The Atmospheric Sciences Entering the Twenty-First Century (1998) / Chapter Skim
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2 Atmospheric Chemistry Research Entering the Twenty-First Century
2 Atmospheric Chemistry Research Entering the Twenty-First Century
Pages 107-168
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From page 107... ...
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.
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From page 108... ...
Major Scientific Questions and Challenge The principal focus for atmospheric chemistry research entering the twentyfirst 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)
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From page 109... ...
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.
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From page 110... ...
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 tonics and nutrients.
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From page 111... ...
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.
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From page 112... ...
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.
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From page 113... ...
, 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.
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From page 114... ...
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.
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From page 115... ...
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
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From page 116... ...
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.
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From page 117... ...
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.
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From page 118... ...
. The photochemical processes, driven largely by chlorine compounds from the photochemical degradation of anthropogenically produced CFCs, give rise to heterogeneous chemical reactions on the PSCs that cause a rapid and dramatic depletion of stratospheric ozone.
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From page 119... ...
. Interestingly, if CFCs had been banned in 1975, one year after the connection between these compounds and stratospheric ozone depletion was first uncovered (Cicerone et al., 1974; Crutzen, 1974; Molina and Rowland, 1974)
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From page 120... ...
SOURCE: Cardelino and Chameides, 1995. atmospheric chemical system can be found in the nation's attempts to control photochemical smog.
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From page 121... ...
Disciplinary Research Challenges The successes of atmospheric chemistry research over the past decades have raised a number of intriguing scientific questions about the workings of the Earth system. The identification and quantification of secular trends in the trace composition of the atmosphere, and the elucidation of complex mechanisms that link atmospheric species to each other and to atmospheric dynamics, are testaments to the vitality and growing technological capabilities of the research programs focusing on atmospheric chemistry and global change.
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From page 122... ...
) FIGURE II.2.6 Conceptual diagram illustrating potential interactions and feedbacks between Photochemical oxidants, greenhouse gases, and stratospheric ozone that might ensue following an initial increase in the emissions of ozone precursor species, that is, volatile organic compounds (VOCs)
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From page 123... ...
elucidation of the coupling between chemistry, dynamics, and radiation in the stratosphere and upper troposphere in order to develop a more reliable predictive capability for stratospheric ozone. Trends in Atmospheric Greenhouse Gases A reliable prediction of future climatic trends demands first and fore most a reliable prediction offuture trends in concentrations of the green house gases.
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From page 124... ...
Note the deviations between predictions and measurements in the midlatitudes of each of the hemispheres. The inability of present-day models to fully explain the levels of ozone depletion seen in the atmosphere indicates the need to remain diligent in monitoring the evolution of the stratosphere under declining burdens of CFCs and to continue to investigate the stratosphere to develop a more reliable predictive capability.
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From page 125... ...
. A reliable prediction of future climatic trends demands first and foremost a reliable prediction of the trend in concentrations of greenhouse gases over time.
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From page 126... ...
Paradoxically, whereas ozone in the stratosphere protects living organisms from the harmful effects of solar ultraviolet radiation, ozone in the lower atmosphere can have adverse effects on plants, animals, and human health. As described below, the critical scientific issues surrounding photochemical oxidants range from more fully understanding the processes involved in the development of urban and rural ozone pollution to elucidating the role of changing concentrations of tropospheric ozone in global environmental change.
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From page 127... ...
ATMOSPHERIC CHEMISTRY RESEARCH A 0.25 0.20 0.05 - 90th Percentile Mean -Median - 10th Percentile 0.00 ' 1 1 1 1 1 1 1 1 1 1 87 88 89 90 91 92 93 94 95 96 600 Sites NAAQS 0.16 0.14 0.12 Q Q .0 ~ 0.08 a) o Cal 0.1 0.06 0.04 0.02 127 Rural (194 sites)
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From page 128... ...
As delineated in Figure II.2.6, a change in the overall oxidizing capacity of the atmosphere could have effects that ripple throughout the atmosphere's chemical system, ultimately affecting greenhouse gas and stratospheric ozone concentrations, as well as those of the photochemical oxidants. Two research challenges emerge from this discussion: It is recommended (l ~ that the observational and computational tools and strategies needed by decision makers to devise more effective urban- and regional-scale ozone pollution abatement strategies and test their efficacy once implemented be developed; and (2)
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From page 129... ...
Chemical and Physical Properties of Atmospheric Aerosols Aerosols play an important role in climate change, stratospheric ozone depletion, and air quality issues. However, large uncertainties in the chemical, physical, and radiative properties of atmospheric aerosols render quantitative assessment of their effects problematic.
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From page 130... ...
Aerosols have the potential to play a critical role in shaping our environment. They affect climate and stratospheric ozone concentrations and may pose a significant health threat in urban and industrial centers.
