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Box II.2.1 Environmentally Important
Atmospheric Species
These species are scientifically interesting and
important to human health and welfare because of their radiative
(e.g., climate changing) and/or chemical properties. They include
the following:
• Stratospheric ozone
• Greenhouse gases
• Photochemical oxidants
• Atmospheric aerosols
• Toxics and nutrients
Documenting the changing concentrations and
distribution of these species, elucidating the processes that
control their concentrations, and assessing their impacts on
important environmental and ecological parameters will define the
principal challenges for atmospheric chemistry in the coming
decades.
The scientific questions facing atmospheric chemistry entering
the twenty-first century are intellectually profound but are also
of vital social and economic importance. They relate to atmospheric
constituents that are fundamentally important to our environment:
stratospheric ozone, greenhouse gases, ozone and photochemical
oxidants in the lower atmosphere, atmospheric aerosols or
particulate matter, and toxics and nutrients (see Box II.2.1). It
is perhaps a measure of the strides made in recent decades, that
the issues of atmospheric chemistry are familiar to the general
public, policy makers, and scientists alike. Continued progress in
the twenty-first century will require an ambitious, but judicious,
commitment of financial, technological, and human resources to
document the changing composition of the atmosphere and elucidate
the causes and potential consequences of these changes.
Major Scientific Questions and
Challenge
The principal focus for atmospheric chemistry research entering
the twenty-first century will be the "Environmentally Important
Atmospheric Species"— species that, by virtue of their
radiative and/or chemical properties, affect climate, key
ecosystems, and living organisms (including humans). From an
intellectual point of view, these species are interesting because
they influence the life support system of our planet. From a
societal point of view, they are also of central importance because
they directly impact human health and welfare.
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The challenge for atmospheric chemistry research in the coming
decades follows:
Development and application of the tools and
scientific infrastructure necessary to document and predict the
concentrations and effects of Environmentally Important Atmospheric
Species on a wide variety of spatial and temporal scales.
To meet this challenge, atmospheric chemistry research should be
formulated around three fundamental questions:
1. What are the shorter-term periodic and longer-term secular
trends in the concentrations of Environmentally Important
Atmospheric Species on local to global scales? What are the causes
of these trends?
2. How will the concentrations of these species change in the
future? What are the most effective and feasible policy options for
managing these changes?
3. What will be the totality of environmental effects of present
and future trends in the concentrations of these species?
Overarching Research Challenges
The scientific strategy for atmospheric chemistry emerges
logically from the application of these fundamental scientific
questions to each of the Environmentally Important Atmospheric
Species. It is a strategy that endeavors to continuously improve
our understanding of the underlying chemical, physical, and
ecological processes that control the concentrations of these
species, while providing timely and relevant input to decision
makers. Toward these ends, the scientific research strategy in
atmospheric chemistry must include the following:
• Document the chemical climatology and meteorology of the
atmosphere, particularly their variability and long-term trends,
through the development and maintenance of diverse and interrelated
arrays of monitoring networks.
• Develop and evaluate predictive tools and models of
atmospheric chemistry through a synthesis of information gathered
from process-oriented field studies, laboratory experiments, and
other observational efforts; their representation in
mathematical/numerical algorithms; and the testing of these
algorithms in well-posed model-evaluation field experiments.
• Provide assessments of the efficacy of environmental
management activities through the gathering and interpretation of
relevant air quality data.
• Be holistic and integrated in the study of the
Environmentally Important Atmospheric Species and of the chemical,
physical, and ecological interactions that couple them
together.
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Disciplinary Research Challenges
The disciplinary challenges listed below focus on the specific,
key scientific issues facing the atmospheric chemistry community in
the twenty-first century:
• Stratospheric Ozone Challenges: Document the
distributions, variability, and trends of stratospheric ozone and
the key species that control its catalytic destruction; elucidate
the coupling between chemistry, dynamics, and radiation in the
stratosphere and upper troposphere.
• Greenhouse Gas Challenges: Elucidate the processes
that control the abundances, variabilities, and long-term trends of
atmospheric CO2 (carbon dioxide),
CH4 (methane), N2O (nitrous oxide), and upper-tropospheric
and lower-stratospheric O3 (ozone)
and water vapor; and expand global monitoring networks to include
upper-tropospheric and lower-stratospheric O3 and water vapor.
• Photochemical Oxidant Challenges: Develop the
observational and computational tools and strategies needed by
decision makers to effectively manage ozone pollution; elucidate
the processes that control, and the interrelations that exist
between, the ozone precursor species, tropospheric ozone, and the
oxidizing capacity of the atmosphere.
• Atmospheric Aerosol Challenges: Document the
chemical, physical, and radiative properties of atmospheric
aerosols, their spatial extent, and long-term trends; elucidate the
chemical and physical processes responsible for determining the
size, concentration, and chemical characteristics of atmospheric
aerosols.
• Toxics and Nutrients Challenges: Document the
rates of chemical exchange between the atmosphere and key
ecosystems of economic and environmental import; elucidate the
extent to which interactions between the atmosphere and biosphere
are influenced by changing concentrations and deposition of harmful
and beneficial compounds.
