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

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.

Temporal evolution of stable water isotopologues in cloud droplets in a hill cap cloud in central Europe (HCCT-2010)

“…Cloud water isotopic signatures could be related to the water vapor source (Scholl et al, 2007). Moreover, Spiegel et al (2012) experimentally confirmed that fog droplets do not differ in isotopic composition with respect to droplet size. As most of the studies did not resolve the temporal trends during a single cloud event, it remains unclear, whether the isotopic composition of cloud water stays constant over time or is subject to large changes that have not been identified yet.…”

“…For details on collection and sampling techniques, we refer to Spiegel et al (2012) and references therein. For the analysis in this work, we use the volume weighted mean values of the three stages from the CASCC.…”

“…Meteorological soundings: Meteorological soundings were launched in Meiningen 30 km South West (for location of the site see Fig. 1) at 00:00 UTC and 12:00 UTC by the German Weather Service (DWD) (Heinold et al, 2005;Tilgner et al, 2012). From these measurements, the dimensionless Froude number was deduced to characterize the overflow over the mountain ridge (Pierrehumbert and Wyman, 1985): .…”

“…We used synoptic charts from the German Weather Service (DWD) with a time resolution of 6 h for the air mass classification and identification of frontal passages (for details see Tilgner et al, 2012).…”

Chemical Composition and Bacterial Community in Size-Resolved Cloud Water at the Summit of Mt. Tai, China

“…Size-resolved cloud water samples were collected using a three-stage Caltech Active Strand Cloud water Collector (CASCC) (Demoz et al, 1996), with 50% collection efficiency cut-off diameters (D 50 ) of 22, 16, and 4 µm for stage 1, stage 2 and stage 3 respectively (Raja et al, 2008;Spiegel et al, 2012). Cloud droplets entering the collector were separated to different size fractions and captured by the given impaction obstacles.…”