Enhanced ice nucleation activity of coal fly ash aerosol particles initiated by ice-filled pores
Authors: Umo Nsikanabasi Silas , Wagner Robert , Ullrich Romy , Kiselev Alexei , Saathoff Harald , Weidler Peter G. , Cziczo Daniel J. , Leisner Thomas , Möhler Ottmar 
Published: July 2019

Abstract. Ice-nucleating particles (INPs), which are precursors for ice
formation in clouds, can alter the microphysical and optical properties of
clouds, thereby impacting the cloud lifetimes and hydrological cycles.
However, the mechanisms with which these INPs nucleate ice when exposed to
different atmospheric conditions are still unclear for some particles.
Recently, some INPs with pores or permanent surface defects of regular or
irregular geometries have been reported to initiate ice formation at cirrus
temperatures via the liquid phase in a two-step process, involving the
condensation and freezing of supercooled water inside these pores. This
mechanism has therefore been labelled pore condensation and freezing
(PCF). The PCF mechanism allows formation and stabilization of ice germs in
the particle without the formation of macroscopic ice. Coal fly ash (CFA)
aerosol particles are known to nucleate ice in the immersion freezing mode
and may play a significant role in cloud formation. In our current ice
nucleation experiments with a particular CFA sample (CFA_UK),
which we conducted in the Aerosol Interaction and Dynamics in the Atmosphere
(AIDA) aerosol and cloud simulation chamber at the Karlsruhe Institute of
Technology (KIT), Germany, we observed a strong increase (at a threshold relative
humidity with respect to ice of 101 %–105 %) in the ice-active fraction
for experiments performed at temperatures just below the homogeneous
freezing of pure water. This observed strong increase in the ice-active
fraction could be related to the PCF mechanism. To further investigate the
potential of CFA particles undergoing the PCF mechanism, we performed a
series of temperature-cycling experiments in AIDA. The temperature-cycling
experiments involve exposing CFA particles to lower temperatures (down to
∼228 K), then warming them up to higher temperatures (238–273 K) before investigating their ice nucleation properties. For the
first time, we report the enhancement of the ice nucleation activity of the
CFA particles for temperatures up to 263 K, from which we conclude that it
is most likely due to the PCF mechanism. This indicates that ice germs
formed in the CFA particles' pores during cooling remain in the pores during
warming and induce ice crystallization as soon as the pre-activated
particles experience ice-supersaturated conditions at higher temperatures;
hence, these pre-activated particles show an enhancement in their ice-nucleating ability compared with
the scenario where the CFA particles are directly probed at higher
temperatures without temporary cooling. The enhancement in the ice
nucleation ability showed a positive correlation with the specific surface
area and porosity of the particles. On the one hand, the PCF mechanism can
play a significant role in mixed-phase cloud formation in a case where the
CFA particles are injected from higher altitudes and then transported to
lower altitudes after being exposed to lower temperatures. On the other
hand, the PCF mechanism could be the prevalent nucleation mode for ice
formation at cirrus temperatures rather than the previously acclaimed
deposition mode.