Infrastructural Initiatives
The following infrastructural initiatives provide the resources
and capabilities recommended to accomplish the disciplinary
challenges:
• Global Observing System: deployment of an
observing system for moderately lived species to complement ongoing
networks and measurement platforms focusing on long-lived species
and stratospheric ozone.
• Ecosystem Exposure Systems: deployment of
monitoring networks capable of assessing ecosystem exposure to
primary and secondary toxics and nutrients.
• Surface Exchange Measurement Systems: development
and deployment of measurement systems capable of quantifying
chemical exchange between the atmosphere and key biological or
ecosystems.
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• Environmental Management Systems: demonstration
and assessment of the feasibility of operational "chemical
meteorology" as a prognostic tool for environmental managers and
regulators.
• Instrument Development and Technology Transfer:
development of programs and facilities to support the evaluation of
new atmospheric chemical instruments and their transfer to the
scientific, regulatory, and private sector communities.
• Fundamental Condensed Phase and Heterogeneous
Chemistry: development and maintenance of laboratory facilities
focused on condensed phase and heterogeneous chemical processes
relevant to the atmosphere.
Expected Benefits and Contribution to
National Well-Being
The scientific questions to be addressed by the atmospheric
chemistry research community entering the twenty-first century are
central to our understanding of the chemical and physical
environment in which we human beings must reside. For this reason,
the science of atmospheric chemistry is highly relevant to the
future development and economic vitality of our society. Today, the
changing chemistry of our atmosphere on local, regional, and global
scales is an observational fact. These changes are impacting human
health and placing economically and environmentally important
resources and ecosystems at risk. At the same time, air quality
management activities in the United States cost tens of billions of
dollars annually. Research in atmospheric chemistry and the
resulting improvements in our predictive capabilities will help us
to maximize the environmental and economic benefits gained from
these sizable investments in air quality management, while also
teaching us how to minimize the deleterious effects of human
activity on the chemical and physical environment.
Introduction and Overview
As the world stands on the threshold of a new millennium, the
atmospheric chemistry community stands at the portal of a new era
of scientific research. During the latter half of the twentieth
century, the discipline of atmospheric chemistry came of age.
Scientific study revealed the crucial role that the chemistry of
the atmosphere plays in the life support system of the planet,
acting as a "connective tissue" by which organisms of the biosphere
interact and exchange materials and energy. It also uncovered a
more disturbing insight: the activities of an increasingly populous
and technological human society are changing the composition of the
atmosphere on local, regional, and even global scales. Experience
has shown that air pollution on local and regional scales can be
environmentally and economically destructive. The consequences of
chemical change on a global scale could be even more damaging.
Thus, the scientific questions
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facing atmospheric chemistry are not only intellectually
challenging but also of vital social and economic importance.
The challenge for atmospheric chemistry as it enters the
twenty-first century will be to build on the discoveries of the
twentieth century by maintaining its scientific vitality and rigor
while also making the results of its scientific and technological
advances available to influence the nation's and the world's social
and economic development. In this Disciplinary Assessment, we
discuss the strategy that will be necessary to address the major
scientific issues of the discipline while also providing decision
makers with the information and tools they require to manage and
maintain environmental and economic vitality. We begin our
discussion with a statement of the mission for atmospheric
chemistry research entering the twenty-first century.
The Mission
Development and application of the tools and
scientific infrastructure necessary to document and predict the
concentrations and effects of Environmentally Important Atmospheric
Species on a wide variety of spatial and temporal scales.
In identifying the mission for atmospheric chemistry research
entering the twenty-first century, we have adopted three basic
premises:
1. The financial and human resources available for research and
development in the coming decades will be limited.
2. The activities of an increasingly populous and technological
society have and will continue to perturb critical environmental
factors that affect the natural resources on which our society
relies.
3. Unraveling the mechanisms that couple the chemistry of the
atmosphere to the life support system of the planet represents one
of the major intellectual and technological challenges of the
coming decades.
Premises 1 and 2 relate to the resource- and policy-relevant
issues that must be considered in defining the mission for
atmospheric chemistry research, whereas premise 3 focuses on the
intellectual or curiosity-based raison d'etre for the discipline.
The prospect of limited resources for research and development
indicated in premise 1 demands that a rigorous prioritization be
applied to any contemporary research program, so that the most
pressing scientific issues can be addressed in the allocation of
public resources to the scientific community. Premise 2 suggests
that priority should be placed on developing a scientifically
robust, predictive, and systematic understanding of the Earth
system, its chemical environment, and the relationships between the
economic and technological growth of the world's nations and the
environmental vitality and natural resources on which they
depend.
