (Smith et al, 2006).…”

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“…Unfortunately, although as expected all studies give values less than unity, there have been widely different results reported for these coefficients. Cappa et al (2005) showed that the evaporation coefficient is less than unity and decreases with temperature, with the activation energy being about 10 kJ mol -1 greater than the enthalpy of evaporation. This is evidence that there is a considerable barrier to free evaporation and therefore condensation, indicating considerable structuring within this surface water.…”