Abstract
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.
Highlights
Understanding the ice nucleation processes remains highly relevant to our knowledge of cloud formation and other applications in cryopreservation, geoengineering, bioengineering, material modifications, aviation, and in agriculture (Kiani and Sun, 2011; Morris and Acton, 2013; Murray, 2017)
Coal fly ash (CFA) aerosol particles inherently nucleate ice in the immersion freezing mode as shown from this investigation and in previous studies. An exposure of these particles to favourable atmospheric conditions such as cold temperatures (∼ 228 K) at ice sub-saturated conditions can induce the formation of ice germs in the pores of the CFA particles by the pore condensation and freezing (PCF) mechanism
This behaviour could be attributed to the degree of surface defects, and porosity of such CFA particles, which differ from sample to sample
Summary
Understanding the ice nucleation processes remains highly relevant to our knowledge of cloud formation and other applications in cryopreservation, geoengineering, bioengineering, material modifications, aviation, and in agriculture (Kiani and Sun, 2011; Morris and Acton, 2013; Murray, 2017). The influence of this quartz content on the particles’ immersion freezing ability can be suppressed in a situation where hydratable components form a layer on the particle surface (Grawe et al, 2018) These hydratable components are chemical compounds (e.g. CaSO4) contained in CFA particles that are capable of taking up water at elevated ambient relative humidity. When we tested the ice nucleation ability of these particles at temperatures just below the homogeneous freezing of pure water, one of the CFA samples showed a high fraction of ice-active particles at a low relative humidity with respect to ice (RHice = 101–105 %), in apparent contrast to its icenucleating ability just above 238 K This result was indicative of a PCF mechanism as put forward by Marcolli (2014), noting that a variety of aerosol particle types showed a sudden increase in their ice-nucleating ability just below the homogeneous freezing temperatures. The article concludes by pointing out some future perspectives for research on this subject
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