5

Changes in the Chemistry of the Atmosphere

SUMMARY

Within the atmospheric chemistry component of the U.S. Global Change Research Program there is a well-defined science focus with a track record of dealing with public policy implications. Development and implementation of the Montreal Protocol rested on a solid scientific foundation, realized through a strong international network of scientists and, within the United States, a multiagency effort led by the National Aeronautics and Space Administration (NASA). In fact, the model of an international, integrated, and periodically repeated assessment was largely formed from the United Nations Environment Programme/World Meteorological Organization Ozone Assessments. Moreover, this research area has a rich history of interaction with the human dimension components at fine spatial scales, as a natural consequence of air pollution studies and policies. Current developments in atmospheric chemistry are revealing the close links between chemistry, radiation, dynamics, and climate. Examples include the powerful role played by aerosol formation in both the boundary layer and the upper troposphere, chemical initiation of subvisible cirrus in the region of the tropopause, the control exerted by water vapor and temperature on the sharply nonlinear partitioning of halogen and hydrogen radicals in the lower stratosphere, and the importance of stratosphere-troposphere exchange on the composition and meteorology of the upper troposphere and lower stratosphere.

However, there are significant lessons to be remembered—lessons resulting from significant research shortcomings. Failure to recognize the Antarctic ozone hole sooner demonstrates the consequences of overreliance on models and how the selected observational strategies are so critically tied to success. This lesson



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GLOBAL ENVIRONMENTAL CHANGE: Research Pathways for the Next Decade 5 Changes in the Chemistry of the Atmosphere SUMMARY Within the atmospheric chemistry component of the U.S. Global Change Research Program there is a well-defined science focus with a track record of dealing with public policy implications. Development and implementation of the Montreal Protocol rested on a solid scientific foundation, realized through a strong international network of scientists and, within the United States, a multiagency effort led by the National Aeronautics and Space Administration (NASA). In fact, the model of an international, integrated, and periodically repeated assessment was largely formed from the United Nations Environment Programme/World Meteorological Organization Ozone Assessments. Moreover, this research area has a rich history of interaction with the human dimension components at fine spatial scales, as a natural consequence of air pollution studies and policies. Current developments in atmospheric chemistry are revealing the close links between chemistry, radiation, dynamics, and climate. Examples include the powerful role played by aerosol formation in both the boundary layer and the upper troposphere, chemical initiation of subvisible cirrus in the region of the tropopause, the control exerted by water vapor and temperature on the sharply nonlinear partitioning of halogen and hydrogen radicals in the lower stratosphere, and the importance of stratosphere-troposphere exchange on the composition and meteorology of the upper troposphere and lower stratosphere. However, there are significant lessons to be remembered—lessons resulting from significant research shortcomings. Failure to recognize the Antarctic ozone hole sooner demonstrates the consequences of overreliance on models and how the selected observational strategies are so critically tied to success. This lesson

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GLOBAL ENVIRONMENTAL CHANGE: Research Pathways for the Next Decade must not be forgotten in studying the complexities of climate, ecosystems, and the chemistry of the troposphere. Today, we have far deeper knowledge about the chemistry of the atmosphere than we did just a decade ago. We also know more clearly what we do not know. These issues are also addressed in a recent National Research Council report (NRC, 1998) that is consistent with the perspective put forward in this chapter. Key challenges to atmospheric chemistry in the coming decade can be expressed in five Research Imperatives, where each Research Imperative combines one or more primary Scientific Questions with the need to know from a human dimensions perspective: Stratospheric ozone and ultraviolet (UV) radiation. Define and predict secular trends in the intensity of UV exposure that the Earth receives. Document the concentrations and distributions of stratospheric ozone and the key chemical species that control its catalytic destruction and elucidate the coupling between chemistry, dynamics, and radiation in the stratosphere and upper troposphere. Greenhouse gases. Determine the fluxes of greenhouse gases into and out of the Earth 's systems and the mechanisms responsible for the exchange and distribution between and within those systems. Expand global detection techniques to elucidate the processes that control the abundances and variability of atmospheric CO2, CH4, N2O, and upper-tropospheric/lowerstratospheric O3 and water vapor. Photochemical oxidants. Develop the observational and computational tools and strategies that policy makers need to effectively manage ozone pollution, and elucidate the processes that control and the relationships that exist among ozone precursor species, tropospheric ozone, and the oxidizing capacity of the atmosphere. Atmospheric aerosols and UV/visible radiation. Document the chemical and physical properties of atmospheric aerosols, and elucidate the chemical and physical processes that determine the size, concentration, and chemical characteristics of atmospheric aerosols. Toxics and nutrients. Document the rates of chemical exchange between the atmosphere and ecosystems of critical economic and environmental import, and elucidate the extent to which interactions between the atmosphere and biosphere are influenced by changing concentrations and depositions of harmful and beneficial compounds. INTRODUCTION The chemistry of the Earth's atmosphere has emerged as a central theme in studies of global change. Atmospheric chemistry provides the scientific foundations to understand a number of phenomena that are part of global change. These

