Abstract
Abstract. Polar mesospheric clouds are the highest water ice clouds occurring in the terrestrial atmosphere. They form in the polar summer mesopause, the coldest region in the atmosphere. It has long been assumed that these clouds form by heterogeneous nucleation on meteoric smoke particles which are the remnants of material ablated from meteoroids in the upper atmosphere. However, until now little was known about the properties of these nanometre-sized particles and application of the classical theory for heterogeneous ice nucleation was impacted by large uncertainties. In this work, we performed laboratory measurements on the heterogeneous ice formation process at mesopause conditions on small (r=1 to 3 nm) iron silicate nanoparticles serving as meteoric smoke analogues. We observe that ice growth on these particles sets in for saturation ratios with respect to hexagonal ice below Sh=50, a value that is commonly exceeded during the polar mesospheric cloud season, affirming meteoric smoke particles as likely nuclei for heterogeneous ice formation in mesospheric clouds. We present a simple ice-activation model based on the Kelvin–Thomson equation that takes into account the water coverage of iron silicates of various compositions. The activation model reproduces the experimental data very well using bulk properties of compact amorphous solid water. This is in line with the finding from our previous study that ice formation on iron silicate nanoparticles occurs by condensation of amorphous solid water rather than by nucleation of crystalline ice at mesopause conditions. Using the activation model, we also show that for iron silicate particles with dry radius larger than r=0.6 nm the nanoparticle charge has no significant effect on the ice-activation threshold.
Highlights
Polar mesospheric clouds (PMCs) are water ice clouds occurring in the terrestrial atmosphere at an altitude of about 83 km (e.g. Rapp and Thomas, 2006; Thomas, 1991; Thomas and Olivero, 2001)
The resulting fits yielded σ = (94±11) mN m−1, which compares very well to extrapolated values of the surface tension of supercooled liquid water (SLW) at these temperatures (σSLW (T = 155 K) = 92 mN m−1 and σSLW (T = 128 K) = 96 mN m−1) and which we recently proposed to use for lack of available surface tension data of amorphous solid water (ASW) (Nachbar et al, 2018c)
We have shown previously that ASW is the initial form of condensed water that deposits on hydrophilic iron silicate nanoparticles at temperatures below 160 K (Nachbar et al, 2018c)
Summary
Polar mesospheric clouds (PMCs) are water ice clouds occurring in the terrestrial atmosphere at an altitude of about 83 km (e.g. Rapp and Thomas, 2006; Thomas, 1991; Thomas and Olivero, 2001). The clouds form during the polar summer in the mesopause and were reported in the literature for the first time at the end of the 19th century (Leslie, 1885). They are optically very thin and can be seen from ground by the naked eye only after the sun has set below the horizon, which is why they are often called noctilucent clouds (NLCs). Recent studies estimated that about 40 t of cosmic material enters the atmosphere each day (Carrillo-Sánchez et al, 2016; Hervig et al, 2017). 20 % of this material ablates in the upper atmosphere with the major elemental species being Fe, Mg, and Si
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