Page 107

2
Atmospheric Chemistry Research Entering the Twenty-First Century1

Summary

Atmospheric chemistry came of age during the latter half of the twentieth century. Through the application of modem analytical and computational techniques, scientists were able to elucidate the critical role the atmosphere plays as the "connective tissue" for life on Earth. In the process, another, more disturbing insight was uncovered: the activities of an increasingly populous and technological human society are changing the composition of the atmosphere on local to regional to global scales. Experience has shown that air pollution on local and regional scales can be environmentally and economically destructive. The consequences of chemical change on a global scale have yet to be fully assessed, but the potential for catastrophic effects exists.

1 Report of the Committee on Atmospheric Chemistry: W.L. Chameides (Chair), Georgia Institute of Technology; J.G. Anderson, Harvard University; M.A. Carroll, University of Michigan, Ann Arbor; J.M. Hales, ENVAIR; D.J. Hofmann, NOAA Aeronomy Laboratory; B.J. Huebert, University of Hawaii; J.A. Logan, Harvard University; A.R. Ravishankara, NOAA Aeronomy Laboratory; D. Schimel, University Corporation for Atmospheric Research; and M.A. Tolbert, University of Colorado, Boulder. The group gratefully acknowledges contributions from C. Ennis, NOAA Aeronomy Laboratory; D. Fahey, NOAA Aeronomy Laboratory; F. Fehsenfeld, NOAA Aeronomy Laboratory; I. Fung. University of Victoria, British Columbia; E.A. Holland, National Center for Atmospheric Research; D. Jacob. Harvard University; C.E. Kolb, Aerodyne Research, Inc.; H. Levy, II, NOAA Goddard Fluid Dynamics Laboratory; S. Liu, Georgia Institute of Technology; P. Reich, University of Minnesota; P. Samson. University of Michigan; and P. Tans, NOAA Aeronomy Laboratory.



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 107
Page 107 2 Atmospheric Chemistry Research Entering the Twenty-First Century1 Summary Atmospheric chemistry came of age during the latter half of the twentieth century. Through the application of modem analytical and computational techniques, scientists were able to elucidate the critical role the atmosphere plays as the "connective tissue" for life on Earth. In the process, another, more disturbing insight was uncovered: the activities of an increasingly populous and technological human society are changing the composition of the atmosphere on local to regional to global scales. Experience has shown that air pollution on local and regional scales can be environmentally and economically destructive. The consequences of chemical change on a global scale have yet to be fully assessed, but the potential for catastrophic effects exists. 1 Report of the Committee on Atmospheric Chemistry: W.L. Chameides (Chair), Georgia Institute of Technology; J.G. Anderson, Harvard University; M.A. Carroll, University of Michigan, Ann Arbor; J.M. Hales, ENVAIR; D.J. Hofmann, NOAA Aeronomy Laboratory; B.J. Huebert, University of Hawaii; J.A. Logan, Harvard University; A.R. Ravishankara, NOAA Aeronomy Laboratory; D. Schimel, University Corporation for Atmospheric Research; and M.A. Tolbert, University of Colorado, Boulder. The group gratefully acknowledges contributions from C. Ennis, NOAA Aeronomy Laboratory; D. Fahey, NOAA Aeronomy Laboratory; F. Fehsenfeld, NOAA Aeronomy Laboratory; I. Fung. University of Victoria, British Columbia; E.A. Holland, National Center for Atmospheric Research; D. Jacob. Harvard University; C.E. Kolb, Aerodyne Research, Inc.; H. Levy, II, NOAA Goddard Fluid Dynamics Laboratory; S. Liu, Georgia Institute of Technology; P. Reich, University of Minnesota; P. Samson. University of Michigan; and P. Tans, NOAA Aeronomy Laboratory.

OCR for page 107
Page 108

Box II.2.1 Environmentally Important Atmospheric Species These species are scientifically interesting and important to human health and welfare because of their radiative (e.g., climate changing) and/or chemical properties. They include the following:   • Stratospheric ozone   • Greenhouse gases   • Photochemical oxidants   • Atmospheric aerosols   • Toxics and nutrients Documenting the changing concentrations and distribution of these species, elucidating the processes that control their concentrations, and assessing their impacts on important environmental and ecological parameters will define the principal challenges for atmospheric chemistry in the coming decades. The scientific questions facing atmospheric chemistry entering the twenty-first century are intellectually profound but are also of vital social and economic importance. They relate to atmospheric constituents that are fundamentally important to our environment: stratospheric ozone, greenhouse gases, ozone and photochemical oxidants in the lower atmosphere, atmospheric aerosols or particulate matter, and toxics and nutrients (see Box II.2.1). It is perhaps a measure of the strides made in recent decades, that the issues of atmospheric chemistry are familiar to the general public, policy makers, and scientists alike. Continued progress in the twenty-first century will require an ambitious, but judicious, commitment of financial, technological, and human resources to document the changing composition of the atmosphere and elucidate the causes and potential consequences of these changes. Major Scientific Questions and Challenge The principal focus for atmospheric chemistry research entering the twenty-first century will be the "Environmentally Important Atmospheric Species"— species that, by virtue of their radiative and/or chemical properties, affect climate, key ecosystems, and living organisms (including humans). From an intellectual point of view, these species are interesting because they influence the life support system of our planet. From a societal point of view, they are also of central importance because they directly impact human health and welfare.

