Special Issue Preface pubs.acs.org/JPCA
James G. Anderson Tribute 1991, DOI: 10.1126/science.252.5010.1260], HOx (OH and HO2) [Wennberg; et al. Science, 1994, DOI: 10.1126/ science.266.5184.398; Wennberg; et al. Science, 1998, DOI: 10.1126/science.279.5347.49] and finally NO2 and the reservoir species ClONO2 [Wennberg; et al. Science, 1998, DOI: 10.1126/science.279.5347.49; Bonne; et al. J. Geophys. Res.: Atmos., 2000, DOI: 10.1029/1999JD900996; Perkins; et al. J. Phys. Chem. A 2001, DOI: 10.1021/jp002519n]. The methodology developed in Anderson’s group starting with balloon flights in the 1970s is to measure in situ the concentrations of free radicals involved in rate-limiting steps of catalytic processes, and to complement these measurements with laboratory measurements of the elementary reaction kinetics under the appropriate conditions (temperature and pressure) for the stratosphere [Donahue; et al. J. Geophys. Res.: Atmos. 1997, DOI: 10.1021/jp002519n; Dransfield; et al. Geophys. Res. Lett. 1999, DOI: 10.1029/1999GL900028]. This is a simple but elegant proposition: because these are elementary, homogeneous, gas-phase reactions, the combined measurements constitute a direct determination of the critical reaction rates, leaving very little wiggle room and enabling a direct test of proposed overall reaction mechanisms. The system is stiff, meaning that important rates are small compared to very large rates for a subset of reactions (e.g., the dissociation and recombination of ozone, O3 → O + O2 or O + O2 + M → O3 + M, is very much faster than the odd-oxygen removal steps), but with a sufficient set of simultaneous, high-frequency measurements, even this stiff system can be completely tested for closure. The audacity of Jim’s vision has been to actually execute this simple proposition. This means pushing the technological envelope to extremes−adapting the techniques of dischargeflow chemical kinetics to ambient measurements by opening the ends of a flowtube and using airspeed rather than a pump to propel gases through the tube, but under controlled conditions; reeling resonance-lamp detectors down a multiple kilometer Kevlar line from a floating balloon to obtain vertical profiles in the stratosphere; flying a frequency-doubled dye laser pumped by a copper vapor laser on a 50 million cubic foot balloon into the middle stratosphere to measure HOx (OH and HO2); and, ultimately, helping to pack the NASA ER-2 full of automated instruments, essentially all of which had to operate simultaneously without supervision for an experiment to succeed. A final element of his vision is to conduct this suite of measurements with sufficient precision and bandwidth to turn the variability of atmospheric composition from a troubling source of noise into a powerful signal. The “smoking gun” figure is a vivid example of this, where the filamentary nature of the polar vortex “collar” region reveals point by point the evolving anticorrelation between ozone and ClO in air masses trapped by orthogonal surfaces of potential vorticity (angular momentum) and potential temperature (entropy). However,
Photo by Shirine Anderson.
T
he most important event in the history of atmospheric chemistry, and arguably environmental science, was the discovery and subsequent diagnosis of the Antarctic ozone hole. The pinnacle of that research was the “smoking gun” figure published by Anderson, Toohey, and Brune [Science, 1991, DOI: 10.1126/science.251.4989.39], reproduced here in Figure 1. That plot, based on measurements of chlorine monoxide (ClO), ozone (O3), and other species on the NASA ER-2 aircraft in 1987, helped galvanize international resolve to remove reactive halogens from the stratosphere, leading first to the London amendments to the Montreal Protocol when the data were known in preliminary form and then to the Copenhagen amendments once the final results were published. Without that action, the global ramifications of ozone loss today would be catastrophic, and increases in the radiative forcing by chlorofluorcarbons would have led to even larger changes in global climate. James G. Anderson of Harvard University is the titanic force behind the research that systematically and conclusively established the quantitative relationship between halogen catalysts and the rapid ozone depletion known as the ozone hole. His unwavering vision is to directly measure the concentrations of free radicals involved in the rate-limiting steps of catalytic ozone removal, thereby establishing without any doubt the rate of these (homogeneous) processes in the atmosphere. This methodology, based on study and direct measurement of elementary reaction kinetics, combined with in situ radical detection, allowed the scientific community to present unequivocal and clear assessments of the science to the international policy community, driving rapid progress toward international agreement at a time when such agreement seemed unimaginable. In situ observations drove progress toward understanding the ozone hole in the late 1980s, arctic ozone loss in the early 1990s, and the effects of civil aviation in the 1990s. Jim played a unique role in the process, systematically leading the developments of instruments to measure ClO [Anderson; et al. Science, 1991, DOI: 10.1126/science.251.4989.39; Brune; et al. Science, © 2016 American Chemical Society
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DOI: 10.1021/acs.jpca.5b11957 J. Phys. Chem. A 2016, 120, 1317−1319
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The Journal of Physical Chemistry A
Figure 1. Evolving anticorrelation of ozone (red) and chlorine monoxide (ClO, blue) observed on the NASA ER2 during the austral spring of 1987. Reprinted with permission from Science, copyright 1991 (DOI: 10.1126/science.251.4989.39).