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GLOBAL ENVIRONMENTAL CHANGE: Research Pathways for the Next Decade phenomena include (1) changes in UV dosage at the Earth's surface owing to the intrinsically chemical nature of the catalytic loss of stratospheric ozone, (2) changes in the dynamics and radiative structure of the climate system through altered thermal forcing by ozone in the upper troposphere, (3) changes in the concentration of highly oxidizing species in urban as well as remote rural regions, 1 and (4) changes in the acid levels of depositions in a variety of ecosystems. In addition, work on the chemistry of the atmosphere provides hard examples of how the scientific method can succeed in guiding public policy. What kind of research can successfully attack global-scale problems, problems that are intrinsically complex yet require reasonably unequivocal answers for international decision making and subsequent enforcement? Addressing this question is the objective of this chapter; the issue is attacked in four steps. First, case studies are presented that illustrate the successful execution of research in which hypotheses are tested by means of observations, leading to identification of cause and effect and thus to identification of the agent of change. This case study approach, while incomplete because of length limitations, helps address a fundamental question: Why is it in the national interest that we have a global change research program to study the planet? This question deserves careful consideration. Have we learned from scientific inquiry facts that constitute a decided reordering in our thinking about how the Earth functions? Have there been notable discoveries? Are there clear links between the discoveries associated with the national program and our economic competitiveness? Second, we identify the key unanswered scientific questions that confront the field of atmospheric chemistry today. There are three categories of such questions: What are the secular and episodic trends in concentrations of environmentally important atmospheric species, on local to global scales? What mechanisms control these concentration changes? How are the concentrations of these species likely to change in the future? What are the most effective and feasible policy options for managing these changes? What are the societal, economic, climatic, and environmental effects of present and future trends in the concentrations of these species? Third, we review lessons learned over the past three decades that bear directly on research strategies for the future. These lessons range from general principles about posing and testing hypotheses regarding the Earth system to more detailed points about how specific observational strategies are selected to establish cause and effect. Fourth, we address what is needed to successfully attack the major unanswered questions confronting the field, including theoretical approaches, observational strategies, instrument and platform development, and data handling and

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GLOBAL ENVIRONMENTAL CHANGE: Research Pathways for the Next Decade storage. We identify five primary research imperatives for atmospheric chemistry in global change research—the research imperatives presented in the opening paragraphs of this chapter. With these disciplinary imperatives come infrastructure initiatives, without which the research cannot be executed. Lessons extracted over the past four decades of research provide substantial guidance for such infrastructure initiatives. CASE STUDIES This section describes selected scientific cases that led to diagnoses central to studies of global change. In describing these developments attention is given to the ways that such transitions in our scientific thinking can be linked to specific public policy initiatives and to how such cases have been related to both environmental decisions and technological developments. Thus, we address the questions: Why is it in the national interest to pursue this research? Are there links between this research and economic competitiveness? The Antarctic Ozone Hole Discovery and diagnosis of the Antarctic Ozone Hole were a major surprise for both scientists and the public policy structure. In worldwide studies extending back to the 1950s, the amount of ozone over the Antarctic was tracked each year through its seasonal cycle. In the late 1970s an anomalous deficit was observed in total ozone amount in the late-winter observations. In 1985 the British Antarctic Survey reported for the first time in the scientific literature2 that dramatic losses were occurring in the ozone concentration over Halley Bay and that the degree of ozone loss was worsening as the decade progressed. Theories about the cause of this unprecedented loss blossomed. Explanations ranged from simple redistribution by atmospheric motion to chemical reactions initiated by magnetic field focusing of solar electrons and protons. Such theories were put forward by serious scientific research groups in an international effort to diagnose the cause of this unexpected development. A number of expeditions were planned to gather more complete information. In 1986 NASA planned an airborne expedition using the ER-2 aircraft to penetrate the region of the stratosphere where ozone was disappearing. The mission, executed in August and September 1987 from Punta Arenas, Chile, and supported by concurrent laboratory and modeling studies, demonstrated unequivocally that ozone was destroyed by chlorine and bromine radicals (see Figure 5.1). The role of chlorofluorocarbons (CFCs)—the molecules that transport chlorine to the stratosphere—in the destruction of ozone over the Antarctic rests on three discoveries from the NASA mission.3 The first discovery was that the region of severe ozone depletion was isolated from the rest of the stratosphere by the polar night jet that defines the perimeter of the Antarctic vortex. This isolation creates

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GLOBAL ENVIRONMENTAL CHANGE: Research Pathways for the Next Decade a continental-scale “containment vessel,” creating a sharp transition in the concentration of key chemical species associated with the destruction of ozone. The existence of this barrier preventing exchange is shown clearly by the high-resolution aircraft data in Figure 5.1. The second discovery from that mission linking CFCs to Antarctic ozone destruction is the documented evolution of the anticorrelation between O3 and ClO occurring in the stratospheric containment vessel. As Figure 5.1 shows, the initial conditions established on 23 August, as sunlight returned to the region, demonstrate that O3 had emerged from the polar night largely unaffected. Three weeks later, on 16 September, ozone had eroded sharply in the presence of high ClO concentrations in the containment vessel. The third confirming discovery emerged from laboratory studies supported by NASA that determined the rates of reactions responsible for destruction of ozone by chlorine and bromine radicals. These laboratory results allowed direct quantitative comparison of the two sets of aircraft observations: (1) observed concentrations of ClO and BrO in the containment vessel and (2) the rate of ozone disappearance in the containment vessel. The case of Antarctic ozone depletion is particularly notable in the context of global change because of the severity of the phenomenon and the isolation afforded by the stratospheric containment vessel. The main elements of the scientific case linking CFC release to ozone destruction, as summarized here, have been extensively covered and critiqued in the international scientific literature. Taken together, the three major findings in the case provide irrefutable evidence that the dramatic reduction in stratospheric ozone over the Antarctic continent would not have occurred had CFCs not been synthesized and added to the atmosphere. There have been additional surprises in the study of stratospheric ozone depletion as well. In 1989 and again in 1991 to 1992, NASA airborne missions staged from Stavanger, Norway, Fairbanks, Alaska, and Bangor, Maine, revealed that the containment vessel over the Arctic contained highly amplified concentrations of the same ClO radical discovered over the Antarctic.4 Because ozone destruction requires high concentrations of chlorine radicals, sunlight, and time, ozone loss over the Arctic is less severe: the slightly higher temperatures over the Arctic allow the system to recover faster, by reducing the length of time that chlorine radicals remain at high concentrations in the containment vessel. In the past five years several northern hemisphere late-winter/early-spring seasons have been marked by dramatic reductions in column ozone at high latitudes.5 High-Speed Civil Transport and Ozone Loss The United States is the world's leader in aircraft design and the development and sale of civilian commercial aircraft. NASA has developed a research program to establish the response of ozone to injections of combustion products (NOx, H2O, particulates) from the proposed High-Speed Civil Transport (HSCT).

