TITLE:
A Review of Thermo- and Diffusio-Phoresis in the Atmospheric Aerosol Scavenging Process. Part 2: Ice Crystal and Snow Scavenging
AUTHORS:
Gianni Santachiara, Franco Prodi, Franco Belosi, Alessia Nicosia
KEYWORDS:
Ice Crystals, Snow Crystals, Ice Nucleating Particles, Aerosol Scavenging, Phoretic Forces
JOURNAL NAME:
Atmospheric and Climate Sciences,
Vol.13 No.4,
October
8,
2023
ABSTRACT: The role of phoretic forces in the identification of
particles acting as ice nuclei in mixed phase cloud is discussed. A method used
to identify the effective ice nucleating particles is to sample ice crystals,
which are afterwards sublimated, and to
examine the particles remaining after evaporation. The procedure takes into
account only crystal with a maximum diameter of 20 μm, by assuming that small
crystals do not scavenge aerosol during growth, and therefore that crystals
contain only the effective nucleating particles. This assumption is
questionable, however, as experiments have shown that even small ice crystals
can scavenge aerosol. Another approach has been to compare the number and
elemental composition of residual particles in small ice crystals and of
aerosol near the cloud. By considering as example soot and black carbon
aerosol, contradictory conclusions on their importance in the processes of ice
nucleation have been reported in the literature. We suggest that, in addition
to physico-chemical properties of soot/carbon aerosol particles, even the microphysical and environmental parameters involved
in the transition of aerosol from gas phase to ice crystals in cloud
should be considered. The contribution of phoretic forces should also be
considered. After initial growth ice crystals can
continue to grow by water vapour diffusion. Laboratory experiments confirm the contribution of diffusiophoresis with Stefan flow in the scavenging by snow
crystals up to 3 mm in diameter. The
particle scavenging efficiency of snow crystals is related to crystalline shape
and depends on air relative humidity and temperature.