OCR for page 107
Page 109 The challenge for atmospheric chemistry research in the coming decades follows: Development and application of the tools and scientific infrastructure necessary to document and predict the concentrations and effects of Environmentally Important Atmospheric Species on a wide variety of spatial and temporal scales. To meet this challenge, atmospheric chemistry research should be formulated around three fundamental questions: 1. What are the shorter-term periodic and longer-term secular trends in the concentrations of Environmentally Important Atmospheric Species on local to global scales? What are the causes of these trends? 2. How will the concentrations of these species change in the future? What are the most effective and feasible policy options for managing these changes? 3. What will be the totality of environmental effects of present and future trends in the concentrations of these species? Overarching Research Challenges The scientific strategy for atmospheric chemistry emerges logically from the application of these fundamental scientific questions to each of the Environmentally Important Atmospheric Species. It is a strategy that endeavors to continuously improve our understanding of the underlying chemical, physical, and ecological processes that control the concentrations of these species, while providing timely and relevant input to decision makers. Toward these ends, the scientific research strategy in atmospheric chemistry must include the following: • Document the chemical climatology and meteorology of the atmosphere, particularly their variability and long-term trends, through the development and maintenance of diverse and interrelated arrays of monitoring networks. • Develop and evaluate predictive tools and models of atmospheric chemistry through a synthesis of information gathered from process-oriented field studies, laboratory experiments, and other observational efforts; their representation in mathematical/numerical algorithms; and the testing of these algorithms in well-posed model-evaluation field experiments. • Provide assessments of the efficacy of environmental management activities through the gathering and interpretation of relevant air quality data. • Be holistic and integrated in the study of the Environmentally Important Atmospheric Species and of the chemical, physical, and ecological interactions that couple them together.

OCR for page 107
Page 110 Disciplinary Research Challenges The disciplinary challenges listed below focus on the specific, key scientific issues facing the atmospheric chemistry community in the twenty-first century: • Stratospheric Ozone Challenges: Document the distributions, variability, and trends of stratospheric ozone and the key species that control its catalytic destruction; elucidate the coupling between chemistry, dynamics, and radiation in the stratosphere and upper troposphere. • Greenhouse Gas Challenges: Elucidate the processes that control the abundances, variabilities, and long-term trends of atmospheric CO2 (carbon dioxide), CH4 (methane), N2O (nitrous oxide), and upper-tropospheric and lower-stratospheric O3 (ozone) and water vapor; and expand global monitoring networks to include upper-tropospheric and lower-stratospheric O3 and water vapor. • Photochemical Oxidant Challenges: Develop the observational and computational tools and strategies needed by decision makers to effectively manage ozone pollution; elucidate the processes that control, and the interrelations that exist between, the ozone precursor species, tropospheric ozone, and the oxidizing capacity of the atmosphere. • Atmospheric Aerosol Challenges: Document the chemical, physical, and radiative properties of atmospheric aerosols, their spatial extent, and long-term trends; elucidate the chemical and physical processes responsible for determining the size, concentration, and chemical characteristics of atmospheric aerosols. • Toxics and Nutrients Challenges: Document the rates of chemical exchange between the atmosphere and key ecosystems of economic and environmental import; elucidate the extent to which interactions between the atmosphere and biosphere are influenced by changing concentrations and deposition of harmful and beneficial compounds. Infrastructural Initiatives The following infrastructural initiatives provide the resources and capabilities recommended to accomplish the disciplinary challenges: • Global Observing System: deployment of an observing system for moderately lived species to complement ongoing networks and measurement platforms focusing on long-lived species and stratospheric ozone. • Ecosystem Exposure Systems: deployment of monitoring networks capable of assessing ecosystem exposure to primary and secondary toxics and nutrients. • Surface Exchange Measurement Systems: development and deployment of measurement systems capable of quantifying chemical exchange between the atmosphere and key biological or ecosystems.

OCR for page 107
Page 111 • Environmental Management Systems: demonstration and assessment of the feasibility of operational "chemical meteorology" as a prognostic tool for environmental managers and regulators. • Instrument Development and Technology Transfer: development of programs and facilities to support the evaluation of new atmospheric chemical instruments and their transfer to the scientific, regulatory, and private sector communities. • Fundamental Condensed Phase and Heterogeneous Chemistry: development and maintenance of laboratory facilities focused on condensed phase and heterogeneous chemical processes relevant to the atmosphere. Expected Benefits and Contribution to National Well-Being The scientific questions to be addressed by the atmospheric chemistry research community entering the twenty-first century are central to our understanding of the chemical and physical environment in which we human beings must reside. For this reason, the science of atmospheric chemistry is highly relevant to the future development and economic vitality of our society. Today, the changing chemistry of our atmosphere on local, regional, and global scales is an observational fact. These changes are impacting human health and placing economically and environmentally important resources and ecosystems at risk. At the same time, air quality management activities in the United States cost tens of billions of dollars annually. Research in atmospheric chemistry and the resulting improvements in our predictive capabilities will help us to maximize the environmental and economic benefits gained from these sizable investments in air quality management, while also teaching us how to minimize the deleterious effects of human activity on the chemical and physical environment. Introduction and Overview As the world stands on the threshold of a new millennium, the atmospheric chemistry community stands at the portal of a new era of scientific research. During the latter half of the twentieth century, the discipline of atmospheric chemistry came of age. Scientific study revealed the crucial role that the chemistry of the atmosphere plays in the life support system of the planet, acting as a "connective tissue" by which organisms of the biosphere interact and exchange materials and energy. It also uncovered a more disturbing insight: the activities of an increasingly populous and technological human society are changing the composition of the atmosphere on local, regional, and even global scales. Experience has shown that air pollution on local and regional scales can be environmentally and economically destructive. The consequences of chemical change on a global scale could be even more damaging. Thus, the scientific questions