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GLOBAL ENVIRONMENTAL CHANGE: Research Pathways for the Next Decade FIGURE 5.1 Rendering of the containment provided by the circumpolar jet that isolates the region of highly enhanced ClO over the Antarctic continent. Evolution of the anticorrelation between ClO and O3 across the vortex transition is traced from (A) the initial condition observed on 23 August 1987 on the southbound leg of the flight; (B) summary of the sequence over the 10-flight series; and (C) imprint on O3 resulting from three weeks of exposure to elevated levels of ClO. Data panels do not include dive segment of trajectory; ClO mixing ratios are in parts per trillion by volume; O3 mixing ratios are in parts per billion by volume. SOURCE: Anderson et al. (1991). Courtesy of the American Association for the Advancement of Science.

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GLOBAL ENVIRONMENTAL CHANGE: Research Pathways for the Next Decade The first direct experiments on the ER-2 aircraft recast our understanding of ozone loss in the lower stratosphere. Commercial aircraft sales represent an international market measured in tens of billions of dollars annually. Development of the HSCT is a main component in the international battle for leadership in this field. A key issue for this development is that the nitrogen oxide/particulate/water vapor effluent from the proposed aircraft could trigger both enhanced ozone loss in the stratosphere and radiative changes linked, through water vapor changes and cloud formation, to climate changes. Senate hearings in the early 1970s hinged significantly on the prospect of damage to the ozone layer by large fleets of supersonic transports resulting from NOx emissions.6 Equally important, however, is recognition that, if an aircraft is detrimental to global ozone and/or climate, business decisions to build such an aircraft are compromised. NASA is carrying out a research effort with airborne missions7 to test fundamental ideas about processes that control tropospheric and stratospheric ozone and, in particular, how the proposed HSCT and subsonic aircraft may alter those processes. The past three years have witnessed two important developments in our understanding of processes that control the catalytic destruction of ozone in the lower stratosphere. The first development emerged out of simultaneous NOx/ NOy observations8 during NASA's research and analysis airborne mission to the Arctic. The mission found that aerosols (minute liquid droplets) have a dramatic impact on the fraction of reactive nitrogen tied up in free radical form (NO and NO2). These ER-2 in situ observations clearly demonstrated that NOx was converted to NOy, thereby providing a natural “sink” for any reactive nitrogen compound added to the lower stratosphere and, in particular, for the combustion effluent from the proposed Mach 2.4 HSCT. This result constitutes the first serious challenge to the two-decades-old premise that catalytic destruction of ozone in the lower stratosphere is dominated by nitrogen radicals (NOx). It was this fundamental tenet—that ozone removal in the lower stratosphere is rate limited by NO 2 —combined with the realization that a significant fleet of supersonic transports would add appreciably to the nitrogen oxide budget of the lower stratosphere, that impugned supersonic transports in the early 1970s.9 The second key development emerged from NASA's Stratospheric Photochemistry, Aerosol, and Dynamics Expedition of May 1993 and has subsequently been confirmed in more recent airborne missions. This ER-2 mission was the first to include a new generation of solid-state laser experiments capable of detecting OH and HO2, thereby completing an ensemble of instruments capable of simultaneous in situ detection of each of the rate-limiting radicals in the dominant catalytic cycles (NO2, ClO, BrO, and HO2) and of the key coupling radicals NO and OH. These ER-2 observations 10 demonstrated the predominance of odd hydrogen HOx) and halogen free radical (ClOx and BrOx) catalysis in determining the rate of removal of ozone in the lower stratosphere. A single catalytic cycle, rate limited by HO2 + O3 → HO + O2 + O2, was found to account for nearly half the total O3 removal in the midlatitude northern hemisphere lower stratosphere. Halogen radical chemistry was found to be responsible for 30 percent of the

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GLOBAL ENVIRONMENTAL CHANGE: Research Pathways for the Next Decade FIGURE 5.2 The O3 removal rate is shown versus NOx. Because of the coupling that exists between the radical families, the response of the total O3 removal rate to changes in NOx is highly nonlinear. At sufficiently low NOx, such as observed during the NASA mission, the removal rates are inversely correlated with NOx. SOURCE: Wennberg et al. (1994). Courtesy of the American Association for the Advancement of Science. catalyzed loss of O3, with reactions involving bromine sustaining half of the halogen catalytic cycles. Of critical importance to the HSCT, this NASA mission demonstrated that in the region sampled by the ER-2 the rate of catalytic ozone destruction is inversely correlated with total NOx loading. The relationship between ozone loss rates and added NOx is most clearly captured in a 1994 figure displayed in Figure 5.2. These two developments changed decidedly the scientific community 's judgment about the expected impact of the NOx component of the HSCT effluent. Specifically, if there were a region of the stratosphere where addition of NOx would actually decrease the rate of ozone catalytic destruction, it becomes plausible, contingent on the design of the aircraft and the dynamical and chemical characteristics of the stratosphere at higher altitudes, that addition of NOx to the lower stratosphere could leave the ozone column virtually unaffected. These ER-2 results decidedly rearranged our thinking on lower-stratospheric ozone photochemistry. There are, of course, a number of other key examples, including the diagnosis by Chameides et al. (1988) showing the coupled impact of volatile organics and NOx on urban ozone production and the demonstration by Charlson et al.

