A. R. Ravishankara
Professor Adjoint (physical). Ph.D., University of Florida, 1975; Postdoctoral Research Associate, University of Maryland, 1976.
Professor Ravishankara's research is aimed at understanding the chemistry of Earth's atmosphere as it is today and as it may be in the future. This research is essential for understanding phenomena such as the Antarctic ozone hole, global warming (greenhouse effect), acid precipitation, urban smog and long-term stratospheric ozone loss. The results of these studies help us elucidate how man's activities affect our air and provide information on how to preserve our environment.
Professor Ravishankara carries out laboratory investigations of gas-phase processes that are important in Earth's atmosphere. Since free radicals are the key players in Earth's oxidative atmosphere, their reactions are studied to obtain the temperature and pressure dependencies of the rate coefficients, to identify products of reactions, and to determine the reaction mechanisms. Since Earth's atmosphere is driven by the input of solar radiation, photochemistry plays a crucial role in determining its composition. Therefore, photochemistry of small molecules is studied to understand how they break apart upon light absorption. Most of the kinetic and photochemical studies are carried out using spectroscopic techniques. Therefore, electronic spectra of small transients are studied to enable their detection. Such characterization also allows monitoring these species in the atmosphere.
Pulsed photolysis or discharge flow methods are used for kinetics experiments. Detection of the reacting species is carried out using laser-induced fluorescence, resonance fluorescence, long-path UV/visible absorption, and photoionization or chemical ionization mass spectrometry. The free radicals are generated by pulsed laser photolysis or by microwave/electrical discharges. Photochemical studies are carried out using mostly optical methods. Quantities such as absorption cross-sections, quantum yields, excitation spectra, fluorescence lifetimes, and quenching rate coefficients are measured.
Recently it has become very clear that reactions between closed shell molecules on surfaces and transport of molecules into drops play critical roles in atmospheric chemistry. Therefore, reactions of closed shell molecules and free radicals are studied on ice, doped ice, water, and sulfuric acid, the four major possible sites for heterogeneous reactions in the stratosphere. In addition, reactive uptake into liquid water and sulfuric acid is also investigated. In some cases, the transport of the molecules from the gas phase into the liquid/solid phase can be the rate limiting step, the bottleneck, in the overall conversion process. Such processes are identified and investigated in the laboratory. A tubular flow reactor whose inside surface is coated with the solid/liquid of interest is used in conjunction with detection methods such as chemical ionization mass spectrometry and laser-induced fluorescence.
Finally, measurements of atmospheric constituents are occasionally carried out using spectroscopic methods.
Recent studies have emphasized reactions and photochemistry of halogen oxides that are important in stratospheric ozone loss, the chemistry of chlorofluoromethane substitutes, the role of sulfur chemistry in controlling cloud formation, and that produce and remove infrared absorbing molecules that can augment global warming by CO2. Transport of gases to the condensed phases is also being investigated to delineate the role of heterogeneous chemistry in acid precipitation formation and polar stratospheric ozone loss.
Selected Publications
A.A. Turnipseed, G.L. Vaghjiani, J.E. Thompson, and A.R. Ravishankara, "Photodissociation of HNO3 at 193, 222, and 248 nm: Products and Quantum Yields, "J. Chem. Phys. 96, 5887 (1992).
J.D. Burkholder, R.L. Mauldin, III, R.J. Yokelson, and A.R. Ravishankara, "Cl2O3: A Kinetic and Spectroscopic Study," J. Phys. Chem.. 97, 7597 (1993).
A.M. Schmoltner, R.K. Talukdar, R. F. Warren, A.M. Mellouki, L. Goldfarb, T. Gierczak, S.A. McKeen, and A.R. Ravishankara,"Rate Coefficients for Reactions of Several Hydrofluoricarbons with OH and (O1D) and their Atmospheric Lifetimes," J. Phys. Chem.. 97, 8976 (1993).
A. R. Ravishankara, A. A. Turnipseed, N. R. Jensen, S. Barone, M. Mills, C. J. Howard, and S. Solomon. "Do Hydrofluorocarbons Destroy Stratospheric Ozone?" Science 263, 71 (1994).
R. D. Hanson, A. A. Ravishankara, and S. Solomon. "Heterogeneous Reactions in Sulfuric Acid Aerosols: A Framework for Model Calculations," J. Geophys. Res. 99, 3615 (1994).