OCR for page 107
Page 112 facing atmospheric chemistry are not only intellectually challenging but also of vital social and economic importance. The challenge for atmospheric chemistry as it enters the twenty-first century will be to build on the discoveries of the twentieth century by maintaining its scientific vitality and rigor while also making the results of its scientific and technological advances available to influence the nation's and the world's social and economic development. In this Disciplinary Assessment, we discuss the strategy that will be necessary to address the major scientific issues of the discipline while also providing decision makers with the information and tools they require to manage and maintain environmental and economic vitality. We begin our discussion with a statement of the mission for atmospheric chemistry research entering the twenty-first century. The Mission Development and application of the tools and scientific infrastructure necessary to document and predict the concentrations and effects of Environmentally Important Atmospheric Species on a wide variety of spatial and temporal scales. In identifying the mission for atmospheric chemistry research entering the twenty-first century, we have adopted three basic premises: 1. The financial and human resources available for research and development in the coming decades will be limited. 2. The activities of an increasingly populous and technological society have and will continue to perturb critical environmental factors that affect the natural resources on which our society relies. 3. Unraveling the mechanisms that couple the chemistry of the atmosphere to the life support system of the planet represents one of the major intellectual and technological challenges of the coming decades. Premises 1 and 2 relate to the resource- and policy-relevant issues that must be considered in defining the mission for atmospheric chemistry research, whereas premise 3 focuses on the intellectual or curiosity-based raison d'etre for the discipline. The prospect of limited resources for research and development indicated in premise 1 demands that a rigorous prioritization be applied to any contemporary research program, so that the most pressing scientific issues can be addressed in the allocation of public resources to the scientific community. Premise 2 suggests that priority should be placed on developing a scientifically robust, predictive, and systematic understanding of the Earth system, its chemical environment, and the relationships between the economic and technological growth of the world's nations and the environmental vitality and natural resources on which they depend. The development of a research program often requires compromises between

OCR for page 107
Page 113 Figure II.2.1 Environmentally Important Atmospheric Species are atmospheric constituents that affect human health and welfare and thus are central to a policy-relevant research program in atmospheric chemistry. Because these species drive the interaction between the atmosphere and the life support system of the planet, they are also central to a curiosity-based research program in atmospheric chemistry. Environmentally Important Atmospheric Species include greenhouse gases (e.g., CO2, CH4, N2O), aerosols, and stratospheric ozone—species that, because of their radiative properties, affect the climate and other physical characteristics of our environment. They also include the photochemical oxidants and tropospheric ozone, acid aerosols, and a wide variety of toxic and nutritive substances that, because of their chemical properties, affect humans and ecosystems of economic and environmental importance when they come in direct contact with them. Although the radiatively important species' effects are more commonly felt on a global scale, the effects of the ecologically important species are most often experienced on local-to-regional scales. Nevertheless, despite the varying scales of their radiative and chemical effects, research has revealed that the atmospheric cycles of these species are coupled together through complex photochemical and dynamical interactions. Unraveling these complex interactions represents one of the major challenges of atmospheric chemistry research in the coming decades. the priorities dictated by policy-relevant issues and those dictated by more theoretical interests. However, in the case of atmospheric chemistry research we find a strong resonance between the two. Atmospheric chemistry research in the coming decades should be focused on documenting and predicting the concentrations and effects of the chemical constituents that most directly affect the physical and biological environment and, by extension, human health and welfare. We refer to these species, here, in the most generic sense, as the Environmentally Important Atmospheric Species that, by virtue of their radiative and/or chemical properties, directly affect living systems and key environmental parameters (see Figure II.2.1).

OCR for page 107
Page 114 For atmospheric chemistry to make significant scientific advances in the coming decades, however, its research focus on these Environmentally Important Atmospheric Species must go well beyond simple observation and documentation of chemical content and change, to a rigorous investigation of the underlying chemical, physical, and ecological processes that determine the atmospheric concentrations of these species. It is, after all, only through understanding these processes that a genuine appreciation of the atmosphere and its relationship to the Earth system can be fostered, and a reliable predictive capability can be achieved and made available for the development of effective public policy. Additionally, because the atmosphere and the stresses placed on it are continually changing, a significant portion of the resources made available for atmospheric chemistry research in the coming decades must be used to develop an enduring research infrastructure that can inform decision makers in an open, effective, and responsive fashion. The mission for atmospheric chemistry research in the coming decades must therefore combine a focus on the Environmentally Important Atmospheric Species with a commitment to the development of a comprehensive, long-term research capability and technological infrastructure. Hence, the mission of atmospheric chemistry in the coming decades: Development and application of the tools and scientific infrastructure necessary to document and predict the concentrations and effects of Environmentally Important Atmospheric Species on a wide variety of spatial and temporal scales. In the following sections, we consider how to accomplish this mission most effectively by first considering what is now known about the atmosphere and then identifying the key unresolved scientific questions surrounding a number of Environmentally Important Atmospheric Species and the research challenges that grow from these questions. Insights of the Twentieth Century By grappling with a number of critical, but largely unforeseen environmental problems in recent decades, scientists have gained fundamental new insights about the atmospheric chemical system. The study of atmospheric chemistry as a quantitative, scientific discipline can be traced to the eighteenth century when world-renowned chemists such as Joseph Priestley, Antoine-Laurent Lavoisier, and Henry Cavendish undertook the investigation of the chemical components of the atmosphere (Farber, 1961; Weeks and Leicester, 1968). It was largely through their efforts, as well as those of a number of prominent chemists and physicists who succeeded them in the nine-

