Anomalous Noble Gas Solubility in Liquid Cloud Water: Possible Implications for Noble Gas Temperatures and Cloud Physics

The noble gas temperature climate proxy is an established tool that has previously been applied to determine the source of groundwater recharge, however, unanswered questions remain. In fractured media (e.g., volcanic islands) recharge can be so rapid that groundwater is significantly depleted in heavy noble gases, indicating that the water has retained noble gas concentrations from higher elevations. Previous studies of rain samples have confirmed a match to patterns seen in fractured‐rock groundwater for heavy noble gases along with a significant helium excess. Snow has been shown to be a credible source for both the helium excess and the observed heavy noble gas pattern. Here, liquid cloud water samples were collected at two mountainous sites and analyzed for noble gas concentrations. A pattern like that of rainwater was found. However, an analysis of diffusive uptake of noble gases into cloud water demonstrates that droplets of 1 mm diameter and smaller should be in constant solubility equilibrium with the atmosphere. To explain this, we present a novel hypothesis that relies on the assumption that liquid water consists of two types of water molecule clusters bounded by hydrogen bonds: a low‐density ice‐like structure and a high‐density condensed structure. In this model, the pressure gradient near the surface of a droplet resulting from surface tension could allow for the formation of a surface layer that is rich in ice‐like low density clusters. This can explain both the helium excess and the heavy noble gas depletion seen in the samples.Plain Language SummaryHighly anomalous solubility values for atmospheric noble gases in small water droplets within clouds were found for samples from Puerto Rico and Virginia, USA. Various possible mechanisms were examined to explain the pattern of helium excess along with highly depleted argon, krypton, and xenon. A hypothesis is presented that assumes that an ice‐like coating forms on micron‐scale droplets due to the presence of large pressure gradients near the surface of small water droplets. It is assumed that these gradients can separate high density water molecule clusters from low density (ice‐like) clusters because of buoyancy forces near the surface of the droplets. If confirmed by further research, this hypothesis has implications for the physics of cloud formation.Key PointsNoble gases are not in solubility equilibrium with liquid cloud waterStandard solubility models suggest that drops small than 1 mm should always be in equilibriumThe discrepancy can be explained if internal droplet pressure gradients favor the formation of an ice‐like surface on the dropletPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/171117/1/wrcr25688_am.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/171117/2/2020WR029306-sup-0002-Supporting_Information_SI-S02.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/171117/3/2020WR029306-sup-0001-Supporting_Information_SI-S01.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/171117/4/wrcr25688.pd