Global Environmental Change Research Pathways for the Next Decade (1999) / Chapter Skim
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5 Changes in the Chemistry of the Atmosphere
5 Changes in the Chemistry of the Atmosphere
Pages 191-236
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From page 191... ...
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.
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From page 192... ...
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)
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From page 193... ...
Second, we identify the key unanswered scientific questions that confront the field of atmospheric chemistry today. There are three categories of such questions: 1.
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From page 194... ...
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.
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From page 195... ...
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.
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From page 196... ...
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.
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From page 197... ...
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.
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From page 198... ...
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 ER2 results decidedly rearranged our thinking on lower-stratospheric ozone photochemistry.
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From page 199... ...
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.
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From page 200... ...
200 GLOBAL ENVIRONMENTAL CHANGE 10 Wave-driven 30 extratopical Large-scale ascent Large-scale Pressure (mb) subsistence "pump" 100 400 380 Some 350 Cbs penetrate 330 stratosphere 300 300 Two-way exchange blocking anticyclones cut-off cyclones tropopause folds 1000 Pole Equator Latitude FIGURE 5.3 Dynamical aspects of stratosphere-troposphere exchange.
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From page 201... ...
However, limited data suggest that tropospheric ozone concentrations may have BOX 5.1 Stratospheric Ozone: Key Scientific Questions 1. Will evolution of the Antarctic stratospheric ozone "hole" proceed as expect ed, with a period of continued increasing intensity, followed by recovery to normal conditions?
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From page 202... ...
6. What are the trends in water vapor in the upper troposphere and lower stratosphere and the causes?
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From page 203... ...
For example, investigations of the distributions and surface exchange rates of N2O, CH4, and the halogenated compounds, as well as O3, are clearly of interest in the study of stratospheric ozone and photochemical oxidants. Photochemical Oxidants Imperative Elevated levels of oxidants on urban and regional scales in the industrialized countries of the world are proving to be among the most intractable of air quality problems.14 To meet the information needs of society, the goals of atmospheric chemistry research in the next two decades must include more complete understanding of the processes determining the distribution and trends of photochemical oxidants and their precursors on urban, regional, and global scales.
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From page 204... ...
? To successfully address these questions, it must be recognized that photochemical oxidants research is truly data poor and measurement limited.
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From page 205... ...
While BOX 5.4 Atmospheric Aerosols: Key Scientific Questions 1. What is the role of natural and anthropogenic aerosols in climate and how might future changes in levels of aerosol precursors affect this role?
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2. More specifically, from the viewpoint of atmospheric chemistry, what are the rates at which biologically important atmospheric trace species are trans ferred from the atmosphere to terrestrial and marine ecosystems through dry and wet deposition?
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From page 207... ...
The degradation of spatial resolution, poor signal-to-noise ratios, the missing of key species in a selected array of simultaneous observations, and the inability to access latitudes, altitudes, solar zenith angles, and seasons are problems that profoundly cripple datasets. The heart of this lesson is remembering the distinction between gathering data, on the one hand, and clearly answering specific questions on the other.
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From page 208... ...
The reason is that, over large regions of the lower stratosphere, dia batic descent brings stratospheric air down from altitudes well above current in situ platform capabilities (~20 km)
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From page 209... ...
• 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.
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From page 210... ...
Continuous cross-calibrated measurements of stratospheric ozone will document the extent of ozone loss and degree of ozone recovery if stratospheric halogen concentrations, water vapor, temperature, and other critical factors return to normal. These measurements will thus provide an essential gauge of the sufficiency of and/or world compliance with the international treaties devised to reverse the ozone depletion of the 1980s and 1990s.
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From page 211... ...
Predicting Future Changes in UV Dosage: Establishing Cause and Effect in the Photochemical Cycles that Control Ozone Loss Rates Accurately determining secular trends in the distribution of stratospheric ozone represents an essential arm of stratospheric ozone research in the coming decades. The need to understand potential changes in stratospheric chemistry in the face of changing sulfur, chlorine, bromine, N2O, methane, CO2, H2O, and other chemical loadings requires understanding the underlying mechanisms.
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From page 212... ...
For example, at low NOx concentrations, ozone loss rate was found to be inversely correlated with NOx. Also shown are actual data obtained by the NASA ER-2 in the stratosphere, demonstrating that the specific slope of ozone loss is directly observable.
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From page 213... ...
Because of the couplings that exist between NOx and the other radical families, the response of ozone loss to a change in NOx concentrations turns out to be a complex function that depends on the individual gradients in each of the rate-limiting steps with respect to NOx. Because of the nature of these gradients, the total ozone loss is smallest at an intermediate NOx concentration.
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From page 214... ...
