CAREER: Effects of Chemical Aging on the Ice Nucleation Properties of Natural and Anthropogenic Atmospheric Particles

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Through this CAREER award funded by the Environmental Chemical Sciences Program in the Chemistry Division at the National Science Foundation, Professor Ryan Sullivan of Carnegie Mellon University is experimentally investigating the effects of atmospheric chemical processing on ice formation initiated (nucleated) by particles in the environment. While chemical reactions are known to alter the freezing properties of ice-nucleating particles, current representations of the process by which water droplets become ice particles in cloud and climate models are largely based on the properties of fresh (or unaged) particles. By determining the changes in the freezing properties of realistic particle systems, this research advances our understanding of how human activities affect cloud evolution and ice formation. The development of educational modules to be loaned to K-12 teachers expose young students to the chemical science and measurement of air pollution, acid rain, and cloud forming reactions. The creation of a series of new hands-on laboratory experiments for the core undergraduate instrumental analysis course at CMU enhances the training of college students in modern chemical analysis.Mineral dust and volcanic ash particles, and biomass burning (wood smoke) aerosol, is chemically aged using a chamber or flow tube reactor to determine the effects of atmospheric processing by anthropogenic pollutants on the contact and immersion freezing properties of these important natural sources of ice-nucleating particles. The effects of chemical aging on particle composition, mixing state, and ice nucleation properties are determined using an extensive combination of online and offline single-particle characterization. This includes online analysis using two single-particle mass spectrometers, and offline spectromicroscopic analysis of particles and ice crystal residues. The contact freezing efficiency of these particle systems is directly measured using a chilled aerosol optical tweezers system. Immersion freezing temperature spectra of particles immersed in droplets is obtained using a droplet in oil cold plate system. This research produces a comprehensive dataset regarding the heterogeneous ice nucleation properties of important types of ice nucleating particles, and the effects of atmospheric processing on their freezing ability. The measurements is parameterized to produce improved representations of the chemical composition and related ice nucleation properties of biomass burning and mineral particles in cloud and atmospheric chemistry models.