Enhanced Ice-nucleating Ability of Coal Fly Ash Aerosol Particles Initiated by Pre-activated Pores

Ice formation by aerosol particles in clouds is an important process in the atmosphere. Cloud life-times, microphysical and optical properties can be significantly altered by their ice nucleation potential. Recently, there has been a renewed interest in understanding the mechanisms with which aerosol particles nucleate ice, especially in cirrus clouds¹. From the classical ice nucleation conception, deposition freezing mode is upheld as the preferred heterogeneous freezing mechanism in the cirrus cloud regime. This involves the formation of ice on an ice-nucleating particle (INP) directly from the vapour phase. In contrast to this definition, some INPs with pores or permanent surface defects with regular or irregular geometries have been reported to initiate ice formation via the liquid phase in a two-step process, involving the condensation and freezing of supercooled water inside these pores. This mechanism has been favourably labelled as the pore condensation and freezing (PCF) mechanism². In this study, we investigated the PCF mechanism on coal fly ash (CFA) aerosol particles using a temperature-cycling procedure³ at the Aerosol Interaction and Dynamics in the Atmosphere (AIDA) cloud simulation chamber. The five samples of CFA particles used in the study, which were collected from electrostatic precipitators of different power plants, showed enhanced ice nucleation abilities up to about 263 K after temporarily cooling them to 228 K. Without temperature cycling, ice nucleation was not observed at these higher temperatures. This suggests that the improved high-temperature ice nucleation ability might have been triggered by ice germs that were formed and preserved in the pores of the CFA INPs via the PCF mechanism when they were temporarily exposed to 228 K where any pore-condensed supercooled water could homogeneously freeze. The ice-filled cavities in the CFA aerosol particles could then account for their improved ice nucleation efficiencies at higher temperatures. This PCF mechanism could be potentially important even for mixed-phase clouds because CFA particles entrained into the upper troposphere with lower temperatures could be pre-activated via the formation of ice-filled pores. By convective atmospheric dynamics, these particles could then be released to lower altitudes and trigger ice formation at warmer temperatures than intrinsically expected. In future studies, it would be useful to constrain the temperature and relative humidity conditions under which ice-filled cavities can survive, which determines the viability of pre-activated INPs to form ice via the PCF mechanism. This could be useful in predicting the behaviour of INPs in different tropospheric conditions.

[1] Marcolli, C. Atmos. Chem. Phys. 14, 2071–2104 (2014).

[2] Marcolli, C. Atmos. Chem. Phys. 17, 1595–1622 (2017).

[3] Wagner, R., et al. Atmos. Chem. Phys. 16, 2025–2042 (2016).