Enhanced ice nucleation activity of coal fly ash aerosol particles initiated by ice-filled pores

Abstract

<p><strong>Abstract.</strong> Ice-nucleating particles (INPs), which are precursors for ice formation in clouds, can alter the microphysical and optical properties of clouds, hence, 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 as 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 CFA particles, which we conducted in the Aerosol Interaction and Dynamics in the Atmosphere (AIDA) aerosol and cloud simulation chamber at the Karlsruhe Institute of Technology, Germany, we partly observed a strong increase in the ice-active fraction for experiments performed at temperatures just below the homogeneous freezing of pure water, which could be related to the PCF mechanism. To further investigate the potential of CFA particles undergoing 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 K–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 remains in the pores during the warming and induces ice crystallization as soon as the pre-activated particles experience ice-supersaturated conditions at warmer temperatures; hence, showing an enhancement in their ice-nucleating ability compared to the scenario where the CFA particles are directly probed at warmer 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 could be the prevalent nucleation mode for intrinsic ice formation at cirrus temperatures rather than the previously acclaimed deposition mode. On the other, the PCF mechanism can also 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.</p>

The Role of Cloud Processing for the Ice Nucleating Ability of Organic Aerosol and Coal Fly Ash Particles

“…This is also consistent with recent results of Umo et al. (2019), who showed that CFA can form ice via PCF at T < HNT . Additionally, a clear distinction is visible for all cirrus temperatures investigated here, suggesting an intrinsic difference in ice nucleation ability depending on dry‐ or wet‐generation of the CFA particles.…”

“…The absence of ice nucleation below water saturation at T = 243 K for the cloud processed CFA samples, seems to be in conflict with the temperature‐cycling measurements of Umo et al. (2019), who report an ice nucleation activity of CFA particles as warm as 263 K for the first time after they had been temperature‐cycled to T ≈ 228 K. Umo et al. (2019) concluded that a few temperature‐cycled CFA particle types can contain ice‐filled pores that cause the improved ice formation through preactivation.…”

“…These physicochemical properties can be altered during atmospheric aging. For instance, during in‐cloud processes and exposure to different temperatures and humidities in the troposphere, the particle size and particle phase can be modified (Adler et al., 2013; Ladino et al., 2014), in turn affecting the INP potential (Mahrt et al., 2020; Umo et al., 2019; Wagner et al., 2012). Even though aerosol particles are on average cycled through multiple clouds during their atmospheric lifetime (Pruppacher & Jaenicke, 1995), it is poorly understood how such cloud processing would impact their INP potential.…”

“…When re‐probing the ice nucleation activity of the cloud processed glassy OA particles in a second cloud‐cycle, they catalyzed ice formation at a lower ice supersaturation compared to the unprocessed particles. Besides the higher particle viscosity, the enhanced ice nucleation activity of cloud processed particles could result from a preactivation effect (Knopf & Koop, 2006; Marcolli, 2017; Pruppacher & Klett, 1997; Umo et al., 2019; Wagner et al., 2014). During preactivation, a microscopic ice phase is retained within pores present on the particle, which can then grow into a macroscopic ice crystal in a subsequent cloud‐cycle (without the need of an ice nucleation process).…”

“…PCF has been implicated in the work of Umo et al. (2019), who measured the ice nucleation potential of coal fly ash (CFA) particles in a temperature‐cycling experiment, and found an increased ice‐active fraction of CFA particles previously exposed to lower temperatures. This enhancement effect was related to preactivation due to initial ice formation in pores by PCF.…”