Computer capabilities have increased so much in recent years that it is now possible to imagine numerically bridging the gap between molecular-level properties of a substance and what would ordinarily be considered the substance’s bulk properties, such as its vapor pressure. One useful approach in this regard is Molecular Dynamics, in which the motion of a group of atoms, molecules, or ions is computed over some time of interest. Nowadays, for example, it takes a few days on a personal computer to obtain simulations of thousands of small molecules over hundreds of nanoseconds.
Research in my laboratory at the University of Puget Sound has lately focused on Molecular Dynamics calculations that simulate the surface of growing and ablating cirrus ice crystals. One motivation for this focus is that cirrus ice crystals exist in the natural atmosphere in a wide range of crystal habits (crystal forms), each with characteristic light-scattering properties, that affect the radiative balance of the planet in significantly different ways. Although we know the general factors that favor certain habits over others (temperature and water vapor concentration are most important), we still don’t really understand what causes a particular ice crystal to grow into a needle, a plate, or a dendrite. If we want to understand how cirrus clouds modulate the earth’s climate well enough to make accurate climate predictions, we need to understand better how ice crystals grow and ablate.
From a physical chemistry perspective, this translates into understanding processes such as sublimation, deposition, and growth of the ice-air interface, at a molecular level. One way of approaching this objective is through Molecular Dynamics, or MD. Students in my research laboratory have been using MD to understand how the quasiliquid layer captures and ejects individual water molecules. Together with a detailed analysis of the distribution of hydrogen bonding networks, this work has provided insight into possible mechanisms of ice crystal growth. This work has been carried out in collaboration with researchers Martina Roeselova and Pavel Jungwirth at the Czech Institute for Organic Chemistry and Biochemistry.
Other work in my lab has addressed questions about the interaction of water with light, either as an isolated gas-phase molecule, as an ice crystal, or as a liquid droplet. With collaborators Penny Rowe and Von Walden at the University of Idaho, we have tried to assess the radiative consequences of recent laboratory results that show that the complex index of refraction of supercooled liquid water is intermediate between that of room-temperature water and ice. One such consequence is that remote-sensing retrieval algorithms that ignore this intermediacy will systematically overestimate ice content in cold clouds.
Students in my research laboratory have also carried out experiments using the university’s low-temperature, variable pressure scanning electron microscope (VPSEM) to examine the interface properties of laboratory-grown ice. We have found that there are characteristic roughness morphologies for basal, prismatic, and other facets of such ice crystals, each capable of scattering light hitting a cirrus cloud crystal in characteristic ways. We are investigating metrics that would quantify these morphological properties, so that the roughness can be represented in cloud radiative transfer calculations. Students are also investigating how these morphological properties depend on impurities at the ice surface, which are considered to be common in the natural atmosphere.
References:
“Arrhenius analysis of anisotropic surface diffusion on the prismatic facet of ice”, Ivan Gladich, William Pfalzgraff, Ondrej Maršálek, Pavel Jungwirth, Martina Roeselová, and Steven Neshyba, Physical Chemistry Chemical Physics, 13 (invited paper), 19960-9 (2011).
“Comparative molecular dynamics study of vapor-exposed basal, prismatic, and pyramidal surfaces of ice”, William Pfalzgraff, Steven Neshyba, and Martina Roeselová, J. Phys. Chem. A, Buch Memorial Issue (invited paper) DOI: 10.1021/jp111359a (2011).
“A responsivity-based criterion for accurate calibration of FTIR spectra: identification of in-band low-responsivity wavenumbers”, Penny M. Rowe, Steven Neshyba, Christopher Cox, and Von P. Walden, Optics Express, 19, 5930-5941 (2011). (see www.opticsinfobase.org/abstract.cfm?uri=oe-19-7-5930.)
“A responsivity-based criterion for accurate calibration of FTIR spectra: theoretical development and bandwidth estimation”, Penny M. Rowe, Steven Neshyba, and Von P. Walden, Optics Express, 19, 5451-5463 (2011). (see www.opticsinfobase.org/abstract.cfm?uri=oe-19-6-5451.)
“Scanning electron microscopy and molecular dynamics of surfaces of growing and ablating hexagonal ice crystals”, William Pfalzgraff, Ryan Hulscher, and Steven Neshyba, Atmos. Chem. Phys., 10, 2927-2935 (2010). (see www.atmos-chem-phys.net/10/2927/2010/; www.atmos-chem-phys-discuss.net/9/20739/2009.html is the discussion paper associated with this article)
“Molecular Dynamics study of ice-vapor interactions via the quasi-liquid layer”, Steven Neshyba, Erin Nugent, Martina Roeselová, Pavel Jungwirth, J. Phys. Chem. C, 113, 4597-4604, doi: 10.1021/jp810589a (2009).