10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Enhancement of the Heterogeneous Ice Nucleation by the Phase State Change of Organic Aerosols
YUE ZHANG, Martin Wolf, Leonid Nichman, Zhenfa Zhang, Avram Gold, John Jayne, Paul Davidovits, Douglas Worsnop, Jason Surratt, Timothy Onasch, Daniel Cziczo, MIT; Aerodyne Research, Inc.; UNC Chapel Hill
Abstract Number: 1649 Working Group: Unraveling the Many Facets of Ice Nucleating Particles and Their Interactions with Clouds
Abstract Cirrus clouds and their effects on earth’s radiative balance are major sources of uncertainties in predicting future climate. These clouds also dehydrate air ascending to the tropopause, thereby reducing water content in the stratosphere. However, the formation of cirrus clouds is not well understood. Data from field sites and campaigns have shown that organic aerosols (OAs) dominate the non-refractory aerosols in the free troposphere where ice cirrus clouds typically form. Measurements by aerosol mass spectrometers over forests indicate a high mass fraction of these OAs are derived from the atmospheric oxidation of isoprene and other volatile organic compounds (VOCs). Despite their abundance, the effects of these OAs on ice nucleation (IN) is controversial. Previously, these OAs were assumed to be homogeneously mixed liquids, which limits their IN properties. Recent studies have shown that depending on the ambient humidity and temperature, OAs can exist in semi-solid or solid phase states, which can potentially increase IN activity. This laboratory study systematically examines the effects of aerosol-phase state on IN properties of OAs by using the spectrometer for ice nucleation (SPIN), a commercially available instrument manufactured by Droplet Measurement Technologies, Inc.
Organic aerosols comprised of representative surrogates and the actual components of isoprene photo-oxidation products were generated and passed through a temperature control apparatus, where the temperature of the aerosols can be varied between -25°C and 13°C before entering the SPIN. A scanning mobility particle sizer (SMPS) measured the number-diameter distribution of the particles upstream of the SPIN. An optical particle counter downstream of the SPIN measured the optical signatures of the ice particles and some of the large bare organic particles. The SPIN operating temperature was between -38°C and -46°C. Our results show that pre-cooling the aerosol particles to -25°C enhances the IN onset relative humidity (RH) and the active fraction of IN when compared with non-pre-cooling conditions. Coupled with viscosity and glass transition temperature calculations, we show that the aerosol phase state changes due to the pre-cooling is the reason for this enhancement.
2-Methyltetrol, a multiphase reaction product in the isoprene secondary organic aerosol (SOA) that exist in ambient aerosols, was aerosolized, temperature controlled, and measured by the SPIN. The glass transition temperature of 2-methyltetrol as a function of relative humidity was parametrized based on measurements from dielectric relaxation spectroscopy. Results show that as the viscosity of 2-methyltetrol increases, the IN onset relative humidity decreases (i.e., IN activity increases).
Our study suggests that the phase state of organic aerosols, governed by their chemical composition, affects their IN properties. As the phase state of the organic aerosols changes from liquid to semi-solid or solid, their IN onset relative humidity decreases, sometimes by 10% RH. The isoprene SOA product molecule used in this study suggests isoprene SOAs may be important ice nuclei in the free troposphere.