Here we discuss another contribution to the INP abundance which is related to the fact that porous particles, when cooled to low temperatures e.g. during atmospheric transport to higher altitudes, can condense supercooled water into pores, capillaries or cracks which then freezes homogeneously at temperatures below about -35°C. This effect is called Pore Condensation and Freezing (PCF). PCF can be a very efficient heterogeneous ice formation process in cirrus and may explain at least in part what was called deposition nucleation so far. However, the PCF pre-activation of porous particles can also contribute to primary ice formation in mixed-phase clouds, e.g. after transport of the same particles to warmer regions, because the pore ice remains stable at temperatures above -35°C and relative humidities below ice saturation and would immediately grow to large ice crystals at ice supersaturated conditions.
Only during the recent years, the PCF mechanism received renewed attention. We have used the AIDA (Aerosol Interaction and Dynamics in the Atmosphere) chamber at the Karlsruhe Institute of Technology (KIT) in order to subject various aerosol types like mineral dust, diatomaceous earth, volcanic ash and soot to temperature changes between -10°C and -50°C in order to investigate their PCF pre-activation behaviour at low temperatures and the related enhanced ice nucleation activity after warming to higher temperatures. In this contribution we will introduce the mechanism of PCF pre-activation, show the results and discuss atmospheric implications.