Heterogeneous nucleation of ice particles on glassy aerosols under cirrus conditions

Ice clouds in the tropical tropopause layer play a key role in dehydrating air as it enters the stratosphere(1,2). However, in situ measurements show that water vapour within these clouds is unexpectedly supersaturated(3-5); normally the growth of ice crystals rapidly quenches supersaturation(3). The high in-cloud humidity may be related to the low number of ice crystals found in these clouds(4,6), but low ice number densities are inconsistent with standard models of cirrus cloud formation involving homogeneous freezing of liquid aerosols(7). Aqueous aerosols rich in organic matter are ubiquitous in the atmosphere(8,9), and under cirrus conditions they are known to become glassy(10,11), 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 matter(12), 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.