OCR for page 107
Page 115 TABLE II.2.1 Important Trace Species of the Atmospherea Species Concentration (Mole Fraction) Principal Sources Methane (CH4) 1.6 × 10-6 Biogenic Carbon Monoxide (CO) (0.5 - 2) × 10-7 Photochemical, Anthropogenic Ozone (O3) 10-8 - 10-6 Photochemical Reactive Nitrogen (NOy) 10-11 - 10-6 Lightning, Anthropogenic Ammonia (NH3) 10-11 - 10-9 Biogenic Particulate Nitrate (NO3-) 10-12 - 10-8 Photochemical, Anthropogenic Particulate Ammonium (NH4-) 10-11 - 10-8 Photochemical, Anthropogenic Nitrous Oxide (N2O) 3 × 10-7 Biogenic, Anthropogenic Hydrogen (H2) 5 × 10-7 Biogenic, Photochemical Hydroxyl (OH) 10-13 - 10-11 Photochemical Peroxyl (HO2) 10-13 - 10-11 Photochemical Hydrogen Peroxide (H2O2) 10-10 - 10-8 Photochemical Formaldehyde (H2CO) 10-10 - 10-9 Photochemical Sulfur Dioxide (SO2) 10-11 - 10-9 Anthropogenic, Volcanic Dimethylsulfide (CH3CCH3) 10-11 - 10-10 Biogenic Carbon Disulfide (CS2) 10-11 - 10-10 Anthropogenic, Biogenic Carbonyl Sulfide (OCS) 10-10 Anthropogenic, Biogenic Particulate Sulfate (SO4-) 10-11 - 10-8 Anthropogenic, Photochemical a After Chameides and Davis (1982). teenth century, that the identity and concentration of the major components of the atmosphere (i.e., nitrogen, oxygen, water, carbon dioxide, and the rare gases) were established. In the late nineteenth and early twentieth centuries, atmospheric chemists shifted their focus from identifying the major atmospheric constituents to consideration of the trace constituents, that is, the gaseous and aerosol atmospheric species having concentrations of less than a few parts per million per volume of air (i.e., ppmv). The application of modem chemical analytical techniques revealed the atmosphere to be a reservoir of a myriad of trace species, whose presence can be attributed to a complex array of geological, biological, chemical, and in many cases, anthropogenic processes (see Table II.2.1). Moreover, these trace species were found to have a disproportionately large impact on our environment. In some instances, they adversely affect plant and animal life because of their toxic properties; in other instances, they benefit these or other organisms because of their nutritive properties; in still other instances, they affect the physical climate because of their radiative properties. The latter half of the twentieth century has seen another major shift in atmospheric chemistry as scientists attempt to grapple with a number of potentially critical environmental problems, including stratospheric ozone depletion, urban

OCR for page 107
Page 116 Figure II.2.2 From an atmospheric chemistry point of view, global change is an observational fact  not a theoretical possibility. (A) Average change per decade in the total atmospheric  ozone column as a function of latitude based on recent Dobson station measurements  (after WMO, 1995). (B-E) Average global concentrations of CO2, CH4, N2O, and CFC-11 since the mid-1700s (after IPCC, 1990). photochemical smog, and rising concentrations of ''greenhouse gases'' (NRC, 1984). In the process, a new, policy-relevant research paradigm for atmospheric chemistry has developed that has profoundly altered its role in society. More importantly, the insights gained from the study of these environmental crises have irrevocably changed our understanding of the atmospheric chemical system in which we as a species must reside. The major aspects of these new insights are outlined below.

OCR for page 107
Page 117 The Chemical State of the Atmosphere Has Changed in the Past and Is Continuing to Change Observations have shown irrefutably that the chemistry of the atmosphere is changing on local, regional, and global scales; indeed, from a chemical point of view, global change is an observational fact not a theoretical possibility. The annual appearance of the Antarctic ozone hole provides striking evidence of the atmosphere's vulnerability to chemical perturbation. Although smaller in magnitude, the depletion of stratospheric ozone in the temperate latitudes over the past decade is perhaps equally disturbing (Figure II.2.2A). Moreover, present-day measurements coupled with analyses of ancient air trapped in ice cores provide a record of dramatic, global increases in the concentrations of a number of long-lived greenhouse gases such as carbon dioxide, methane, nitrous oxide, various chlorofluorocarbons (CFCs), and other halocarbons (Figure II.2.2B-E). Although secular trends in shorter-lived species are more difficult to document, a strong case can be made that the abundances of tropospheric ozone and sulfate and carbonaceous aerosols have also increased significantly in the Northern Hemisphere during the past century (NRC, 1993). Humans Are a Significant Driving Force in Global Chemical Change Many of the recent changes in atmospheric composition can be traced to anthropogenic causes. A classic example is that of atmospheric CO2, whose increasing rates of production from the burning of fossil fuels and biomass closely mimic its rising atmospheric abundance since the Industrial Revolution (see Figure II.2.3). In other examples, the forcing from anthropogenic activities is less obvious, largely arising when the photochemical oxidation and degradation processes of anthropogenic emissions lead to the production of secondary products that perturb important environmental parameters. Examples of these indirect perturbations include the release of chlorofluorocarbons that cause stratospheric ozone depletion, the emission of sulfur oxides that result in increasing concentrations of radiatively important and health-damaging sulfate aerosols, and the emissions of nitrogen oxides and volatile organic compounds that lead to the production of tropospheric ozone and other photochemical oxidants. Chemical Emissions into the Atmosphere Can Have Long-Term Environmental Consequences That May Be Difficult to Reverse Because of the long time scales associated with many of the processes that affect atmospheric composition, the chemicals we put into the atmosphere and the environmental effects they engender can persist for decades or even centuries. A prime example is the long-term impact of anthropogenic CFCs on stratospheric ozone.

