Ice clouds in the tropical tropopause layer play a key role in dehydrating air as it enters the stratosphere1,2. However, in situ measurements show that water vapour within these clouds is unexpectedly supersaturated3,4,5; normally the growth of ice crystals rapidly quenches supersaturation3. The high in-cloud humidity may be related to the low number of ice crystals found in these clouds4,6, but low ice number densities are inconsistent with standard models of cirrus cloud formation involving homogeneous freezing of liquid aerosols7. Aqueous aerosols rich in organic matter are ubiquitous in the atmosphere8,9, and under cirrus conditions they are known to become glassy10,11, that is, amorphous, non-crystalline solids. Here we report experiments in a cloud simulation chamber that demonstrate heterogeneous nucleation of ice on glassy solution droplets. Cirrus residues measured in situ showed ice nuclei rich in oxidized organic matter12, consistent with heterogeneous nucleation on glassy aerosols. In addition, using a one-dimensional cirrus model, we show that nucleation on glassy aerosols may explain low ice crystal numbers and high in-cloud humidity in the tropical tropopause layer. We propose that heterogeneous nucleation on glassy aerosols is an important mechanism for ice nucleation in the tropical tropopause layer.
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B.J.M. thanks the Natural Environment Research Council (NE/D009308/1) and the European Research Council (FP7, 240449—ICE) for fellowships. We acknowledge the FP6 European Network of Excellence Atmospheric Composition Change ‘ACCENT’ for travel funds (GOCE CT-2004—505337). T.W.W. thanks the Charles Brotherton Trust for a Studentship and the Aerosol Society for further financial support. We thank the AIDA operators and technicians for their support during the experiments. The water vapour measurements were partially funded by the Helmholtz Virtual Institute Aerosol Cloud Interaction (VIACI). We thank T. Leisner for helpful discussions and M. Pilling for commenting on this manuscript.
The authors declare no competing financial interests.
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Murray, B., Wilson, T., Dobbie, S. et al. Heterogeneous nucleation of ice particles on glassy aerosols under cirrus conditions. Nature Geosci 3, 233–237 (2010). https://doi.org/10.1038/ngeo817
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