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GLOBAL ENVIRONMENTAL CHANGE: Research Pathways for the Next Decade (1995) of the importance to climate forcing of anthropogenic aerosols. There have also been key examples of direct sampling in supersonic aircraft exhaust in the stratosphere.11 A RESEARCH AGENDA FOR THE NEXT DECADE The Scientific Questions facing atmospheric chemistry today are intellectually profound but 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. It is perhaps a measure of the strides made in recent decades that the issues of atmospheric chemistry are familiar now to the general public, policy makers, and scientists alike. Continued progress will require an ambitious and 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. Key Scientific Questions The principal focus for atmospheric chemistry research will be on the environmentally important atmospheric 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 are central to the life support system of our planet. From a societal point of view they are of interest because they directly impact human health and welfare. Out of this focus emerges the challenge for atmospheric chemistry research in the coming decades: development and application of the tools and scientific infrastructure required to document and predict the concentrations and effects of environmentally important atmospheric species on local, regional, and global spatial scales and on daily to decadal timescales. Stratospheric Ozone and UV Radiation Imperative The stratosphere is a dynamical/radiative system12 that exports ozone from the high-altitude tropics to mid/high latitudes along downward-sloping surfaces, defined by constant mixing ratios of tracers such as N2O and CH4. The coherence of these tracer surfaces as a vertical coordinate, revealed by tight regression relationships, is a dramatic and simplifying feature of the stratosphere.13 Recognition of this fact has profound implications for research strategies in the next decade. Material enters the stratosphere primarily in the cold inner tropics in a process that desiccates the air, confines its point of entry, and establishes a “leaky chimney” that persists well into the middle stratosphere, dictating poleward motion from the tropics, as shown in Figure 5.3.

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GLOBAL ENVIRONMENTAL CHANGE: Research Pathways for the Next Decade FIGURE 5.3 Dynamical aspects of stratosphere-troposphere exchange. The tropopause is shown by the thick line. Thin lines are isentropic or constant potential temperature surfaces (in degrees Kelvin). The heavily shaded region is “lowermost stratosphere,” in which isentropic surfaces span the tropopause and isentropic exchange by tropopause folding occurs. The region above the 380 K surface is the “overworld,” in which isentropes lie entirely in the stratosphere. Light shading in the overworld denotes wave-induced forcing (the extratropical “pump”). The wavy double-headed horizontal arrows denote meridional transport by eddy motions, which include tropical upper-tropospheric troughs and their cutoff cyclones, as well as their midlatitude counterparts, including folds. The broad vertical arrows show transport by the global-scale circulation, which consists of tropical upwelling and extratropical downwelling, driven nonlocally by the extratropical pump. This large-scale circulation is the primary contribution to exchange across isentropic surfaces (e.g., the ~400 K surface) that are entirely in the overworld. Exchange between the extratropical boundary of the chimney and mid-latitudes occurs on timescales of a very few months; this meridional exchange may be highly seasonally dependent. Transfer through the confines of the chimney is largely uncharacterized. Vertical exchange in the extratropics occurs via a sequence of equatorward-vertical-poleward motions. Polar regimes are charac-

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GLOBAL ENVIRONMENTAL CHANGE: Research Pathways for the Next Decade terized by rapid cooling in fall, with subsidence of many kilometers associated with the establishment of a strong polar jet that restricts exchange. This jet confines the winter polar stratosphere, most acutely in the southern hemisphere, and plays a significant role in the annual dynamical cycle of the stratosphere. The subtropical jet, the respective polar jets, and the tropopause constitute barriers to exchange; the low and high latitudes in each hemisphere are to a degree dynamically coupled in the lower stratosphere. Predicting accurately the path taken by material from a given point in the stratosphere in a given season is a central unanswered question. Our understanding of the response of the stratosphere to natural and inflicted changes is seriously compromised by this lack of understanding. In formulating a strategy for studying the stratosphere, we have identified five basic scientific questions that we believe will motivate research on stratospheric ozone in the coming decades (see Box 5.1). The essential research activities that will be required to address these questions are outlined later in the section on research imperatives. Atmospheric Greenhouse Gases Imperative Observed increases in the concentrations of CO2, CH4, N2O, and CFCs provide one of the clearest manifestations of global change in the atmosphere. Historical trends in H2O and O3 have yet to be quantitatively characterized. However, limited data suggest that tropospheric ozone concentrations may have BOX 5.1 Stratospheric Ozone: Key Scientific Questions Will evolution of the Antarctic stratospheric ozone “hole” proceed as expected, with a period of continued increasing intensity, followed by recovery to normal conditions? Will the Arctic emulate the Antarctic? How will the midlatitude ozone depletion evolve? What mechanisms are controlling this erosion? How sensitive is this ozone erosion to temperature, water vapor partial pressure, sulfate/nitrate concentration, and aerosol loading and can we develop models to correctly simulate this evolution? What is the role of the largely unexplored tropical region of the stratosphere in global ozone change? What are the interactions between stratospheric ozone depletion and climate change? What are the consequences of current and future perturbations, such as aircraft emissions and volcanic eruptions, on stratospheric ozone concentrations?