The development of a research program often requires compromises
between
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Figure II.2.1
Environmentally Important Atmospheric Species are atmospheric
constituents that affect human health and welfare and thus are
central to a policy-relevant research program in atmospheric
chemistry. Because these species drive the interaction between the
atmosphere and the life support system of the planet, they are also
central to a curiosity-based research program in atmospheric
chemistry. Environmentally Important Atmospheric Species include
greenhouse gases (e.g., CO2, CH4, N2O),
aerosols, and stratospheric ozone—species that, because of
their radiative properties, affect the climate and other physical
characteristics of our environment. They also include the
photochemical oxidants and tropospheric ozone, acid aerosols, and a
wide variety of toxic and nutritive substances that, because of
their chemical properties, affect humans and ecosystems of economic
and environmental importance when they come in direct contact with
them. Although the radiatively important species' effects are more
commonly felt on a global scale, the effects of the ecologically
important species are most often experienced on local-to-regional
scales. Nevertheless, despite the varying scales of their radiative
and chemical effects, research has revealed that the atmospheric
cycles of these species are coupled together through complex
photochemical and dynamical interactions. Unraveling these complex
interactions represents one of the major challenges of atmospheric
chemistry research in the coming decades.
the priorities dictated by policy-relevant issues and those
dictated by more theoretical interests. However, in the case of
atmospheric chemistry research we find a strong resonance between
the two. Atmospheric chemistry research in the coming decades
should be focused on documenting and predicting the concentrations
and effects of the chemical constituents that most directly affect
the physical and biological environment and, by extension, human
health and welfare. We refer to these species, here, in the most
generic sense, as the Environmentally Important Atmospheric Species
that, by virtue of their radiative and/or chemical properties,
directly affect living systems and key environmental parameters
(see Figure II.2.1).
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For atmospheric chemistry to make significant scientific
advances in the coming decades, however, its research focus on
these Environmentally Important Atmospheric Species must go well
beyond simple observation and documentation of chemical content and
change, to a rigorous investigation of the underlying chemical,
physical, and ecological processes that determine the atmospheric
concentrations of these species. It is, after all, only through
understanding these processes that a genuine appreciation of the
atmosphere and its relationship to the Earth system can be
fostered, and a reliable predictive capability can be achieved and
made available for the development of effective public policy.
Additionally, because the atmosphere and the stresses placed on it
are continually changing, a significant portion of the resources
made available for atmospheric chemistry research in the coming
decades must be used to develop an enduring research infrastructure
that can inform decision makers in an open, effective, and
responsive fashion.
The mission for atmospheric chemistry research in the coming
decades must therefore combine a focus on the Environmentally
Important Atmospheric Species with a commitment to the development
of a comprehensive, long-term research capability and technological
infrastructure. Hence, the mission of atmospheric chemistry in the
coming decades:
Development and application of the tools and
scientific infrastructure necessary to document and predict the
concentrations and effects of Environmentally Important Atmospheric
Species on a wide variety of spatial and temporal scales.
In the following sections, we consider how to accomplish this
mission most effectively by first considering what is now known
about the atmosphere and then identifying the key unresolved
scientific questions surrounding a number of Environmentally
Important Atmospheric Species and the research challenges that grow
from these questions.
Insights of the Twentieth Century
By grappling with a number of critical, but
largely unforeseen environmental problems in recent decades,
scientists have gained fundamental new insights about the
atmospheric chemical system.
The study of atmospheric chemistry as a quantitative, scientific
discipline can be traced to the eighteenth century when
world-renowned chemists such as Joseph Priestley, Antoine-Laurent
Lavoisier, and Henry Cavendish undertook the investigation of the
chemical components of the atmosphere (Farber, 1961; Weeks and
Leicester, 1968). It was largely through their efforts, as well as
those of a number of prominent chemists and physicists who
succeeded them in the nine-
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TABLE II.2.1 Important Trace Species of the
Atmospherea
Species
Concentration (Mole Fraction)
Principal Sources
Methane (CH4)
1.6 × 10-6
Biogenic
Carbon Monoxide (CO)
(0.5 - 2) × 10-7
Photochemical, Anthropogenic
Ozone (O3)
10-8 - 10-6
Photochemical
Reactive Nitrogen (NOy)
10-11 - 10-6
Lightning, Anthropogenic
Ammonia (NH3)
10-11 - 10-9
Biogenic
Particulate Nitrate (NO3-)
10-12 - 10-8
Photochemical, Anthropogenic
Particulate Ammonium (NH4-)
10-11 - 10-8
Photochemical, Anthropogenic
Nitrous Oxide (N2O)
3 × 10-7
Biogenic, Anthropogenic
Hydrogen (H2)
5 × 10-7
Biogenic, Photochemical
Hydroxyl (OH)
10-13 - 10-11
Photochemical
Peroxyl (HO2)
10-13 - 10-11
Photochemical
Hydrogen Peroxide (H2O2)
10-10 - 10-8
Photochemical
Formaldehyde (H2CO)
10-10 - 10-9
Photochemical
Sulfur Dioxide (SO2)
10-11 - 10-9
Anthropogenic, Volcanic
Dimethylsulfide (CH3CCH3)
10-11 - 10-10
Biogenic
Carbon Disulfide (CS2)
10-11 - 10-10
Anthropogenic, Biogenic
Carbonyl Sulfide (OCS)
10-10
Anthropogenic, Biogenic
Particulate Sulfate (SO4-)
10-11 - 10-8
Anthropogenic, Photochemical
a After
Chameides and Davis (1982).
teenth century, that the identity and concentration of the major
components of the atmosphere (i.e., nitrogen, oxygen, water, carbon
dioxide, and the rare gases) were established.
