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

ZHENLI LAI, Isabelle Steinke, Martin Wolf, Jiayun Zhao, Carolin Roesch, Xiaohong Liu, Zhenfa Zhang, Jason Surratt, Daniel Cziczo, Susannah Burrows, Yue Zhang, Texas A&M University

     Abstract Number: 340
     Working Group: Aerosol Physical Chemistry and Microphysics

Abstract
Aerosol-cloud interactions, including the formation of ice clouds, are among the largest uncertainties in predicting future climate based on the reports from Intergovernmental Panel on Climate Change (IPCC). Ice clouds are often formed through heterogeneous ice nucleation when aided by ice nucleating particles (INPs). Recent studies show that secondary organic aerosol (SOA), a major component of atmospheric fine particulate matter, may facilitate heterogeneous ice nucleation when its phase state changes from liquid to semi-solid or glass. However, few model parameterizations have been established to describe such heterogeneous ice nucleation process. 

In this study, we develop a parameterization to describe the deposition ice nucleation efficiency of SOA particles for various phase states and environmental conditions. This framework takes the experimentally measured ice activation fraction and outputs the heterogeneous ice nucleation rate coefficient J_het, as a function of temperature, ice supersaturation, and aerosol phase state (viscosity). The simulated J_het 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 upper troposphere (UT) aerosol to explore the importance of SOA INPs. Our simulated results suggest that certain SOA species are potentially important cirrus INP sources, and the updraft velocity of air may also alter the ice nucleation ability of SOA by affecting the cooling rate, and by extension, viscosity. 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 -46 ^oC and 1.1 ice supersaturation — where cirrus clouds typically form. These results imply that the interplay between INPs and aerosol phase state can be important under specific conditions and may require investigation in regional and global scale climate model to further understand the aerosol-cloud interactions and global radiative forcing.