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During the FROST-2 (FReezing Of duST) measurement campaign conducted at theLeipzig Aerosol Cloud Interaction Simulator (LACIS), we investigated changesin the ice nucleation properties of 300 nm Arizona Test Dust mineralparticles following thermochemical processing by varying amounts andcombinations of exposure to sulphuric acid vapour, ammonia gas, watervapour, and heat. The processed particles' heterogeneous ice nucleationproperties were determined in both the water subsaturated and supersaturatedhumidity regimes at −30 °C and −25 °C using Colorado StateUniversity's continuous flow diffusion chamber. The amount of sulphuric acidcoating material was estimated by an aerosol mass spectrometer and fromCCN-derived hygroscopicity measurements. The condensation of sulphuric aciddecreased the dust particles' ice nucleation ability in proportion to theamount of sulphuric acid added. Heating the coated particles in athermodenuder at 250 °C – intended to evaporate the sulphuric acidcoating – reduced their freezing ability even further. We attribute thisbehaviour to accelerated acid digestion of ice active surface sites by heat.Exposing sulphuric acid coated dust to ammonia gas produced particles withsimilarly poor freezing potential; however a portion of their ice nucleationability could be restored after heating in the thermodenuder. In no case didany combination of thermochemical treatments increase the ice nucleationability of the coated mineral dust particles compared to unprocessed dust.These first measurements of the effect of identical chemical processing ofdust particles on their ice nucleation ability under both water subsaturatedand mixed-phase supersaturated cloud conditions revealed that ice nucleationwas more sensitive to all coating treatments in the water subsaturatedregime. The results clearly indicate irreversible impairment of icenucleation activity in both regimes after condensation of concentratedsulphuric acid. This implies that the sulphuric acid coating causedpermanent chemical and/or physical modification of the ice active surfacesites; the possible dissolution of the coating during droplet activation didnot restore all immersion/condensation-freezing ability

Irreversible loss of ice nucleation active sites in mineral dust particles caused by sulphuric acid condensation