Enhancement of the Heterogeneous Ice Nucleation by the Changing Phase State of Secondary Organic Aerosols

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) is a major component of the non-refractory aerosols in the free troposphere where ice cirrus clouds typically form. Measurements by aerosol mass spectrometers in the free troposphere above 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 INabilities. 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 INactivity. This laboratory study systematically examines the effects of aerosol-phase state on IN properties of secondary organic aerosols (SOA) produced by the environmental chamber by simultaneously measuring their chemical composition and ice nucleation properties.

Selected types of SOA particles were generated by reacting the respective volatile organic compounds (VOCs) with either ozone and/or OH radicals in the MIT environmental chamber and a potential aerosol mass (PAM) oxidation flow reactor. Four kinds of SOA, namely a-pinene SOA, toluene SOA, -caryophyllene SOA, and IEPOX-derived SOA were generated and passed through a temperature control apparatus, where the temperature of the aerosols can be varied between -42°C and 20°C before entering the spectrometer for ice nucleation (SPIN, Droplet Measurement Technologies, Inc.) for determining ice nucleation activity. A scanning mobility particle sizer (SMPS) and an aerosol mass spectrometer (AMS, Aerodyne Inc.) measured the number-diameter distribution and chemical composition 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 to -42°C enhances the IN onset relative humidity (RH) and the active fraction of IN when compared with non-pre-cooling conditions only for -caryophyllene SOA and IEPOX-derived SOA. Coupled with viscosity and glass transition temperature calculations, we show that the aerosol phase state changes due to the pre-cooling explains this enhancement.

By combining the ice nucleation results with chemical analysis of the SOA, our study suggests that the chemical composition influences of these organic aerosols alter the hygroscopicity and the phase state of these organic aerosols, which eventually affects their INproperties. As the phase state of the organic aerosols changes from liquid to semi-solid or solid, their INonset relative humidity decreases, suggesting certain types of SOA (including b-caryophyllene and isoprene SOA) could be potentially important ice nuclei in the free troposphere.