OCR for page 107
Page 158 1. What determines the ability of the atmosphere to cleanse itself of pollutants via free-radical oxidation, both now and in the coming decades? More specifically: • To what extent does our current understanding explain simultaneous measured OH concentrations and the principal OH chemical production and loss processes? • Can the oxidation of compounds or the appearance of their oxidation products be successfully used to infer concentrations of OH? • To what extent do oxidants other than OH (O3, NO3, H2O2, halogen atoms, etc.) play significant roles in atmospheric chemistry? • To what extent do changes in stratospheric ozone, climate, and/or cloud cover affect the oxidizing capacity of the lower atmosphere? 2. What determines the distribution of ozone in the troposphere and how will this distribution change in the coming decades? More specifically: • What fraction of tropospheric O3 can be attributed to transport from the stratosphere, and how does this change with meteorology and season? • What portion of O3 precursors is emitted from biogenic sources, and how will these emissions change with natural (e.g., meteorological variability) and human-induced (e.g., land use, climate change) perturbations? • What is the contribution of urban pollution to rural and regional O3, and conversely, what is the impact of rural or regional O3 on urban pollution? • How does meteorological variability affect the trends of O3 and/or its precursors? • What are the major sources of the oxides of nitrogen in each region of the atmosphere over various geographic regions? What are the rates of emission of NOx from these sources? • Which major reservoir and oxidizing species and which gas-phase and heterogeneous chemical processes are responsible for partitioning within the NOy family? • Where and when is the production of O3 limited by the availability of volatile organic compounds (VOCs) or NOx? • What are the trends in regional and local O3 precursors (NOx, VOCs, carbon monoxide)? 3. How can atmospheric models be improved to better represent current atmospheric oxidants and better predict the atmosphere's response to future levels of pollutants? More specifically: • What laboratory research is required to provide sufficient understanding of the fundamental chemical processes (heterogeneous as well as gas phase) involved in tropospheric oxidant formation?

OCR for page 107
Page 159 • What atmospheric measurements are required, and with what precision and accuracy, to apply diagnostic and predictive models of tropospheric oxidant chemistry? • What are the quantitative uncertainties associated with the estimates from diagnostic and predictive models of tropospheric oxidant chemistry? • How can models of tropospheric oxidant chemistry be improved to incorporate direct and indirect effects of multiple, interacting forcing agents (e.g., climate change, stratospheric ozone depletion, anthropogenic perturbations)? 4. What will enable us to evaluate and improve our air quality management strategies for photochemical oxidants? More specifically: • What design and implementation strategies will provide monitoring networks capable of determining if control measures for photochemical oxidants are having the intended impact? • What design and implementation strategies will yield monitoring networks capable of determining, for a particular air quality problem, what portion of the problem is essentially irreducible (i.e., natural emissions of ozone precursors and stratospheric influx of ozone) and what portion of the ozone problem is potentially controllable (i.e., human-made precursor emissions)? To successfully address these questions in the coming decades, it must be recognized that research on photochemical oxidants is truly ''data poor'' and "measurement limited." As a result, significant progress in this area will require a commitment to acquire high-quality, observational data sets that, collectively, are global in coverage but, individually, are of high enough spatial and temporal resolution to elucidate the important chemical and physical processes responsible for the production, transport, and removal of photochemical oxidants. To accomplish this, a research strategy that is both evolutionary and revolutionary will be required. The beginning of such a strategy focused on the management of urban-and regional-scale photochemical oxidant pollution in North America has recently been developed [see North American Research Strategy on Troposphere Ozone (NARSTO) and charter available from NARSTO Home Page at URL: http://narsto.owt.com/Narsto/]. The research strategy outlined below and in Box II.2.8 is similar in many respects to this previous work but also addresses longer-term and globally relevant issues. Continue Development and Validation of Chemical Instrumentation Instrument development and validation should aim at improving the sensitivity, specificity, and sampling rates of instruments needed to measure the compounds of interest throughout the atmosphere from the measurement platforms of choice (Albritton et al., 1990). The focus should be on (1) the development of

OCR for page 107
Page 160

Box II.2.8 Recommended Research Tasks for Photochemical Oxidants 1. Continue development and validation of chemical instrumentation to   • provide techniques for long-term monitoring;   • provide continuous, fast-response techniques for flux divergence methods;   • provide miniaturized techniques for airbome platforms; and   • provide long-path spatially resolved techniques for making multidimensional measurements. 2. Continue implementation of integrated field campaigns to   • elucidate fundamental processes;   • document key species' trends, sources, and sinks; and   • evaluate air quality and chemical transport models. 3. Carry out observation-based studies to   • elucidate trends and distribution in short-lived radical species;   • independently infer emission inventories; and . • infer ozone precursor relationships. 4. Develop and deploy monitoring networks to   • document the chemical climatology of photochemical oxidants, and   • document the response of ozone to changes in precursor concentrations (e.g., as a result of emission controls). 5. Develop analytical models and tools to support integrated assessments. simpler and more reliable instruments to be used in long-term monitoring; (2) the miniaturization of instruments to accommodate a wide array of measurements on airborne platforms; (3) the development of continuous, fast-response instruments to be used for flux measurements and airborne applications; and (4) the use of spatially resolved, long-path methods (e.g., Lidar) that can be operated from airborne and mobile platforms to determine distributions of compounds of interest over considerable distances. Continue Implementation of Integrated Field Campaigns Integrated field campaigns are undertaken to increase our understanding of fundamental atmospheric processes; elucidate the distributions, sources, and sinks of key species; and provide data for the evaluation of air quality and chemical transport models. Scientific guidance is required to carefully define how key uncertainties are going to be reduced and what key science questions will be addressed in a specific field campaign. Atmospheric chemistry and meteorology must be integrated in the planning and deployment of air quality measurements and monitoring. The questions that are presently before us will require multi-

