Bingbing Wang^a, Daniel A. Knopf^b, Swarup China^a, Bruce W. Arey^a, Tristan H. Harder^c,d, Mary K. Gilles^c, Alexander Laskin^a, Direct observation of ice nucleation events on individual atmospheric particles, Phys. Chem. Chem. Phys., 2016, 18, 29721-29731 http://dx.doi.org/10.1039/C6CP05253C.

^aWilliam. R. Wiley Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, WA 99352, USA

^bInstitute for Terrestrial and Planetary Atmospheres, School of Marine and Atmospheric Sciences, Stony Brook University, Stony Brook, NY 11794, USA

^cChemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA

^dDepartment of Chemistry, University of California, Berkeley, California 94720, USA

Funding:

Laboratory Directed Research and Development funds of Pacific Northwest National Laboratory. U.S. Department of Energy, Office of Science (OBER). Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy.

Aerosol particles, tiny suspension of liquid or condensed phase material in air, in sizes of a few nanometers to ~100s of micrometers (a hair is about 10s of micrometers in diameter), are ubiquitously present in the atmosphere. Aerosol-cloud interactions are responsible for the largest uncertainties predicting future climate. In particular, how aerosol particles initiate ice crystal formation is little understood and not implemented in current climate models. Hence, atmospheric ice nucleation is regarded as a grand challenge in the atmospheric sciences. Ice crystals do not only impact the climate but also the distribution of water vapor, the strongest greenhouse gas, and the hydrological cycle and thus precipitation. For these reasons, there is strong interest in improving our predictive understanding of ice formation in the atmosphere.

Atmospheric ice formation by, e.g. an insoluble aerosol particle such as a mineral dust particle, remains insufficiently understood, partially due to the lack of experimental methods capable of obtaining in situ microscopic details of ice nucleation over ice forming particles. We developed a novel instrumentation that utilizes a custom-built ice nucleation cell that is attached to an Environmental Scanning Electron Microscope (termed IN-ESEM platform). Cloud formation can be observed in situ for temperatures as low as 200 K for relative humidity (RH) up to water saturation covering, essentially, all atmospheric thermodynamic conditions. The rate of temperature or RH change can be adjusted to simulate relevant cloud forming conditions (including slow updrafts for cirrus formation and conditions typical of deep convection). Observations of ice nucleation events on kaolinite particles on the nanoscale are conducted and we demonstrate the capability of direct tracking and micro-spectroscopic characterization of individual ice nucleating particles (INPs) in an authentic atmospheric aerosol particle sample.

The high resolution observational capability demonstrates that ice crystals preferably nucleate at the edges of the stacked kaolinite platelets instead on their basal planes (see e.g. Fig. 1 and movie) improving our understanding of the physicochemical features on a particle surface that make good ice nuclei.

This novel technique allows to examine the physical mechanisms of different ice formation pathways that, e.g. i) require the presence of condensed liquid water prior to freezing and ii) the presence of nano-pores that may be involved in deposition ice nucleation. Furthermore, this instrumentation will also serve the applied sciences, where the heterogeneous nucleation of ice plays an important role in technologies of cryopreservation, freeze-drying in biomedical research and the food industry, and the development of anti-icing coatings for aircraft.

Wang, B., Knopf, D. A., China, S., Arey, B., Harder, T. H., Gilles, M. K., & Laskin, A. (2016). Direct observation of ice nucleation events on individual atmospheric particles, Phys. Chem. Chem. Phys.,, 18, 29721-29731