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GLOBAL ENVIRONMENTAL CHANGE: Research Pathways for the Next Decade lyze inorganic chlorine and bromine to free radical precursors. The intrinsic dynamic instability of the troposphere interacts with these regions on timescales of weeks, such that observational trajectories must be tactically selected to track volume elements in a Lagrangian frame. Moreover, chemical time constants linking species are typically 10 days or more. Thus, observations must be pursued in the context of local trajectory patterns defined by meteorological analysis. As altitudes approaching the tropopause are investigated, there is increasing opportunity to use strategically designed satellite observations to simultaneously observe O3, H2O, and associated tracers such as N2 O, CH4, and CFC-11. 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. Development should focus on several areas: simpler and more reliable instruments to be used in long-term monitoring; miniaturization of instruments, to accommodate a wide array of measurements on airborne platforms; continuous, fast-response instruments to be used for flux measurements and in airborne applications; the use of spatially resolved long-path methods (e.g., LIDAR (light detection and ranging instrument)) that can be operated from airborne and mobile platforms to determine one- and two-dimensional distributions of compounds of interest over considerable distances from the emitter/detector units; innovative aircraft platforms that can follow specific trajectories using fast-response instruments; and innovative small satellites. Integrated Field Campaigns: The Union of Chemical, Radiation, Dynamics, and Technology Integrated field campaigns increase our understanding of fundamental atmospheric processes; elucidate the distributions, sources, and sinks of key species; and provide the data to evaluate air quality and chemical transport models. Any specific field campaign must be designed carefully with regard to the scientific questions it addresses and the uncertainties it must minimize. Atmospheric chemistry and meteorology must be integrated in the planning and deployment of air quality measurements and monitoring. The questions now before us will require multidisciplinary teams to consider 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 to make these studies feasible.

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GLOBAL ENVIRONMENTAL CHANGE: Research Pathways for the Next Decade Carefully designed observations (with or without specific tracer compounds or suites of tracer compounds) can be used in conjunction with diagnostic and/or observation-based models to independently infer a number of phenomena: long-term trends and regional and seasonal variability in short-lived free radical species not amenable to continuous, spatially extensive monitoring; urban, regional, and global-scale emission inventories of ozone precursors; and the sensitivity of ozone and other photochemical oxidants to ozone precursor compounds. At the same time, the diagnostic and observation-based interpretation of field measurements will require adequate laboratory definition of the fundamental mechanisms involved in atmospheric processes. The development and deployment of monitoring networks, along with analysis of resulting data, are needed to establish the chemical climatology of ozone, other photochemical oxidants, and their precursors. This climatology will help establish temporal and spatial trends and shorten the time required to unequivocally observe a response in ozone to changes in the concentration of its precursor compounds. These networks must include components capturing the roles that meteorology and dynamics play in the redistribution of airborne chemicals. Moreover, a comprehensive chemical climatology for photochemical oxidants must include data from the free troposphere as well as the surface. It is thus likely that these networks will require the use of balloon sondes, robotic aircraft, and spacebased platforms in conjunction with newly developed instrumentation based on small, lightweight, low-power technology. Linking Scientific Results with Integrated Assessments Integrated assessments draw from a wide range of scientific information and disciplines to provide more comprehensive guidance on scientific and technical matters to the decision-making community. The research strategy in atmospheric chemistry should support these assessments by providing analytical and modeling tools that can readily support these integrated assessments. Atmospheric Aerosols and UV/Visible Radiation Compelling evidence has emerged that aerosols play a central role in control of both the UV/visible exposure at the Earth's surface and the balance between albedo and retention of infrared radiation in the atmosphere. Aerosols demand specific attention. 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. The atmospheric haze associated with industrial activity near major cities is now believed to partially mask the expected increase in surface temperature associated with greenhouse gas increases. Atmospheric aerosols also

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GLOBAL ENVIRONMENTAL CHANGE: Research Pathways for the Next Decade BOX 5.8 Critical Scientific Questions for Atmospheric Aerosols 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? How will future natural and anthropogenic aerosols affect stratospheric and tropospheric ozone and the cleansing capacity of the atmosphere? What is the role of atmospheric chemistry in changing the composition of aerosols that affect human health, the environment, visibility, and infrastructural materials? have important impacts on human health, quality of life, and materials degradation. Yet despite these recent advances in appreciating the importance of atmospheric aerosols, our understanding of these critical species is in its infancy. We do not comprehend the impacts of aerosols now and cannot now predict how those impacts will change in the future through human activities. Several important questions must be addressed about the effects of atmospheric aerosols on climate, atmospheric chemistry, and human health and well-being (see Box 5.8). To answer these questions, we must go far beyond our current state of knowledge of atmospheric aerosols. Further details of the needed research strategy can be found in A Plan for a Research Program on Aerosol Radiative Forcing and Climate Change (NRC 1996). Stratospheric Aerosols 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. Combined with in situ measurements of particle size distributions from balloons and stratospheric aircraft for validation, satellite multiwavelength extinction measurements have determined the surface areas of stratospheric aerosol particles 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 future. Tropospheric Aerosols The complexity of the tropospheric aerosol presents a considerably more difficult problem. Past in situ measurements focused on determining the size

