Abstract View
Ice Nucleation on Soot Particles: Measurements, Predictions and Implications
FABIAN MAHRT, Bernd Kärcher, Claudia Marcolli, University of British Columbia
Abstract Number: 613
Working Group: Aerosols, Clouds and Climate
Abstract
Soot particles are emitted as a by-product of incomplete combustion processes. They play a critical role for air quality and human health. Soot particles also impact climate, directly by absorbing radiation and indirectly by acting as ice nucleating particles. Despite extensive previous work, the understanding of the climate impacts of soot remains incomplete, in large parts because a coherent understanding of the ice nucleation mechanism and ice activity of soot particles has not emerged. As a consequence, there remain large uncertainties associated with the soot-cloud interactions and associated feedbacks. A key example is aviation, where it has been hypothesized that the interaction of emitted soot particles with natural cirrus affects their microphysical properties and might lead to a large radiative forcing. However, such impacts are associated with large uncertainty due mainly to the lack of understanding the conditions in which aircraft-emitted soot particles nucleate ice.
Here, we integrate laboratory measurements, theory and cloud modelling to address the ice nucleation activity and mechanism of soot particles. Starting from soot ice nucleation laboratory measurements that evidenced a pore condensation and freezing (PCF) process, we developed a novel theoretical framework that incorporates the aggregate structure of soot particles in the description of ice nucleation by PCF. The soot-PCF framework treats the primary soot particles as spheres that aggregate together forming pores that are characterized by the primary particle size and overlap, as well as the contact angle between the particle surface and water. We used the soot-PCF framework to derive a new parameterization to describe the ice formation of aviation soot particles. The parameterization is well constrained by experimentally determined soot particle properties but is versatile and can easily be adjusted to soot particles with other characteristics, e.g. from different emission sources. Implementing the new parameterization into a high-resolution cirrus column model to study how aircraft-emitted soot particles, released from sublimating contrails, perturb natural cirrus clouds, we find that at most 1 % of the soot particles form ice crystals alongside homogeneous freezing. Our results have important implications for the climate impact of aviation.