Margaret Tolbert Group
We are a research group in the department of Chemistry and CIRES at the University of Colorado at Boulder. Our research is aimed at contributing to a better understanding of the Earth's complex atmosphere. Specifically, work in our group specializes in understanding atmospheric heterogeneous chemistry. For example, the importance of heterogeneous chemistry in catalyzing stratospheric ozone loss has been firmly established. In the case of the ozone hole, reactions on polar stratospheric clouds (PSCs) are responsible for repartitioning chlorine reservoir species into photochemically active species capable of catalytically destroying ozone. However, significant questions still remain as to the composition, phase, nucleation mechanisms, and surface chemistry of PSCs. Traditionally, work in our group has been aimed at answering these questions.
Today, the research in our group has expanded in an attempt to answer similar questions about cirrus clouds and other particulate matter that exist in the troposphere. Currently, our research explores the chemistry of tropospheric aerosols, and the impact of such aerosols on climate and visibility. Finally, we are also probing aerosols in other planetary atmospheres and studying the possible role of aerosols on early Earth as life was developing. Research in our group is funded primarily through NASA and NSF.
Heterogeneous nucleation studied in a long working-distance optical trap
The phase state of an atmospheric particle is critical in determining its optical properties, ability to serve as cloud-condensation nuclei, heterogeneous chemical reactivity, and atmospheric lifetime. The ability to predict when a particle will exist as a liquid, solid, or amorphous (semi)-solid is therefore essential for accurate global climate and air quality models. In the atmosphere, nucleation of a solid or crystalline phase from a liquid precursor is complicated by the wide variety of chemical components that exist and the ability for multiple particles to interact.
Laboratory Studies of Titan and Early Earth Tholins
Titan, Saturn's largest moon, is the most Earth-like planetary body in the solar system. It has volcanoes, lakes filled with liquid hydrocarbons, and a thick nitrogen-rich atmosphere. About 2% of Titan's atmosphere is methane, which undergoes photodissociation at the alpha-Lyman wavelength (121.6 nm). Methane fragments recombine into heavier organic molecules such as aliphatic and polycyclic aromatic hydrocarbons. The larger hydrocarbons condense into aerosols, particles suspended in air. I use a gas mixing chamber, a UV lamp, and a flow-tube setup (figure below) to create aerosol that have analogous properties to those found on Titan. Carl Sagan has termed these planetary aerosol analogues "tholins."
Liquid water on Mars? The formation and stability of aqueous salt solutions on present day Mars
Mars is known to have abundant H2O in the gas (water vapor) or solid (water ice) phases, but until very recently it was not thought that liquid water could exist on Mars. In the past few years, however, orbiting spacecraft and rovers on Mars have collected data suggesting that liquid water may exist on the surface or in the shallow subsurface for at least some portion of the Martian year. Pure liquid water is not thought to be stable due to the low pressures and temperatures found on the surface of Mars; however, salts such as perchlorates and chlorides are present in the Martian soil and may play a key role in stabilizing liquid water.
Optical Properties of Secondary Organic Aerosol
Atmospheric aerosols play a significant but poorly understood role in affecting the global radiative balance. Aerosols interact with light indirectly by nucleating clouds or directly by scattering and absorbing incoming light. Predicting the magnitude of the last two effects requires an accurate knowledge of optical properties of aerosols, but there is much uncertainty in this area.
Understanding oxygen incorporation into planetary atmospheric aerosols
Measurements of Titan’s atmosphere from the Cassini-Huygens Mission indicate that our understanding of complex organic chemistry and aerosol formation in planetary atmospheres is still in its infancy. Complex organic aerosols are important both for the radiative balance in an atmosphere and as a potential source of biologically interesting molecules, both of which have implications for the habitability of planets.