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GLOBAL ENVIRONMENTAL CHANGE: Research Pathways for the Next Decade 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 measurements are essentially point measurements, with little information about spatial and temporal variability. Moreover, methods for analyzing the composition of organic aerosols, arising, for example, from biomass burning or urban pollution, are incomplete. Clearly, new in situ instrumentation is needed to quantitatively document the complex chemical composition of tropospheric aerosols over their full size range in the various regions of the globe of interest for atmospheric chemistry. Current remote sensing technology allows the measurement of gross tropospheric aerosol parameters over large spatial regions but not such features as composition or a complete size distribution. Technologies such as scanning polarimeters in the visible and near infrared appear able to retrieve tropospheric aerosol scattering characteristics from measurements of multispectral radiance and polarization by resolving aerosols from clouds and thus hold promise. Moreover, surface and airborne LIDAR can be used to map tropospheric aerosol backscatter and, combined with Raman measurement of scattering, can provide limited information about aerosol characteristics. Preliminary measurements with nadir-viewing LIDAR from the U.S. Space Shuttle show promise for obtaining detailed gross features of tropospheric aerosols on a global basis. With the development of new instrumentation, monitoring networks will be needed to document the spatial and temporal trends in key aerosol characteristics. These characteristics include aerosol number, size, distribution, chemical (and toxic) composition, and radiative properties. Moreover the networks must be designed to address a variety of issues on urban, regional, and global scales. For example, on urban scales, monitoring networks are needed to uncover the characteristics of aerosols that lead to pulmonary health effects in humans. On regional scales they are needed to clarify the relationships between aerosol precursor species and visibility, and on global scales they are needed to better characterize quantitative relationships between aerosols and climatic effects. The Strategy of Observations and Calculations To predict how future human activities are likely to affect atmospheric aerosols and the related impacts on climate, chemistry, 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 advance will require the design and implementation of intensive field programs that bring together chemical and physical aerosol measurements and precursor gas studies using surface, aircraft, and ship measurements. In this regard two novel experimental strategies have emerged for resolving some of the most important questions concerning tropospheric aerosols and their effects. The first is the “closure experiment, ” in which an overdetermined set of

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GLOBAL ENVIRONMENTAL CHANGE: Research Pathways for the Next Decade variables is measured. A subset of the observations and the relevant theories is then used to predict values for a “closure variable,” which is also measured independently. The result is a test of both measurements and theory and an opportunity to evaluate the quality of our understanding. With the instrumentation now available, closure experiments can be performed on aerosol number concentration (using a variety of sizing instruments), mass (based on measurements 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 long-wave radiation. Closure experiments on aerosol mass can help answer questions about chemical composition, since missing species will make closure impossible. Theories about the impacts of aerosols on radiative climate forcing 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 because it offers a rigorous test of both measurements and the process models on which more comprehensive models depend. The other new observational 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 occasionally. Recently, however, there has been considerable work on tagging air masses with balloons and chemical tracers, so that aircraft carrying large suites of instruments can revisit the air mass over a period of days to observe changes with time. 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, entrapment of air from other strata, and mixing through the sides of the “air mass” (dispersion). These experiments tend to be complex and expensive (at least one ship and one or two aircraft are required), but they offer the potential to test the aerosol models that now exist and that will be developed from future laboratory work and other process studies. The overall strategic goal for the next two decades should be a predictive model to calculate atmospheric temperature and chemical species concentration fields and from that information to derive new aerosol particle formation rates and predict the chemical content and size distribution of the aerosol fields. Because current atmospheric models generally impose rather than predict aerosol distributions, significantly more sophistication will be needed in future models to represent precursor gas and gas/particle kinetics, nucleation and agglomeration kinetics, and vapor/particle interactions. One way to stimulate the needed improvements in aerosol modeling is to encourage the modeling community to participate directly in the planning, execution, and data analysis for the strategic field measurement programs described above. Furthermore, predictive aerosol models will require currently unavailable quantitative mechanistic and kinetic input data describing a large number of

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GLOBAL ENVIRONMENTAL CHANGE: Research Pathways for the Next Decade heterogeneous growth, nucleation, agglomeration, and accommodation/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 those of economic import (e.g., those dedicated to agriculture and forestry), can be profoundly affected by the wet and dry deposition of both toxic and nutritive atmospheric substances. While many of the atmosphere 's naturally occurring components can have toxic and/or nutritive effects on the biosphere, there are myriad toxic and nutritive substances in the atmosphere that are significantly shaped by industrial and agricultural activities. Moreover, while we are beginning to identify more acute cases of atmospheric toxicity and overfertilization for critical ecosystems, our understanding is far too limited to assess the extent of these problems now and to predict future ones. In its most general form the motivating scientific question for the study of toxics and nutrients is: How are interactions between the atmosphere and biosphere influenced by the changing atmospheric concentrations of these substances and by the deposition of harmful and beneficial compounds? More specifically, from the viewpoint of atmospheric chemistry, this question can be posed as: 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 this question are outlined below. Toxic and Nutrient Impacts: The Measurement of Deposition Fluxes Many of the key issues for toxics and nutrients cannot be satisfactorily answered yet because we lack methods to measure deposition fluxes on appropriate spatial and temporal scales. This problem is most severe for dry deposition, where technologies for reliably measuring many of the most biologically important fluxes do not yet exist. Adequate support for technique development in this area is thus a critical need; relaxed eddy accumulation, eddy correlation, and gradient methods offer particular promise. In the case of wet deposition, reliable techniques have been developed in principle, but serious questions remain about sampling representativeness and contamination. The problem is most acute for measuring wet deposition fluxes over the ocean, where it is virtually impossible to collect uncontaminated rain samples from a buoy in midocean, while samples from shipboard platforms are necessarily intermittent. Present marine deposition estimates, often the result of comparing model calculations with a very small set of shipboard and island