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From page 131... ...
document the chemical and physical properties of atmospheric aerosols to provide more accurate assessments of the effects of aerosols on climate, stratospheric ozone, tropospheric oxidation, human health and welfare, and ecosystem functioning; and (2) elucidate the chemical and physical processes responsible for determining the size, concentration, and chemical characteristics of atmospheric aerosols in order to develop a more robust predictive capability.
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From page 132... ...
The discussions in the preceding sections have identified a series of disciplinary research challenges aimed at addressing the major scientific issues concerning the Environmentally Important Atmospheric Species. These challenges are summarized in Box II.2.2.
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From page 133... ...
Overarching Research Challenge 1: Document the Chemical Climatology of the Atmosphere The first priority of atmospheric chemistry research must be to establish the present chemical climatology of the atmosphere by documenting the spatial dis
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From page 134... ...
address a variety of environmental issues related to atmospheric chemistry, including urban ozone pollution, acid rain, air tonics, and stratospheric ozone depletion. Although the environmental problems that the CAAA-90 address pose significant economic as well as environmental costs to the nation, management of these problems will also be quite costly.
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From page 135... ...
Overarching Research Challenge 4: Develop a Holistic Research Strategy Thus far, our discussions have focused on the activities needed to address the uncertainties associated with each of the Environmentally Important Atmospheric Species. However, these species and the processes that cause their appearance, transport, transformation, and ultimate removal from the atmosphere operate within a highly coupled and interactive system of chemical and physical processes.
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From page 136... ...
Infrastructural Initiative 1: Deployment of an Observing System for Moderately Lived Species The need for long-term measurements of atmospheric species has been a recurring theme in our discussions. To meet this critical need, a new observing system must be designed and deployed that is able to establish the regional and global distributions and temporal trends in moderately lived species of environmental import and the chemical and physical properties of atmospheric aerosols.
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From page 137... ...
Infrastructural Initiative 2: Deployment of Exposure Assessment Networks Current monitoring networks have largely been designed to characterize the chemical climatology of the atmosphere or to monitor compliance with specific environmental regulations; they are not well suited to examining interactions between the biosphere and the atmosphere or to characterizing the exposure of ecosystems to toxics and nutrients. A new infrastructural initiative that is specifically aimed at assessing exposures of targeted populations and blames through the deployment of exposure assessment networks is therefore critically needed.
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From page 138... ...
To aid in the development of predictive models and to better understand biospheric-atmospheric interactions, a new initiative is needed that focuses on the development and evaluation of measurement platforms designed to quantify the surface emission and deposition rates of Environmentally Important Atmospheric Species at a wide variety of spatial scales. Infrastructural Initiative 4: Demonstration of an Operational Chemical Meteorology System The standard approach in atmospheric chemistry research generally means that months or even years go by between initial observations and the analysis of these observations and model simulations.
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From page 139... ...
To facilitate this process and thereby aid in the development of global sampling systems and data bases (Overarching Research Challenges 1 and 2) and in the support of environmental management activities (Overarching Research Challenge 3)
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From page 140... ...
THE ENVIRONMENTALLY IMPORTANT ATMOSPHERIC SPECIES: SCIENTIFIC QUESTIONS AND RESEARCH STRATEGIES Stratospheric Ozone In the past couple of decades, column ozone abundance has decreased substantially over major portions of the globe, and the scientific evidence overwhelmingly points to anthropogenically produced chlorofluorocarbons and other halogenated compounds as the main cause of this ozone loss (WMO, 1995~. The consequences of this reduction are serious; hence, steps have been taken by the international community to reverse the loss.
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From page 141... ...
141 Cal a' .s .= ·_4 C)
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From page 142... ...
The essential research activities that will be required to address these questions are outlined in Box II.2.3. Monitoring the Distribution of Stratospheric Ozone The centerpiece of the strategy for research on stratospheric ozone depletion must be uninterrupted observations of the total concentration and vertical distribution of ozone made with high temporal resolution and accuracy using a combination of intercalibrated instruments on space-based and ground-based platforms as well as small balloon sondes.
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From page 143... ...
The total rate of stratospheric ozone destruction is governed by several catalytic loss processes whose individual rates are limited by the abundances of specific free-radical species such as hydroxyl radical (OH) , hydroperoxyl radical (HO2)
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From page 144... ...
However, to be able to define the full spectra of stratospheric ozone responses to the myriad of current and possible future perturbations of the stratosphere (e.g., volcanic injections of sulfur; aerosols, nitrogen oxides, and water vapor from subsonic and supersonic aircraft; emissions of bromine and chlorine compounds; temperature changes resulting from ozone depletion and greenhouse gas forcings) , it is essential that we map the specific rates of change of rate-limiting reactions with respect to the variables on which the concentrations depend, as well as the radical concentrations themselves.
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From page 145... ...