OCR for page 107
Page 161 disciplinary teams that can address chemistry, transport, and ecosystem feedbacks. Modeling tools adequate to depict or simulate these processes must be available to guide the planning of measurements as well as the interpretation of results. Moreover, an adequate fleet of research aircraft must be available to the atmospheric sciences community in order to make these studies feasible. Carry out Inferential Observation-Based Studies Carefully designed observations of specific tracer compounds or suites of tracer compounds can be used in conjunction with diagnostic and/or observation-based models to independently infer (1) the long-term trends, seasonal variability, and regional distribution of short-lived free-radical species not amenable to continuous, spatially extensive monitoring; (2) urban-, regional-, and global-scale emission inventories of ozone precursors; and (3) the sensitivity of ozone and other photochemical oxidants to precursor compounds. It should be noted however that interpretation of field measurements will require a solid understanding of the fundamental mechanisms involved in related atmospheric processes. Develop and Deploy Monitoring Networks The development and deployment of monitoring networks are necessary to establish the chemical climatology of ozone, other photochemical oxidants, and their precursors. This climatology will help shorten the time required to unequivocally observe a response in ozone to changes in the concentration of its precursor compounds. These networks must include a meteorological component that captures the role of meteorology and dynamics in the redistribution of airborne chemicals. Moreover, a comprehensive chemical climatology for the photochemical oxidants must include data from the free troposphere as well the surface. It is thus likely that these networks will require the use of balloon sondes; robotic, pilotless aircraft; and space-based platforms, in conjunction with newly developed instrumentation based on small, lightweight, low-power technology. Support Integrated Assessments Integrated assessments draw from a wide range of scientific information and disciplines in order to provide more comprehensive guidance on scientific and technical matters to the decision-making community. A thorough understanding of the distributions and trends in photochemical oxidants and the processes that determine their production and removal is not yet in hand, and this seriously limits our ability to conduct a rigorous integrated assessment of global change (Logan, 1994; IPCC, 1995). The research strategy in atmospheric chemistry should support these assessments by providing analytical and modeling tools that can be readily applied to these integrated assessments.

OCR for page 107
Page 162 Atmospheric Aerosols Atmospheric aerosols play a critical role in the chemistry and radiative transfer of the atmosphere. Minute amounts of particulate matter in the stratosphere, along with increased levels of anthropogenic chlorine, are responsible for the Antarctic ozone hole and probably for the less dramatic but nevertheless significant global-scale ozone depletion (WMO, 1995). Aerosols emitted by industrial activity and biomass burning are now believed to be responsible for partially masking the expected increase in surface temperature associated with greenhouse gas radiative forcing (IPCC, 1995; NRC, 1996a). Atmospheric aerosols also have important impacts on human health and materials degradation (American Thoracic Society, 1996a,b). Despite our recent advances in appreciating the importance of aerosols, our understanding of this critical class of atmospheric species is in its infancy. Why is this the case? An outstanding reason is the complex nature of aerosols and the forces they exert. Unlike the atmospheric gases, aerosols have an infinite number of sizes and a variable, mixed composition. We are not able to fully comprehend the impacts of aerosols now and are not in a position to make predictions about how these impacts will change in the future due to mankind's activities. The important questions that must be addressed in the twenty-first century involve the effects of atmospheric aerosols on climate, atmospheric chemistry, and human health and well-being and in a fundamental form can be stated as follows: 1. What is the role of natural and anthropogenic aerosols in climate, and how will future changes in the levels of aerosol precursors affect this role? 2. How will future natural and anthropogenic aerosols impact stratospheric and tropospheric ozone and the oxidizing capacity of the atmosphere? 3. What is the role of atmospheric chemistry in changing the composition of aerosols that impact human health, the environment, visibility, and infrastructural materials? To answer these questions, we must go far beyond our current state of knowledge of atmospheric aerosols. The essential elements of the research strategy that will be needed are outlined below and in Box II.2.9. A more detailed discussion of many aspects of this strategy can be found in Aerosol Radiative Forcing and Climate Change (NRC, 1996a). Maintain and Expand Stratospheric Aerosol Measurement Capability Limb scanning of solar extinction from satellites has been very successful in monitoring the global stratospheric sulfate layer and its spatial and temporal response to volcanic perturbations. When validated by in situ measurements of

