Modeling the dynamic behavior of a droplet evaporation device for the delivery of isotopically calibrated low-humidity water vapor

Erik Kerstel

doi:10.5194/amt-14-4657-2021

Abstract

Abstract. A model is presented that gives a quantitative description of the dynamic behavior of a low-humidity water vapor generator in terms of water vapor concentration (humidity) and isotope ratios. The generator is based on the evaporation of a nanoliter-sized droplet produced at the end of a syringe needle by balancing the inlet water flow and the evaporation of water from the droplet surface into a dry-air stream. The humidity level is adjusted by changing the speed of the high-precision syringe pump and, if needed, the dry-air flow. The generator was developed specifically for use with laser-based water isotope analyzers in Antarctica, and it was recently described in Leroy-Dos Santos et al. (2021). Apart from operating parameters such as temperature, pressure, and water and dry-air flows, the model has as “free” input parameters: water isotope fractionation factors and the evaporation rate. We show that the experimental data constrain these parameters to physically realistic values that are in reasonable to good agreement with available literature values. With the advent of new ultraprecise isotope ratio spectrometers, the approach used here may permit the measurement of not only the evaporation rate but also the effective fractionation factors and isotopologue-dependent diffusivity ratios, in the evaporation of small droplets.

Highlights

Water is arguably the most important molecule in Earth’s atmosphere
As we are interested in the dynamic behavior of the water vapor source that feeds the spectrometer, it is necessary to take the response time of the spectrometer into account
The standard waters used were working standards of the Groningen Center for Isotope Research (CIO), known as GS-48 (δ18O = −6.3 ‰, δ2H = −43 ‰) and BEW-2 (δ18O = 795 ‰, δ2H = 5983 ‰). Despite careful storage, these absolute isotopic compositions can no longer be guaranteed with the precision specified by the CIO, as the standards had previously been used for other experiments

Summary

Introduction

Water is arguably the most important molecule in Earth’s atmosphere. The large enthalpy change associated with the evaporation and condensation of water causes it to dominate the global redistribution of energy by tropospheric transport of latent heat. About 75 % of this temperature increase is generated by water vapor and clouds, as a feedback effect driven by the non-condensable greenhouse agents – foremost carbon dioxide (Lacis et al, 2010). This feedback effect, in turn, is a superposition of a multitude of large and (especially as clouds are involved) complex individual processes that partially cancel each other. Due to this complexity, water, in the form of water vapor as well as liquid- and crystal-phase water inside clouds, is by far the largest unknown in current climate models (IPCC, 2013). In the journal Nature, the climate researcher Gavin Schmidt called the water isotopes “the most super-duper fantastic thing ever” (Tollefson, 2008)

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Discussion

Conclusion