this also means that the vast data set from stratospheric flights can be examined as a partial derivative surface of the coupled chemical system. The measurements thus directly reveal how the system will respond to changes, further enriching the information that can be given to policy makers. This includes intentional or inadvertent changes to the catalytic species (i.e., halogens) or climate driven changes (temperature, water vapor, etc.). Both the conception and realization of these measurement campaigns were only possible via the collaborative work of a large team. That includes both an extended network of research groups in academic institutions and national laboratories and also a highly talented group of researchers in Jim’s laboratory at Harvard. Many of those researchers have gone on to independent careers, whereas several have remained as critical elements of the Harvard University Atmospheric Research Project, also known as Jim’s group. This issue contains a sampling of recent research from those researchers as well as collaborators from though out Jim’s career. As important as describing ozone depletion was, more important may be the resonance of that work with current research and policy associated with climate change. The decade preceding the discovery and understanding of the ozone hole was one filled with recalcitrance, denial, and public scorn of the scientists claiming that Freon destroyed ozone. Almost every argument in the denial of ozone depletion is resonant with arguments raised by climate deniers today (sadly, several of the
major players are even the same). Before the emergence of the ozone hole, ozone depletion was a phenomenon understood for the most part through theory, laboratory experiments, and large-scale atmospheric models. It was thought to be diffuse, occurring largely in the upper stratosphere, and gradual, predicted to cause column losses of a few percent per decade. That gradual, homogeneous, tropical depletion has occurred, as predicted in the 1970s, and it is now slowly abating as the effects of the Montreal protocol and amendments literally diffuse into the stratosphere. This relatively small signal, based largely on theory and computer models, is directly analogous to the situation with climate. The global climate signal is weak though significant, but our theoretical understanding of the climate system is robust, albeit with important uncertainties remaining in the exact sensitivity of climate to increased forcing (just as the exact degree of expected ozone removal was uncertain in the late 1970s). Surprises in the climate sensitivity remain possible even likely. The estimated probability distribution in climate sensitivity is skewed toward high sensitivity; it is more likely that climate is more sensitive than the consensus estimate to forcing changes than it is less sensitive to that forcing. Jim has continued to develop and carry out high-fidelity observations of the climate system. In particular, his recent analysis of stratospheric water enhancement by storms highlights the tight coupling between climate change and enhanced ozone depletion and increased storm frequency. Jim’s 1318
DOI: 10.1021/acs.jpca.5b11957 J. Phys. Chem. A 2016, 120, 1317−1319
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The Journal of Physical Chemistry A science continues to drive policy and he takes an proactive intergenerational role in education, research, and outreach. Noteworthy is his highly regarded undergraduate class in energy and environment at Harvard that is training the next generation of politicians, lawyers, scientists, artists, doctors, and engineers to confront and solve the climate-change problem. We must not forget that the ozone hole was a complete surprise. If any of us were asked before 1985 what the worstcase scenario might be for an unknown process, it might well have been something that simultaneously converted all of the potential reactive halogen catalysts into an active form and removed the nitrogen oxides (which play a dual role as catalysts in some cases but as inhibitors for halogen catalysts in others). To our shock and dismay, this nightmare turned out to be true. The analogies in climate are phenomena such as much more rapid loss of continental ice sheets (i.e., Greenland) or sudden changes to ocean circulation. These are not thought to be likely, but the ozone hole was not thought of at all. With ozone, the international community was able to take action, and as far as we know, extensive and long-lasting ecological damage has been avoided (though the system will take a century to clean out completely). With climate, it is not at all clear that nature will be so kind as to grant us a near miss if one of these nightmare scenariosor others as yet unimaginedemerge as reality. This time we know that, absent some as yet unknown method to economically remove CO2 from the atmosphere, the system will take thousands of years to clean out completely.
Neil M. Donahue*,† Manvendra K. Dubey‡ Paul O. Wennberg§ William H. Brune∥ †
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Center for Atmospheric Particle Studies, Carnegie Mellon University ‡ Earth and Environmental Sciences Division, Los Alamos National Laboratory § Division of Geological and Planetary Sciences and Division of Engineering and Applied Science, California Institute of Technology ∥ Department of Meteorology, Pennsylvania State University
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: (412) 268-4415.
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DOI: 10.1021/acs.jpca.5b11957 J. Phys. Chem. A 2016, 120, 1317−1319