OCR for page 107
Page 163

Box II.2.9 Recommended Research Tasks for Atmospheric Aerosois 1. Maintain and expand stratospheric aerosol measurements to   • document aerosol chemical effects on stratospheric ozone, and   • monitor impact of volcanic injections. 2. Develop new suite of tropospheric aerosol measurements to   • document complex chemical and physical aerosol properties, and   • expand remote sensing capability. 3. Deploy monitoring networks to document spatial and temporal frends in aerosol characteristics and their impact on climate, human health, and so forth. 4. Design and implement intensive field campaigns to better understand processes that control aerosol formation, transformation, transport, and loss. 5. Develop and evaluate models to provide predictive capability. particle size distributions from balloons and stratospheric aircraft for validation, satellite multiwavelength extinction measurements have provided stratospheric aerosol particle surface areas with an accuracy adequate for heterogeneous chemical applications. New instruments with higher wavelength resolution, possibly deployed on small satellites, will be the main monitoring tool for this component in the twenty-first century. Design and Implement New Suite of Measurement Technologies for Tropospheric Aerosols The complexity of tropospheric aerosol presents a considerably more difficult problem. Past in situ measurements have focused on determining the size distribution or chemical composition of aerosols at specific locations. Several new techniques under development are probing the chemical composition of single aerosol particles. However, these are essentially point measurements that yield little information about spatial and temporal variability. Moreover, there are few methods for analyzing the composition of organic aerosols, which are emitted from biomass burning and industrial activity. Clearly, a new suite of in situ instrumentation is needed that can quantitatively document the complex chemical composition of tropospheric aerosols in regions of the globe that are of interest for atmospheric chemistry. Current remote sensing technology allows the measurement of gross tropospheric aerosol parameters over large spatial regions, but features such as composition and a complete size distribution cannot be measured yet. Technologies

OCR for page 107
Page 164 such as scanning polarimeters in the visible and near infrared appear to hold promise because they are able to retrieve tropospheric aerosol scattering characteristics from measurements of multispectral radiance and polarization by resolving aerosols from clouds. Moreover, surface and airborne lidars can be used to map tropospheric aerosol backscatter and, combined with Raman scattering techniques, can provide limited information on aerosol characteristics. Preliminary measurements with nadir-viewing lidars from the space shuttle show promise for obtaining detailed gross features of the tropospheric aerosol on a global basis. However, adequate opportunities for deployment of such instruments do not presently exist and must be a priority for the twenty-first century. Design and Deploy Networks to Document Aerosol Climatology With the development of new instrumentation, monitoring networks can be deployed to document the spatial and temporal trends in key aerosol characteristics. These characteristics include aerosol number, size distribution, chemical composition, and radiative properties. Moreover these networks must be designed in such a way that they can address issues on varying spatial scales. For example, urban-scale monitoring networks are needed to uncover the characteristics of aerosols that lead to pulmonary health effects in humans; regional-scale networks are needed to better establish the relationships between aerosol precursor species and visibility; and global-scale networks are needed to better quantify the role of aerosols in climate change. Design and Implement Intensive Field Programs To be able to predict how future anthropogenic activities will affect aerosols, and their consequent impacts on climate, chemistry, the environment, and human health, we must go beyond an aerosol climatology to a deeper understanding of the processes that control aerosol formation, transformation, and removal. This will require the design and implementation of intensive field programs that bring together chemical and physical aerosol measurements and precursor gas studies utilizing surface, aircraft, and ship measurements. It is relevant to note in this regard two novel experimental strategies that have emerged for resolving some of the key questions concerning tropospheric aerosols and their effects [see, for example, the ACE-1 Science and Implementation Plan (IGAC, 1995)]. The first of these is the "closure experiment," in which an overdetermined set of variables is measured. A subset of the observations and the relevant theories are then used to predict the "closure variable," which is also measured independently. The result is a test of both measurements and theory, with an opportunity to evaluate the quality of our understanding in each experiment. With instrumentation now available, it is possible to perform closure experiments on aerosol number concentration (using a variety of sizing instruments), mass (based on measurements

OCR for page 107
Page 165 of relevant inorganic and organic species), radiative properties (using chemical composition, relative humidity, and Mie theory), and the integrated column effect of aerosols on short- and longwave radiation. Closure experiments on aerosol mass can help answer questions about chemical composition, since missing species will make closure impossible. Theories concerning the impact of aerosols on radiative forcing of climate can also be tested by local and column closure experiments. Most of the aerosol experiments planned for the next decade depend heavily on this strategy, since it offers a rigorous test of both measurements and the process models on which more comprehensive models depend. The other new strategy is to observe the evolution of aerosols and their precursor gases in a Lagrangian reference frame. The idea of Lagrangian experiments is not new, and variations on this theme have been used from time to time. Recently, however, there has been considerable work on tagging airmasses with balloons and chemical tracers, so that aircraft carrying large suites of instruments can revisit the airmass over a period of days to observe changes with time (Huebert, 1993; Draxler and Hefter, 1989). Although these experiments cannot eliminate the effects of dispersion and vertical mixing on concentrations, with ample dynamical measurements they make it possible to sort out the chemical and physical processes that cause changes in aerosols. These processes include gas-to-particle conversion, chemical transformations, wet and dry deposition, entrainment of air from other strata, and mixing through the sides of the "airmass" (dispersion). Although these experiments tend to be complex and expensive (at least one ship and one or two aircraft are required), they offer the potential to test the aerosol models that presently exist or will be developed from future laboratory work and other process studies. Develop Predictive Model Capability The overall strategic goal for the twenty-first century should be development of a predictive model that can be used to calculate atmospheric temperature and chemical species concentration fields and, from this information, to derive aerosol formation rates, predict the chemical content and size distribution of the aerosol fields, and determine their concomitant influence on atmospheric radiation and the reflectivity and lifetime of clouds. Since current atmospheric models generally impose, rather than predict, aerosol distributions, it will be necessary to achieve significantly more sophistication in representing precursor gas and gas-particle kinetics, nucleation and agglomeration kinetics, and vapor-particle interactions in future models. One way to naturally stimulate the necessary improvements in aerosol modeling capabilities is to encourage the modeling community to participate directly in the planning, execution, and data analysis portions of the strategic field measurements programs described above. Furthermore, predictive aerosol models will require currently unavailable quantitative mechanistic and kinetic input data describing a large number of

