Mario J. Molina: Creative Science that Redefined Human Responsibility to the Planet While Uniting Chemistry and Meteorology (Invited Keynote Presentation)

Mario Molina's research, extending over more than four decades, stands as a triumph of human thought bridging from a molecular level understanding of kinetics, photochemistry and catalysis to a series of globally critical insights central to our understanding of stratospheric ozone that underscored the delicate nature of life at the planet's surface. As a result of the now recognized sensitivity of human health to the column concentration of stratospheric ozone, Mario's initial work on chlorofluorocarbons (CFCs) with Sherry Rowland redefined an understanding of global toxicology. But his insight into chemical mechanisms that lead to the Molina chlorine dimer mechanism provided the key foundation for dissecting the cause of the Antarctic Ozone Hole. This led directly to the formulation and international acceptance of the Montreal Protocol banning the production of CFCs. It was subsequently demonstrated that the chlorine dimer mechanism is responsible for the dramatic loss of ozone over the Arctic in late winter and early spring. However, Mario's contributions now link catalytic chemistry to meteorology through a combination of factors that place the summertime lower stratosphere over the U.S. in a unique position at the intersection of climate forcing and potential ozone loss in the stratosphere. The factors that conspire to establish this circumstance engage four independent considerations. First is the occurrence of severe storms in the U.S. Mid-West caused by the intersection of moist airflow from the Gulf of Mexico with the subtropical jet in late spring and summer. These storms are characterized by a combination of tornadoes, hail, heavy precipitation and high winds, the frequency and intensity of which are increasing with increased forcing of the climate system by the addition of CO2, CH4, N2O CFCs, and other infrared active species to the atmosphere associated with human activity. Second is the recognition that these severe storms are capable of injecting water vapor deep into the stratosphere over the U.S., with injection depths reaching the altitude of increasing inorganic halogen species formed by the photolytic breakdown of organic chorine and bromine transported into the stratosphere. Third is the recognition that the catalytic conversion of inorganic halogen species to chlorine and bromine radicals occurs on ubiquitous sulfate-water aerosols wherever and whenever the temperature-water vapor conditions are met. These conditions are met in the summertime lower stratosphere over the U.S. by virtue of temperatures between 200 and 205K in combination with convectively injected water vapor concentrations that reach 15 ppmv or greater. Fourth, is the recognition that the flow pattern of the lower stratosphere over the central U.S. in summer is repeatedly under the control of the North American monsoon that forms an anti-cyclonic (clockwise) gyre that captures the combination of convectively injected water vapor and boundary layer radical precursors within it, retaining that chemical mix for periods up to three weeks during the months of July and August. It is within this gyre over the U.S. that the catalytic reactions can occur potentially reducing the column concentration of ozone in summer as they do over the Arctic in winter. Recent developments linking observations from in situ aircraft, satellite and NEXRAD weather radar are reviewed and placed in the context of Mario Molina's seminal contributions linking the physical sciences to societal objectives.