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Volume 12, issue 20
Atmos. Chem. Phys., 12, 9855–9863, 2012
https://doi.org/10.5194/acp-12-9855-2012
© Author(s) 2012. This work is distributed under
the Creative Commons Attribution 3.0 License.

Special issue: HCCT-2010: a complex ground-based experiment on aerosol-cloud...

Atmos. Chem. Phys., 12, 9855–9863, 2012
https://doi.org/10.5194/acp-12-9855-2012
© Author(s) 2012. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 29 Oct 2012

Research article | 29 Oct 2012

Stable water isotopologue ratios in fog and cloud droplets of liquid clouds are not size-dependent

J. K. Spiegel1, F. Aemisegger2, M. Scholl3, F. G. Wienhold2, J. L. Collett Jr.4, T. Lee4, D. van Pinxteren5, S. Mertes5, A. Tilgner5, H. Herrmann5, R. A. Werner1, N. Buchmann1, and W. Eugster1 J. K. Spiegel et al.
  • 1Institute of Agricultural Sciences, ETH Zurich, Zurich, Switzerland
  • 2Institute for Atmospheric and Climate Science, ETH Zurich, Zurich, Switzerland
  • 3US Geological Survey, Reston, VA, USA
  • 4Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado, USA
  • 5Leibniz Institute for Tropospheric Research, Leipzig, Germany

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.

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