OCR for page 107
Page 166 heterogeneous growth, nucleation, agglomeration, and accommodation or evaporation processes. These quantitative input data will have to come from a vigorous laboratory program in heterogeneous kinetics and aerosol microphysics. Toxics and Nutrients The atmosphere and biosphere are fundamentally coupled through the exchange of gases and aerosols. Ecological systems, including economically important ones such as those dedicated to agriculture and forestry, can be profoundly impacted by the wet and dry deposition of both toxic and nutritive atmospheric substances (e.g., Ridley et al., 1977; Duce, 1986; Aber et al., 1989; Schulze, 1989; Van Dijk et al., 1990; Lindquist et al., 1991; Benjamin and Honeyman, 1992; Vitousek et al., 1993; Shannon and Voldner, 1995). Although many of the atmosphere's naturally occurring components can have toxic and/or nutritive effects on the biosphere, there are a myriad of toxic and nutritive substances in the atmosphere that are significantly influenced by anthropogenic activities. These include nutrients such as sulfur and nitrogen compounds; heavy metals such as mercury, cadmium, and lead; and toxic organic compounds such as pesticides, polychlorinated biphenyls (PCBs), plasticizers, dioxins, and furans. Although we are beginning to be able to identify the more acute effects of atmospheric toxicity and overfertilization on key ecosystems, our understanding is far too limited for us to assess the current extent of these problems or to predict future ones. Overall, the motivating scientific questions for the study of toxics and nutrients are as follows: 1. How are interactions between the atmosphere and biosphere influenced by changing atmospheric concentrations and by the deposition of harmful and beneficial compounds? 2. What are the rates at which biologically important atmospheric trace species are transferred from the atmosphere to terrestrial and marine ecosystems through dry and wet deposition? The essential elements of a research strategy to address these questions are outlined below and in Box II.2.10. Develop and Evaluate Techniques for Measuring Deposition Fluxes Many of the key questions about toxics and nutrients cannot yet be answered comprehensively because we lack the necessary methods for measuring deposition fluxes on the appropriate spatial and temporal scales. This problem is most severe in the case of dry deposition, where technologies for reliably measuring many of the most biologically important fluxes do not yet exist. Adequate support for development of the necessary techniques in this area is thus critical; relaxed eddy accumulation, eddy correlation, and gradient methods offer particular promise.

OCR for page 107
Page 167

Box II.2.10 Recommended Research Tasks for Toxics and Nutrients 1. Develop and evaluate deposition flux measurement techniques to   • provide new methods for measuring dry deposition rates, and   • provide methods for obtaining more spatially comprehensive deposition data. 2. Design and implement ecosystem exposure monitoring networks to develop long-term record of stresses and benefits to ecosystems of economic and/or environmental import. 3. Carry out process-oriented field studies to   • developed and evaluate deposition flux algorithms,   • contribute to the development of coupled ecosystem-atmospheric chemistr model, and   • provide tools for integrated assessments. In the case of wet deposition, reliable techniques have in principle been developed, but serious questions exist about sampling representativeness and contamination problems. The problem is most severe for measuring wet deposition fluxes over the ocean, where it is virtually impossible to collect uncontaminated rain samples from a buoy in midocean and samples from shipboard platforms are necessarily intermittent. Present marine deposition estimates, often the result of comparing model calculations with a very small suite of shipboard and island observations, are typically subject to uncertainties of factors of three or more (Duce et al., 1991). The development of new techniques that will allow for more representative determination of wet as well as dry deposition fluxes, perhaps from a low-flying airborne platform, must therefore also be considered a high priority. In some cases, such as high-altitude forests and foggy regions, the deposition of cloud droplets may be the primary avenue by which toxics and nutrients are delivered to the Earth's surface (Vong et al., 1991). It is extremely difficult to measure such fluxes, because the droplets are so transient that their flux is easily altered by the presence of measuring devices. Thus, new methodologies should be developed to assess the importance of droplet deposition and allow reliable flux measurements. Design and Implement Ecosystem Exposure Monitoring Networks In the recent past, deposition monitoring networks have proved useful for assessing the ecological impacts of atmospheric deposition (e.g., Cooperative

OCR for page 107
Page 168 Programme for the Monitoring and Evaluation of Long Range Air Pollutants in Europe, National Crop Loss Assessment Network). However, these networks have been largely limited to monitoring the deposition of a specific chemical or class of compounds (e.g., acid deposition, ozone). For this reason, they have provided very limited information on the full suite of stresses and benefits experienced by an ecosystem from atmospheric deposition and, thus, on the long-term effects of this deposition. With the development of new deposition measurement techniques, it should be possible to design more comprehensive atmospheric deposition and exposure monitoring networks. Implementation of these networks for key ecosystems and biomes (e.g., at Long-Term Ecological Research sites) would provide a long-term record of atmospheric deposition; with co-located ecological monitoring, this record would no doubt prove useful in establishing causal relationships between atmospheric deposition and ecosystem vitality and succession. Carry out Process-Oriented Field Studies for Algorithm Development and Evaluation Even with reliable and fully evaluated deposition measurement techniques, it will never be possible to measure dry and wet fluxes for all species of interest over all ecosystems of interest, over all time. For this reason, process-oriented field studies, involving observations of fluxes under a carefully selected range of conditions, have to be undertaken in order to identify the factors that control such fluxes. With these factors identified, algorithms and parameterizations describing deposition fluxes can be developed, tested by further observations, and incorporated into regional and global atmospheric chemistry models, as well as integrated atmospheric-biospheric response models.