In the late nineteenth and early twentieth centuries,
atmospheric chemists shifted their focus from identifying the major
atmospheric constituents to consideration of the trace
constituents, that is, the gaseous and aerosol atmospheric species
having concentrations of less than a few parts per million per
volume of air (i.e., ppmv). The application of modem chemical
analytical techniques revealed the atmosphere to be a reservoir of
a myriad of trace species, whose presence can be attributed to a
complex array of geological, biological, chemical, and in many
cases, anthropogenic processes (see Table II.2.1). Moreover, these
trace species were found to have a disproportionately large impact
on our environment. In some instances, they adversely affect plant
and animal life because of their toxic properties; in other
instances, they benefit these or other organisms because of their
nutritive properties; in still other instances, they affect the
physical climate because of their radiative properties.
The latter half of the twentieth century has seen another major
shift in atmospheric chemistry as scientists attempt to grapple
with a number of potentially critical environmental problems,
including stratospheric ozone depletion, urban
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Figure II.2.2
From an atmospheric chemistry point of view, global change is an observational fact
not a theoretical possibility. (A) Average change per decade in the total atmospheric
ozone column as a function of latitude based on recent Dobson station measurements
(after WMO, 1995). (B-E) Average global concentrations of CO2, CH4,
N2O, and CFC-11 since the mid-1700s (after IPCC, 1990).
photochemical smog, and rising concentrations of ''greenhouse
gases'' (NRC, 1984). In the process, a new, policy-relevant
research paradigm for atmospheric chemistry has developed that has
profoundly altered its role in society. More importantly, the
insights gained from the study of these environmental crises have
irrevocably changed our understanding of the atmospheric chemical
system in which we as a species must reside. The major aspects of
these new insights are outlined below.
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The Chemical State of the Atmosphere
Has Changed in the Past and Is Continuing to Change
Observations have shown irrefutably that the chemistry of the
atmosphere is changing on local, regional, and global scales;
indeed, from a chemical point of view, global change is an
observational fact not a theoretical possibility. The annual
appearance of the Antarctic ozone hole provides striking evidence
of the atmosphere's vulnerability to chemical perturbation.
Although smaller in magnitude, the depletion of stratospheric ozone
in the temperate latitudes over the past decade is perhaps equally
disturbing (Figure II.2.2A). Moreover, present-day measurements
coupled with analyses of ancient air trapped in ice cores provide a
record of dramatic, global increases in the concentrations of a
number of long-lived greenhouse gases such as carbon dioxide,
methane, nitrous oxide, various chlorofluorocarbons (CFCs), and
other halocarbons (Figure II.2.2B-E). Although secular trends in
shorter-lived species are more difficult to document, a strong case
can be made that the abundances of tropospheric ozone and sulfate
and carbonaceous aerosols have also increased significantly in the
Northern Hemisphere during the past century (NRC, 1993).
Humans Are a Significant Driving Force
in Global Chemical Change
Many of the recent changes in atmospheric composition can be
traced to anthropogenic causes. A classic example is that of
atmospheric CO2, whose increasing
rates of production from the burning of fossil fuels and biomass
closely mimic its rising atmospheric abundance since the Industrial
Revolution (see Figure II.2.3). In other examples, the forcing from
anthropogenic activities is less obvious, largely arising when the
photochemical oxidation and degradation processes of anthropogenic
emissions lead to the production of secondary products that perturb
important environmental parameters. Examples of these indirect
perturbations include the release of chlorofluorocarbons that cause
stratospheric ozone depletion, the emission of sulfur oxides that
result in increasing concentrations of radiatively important and
health-damaging sulfate aerosols, and the emissions of nitrogen
oxides and volatile organic compounds that lead to the production
of tropospheric ozone and other photochemical oxidants.
Chemical Emissions into the Atmosphere
Can Have Long-Term Environmental Consequences That May Be Difficult
to Reverse
Because of the long time scales associated with many of the
processes that affect atmospheric composition, the chemicals we put
into the atmosphere and the environmental effects they engender can
persist for decades or even centuries. A prime example is the
long-term impact of anthropogenic CFCs on stratospheric ozone.
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1. What determines the ability of the atmosphere to cleanse
itself of pollutants via free-radical oxidation, both now and in
the coming decades? More specifically:
• To what extent does our current understanding explain
simultaneous measured OH concentrations and the principal OH
chemical production and loss processes?
• Can the oxidation of compounds or the appearance of their
oxidation products be successfully used to infer concentrations of
OH?
• To what extent do oxidants other than OH (O3, NO3,
H2O2,
halogen atoms, etc.) play significant roles in atmospheric
chemistry?
