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

Abstract

The effect of organic coating on the heterogeneous ice nucleation (IN) efficiency of dust particles was investigated at simulated cirrus cloud conditions in the AIDA cloud chamber of Forschungszentrum Karlsruhe. Arizona test dust (ATD) and the clay mineral illite were used as surrogates for atmospheric dust aerosols. The dry dust samples were dispersed into a 3.7 m 3 aerosol vessel and either directly transferred into the 84 m 3 cloud simulation chamber or coated before with the semi-volatile products from the reaction of α-pinene with ozone in order to mimic the coating of atmospheric dust particles with secondary organic aerosol (SOA) substances. The ice-active fraction was measured in AIDA expansion cooling experiments as a function of the relative humidity with respect to ice, RHi, in the temperature range from 205 to 210 K. Almost all uncoated dust particles with diameters between 0.1 and 1.0 μm acted as efficient deposition mode ice nuclei at RHi between 105 and 120%. This high ice nucleation efficiency was markedly suppressed by coating with SOA. About 20% of the ATD particles coated with a SOA mass fraction of 17 wt% were ice-active at RHi between 115 and 130%, and only 10% of the illite particles coated with an SOA mass fraction of 41 wt% were ice-active at RHi between 160 and 170%. Only a minor fraction of pure SOA particles were ice-active at RHi between 150 and 190%. Strong IN activation of SOA particles was observed only at RHi above 200%, which is clearly above water saturation at the given temperature. The IN suppression and the shift of the heterogeneous IN onset to higher RHi seem to depend on the coating thickness or the fractional surface coverage of the mineral particles. The results indicate that the heterogeneous ice nucleation potential of atmospheric mineral particles may also be suppressed if they are coated with secondary organics.

A new temperature- and humidity-dependent surface site density approach for deposition ice nucleation

“…Aerosol number concentrations are measured with a condensation particle counter (CPC3010, TSI), whereas the aerosol size distribution was measured by combining SMPS (Scanning Mobility Particle Sizer -TSI) and APS (Aerodynamical Particle Sizer -TSI) measurements. From these data, the total aerosol surface area concentration can be inferred by translating the size distribution into a surface distribution after converting mobility and aerodynamic diameters into equivalent sphere diameters (Möhler et al, 2008). To this surface distribution a lognormal fit is applied from which the total aerosol surface area concentration can be estimated through integrating the distribution.…”

Different contact angle distributions for heterogeneous ice nucleation in the Community Atmospheric Model version 5

“…Suppression of heterogeneous ice nucleation is dependent on coating thickness or the fractional soluble mass coverage. Generally we assume that, if a potential IN is covered by more than one monolayer, its heterogeneous nucleation behavior in the deposition and contact modes will be suppressed completely due to a shift to the higher-onset relative humidity with respect to ice, RHi, and to the colder onset temperature (Hoose et al, 2010;Möhler et al, 2008). Therefore, only uncoated particles will participate in ice nucleation.…”

Aerosols that form subvisible cirrus at the tropical tropopause

“…It is likely that effective IN were removed from the aerosol population lower in the TTL. Many of the dust spectra showed evidence of organic and sulfate coatings, which probably reduced their effectiveness as heterogeneous IN (Mohler et al, 2008;Cziczo et al, 2009). Acidic sulfate aerosols and meteoric material from the stratosphere were not found in either the SVC residue or the nearby unfrozen aerosol.…”

“…Although PALMS cannot determine the phase of these ammonium sulfate aerosols, impactor samples from the subtropical stratosphere have occasionally yielded electron microscope images indicative of dry ammonium sulfate (Gras, 1978;Bigg, 1986). All the ice residuals also contained organics, which will likely suppress heterogeneous freezing (Mohler et al, 2008). However, ammonium sulfate aerosols that are only partially coated and have exposed surface sites may still be IN-active.…”