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A recent study by Knopf Lab Alum Dr. Bingbing Wang was featured on the back cover of Physical Chemistry Chemical Physics, issue 43.

This study is by ITPA/SoMAS alum Dr. Bingbing Wang, 2011 (advised by Daniel Knopf), now Professor at Xiamen University in the State Key Lab of Marine and Environmental Science and College of Ocean and Earth Sciences and the Alexander Laskin (William. R. Wiley Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory) and Mary Gilles (Chemical Sciences Division, Lawrence Berkeley National Laboratory) research groups.

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.

Figure 1. Ice nucleation on an individual kaolinite particle at 205 K. Kaolinite is a clay mineral that serves as a surrogate of atmospheric mineral dust. Hexagonal ice crystals are false colored in blue and the Kaolinite mineral in brown. Scale bar represents 5 μm.

Bingbing Wanga, Daniel A. Knopfb, Swarup Chinaa, Bruce W. Areya, Tristan H. Harderc,d, Mary K. Gillesc, Alexander Laskina, Direct observation of ice nucleation events on individual atmospheric particles, Phys. Chem. Chem. Phys., 2016, 18, 29721-29731

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

About the Author

Daniel Knopf, Associate Professor at the Institute for Terrestrial and Planetary Atmospheres/School of Marine and Atmospheric Sciences, interested in the role of airborne particulate matter in air quality related issues and cloud formation processes. He joined Stony Brook University in 2007, received his M.Sc. 1999 with major in physics from the University of Heidelberg/Max-Planck Institute for Nuclear Physics and conducted his Ph.D. research at the Institute for Atmospheric and Climate Science at the Swiss Federal Institute of Technology followed by post-doctoral studies at the Chemistry Department of the University of British Columbia.

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