• To what extent do changes in stratospheric ozone,
climate, and/or cloud cover affect the oxidizing capacity of the
lower atmosphere?
2. What determines the distribution of ozone in the troposphere
and how will this distribution change in the coming decades? More
specifically:
• What fraction of tropospheric O3 can be attributed to transport from the
stratosphere, and how does this change with meteorology and
season?
• What portion of O3
precursors is emitted from biogenic sources, and how will these
emissions change with natural (e.g., meteorological variability)
and human-induced (e.g., land use, climate change)
perturbations?
• What is the contribution of urban pollution to rural and
regional O3, and conversely, what is
the impact of rural or regional O3
on urban pollution?
• How does meteorological variability affect the trends of
O3 and/or its precursors?
• What are the major sources of the oxides of nitrogen in
each region of the atmosphere over various geographic regions? What
are the rates of emission of NOx
from these sources?
• Which major reservoir and oxidizing species and which
gas-phase and heterogeneous chemical processes are responsible for
partitioning within the NOy
family?
• Where and when is the production of O3 limited by the availability of volatile
organic compounds (VOCs) or NOx?
• What are the trends in regional and local O3 precursors (NOx, VOCs, carbon monoxide)?
3. How can atmospheric models be improved to better represent
current atmospheric oxidants and better predict the atmosphere's
response to future levels of pollutants? More specifically:
• What laboratory research is required to provide
sufficient understanding of the fundamental chemical processes
(heterogeneous as well as gas phase) involved in tropospheric
oxidant formation?
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• What atmospheric measurements are required, and with what
precision and accuracy, to apply diagnostic and predictive models
of tropospheric oxidant chemistry?
• What are the quantitative uncertainties associated with
the estimates from diagnostic and predictive models of tropospheric
oxidant chemistry?
• How can models of tropospheric oxidant chemistry be
improved to incorporate direct and indirect effects of multiple,
interacting forcing agents (e.g., climate change, stratospheric
ozone depletion, anthropogenic perturbations)?
4. What will enable us to evaluate and improve our air quality
management strategies for photochemical oxidants? More
specifically:
• What design and implementation strategies will provide
monitoring networks capable of determining if control measures for
photochemical oxidants are having the intended impact?
• What design and implementation strategies will yield
monitoring networks capable of determining, for a particular air
quality problem, what portion of the problem is essentially
irreducible (i.e., natural emissions of ozone precursors and
stratospheric influx of ozone) and what portion of the ozone
problem is potentially controllable (i.e., human-made precursor
emissions)?
To successfully address these questions in the coming decades,
it must be recognized that research on photochemical oxidants is
truly ''data poor'' and "measurement limited." As a result,
significant progress in this area will require a commitment to
acquire high-quality, observational data sets that, collectively,
are global in coverage but, individually, are of high enough
spatial and temporal resolution to elucidate the important chemical
and physical processes responsible for the production, transport,
and removal of photochemical oxidants. To accomplish this, a
research strategy that is both evolutionary and revolutionary will
be required. The beginning of such a strategy focused on the
management of urban-and regional-scale photochemical oxidant
pollution in North America has recently been developed [see North
American Research Strategy on Troposphere Ozone (NARSTO) and
charter available from NARSTO Home Page at URL:
http://narsto.owt.com/Narsto/]. The research strategy outlined
below and in Box II.2.8 is similar in many respects to this
previous work but also addresses longer-term and globally relevant
issues.
Continue Development and Validation of
Chemical Instrumentation
Instrument development and validation should aim at improving
the sensitivity, specificity, and sampling rates of instruments
needed to measure the compounds of interest throughout the
atmosphere from the measurement platforms of choice (Albritton et
al., 1990). The focus should be on (1) the development of
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Box II.2.8 Recommended Research Tasks for
Photochemical Oxidants
1. Continue development and validation of chemical
instrumentation to
• provide techniques for long-term
monitoring;
• provide continuous, fast-response
techniques for flux divergence methods;
• provide miniaturized techniques for airbome
platforms; and
• provide long-path spatially resolved
techniques for making multidimensional measurements.
2. Continue implementation of integrated field
campaigns to
• elucidate fundamental processes;
• document key species' trends, sources, and
sinks; and
• evaluate air quality and chemical transport
models.
3. Carry out observation-based studies to
• elucidate trends and distribution in
short-lived radical species;
• independently infer emission inventories;
and
.
• infer ozone precursor relationships.
4. Develop and deploy monitoring networks to
• document the chemical climatology of
photochemical oxidants, and
• document the response of ozone to changes
in precursor concentrations (e.g., as a result of emission
controls).
5. Develop analytical models and tools to support
integrated assessments.
simpler and more reliable instruments to be used in long-term
monitoring; (2) the miniaturization of instruments to accommodate a
wide array of measurements on airborne platforms; (3) the
development of continuous, fast-response instruments to be used for
flux measurements and airborne applications; and (4) the use of
spatially resolved, long-path methods (e.g., Lidar) that can be
operated from airborne and mobile platforms to determine
distributions of compounds of interest over considerable
distances.
