Margaret A. Tolbert
Ph.D. California Institute of Technology, 1986
Professor, Chemistry and Biochemistry
Atmospheric chemistry; heterogeneous reactions, polar stratospheric clouds and cirrus clouds, nucleation, and planetary atmospheres.
Current Research: Laboratory Studies of Clouds and Aerosols
Cirrus clouds, composed of water ice, cover up to 30 percent of the Earth’s surface at any time and subvisible cirrus are almost always present in parts of the tropics. Cirrus and subvisible cirrus clouds play an important role in the climate system as well as in controlling the amount of water getting into the stratosphere. The clouds are usually optically thin in visible wavelengths, allowing most, but not all, sunlight to reach the Earth’s surface. In contrast, the outgoing infrared radiation is efficiently absorbed by cirrus ice particles. While the net effect of cirrus clouds on climate is usually a warming at the surface, the microphysical properties of the clouds dictate the overall climatic impact. The microphysical properties, in turn, depend on the nucleation mechanism of ice in the atmosphere. In laboratory studies, our research group is examining ice nucleation on a wide range of possible atmospheric aerosols including organics, minerals, sulfates, and combinations of these species.
To study ice nucleation, we are using a combination of optical and Raman microscopy. In an environmental cell, we expose aerosols to increasing relative humidity at low temperature and detect ice nucleation using optical microscopy. We then evaporate the ice and use Raman spectroscopy to identify the chemical nature of the particles that nucleated ice. In this way, we can identify the species most likely to nucleate ice, and also determine the atmospheric conditions necessary for ice nucleation.
Work in our laboratory to date has shown that the chemical composition and morphology of the nucleating particles is more important than the particle size in controlling ice nucleation.
In addition to laboratory studies on well-defined particles generated with known composition, additional studies are probing heterogeneous ice nucleation on samples collected in the field. Graduate student Kelly Baustian has participated in two field experiments at Storm Peak Laboratory in Steamboat Springs, CO, collecting particles for analysis using our Raman technique. Here we examine unknown samples to determine which particles are the best nuclei, followed by a detailed chemical analysis of those particles that nucleate ice. These studies are giving us new insight into atmospheric ice nucleation.
University of Colorado Sponsored Research 2003-04 Feature Highlighting Some of CU's outstanding Women Scholars, "Contemplating the Clouds"
Dr. Tolbert is a member of the CIRES Professor..