Seminar

Abstract

Heterogeneous ice nucleation has been shown to play an important role in the formation of cirrus and mixed-phase clouds. Although the importance of atmospheric ice formation is widely acknowledged, the actual effects of aerosol particles on heterogeneous ice nucleation are still not sufficiently understood. Here, we present laboratory studies investigating the ice nucleation efficiency of organic dominated anthropogenic particles impacted by different degrees of photochemical aging collected in and around Mexico City, organic containing particles sampled in Los Angeles during the CalNex campaign, particles composed of humic like substances exposed to O3, and biogenic marine particles. The results of experiments on the propensity of these particles to induce ice formation via immersion and deposition freezing (i.e. the ice nuclei (IN) nucleates ice in a supercooled aqueous aerosol particle and directly from the supersaturated vapor phase, respectively), using a cryo-cooling stage/optical microscope system are summarized. Heterogeneous freezing temperatures and corresponding nucleation rates are derived. One focus of this talk concerns the different methods used to describe and parameterize ice nucleation data. The application of classical nucleation theory, singular hypothesis, ice nuclei activated fraction, and the water activity based ice nucleation approach for interpretation of the experimentally derived freezing data will be discussed.

The physical and chemical characteristics of the field collected particles used in our experiments were determined by CCSEM/EDX analysis (computer controlled scanning electron microscopy with energy dispersed analysis of X-rays and STXM/NEXAFS (scanning transmission X-ray microscopy combined with near edge X-ray absorption fine structure spectroscopy). Particles sampled in urban areas can induce ice formation at conditions typical for cirrus and mixed- phase clouds. Solid humic and fulvic acid particles nucleate ice via immersion and deposition modes at atmospherically relevant conditions. Oxidation by O₃ affects the particles' IN efficiency, however, no clear relationship between increased particle hygroscopicity due to oxidation and IN efficiency was found. Various phytoplankton species are the first unambiguously identified marine organisms which act as efficient IN in both immersion and deposition modes. These findings can help resolve observations of elevated atmospheric IN concentrations observed over surface waters containing blooms of phytoplankton. Respective freezing temperatures, nucleation rates, and parameterizations for immersion and deposition freezing will be discussed.