Continue Implementation of Integrated
Field Campaigns
Integrated field campaigns are undertaken to increase our
understanding of fundamental atmospheric processes; elucidate the
distributions, sources, and sinks of key species; and provide data
for the evaluation of air quality and chemical transport models.
Scientific guidance is required to carefully define how key
uncertainties are going to be reduced and what key science
questions will be addressed in a specific field campaign.
Atmospheric chemistry and meteorology must be integrated in the
planning and deployment of air quality measurements and monitoring.
The questions that are presently before us will require multi-
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disciplinary teams that can address chemistry, transport, and
ecosystem feedbacks. Modeling tools adequate to depict or simulate
these processes must be available to guide the planning of
measurements as well as the interpretation of results. Moreover, an
adequate fleet of research aircraft must be available to the
atmospheric sciences community in order to make these studies
feasible.
Carry out Inferential
Observation-Based Studies
Carefully designed observations of specific tracer compounds or
suites of tracer compounds can be used in conjunction with
diagnostic and/or observation-based models to independently infer
(1) the long-term trends, seasonal variability, and regional
distribution of short-lived free-radical species not amenable to
continuous, spatially extensive monitoring; (2) urban-, regional-,
and global-scale emission inventories of ozone precursors; and (3)
the sensitivity of ozone and other photochemical oxidants to
precursor compounds. It should be noted however that interpretation
of field measurements will require a solid understanding of the
fundamental mechanisms involved in related atmospheric
processes.
Develop and Deploy Monitoring
Networks
The development and deployment of monitoring networks are
necessary to establish the chemical climatology of ozone, other
photochemical oxidants, and their precursors. This climatology will
help shorten the time required to unequivocally observe a response
in ozone to changes in the concentration of its precursor
compounds. These networks must include a meteorological component
that captures the role of meteorology and dynamics in the
redistribution of airborne chemicals. Moreover, a comprehensive
chemical climatology for the photochemical oxidants must include
data from the free troposphere as well the surface. It is thus
likely that these networks will require the use of balloon sondes;
robotic, pilotless aircraft; and space-based platforms, in
conjunction with newly developed instrumentation based on small,
lightweight, low-power technology.
Support Integrated Assessments
Integrated assessments draw from a wide range of scientific
information and disciplines in order to provide more comprehensive
guidance on scientific and technical matters to the decision-making
community. A thorough understanding of the distributions and trends
in photochemical oxidants and the processes that determine their
production and removal is not yet in hand, and this seriously
limits our ability to conduct a rigorous integrated assessment of
global change (Logan, 1994; IPCC, 1995). The research strategy in
atmospheric chemistry should support these assessments by providing
analytical and modeling tools that can be readily applied to these
integrated assessments.
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Atmospheric Aerosols
Atmospheric aerosols play a critical role in the chemistry and
radiative transfer of the atmosphere. Minute amounts of particulate
matter in the stratosphere, along with increased levels of
anthropogenic chlorine, are responsible for the Antarctic ozone
hole and probably for the less dramatic but nevertheless
significant global-scale ozone depletion (WMO, 1995). Aerosols
emitted by industrial activity and biomass burning are now believed
to be responsible for partially masking the expected increase in
surface temperature associated with greenhouse gas radiative
forcing (IPCC, 1995; NRC, 1996a). Atmospheric aerosols also have
important impacts on human health and materials degradation
(American Thoracic Society, 1996a,b). Despite our recent advances
in appreciating the importance of aerosols, our understanding of
this critical class of atmospheric species is in its infancy. Why
is this the case? An outstanding reason is the complex nature of
aerosols and the forces they exert. Unlike the atmospheric gases,
aerosols have an infinite number of sizes and a variable, mixed
composition. We are not able to fully comprehend the impacts of
aerosols now and are not in a position to make predictions about
how these impacts will change in the future due to mankind's
activities.
The important questions that must be addressed in the
twenty-first century involve the effects of atmospheric aerosols on
climate, atmospheric chemistry, and human health and well-being and
in a fundamental form can be stated as follows:
1. What is the role of natural and anthropogenic aerosols in
climate, and how will future changes in the levels of aerosol
precursors affect this role?
2. How will future natural and anthropogenic aerosols impact
stratospheric and tropospheric ozone and the oxidizing capacity of
the atmosphere?
3. What is the role of atmospheric chemistry in changing the
composition of aerosols that impact human health, the environment,
visibility, and infrastructural materials?
To answer these questions, we must go far beyond our current
state of knowledge of atmospheric aerosols. The essential elements
of the research strategy that will be needed are outlined below and
in Box II.2.9. A more detailed discussion of many aspects of this
strategy can be found in Aerosol Radiative Forcing and Climate
Change (NRC, 1996a).
Maintain and Expand Stratospheric
Aerosol Measurement Capability
Limb scanning of solar extinction from satellites has been very
successful in monitoring the global stratospheric sulfate layer and
its spatial and temporal response to volcanic perturbations. When
validated by in situ measurements of
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Box II.2.9 Recommended Research Tasks for
Atmospheric Aerosois
1. Maintain and expand stratospheric aerosol
measurements to
• document aerosol chemical effects on
stratospheric ozone, and
• monitor impact of volcanic injections.
