Modeling the Effects of Aerosol Phase States on the Deposition Ice Nucleation Ability of Biogenic Secondary Organic Aerosols (SOA)

Abstract

Aerosol-cloud interactions, including the formation of ice clouds, are among the largest uncertainties in predicting future climate based on the reports from the Intergovernmental Panel on Climate Change (IPCC). Recent studies show that organic aerosol (OA) may facilitate heterogeneous ice nucleation when its phase state changes from liquid to semi-solid or glass. In this study, we characterized the deposition ice nucleation abilities of aerosolized 2-methyltetrols (2-MT), a key component from isoprene epoxydiol (IEPOX) derived secondary organic aerosol, as a function of its phase state by using the SPectrometer for Ice Nucleation (SPIN). Our results demonstrate that as the phase state of 2-MT aerosols changes from liquid-like to semi-solid, its deposition ice nucleation ability is enhanced. Based on such ice nucleation data, a parameterization describing the deposition ice nucleation efficiency of SOA particles as a function of phase states and environmental conditions is developed. The heterogeneous ice nucleation rate coefficient, Jhet, is expressed as a function of temperature, ice supersaturation, and aerosol phase state (viscosity). The simulated Jhet isolines follow u-shaped curves in the ice supersaturation–temperature diagram, agreeing well with experimental data. We further apply our parameterization to scenarios representative of the upper troposphere (UT) aerosol to explore the potential importance of SOA INPs. Our simulated results suggest that certain SOA species are potentially important cirrus INP sources in the tropical region and that the updraft velocity of air may also alter the ice nucleation ability of SOA by affecting the cooling rate through viscosity changes. Based on field measurements over the Amazon rainforest, isoprene-epoxydiol-derived SOA (IEPOX-SOA) is estimated to increase the INP number concentration by 30 L-1 at altitudes of 10-12 km at -46oC and 1.1 ice supersaturation— where cirrus clouds typically form. These results imply that the interplay between INPs and aerosol phase state may need to be considered in regional and global scale climate to further understand the aerosol-cloud interactions and global radiative forcing.