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GLOBAL ENVIRONMENTAL CHANGE: Research Pathways for the Next Decade observations, are typically subject to uncertainties of a factor of three or more. The development of new techniques 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 instances, such as in 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.26 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 need to be developed to assess the importance of droplet deposition and to provide reliable flux measurements. In the recent past, deposition monitoring networks have proven useful in determining the ecological impacts of atmospheric deposition (e.g., the National Acid Deposition Program/National Trends Network). However, these networks have mostly been limited to monitoring the deposition of a specific chemical or class of compounds (e.g., acid deposition, ozone). For this reason these networks have provided very limited information on the stresses and benefits experienced by an ecosystem from atmospheric deposition and thus on the long-term effects of this deposition as well. With the development of new deposition measurement techniques, it should be possible to design more comprehensive atmospheric deposition/exposure monitoring networks. Implementation of these networks for key ecosystems would provide a long-term record of atmospheric deposition; with colocated ecological monitoring, this record would no doubt prove useful in establishing causal relationships between atmospheric deposition and ecosystems' vitality and succession. Toxics and Nutrients: Processes and Mechanisms Leading to Deposition 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. Consequently, process-oriented field studies, which make observations of fluxes under carefully selected ranges of conditions, must be undertaken to identify the factors that control fluxes. With these factors identified, algorithms and parameterizations describing deposition fluxes could be developed, tested by further observations, and incorporated into regional and global atmospheric chemistry models, as well as integrated atmospheric/biospheric response models. NOTES 1. NRC (1992). 2. Farman et al. (1985). 3. Anderson et al. (1991).

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GLOBAL ENVIRONMENTAL CHANGE: Research Pathways for the Next Decade 4. Brune et al. (1991). 5. Weinheimer et al. (1998). 6. Johnson (1971). 7. Climate Impact Assessment Program (1975). 8. Fahey et al. (1995). 9. Johnson (1971). 10. Wennberg et al. (1994). 11. Fahey et al. (1995), Hanisco et al. (1997). 12. See review by Holton et al. (1995). 13. Plumb and Ko (1992). 14. NRC (1992). 15. Ibid. 16. Seinfeld and Pandis (1998). 17. Prather (1985, 1986). 18. Ibid. 19. Prather et al. (1987). 20. E.g., Baldocci et al. (1996). 21. Tans et al. (1996). 22. Businger and Oncley (1990). 23. Fishman et al. (1990). 24. McCormick (1993). 25. Ibid., Rind et al. (1993). 26. Vong et al. (1991). REFERENCES AND BIBLIOGRAPHY Anderson, J.G., D.W. Toohey, and W.H. Brune. 1991. Free radicals within the Antarctic vortex: The role of CFCs in Antarctic ozone loss. Science 251:39-46. Baldocchi, D., R. Valentini, S. Running, W. Oechel, and R. Dahlman. 1996. Strategies for measuring and modeling carbon-dioxide and water-vapor fluxes over terrestrial ecosystems. Global Change Biology 2(2):159-168. Baughcum, S.L., S.C. Henderson, and D.J. Sutkus, 1998. Scheduled civil aircraft emission inventories projected for 2015: Database development and aerosols. NASA CR-1998-207638. National Aeronautics and Space Administration, Washington, D.C. Brune, W.H., J.G. Anderson, D.W. Toohey, D.W. Fahey, S.R. Kawa, R.L. Jones, D.S. McKenna, and L.R. Poole. 1991. The potential of ozone depletion in the Arctic polar stratosphere Science 252:1260. Businger, J.A., and S.P. Oncley. 1990. Flux measurement with conditional sampling. Journal of Atmospheric and Oceanic Technology 7(2):349-352. California Air Resources Board. 1989. Information on substances for review as toxic air contaminants. Report No. ARB/SSD/89-01. California Air Resources Board, Sacramento. Chameides, W.L., R.W. Lindsay, J. Richardson, and C.S. Kiang. 1988. The role of biogenic hydrocarbons in urban photochemical smog: Atlanta as a case study. Science 241:1473. Charlson, R.J., and J. Heintzenberg, eds. 1995. Aerosol forcing of climate. In Report of the Dahlem Workshop on Aerosol Forcing, Berlin, April 24-29. Wiley, Chichester, U.K. Climate Impact Assessment Program (CIAP). 1975. DOT-TST-75-51. U.S. Department of Transportation, Washington, D.C. Crutzen, P.J. 1995. Ozone in the troposphere. In Composition, Chemistry, and Climate of the Atmosphere, H.B. Singh, ed. Van Nostrand Reinhold, New York