2. Develop new suite of tropospheric aerosol
measurements to
• document complex chemical and physical
aerosol properties, and
• expand remote sensing capability.
3. Deploy monitoring networks to document spatial
and temporal frends in aerosol characteristics and their impact on
climate, human health, and so forth.
4. Design and implement intensive field campaigns
to better understand processes that control aerosol formation,
transformation, transport, and loss.
5. Develop and evaluate models to provide
predictive capability.
particle size distributions from balloons and stratospheric
aircraft for validation, satellite multiwavelength extinction
measurements have provided stratospheric aerosol particle surface
areas with an accuracy adequate for heterogeneous chemical
applications. New instruments with higher wavelength resolution,
possibly deployed on small satellites, will be the main monitoring
tool for this component in the twenty-first century.
Design and Implement New Suite of
Measurement Technologies for Tropospheric Aerosols
The complexity of tropospheric aerosol presents a considerably
more difficult problem. Past in situ measurements have focused on
determining the size distribution or chemical composition of
aerosols at specific locations. Several new techniques under
development are probing the chemical composition of single aerosol
particles. However, these are essentially point measurements that
yield little information about spatial and temporal variability.
Moreover, there are few methods for analyzing the composition of
organic aerosols, which are emitted from biomass burning and
industrial activity. Clearly, a new suite of in situ
instrumentation is needed that can quantitatively document the
complex chemical composition of tropospheric aerosols in regions of
the globe that are of interest for atmospheric chemistry.
Current remote sensing technology allows the measurement of
gross tropospheric aerosol parameters over large spatial regions,
but features such as composition and a complete size distribution
cannot be measured yet. Technologies
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such as scanning polarimeters in the visible and near infrared
appear to hold promise because they are able to retrieve
tropospheric aerosol scattering characteristics from measurements
of multispectral radiance and polarization by resolving aerosols
from clouds. Moreover, surface and airborne lidars can be used to
map tropospheric aerosol backscatter and, combined with Raman
scattering techniques, can provide limited information on aerosol
characteristics. Preliminary measurements with nadir-viewing lidars
from the space shuttle show promise for obtaining detailed gross
features of the tropospheric aerosol on a global basis. However,
adequate opportunities for deployment of such instruments do not
presently exist and must be a priority for the twenty-first
century.
Design and Deploy Networks to Document
Aerosol Climatology
With the development of new instrumentation, monitoring networks
can be deployed to document the spatial and temporal trends in key
aerosol characteristics. These characteristics include aerosol
number, size distribution, chemical composition, and radiative
properties. Moreover these networks must be designed in such a way
that they can address issues on varying spatial scales. For
example, urban-scale monitoring networks are needed to uncover the
characteristics of aerosols that lead to pulmonary health effects
in humans; regional-scale networks are needed to better establish
the relationships between aerosol precursor species and visibility;
and global-scale networks are needed to better quantify the role of
aerosols in climate change.
Design and Implement Intensive Field
Programs
To be able to predict how future anthropogenic activities will
affect aerosols, and their consequent impacts on climate,
chemistry, the environment, and human health, we must go beyond an
aerosol climatology to a deeper understanding of the processes that
control aerosol formation, transformation, and removal. This will
require the design and implementation of intensive field programs
that bring together chemical and physical aerosol measurements and
precursor gas studies utilizing surface, aircraft, and ship
measurements. It is relevant to note in this regard two novel
experimental strategies that have emerged for resolving some of the
key questions concerning tropospheric aerosols and their effects
[see, for example, the ACE-1 Science and Implementation Plan (IGAC,
1995)]. The first of these is the "closure experiment," in which an
overdetermined set of variables is measured. A subset of the
observations and the relevant theories are then used to predict the
"closure variable," which is also measured independently. The
result is a test of both measurements and theory, with an
opportunity to evaluate the quality of our understanding in each
experiment. With instrumentation now available, it is possible to
perform closure experiments on aerosol number concentration (using
a variety of sizing instruments), mass (based on measurements
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of relevant inorganic and organic species), radiative properties
(using chemical composition, relative humidity, and Mie theory),
and the integrated column effect of aerosols on short- and longwave
radiation. Closure experiments on aerosol mass can help answer
questions about chemical composition, since missing species will
make closure impossible. Theories concerning the impact of aerosols
on radiative forcing of climate can also be tested by local and
column closure experiments. Most of the aerosol experiments planned
for the next decade depend heavily on this strategy, since it
offers a rigorous test of both measurements and the process models
on which more comprehensive models depend.