This action, in turn, restricts mixing with the midlatitudes, thus isolating the winter polar stratosphere from the rest of the stratosphere during the winter months, especially in the southern hemisphere. The restriction of exchange with the wintertime polar stratosphere plays a significant role in the annual dynamical cycle of the stratosphere and is an essential element in the formation of large regions of severe ozone loss over the polar regions.
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From page 215... ...
Analysis of climate changes in response to secular trends in infrared species requires accurate knowledge of how the major infrared-active gases evolve over time and of the processes controlling the production and removal rates for those species in the atmosphere. These species include the primary infrared active molecules emitted directly into the stratosphere (e.g., CO2, CH4, N2O, CFCs)
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From page 216... ...
Note that the research described here is directly relevant to other central issues in atmospheric chemistry identified in this report. 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 ultraviolet exposure and photochemical oxidants.
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From page 217... ...
What are the trends in water vapor in the upper troposphere and lower stratosphere? providing a clear distinction between these compounds and biogenic greenhouse gases.
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From page 218... ...
An atmospheric transport model based on observed winds would play a key part in analyzing the data. Developing a Systematic Method for Multiyear Flux Measurements and Vertical Profile Measurements over Continents A predictive capability for infrared active species requires understanding how the surface exchange rates of these gases behave as a function of such factors as season, rainfall, and local, regional, and global climate variations.
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From page 219... ...
While multiyear flux measurements provide insight into the mechanisms responsible for gas exchange on scales of a few hundred kilometers, larger-scale studies are needed to establish the methodology and the validity of extrapolating this flux information to regional and global scales. This type of study, such as that planned for Brazil in 1998, represents a natural progression from the mesoscale studies carried out in the 1980s and early 1990s (e.g., the ABLE campaigns)
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From page 220... ...
A promising near-term approach is a variant of the eddy correlation method, namely, the conditional sampling method, which slowly accumulates samples from the upward-moving eddies in one container and samples from the downward eddies in another.22 After careful conditioning, the differences between the two containers can be measured with conventional slowresponse instruments. For many of the questions about the mechanisms controlling global sources and sinks of infrared active gases, time-resolved full eddy correlation measurements are required.
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From page 221... ...
Ultimately, coupled biospheric, oceanic, and atmospheric models that allow for the proper feedbacks must be developed to reach the goal of predicting trends in biogenic greenhouse gases. Establishing a Consistent Strategy for Determining Secular Trends in Ozone as an Infrared Active Species in the Mid- to Upper Troposphere and Lower Stratosphere Ozone has a critical role as absorber of ultraviolet radiation in the stratosphere and in the oxidant chemistry of the troposphere, but additionally its absorption feature at 9.6 µm makes it an effective greenhouse gas when present in the upper troposphere and lower stratosphere.
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From page 222... ...
Measurements from the Stratospheric Aerosol and Gas Experiment (SAGE) II have provided valuable information on ozone trends above 17 km since 1984, with the exception of the period after the Pinatubo volcanic eruption, when data could not be obtained in the lower stratosphere.
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From page 223... ...
offer the advantage of high vertical resolution and provide a test of existing satellite data. Instruments such as SAGE II,24 the Halogen Occultation Experiment, and the Microwave Limb Sounder on the Upper Atmosphere Research Satellite have demonstrated that stratospheric water vapor can be measured from satellites with adequate precision to characterize temporal trends at some levels.
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From page 224... ...
One goal of atmospheric chemistry research must be the better definition of mechanisms that determine the distribution and secular trends of these photochemical oxidants. This imperative has two principal objectives: (1)
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From page 225... ...
In short, a research strategy that is both evolutionary and revolutionary will be required -- a strategy that is outlined below. The key reactions relating to photochemical oxidants take place under a wide range of conditions: on salt spray in the presence of high fluxes of solar ultraviolet radiation; in biomass-burning plumes with a rich variety of organics, sulfur, aerosols, moisture, and other constituents; in clean marine regions nearly devoid of NOx but with very high levels of water vapor and ultraviolet; and in cold highlatitude regions, where heterogeneous reactions on micron-sized ice crystals cata
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From page 226... ...
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.
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From page 227... ...
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.
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From page 228... ...
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 wellbeing (see Box 5.8)
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From page 229... ...
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.
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From page 230... ...
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.
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From page 231... ...
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.
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From page 232... ...
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.
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From page 233... ...
1991. Free radicals within the Antarctic vortex: The role of CFCs in Antarctic ozone loss.
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From page 234... ...
1993. Annual variations of water vapor in the stratosphere and upper troposphere observed by the Stratospheric Aerosol and Gas Experiment II.
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From page 235... ...
1994. Removal of stratospheric O3 by radicals: In situ measurements of OH, HO2, NO, NO2, CIO, and BrO.
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