The Role of Atmospheric Organic Aerosols as Ice-nucleating Particles

Knopf, D., Stony Brook University

Cloud-Aerosol-Precipitation Interactions

Aerosol Processes

Knopf D, P Alpert, and B Wang. 2018. "The Role of Organic Aerosol in Atmospheric Ice Nucleation: A Review." ACS Earth and Space Chemistry, 2(3), doi:10.1021/acsearthspacechem.7b00120.

Science

Airborne particles can be dominated by organic material, which is ubiquitous in the atmosphere, from the Earth’s surface to many kilometers above. We gathered the most up-to-date findings and assessed the impact of organic material on ice cloud formation, but much more research is needed.

Impact

Organic aerosol particles can be amorphous (non-crystalline) in nature, altering their phase-state between liquid, semisolid, and solid (glassy) in response to changes in humidity and temperature. Although difficult to predict how these phase-state changes affect atmospheric ice formation, the compiled data illustrate associated challenges in field and laboratory measurements, ultimately improving our fundamentally understanding of these coupled phenomena.

Summary

Heterogeneous ice nucleation can proceed on aerosol particles ranging from a few nanometers to micrometers in size, commonly referred to ice-nucleating particles. Organic matter can be present as organic aerosol particles or as coatings on other particle types. The physicochemical properties of organic matter impact how organic aerosol form ice crystals in the atmosphere and contribute to the ice-nucleating particle population. Examination of ice residuals and ice-nucleating particles with advanced analytical techniques demonstrates that organic matter participates in atmospheric ice crystal formation. The phase-state of organic matter in the supercooled and metastable regime is identified as a key factor in assessing particle nucleation pathways and rates. Two competing kinetic regimes are identified, which are the time scales under which a particle changes its phase-state governed by condensed-phase diffusion processes, and the associated ice nucleation rate for given thermodynamic conditions. A theoretical model is advanced accounting for changes in humidity and temperature based on particle water activity that predicts amorphous phase changes and ice nucleation of particles coated by organic matter. This review synthesizes our current understanding and proposes future research directions needed to fully evaluate how organic aerosol contributes to atmospheric ice nucleation.