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GLOBAL ENVIRONMENTAL CHANGE: Research Pathways for the Next Decade Fahey, D.W., E.R. Keim, K.A. Boering, C.A. Brook, J.C. Wilson, H.H. Jonsson, S. Anthony, T.F. Hanisco, P.O. Wennberg, R.C. Miakelye, R.J. Salawitch, N. Louisnard, E.L. Woodbridge, R.S. Gao, S.G. Donnelly, R.C. Wamsley, L.A. Delnegro, S. Solomon, B.C. Daube, S.C. Wofsy, C.R. Webster, R.D. May, K.K. Kelly, M. Loewenstein, J.R. Podolske, and K.R. Chan. 1995. Emission Measurements of the Concorde supersonic aircraft in the lower stratosphere. Science 270:(5233)70-74. Farman, J.C., B.G. Galdiner, and J.D. Shanklin. 1985. Large losses of total ozone in Antarctica reveal seasonal C1Ox/NOx interaction. Nature 315:207. Fishman, J., C.E. Watson, J.C. Larsen, and J.A. Logan. 1990. Distribution of tropospheric ozone determined from satellite data Journal of Geophysical Research 95:3599-3617. Friedl, R.R. ed. 1997. Atmospheric effects of subsonic aircraft: Interim assessment report of the advanced subsonic technology program. NASA-RP-1400. National Aeronautics and Space Administration, Washington, D.C. Hanisco, T.F., P.O. Wennberg, R.C. Cohen, J.G. Anderson, D.W. Fahey, E.R. Keim, R.S. Gao, R.C. Wamsley, S.G. Donnelly, L.A. Del Negro, R.J. Salawitch, K. Kelly, and M.H. Proffitt. 1997. The role of HOx in super- and subsonic aircraft exhaust plumes. Geophysical Research Letters 24(1):65-68. Holton, J.R., P.N. Haynes, M.E. McIntyre, A.R. Douglass, R.B. Rood, and L. Pfister. 1995. Stratosphere-troposphere exchange. Reviews in Geophysics 33:403. Johnson, H.S. 1971. Reduction of stratospheric ozone by nitrogen oxide catalysts from supersonic transport exhaust. Science 173:517. Keeling, C.D. 1995. Halogens in the atmospheric environment. In Composition, Chemistry, and Climate of the Atmosphere, H.B. Singh, ed. Van Nostrand Reinhold, New York. Logan, J.A. 1994. Trends in the vertical distribution of ozone: An analysis of ozonesonde data. Journal of Geophysical Research 99:25,553-25,585. McCormick, M.P. 1993. Annual variations of water vapor in the stratosphere and upper troposphere observed by the Stratospheric Aerosol and Gas Experiment II. Journal of Geophysical Research 98:4867-4874. National Research Council. 1992. Rethinking the Ozone Problem in Urban and Regional Air Pollution. National Academy Press, Washington, D.C. National Research Council. 1996. A Plan for a Research Program on Aerosol Radiative Forcing and Climate Change. National Academy Press, Washington, D.C. National Research Council. 1998. Decade- to Century-Scale Climate Variability and Change: A Science Strategy. National Academy Press, Washington, D.C. Plumb, R.A. 1996. A “tropical pipe” model of stratospheric transport. Journal of Geophysical Research 101:39, 57. Plumb, R.A., and M.K.W. Ko. 1992. Interrelationships between mixing ratios of long-lived stratospheric constituents. Journal of Geophysical Research 97:10, 145. Prather, M.J. 1985. Continental sources of halocarbons and nitrous oxide. Nature 317: 221-225. Prather, M.J. 1986. European sources of halocarbons and nitrous oxide: Update. Journal of Atmospheric Chemistry 6:375-406. Rind, D., E-W. Chiou, W. Chu, J. Larsen, S. Oltmans, J. Lerner, M.P. McCormick, and L. McMasters. 1993. Overview of the SAGE II water vapor observations: Method, validation and data characteristics. Journal of Geophysical Research 98:4835-4856. Prather, M., M. McElroy, S. Wofsy, G. Russell, and D. Rind. 1987. Chemistry of the global troposphere: Fluorocarbons as tracers of air motion. Journal of Geophysical Research 92:6579-6613. Seinfeld, J.H., and S.N. Pandis. 1998. Atmospheric Chemistry and Physics. John Wiley & Sons, New York. Tans, P.P., P.S. Bakwin, and D.W. Guenther. 1996. A feasible global carbon cycle observing system: A plan to decipher oday's carbon cycle based on observations. Global Change Biology 2:309-318.

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GLOBAL ENVIRONMENTAL CHANGE: Research Pathways for the Next Decade Vong, R.J, J.T. Sigmon, and S.F. Mueller. 1991. Cloud water deposition to Appalachian forests. Environmental Science and Technology 26:1014-1021. Weinheimer, A.J., D.D. Montzka, T.L. Campos, J.G. Walega, B.A. Ridley, S.G. Donnelly, E.R. Keim, L.A. Del Negro, M.H. Proffitt, J.J. Margitan, K.A. Boering, A.E. Anderws, B.C. Daube, S.C. Wofsy, B.E. Anderson, J.E. Collins, G.W. Sachse, S.A. Vay, J.W. Elkins, P.R. Wamsley, E.L. Atlas, F. Flcke, S. Schauffler, C.R. Webster, R.E. May, M. Loewenstein, J.R. Podolske, T.P. Bui, K.R. Chan, S.W. Bowen, M.R. Schoeberl, L.R. Lait, and P.A. Newman. 1998. Comparison between DC-8 and ER-2 species measurements in the tropical middle troposphere: NOy, O3, CO2, CH4, and N2O. Journal of Geophysical Research—Atmospheres 103:22,807-22,096. Wennberg, P.O., R.C. Cohen, R.M. Stimpfle, J.P. Koplow, J.G. Anderson, R.J. Salawitch, D.W. Fahey, E.L.Woodbridge, E.R. Keim, R.S. Gao, C.R. Webster, R.D. May, D.W. Toohey, L.M. Avallone, M.H. Proffitt, M. Loewenstein, J.R. Podolske, K.R. Chan, and S.C. Wofsy. 1994. Removal of stratospheric O3 by radicals: In situ measurements of OH, HO2, NO, NO2, CIO, and BrO. Science 266:398-404.

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