The coherence of these tracer surfaces as a vertical coordinate is revealed by the tight relations between them and other reactive species; this dramatic feature has led to important insights into the dynamics of the stratosphere and the response of stratospheric ozone to chemical and physical perturbations. Air enters the stratosphere primarily in the cold inner tropics through a process that desiccates the air and confines the upwelling flow to the middle stratosphere, as shown in Figure II.2.15.
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From page 146... ...
, highly accurate, in situ observations of the phase relationships between CO2 and H2O; measurements of the seasonal changes in the relationships between N2O, CH4, SF6 (sulfur hexafluoride) , CFC-ll, CFC-12, O3, and NOy; and determination of the age of the air mass in which the measurements are made.
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From page 147... ...
Quantification and Characterization of Critical Gas-Phase and Heterogeneous Mechanisms Predicting future changes in stratospheric ozone concentrations will require a more thorough understanding of the fundamental processes, both chemical and physical, that govern the balances between formation, transport, and destruction of ozone in the stratosphere. The chemical transformations that take place in the stratosphere either in the gas phase or on condensed matter ultimately determine the composition of this region.
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From page 148... ...
, whereas stratospheric ozone concentrations have decreased over the past 20 years (see Figure II.2.2~. In addition there are preliminary indications that stratospheric H2O concentrations are currently on the rise.
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From page 149... ...
It should be noted that many of the research activities described here are relevant to other key issues in atmospheric chemistry highlighted in this Disciplinary Assessment. For example, investigations of the distributions and surface exchange rates of N2O, CH4, and the halogenated compounds, as well as O3, are clearly relevant to the study of stratospheric ozone and photochemical oxidants.
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From page 150... ...
Recommended research tasks are listed in Box II.2.5 and discussed below. Maintain Current Concentration Monitoring Networks The most robust large-scale signature of sources and sinks of the greenhouse gases and their time dependence will be variations in the mixing ratios of CO2, CH4, N2O; in their isotonic ratios.
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From page 151... ...
Conduct Multlyear Flux Measurements over Different Ecosystems Predicting future greenhouse gas concentrations requires understanding how the surface exchange rates of these gases will respond to local, regional, and global climate variations. For the biogenic greenhouse gases, this will require surface flux measurements, in concert with hydrological and climatic observations, over a variety of blames and climate regimes for a multiyear period (see, for example, Baldocchi et al., 1996~.
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From page 152... ...
Thus, the implementation of a monitoring network to document changing greenhouse gas concentrations downwind of major source regions would provide critical information about the magnitude of greenhouse gas emissions from these regions. Such information would prove highly valuable for constraining atmospheric budgets for these gases and could ultimately represent a means of verifying compliance with any future international emissions agreements.
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From page 153... ...
Application of this atmospheric measurement over the oceans would enable us to determine some of the factors driving the kinetics of air-sea exchange, whereas for other factors, time-resolved full eddy correlation measurements are required. Devise New Systems to Make Accurate Concentration Measurements Because of their global coverage, satellites represent ideal platforms for measuring concentrations of greenhouse gases on the meso- and global scales.
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From page 154... ...
Ozone In addition to its critical role as an absorber of ultraviolet radiation in the stratosphere and in the oxidant chemistry of the troposphere, ozone's infrared absorption (at 9.6 ,um) makes it an effective greenhouse gas, especially in the upper troposphere and lower stratosphere (Lacis et al., 1990~.
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From page 155... ...
Maintain and Expand Space-Based Ozone Observations Although sonde measurements provide data with high vertical resolution, space-based measurements provide true global coverage. For this reason, maintenance and enhancement of satellite measurements of the ozone vertical profile are also critical.
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From page 156... ...
Water Vapor As in the case of ozone, elucidating the role of changes in water vapor concentration in an enhanced greenhouse effect most critically requires characterization of the species' trend in the upper troposphere and lower stratosphere (IPCC, 1995~. Strategies for accomplishing this are described below and in Box II.2.7.
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From page 157... ...
Photochemical Oxidants Elevated concentrations of oxidants on urban and regional scales in the industrialized countries of the world are proving to be among the most intractable air quality problems (NRC, 1991~. Thus, the goals of atmospheric chemistry research in the twenty-first century must include the development of a more complete understanding of the chemical processes occurring in the boundary layer and troposphere that determine the distribution and trends of the photochemical oxidants and their precursors on urban, regional, and global scales.
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From page 158... ...
. To what extent do changes in stratospheric ozone, climate, and/or cloud cover affect the oxidizing capacity of the lower atmosphere?
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From page 159... ...
· 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)
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From page 160... ...
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.
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From page 161... ...
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.
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From page 162... ...
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.
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From page 163... ...
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.
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From page 164... ...
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.
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From page 165... ...
, 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.
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From page 166... ...
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.
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From page 167... ...
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
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From page 168... ...
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 blames (e.g., at Long-Term Ecological Research sites)
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