Enhancement of the Heterogeneous Ice Nucleation by the Phase State Change of Organic Aerosols

Cirrus clouds and their effects on earth's radiative balance are major sources of uncertainties in predicting future climate. However, the formation of cirrus clouds is not well understood. Data from field studies have shown that organic aerosols (OAs) dominate the non-refractory aerosols in the free troposphere where ice cirrus clouds typically form. Recent studies have shown that depending on the ambient humidity and temperature, OAs can exist in semi-solid or solid phase states, which may 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 (Droplet Measurement Technologies, Inc).

OAs comprised of representative surrogates and the actual secondary organic aerosols (SOA) from various precursors were generated and passed through a temperature control apparatus, where the temperature of the aerosols was kept between -25 20°C before entering the SPIN. A scanning mobility particle sizer measured the number-diameter distribution of the particles upstream of the SPIN. An optical particle counter downstream of the SPIN measured the number and optical signatures of the particles. Our results show that pre-cooling the aerosol particles enhances the IN onset relative humidity (RH) and the active fraction of IN. Coupled with viscosity and glass transition temperature calculations, the aerosol phase state changes is shown to be the reason for the enhanced ice nucleation.

2-Methyltetrol, a multiphase reaction product from the isoprene SOA that exist in ambient aerosols, was also measured by the SPIN with pre-cooling. The glass transition temperature of 2-methyltetrol as a function of RH and temperature was parametrized based on measurements from dielectric relaxation spectroscopy. Results show that the IN activity of 2-methyltetrol enhances with increasing viscosity.

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 RH decreases up to 10% RH. The 2-methyltetrol used in this study suggests isoprene SOAs may be important ice nuclei in the free troposphere.