Abstract
Abstract. In this work, we present the first observations of stable water isotopologue ratios in cloud droplets of different sizes collected simultaneously. We address the question whether the isotope ratio of droplets in a liquid cloud varies as a function of droplet size. Samples were collected from a ground intercepted cloud (= fog) during the Hill Cap Cloud Thuringia 2010 campaign (HCCT-2010) using a three-stage Caltech Active Strand Cloud water Collector (CASCC). An instrument test revealed that no artificial isotopic fractionation occurs during sample collection with the CASCC. Furthermore, we could experimentally confirm the hypothesis that the δ values of cloud droplets of the relevant droplet sizes (μm-range) were not significantly different and thus can be assumed to be in isotopic equilibrium immediately with the surrounding water vapor. However, during the dissolution period of the cloud, when the supersaturation inside the cloud decreased and the cloud began to clear, differences in isotope ratios of the different droplet sizes tended to be larger. This is likely to result from the cloud's heterogeneity, implying that larger and smaller cloud droplets have been collected at different moments in time, delivering isotope ratios from different collection times.
Highlights
In order to use stable water isotopologues (1H216O, 1H2H16O, and 1H218O) as a tool to assess paleoclimatical (e.g. Dansgaard et al, 1993; Petit et al, 1999), ecological (e.g. Yakir and Sternberg, 2000; Farquhar et al, 2007) and hydrological questions (e.g. Dansgaard, 1964), a quantitative understanding of processes involving stable water isotopologues in the hydrosphere is needed
An important assumption used in cloud models is that the cloud droplets are in isotopic equilibrium with the surrounding water vapor, leading to cloud droplets which do not differ in isotope ratios for different droplet sizes (Jouzel, 1986)
liquid water content (LWC) is the sum of the volumes of cloud droplets per unit air volume, and Reff is defined as the radius yielding the same volume to surface ratio as the ambient cloud droplet size distribution
Summary
In order to use stable water isotopologues (1H216O, 1H2H16O, and 1H218O) as a tool to assess paleoclimatical (e.g. Dansgaard et al, 1993; Petit et al, 1999), ecological (e.g. Yakir and Sternberg, 2000; Farquhar et al, 2007) and hydrological questions (e.g. Dansgaard, 1964), a quantitative understanding of processes involving stable water isotopologues in the hydrosphere is needed. In recent years, modeling efforts focused on the explanation of observed trends of isotope ratios in precipitation, both on a local and a global scale (Lee and Fung, 2007; Risi et al, 2008), aiming for a better understanding of the “amount effect” This effect describes the on-going depletion of precipitation water in heavy isotopologues with increasing rain intensity. An important assumption used in cloud models is that the cloud droplets are in isotopic equilibrium with the surrounding water vapor, leading to cloud droplets which do not differ in isotope ratios for different droplet sizes (Jouzel, 1986) This assumption was first presented by Bolin (1958) who carried out a theoretical analysis of tritium isotope exchange between single freely falling rain droplets and their environment, based on the laboratory work by Kinzer and Gunn (1951).
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