Unravelling the microphysics of polar mesospheric cloud formation

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 S h = 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.

The impact of solar radiation on polar mesospheric ice particle formation

“…The particle heating will be much less for other MSP materials and at lower altitudes due to the higher collisional cooling rate at higher pressures. In general, the particle temperature offset reported here is about 5 times less than previous es-timates (Asmus et al, 2014) for two main reasons: (1) the uptake of water molecules increases the particle surface area and therefore the collisional cooling rate, and (2) the thermal accommodation coefficient of α = 0.5 used in previous calculations (see also Jutt, 2002, andGrams andFiocco, 1977) is very likely an underestimation. We use a value of 1 based on recent results of laboratory experiments, which increases the collisional cooling rate by a factor of 2 (see Appendix B for more details).…”

“…Here, I (λ) (blackbody radiation assuming T = 5780 K), P a env , and P e rad were calculated as presented in Asmus et al (2014). The thermal accommodation coefficient α, which is typically used in literature to describe the heating of MSPs or NLC particles, is 0.5 (e.g., Asmus et al, 2014;Espy and Jutt, 2002). This value seems to originate from the work of Grams and Fiocco (1977) and was chosen due to a lack of relevant measurements determining α at realistic mesopause conditions.…”

The influence of surface charge on the coalescence of ice and dust particles in the mesosphere and lower thermosphere

“…In this paper, we investigate the influence of surface charge on the particle agglomeration processes. We apply models that are developed to describe electrostatic interactions between charged dielectric spheres and are based on solutions presented by Bichoutskaia et al (2010) and Filippov et al (2019). These theories predict collision outcomes according to the variables of particle size, charge, dielectric constant, relative kinetic energy, collision geometry and the coefficient of restitution.…”

“…As these particles typically possess a low charge (or single charge arising, for example, from either a photoionisation event that removes a single electron from a molecule on the particle or the attachment of an ambient air ion) the charge distribution is best represented by a point free charge residing on the surface. For this case, we have extended the numerical method developed in Lindgren et al (2018a) to allow for description of particle charge in the form of point charge(s) residing on its surface, similar to a solution proposed in Filippov et al (2019) but based on a numerical method. Comparisons with a uniform distribution of free surface charge, as described in Bichoutskaia et al (2010), shows that, for particles with radii greater than 10 nm, the choice of a specific form of surface charge distribution does not affect the calculated electrostatic energy between particles; however, the difference does become important for sub-nanometre particles.…”

The influence of surface charge on the coalescence of ice and dust particles in the mesosphere

“…Hervig et al (2012) describe PMC extinction measurements for a mixture of ice and meteoric smoke and suggest wüstite and magnesiowüstite as possible smoke materials. Plane et al (2015) consider olivine and pyroxene and Duft et al (2019) iron silicate.…”