The effect of organic coating on the heterogeneous ice nucleation efficiency of mineral dustaerosols

Abstract

The effect of organic coating on the heterogeneous ice nucleation (IN) efficiency of dustparticles was investigated at simulated cirrus cloud conditions in the AIDA cloud chamberof Forschungszentrum Karlsruhe. Arizona test dust (ATD) and the clay mineral illite were usedas surrogates for atmospheric dust aerosols. The dry dust samples were dispersed into a3.7 m3 aerosol vessel and either directly transferred into the84 m3 cloud simulation chamber or coated before with the semi-volatile products from the reaction ofα-pinene with ozone in order to mimic the coating of atmospheric dust particleswith secondary organic aerosol (SOA) substances. The ice-active fraction wasmeasured in AIDA expansion cooling experiments as a function of the relativehumidity with respect to ice, RHi, in the temperature range from 205 to210 K. Almost all uncoated dust particles with diameters between 0.1 and1.0 µm acted as efficient deposition mode ice nuclei at RHi between 105 and 120%. This high icenucleation efficiency was markedly suppressed by coating with SOA. About 20% of theATD particles coated with a SOA mass fraction of 17 wt% were ice-active at RHi between115 and 130%, and only 10% of the illite particles coated with an SOA massfraction of 41 wt% were ice-active at RHi between 160 and 170%. Only a minorfraction of pure SOA particles were ice-active at RHi between 150 and 190%. StrongIN activation of SOA particles was observed only at RHi above 200%, which isclearly above water saturation at the given temperature. The IN suppression andthe shift of the heterogeneous IN onset to higher RHi seem to depend on thecoating thickness or the fractional surface coverage of the mineral particles. Theresults indicate that the heterogeneous ice nucleation potential of atmosphericmineral particles may also be suppressed if they are coated with secondary organics.