The other new strategy is to observe the evolution of aerosols
and their precursor gases in a Lagrangian reference frame. The idea
of Lagrangian experiments is not new, and variations on this theme
have been used from time to time. Recently, however, there has been
considerable work on tagging airmasses with balloons and chemical
tracers, so that aircraft carrying large suites of instruments can
revisit the airmass over a period of days to observe changes with
time (Huebert, 1993; Draxler and Hefter, 1989). Although these
experiments cannot eliminate the effects of dispersion and vertical
mixing on concentrations, with ample dynamical measurements they
make it possible to sort out the chemical and physical processes
that cause changes in aerosols. These processes include
gas-to-particle conversion, chemical transformations, wet and dry
deposition, entrainment of air from other strata, and mixing
through the sides of the "airmass" (dispersion). Although these
experiments tend to be complex and expensive (at least one ship and
one or two aircraft are required), they offer the potential to test
the aerosol models that presently exist or will be developed from
future laboratory work and other process studies.
Develop Predictive Model
Capability
The overall strategic goal for the twenty-first century should
be development of a predictive model that can be used to calculate
atmospheric temperature and chemical species concentration fields
and, from this information, to derive aerosol formation rates,
predict the chemical content and size distribution of the aerosol
fields, and determine their concomitant influence on atmospheric
radiation and the reflectivity and lifetime of clouds. Since
current atmospheric models generally impose, rather than predict,
aerosol distributions, it will be necessary to achieve
significantly more sophistication in representing precursor gas and
gas-particle kinetics, nucleation and agglomeration kinetics, and
vapor-particle interactions in future models. One way to naturally
stimulate the necessary improvements in aerosol modeling
capabilities is to encourage the modeling community to participate
directly in the planning, execution, and data analysis portions of
the strategic field measurements programs described above.
Furthermore, predictive aerosol models will require currently
unavailable quantitative mechanistic and kinetic input data
describing a large number of
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heterogeneous growth, nucleation, agglomeration, and
accommodation or evaporation processes. These quantitative input
data will have to come from a vigorous laboratory program in
heterogeneous kinetics and aerosol microphysics.
Toxics and Nutrients
The atmosphere and biosphere are fundamentally coupled through
the exchange of gases and aerosols. Ecological systems, including
economically important ones such as those dedicated to agriculture
and forestry, can be profoundly impacted by the wet and dry
deposition of both toxic and nutritive atmospheric substances
(e.g., Ridley et al., 1977; Duce, 1986; Aber et al., 1989; Schulze,
1989; Van Dijk et al., 1990; Lindquist et al., 1991; Benjamin and
Honeyman, 1992; Vitousek et al., 1993; Shannon and Voldner, 1995).
Although many of the atmosphere's naturally occurring components
can have toxic and/or nutritive effects on the biosphere, there are
a myriad of toxic and nutritive substances in the atmosphere that
are significantly influenced by anthropogenic activities. These
include nutrients such as sulfur and nitrogen compounds; heavy
metals such as mercury, cadmium, and lead; and toxic organic
compounds such as pesticides, polychlorinated biphenyls (PCBs),
plasticizers, dioxins, and furans.
Although we are beginning to be able to identify the more acute
effects of atmospheric toxicity and overfertilization on key
ecosystems, our understanding is far too limited for us to assess
the current extent of these problems or to predict future ones.
Overall, the motivating scientific questions for the study of
toxics and nutrients are as follows:
1. How are interactions between the atmosphere and biosphere
influenced by changing atmospheric concentrations and by the
deposition of harmful and beneficial compounds?
2. What are the rates at which biologically important
atmospheric trace species are transferred from the atmosphere to
terrestrial and marine ecosystems through dry and wet
deposition?
The essential elements of a research strategy to address these
questions are outlined below and in Box II.2.10.
Develop and Evaluate Techniques for
Measuring Deposition Fluxes
Many of the key questions about toxics and nutrients cannot yet
be answered comprehensively because we lack the necessary methods
for measuring deposition fluxes on the appropriate spatial and
temporal scales. This problem is most severe in the case of dry
deposition, where technologies for reliably measuring many of the
most biologically important fluxes do not yet exist. Adequate
support for development of the necessary techniques in this area is
thus critical; relaxed eddy accumulation, eddy correlation, and
gradient methods offer particular promise.
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Box II.2.10 Recommended Research Tasks for
Toxics and Nutrients
1. Develop and evaluate deposition flux
measurement techniques to
• provide new methods for measuring dry
deposition rates, and
• provide methods for obtaining more
spatially comprehensive deposition data.
2. Design and implement ecosystem exposure
monitoring networks to develop long-term record of stresses and
benefits to ecosystems of economic and/or environmental import.
3. Carry out process-oriented field studies to
• developed and evaluate deposition flux
algorithms,
• contribute to the development of coupled
ecosystem-atmospheric chemistr model, and
• provide tools for integrated
assessments.
In the case of wet deposition, reliable techniques have in
principle been developed, but serious questions exist about
sampling representativeness and contamination problems. The problem
is most severe for measuring wet deposition fluxes over the ocean,
where it is virtually impossible to collect uncontaminated rain
samples from a buoy in midocean and samples from shipboard
platforms are necessarily intermittent. Present marine deposition
estimates, often the result of comparing model calculations with a
very small suite of shipboard and island observations, are
typically subject to uncertainties of factors of three or more
(Duce et al., 1991). The development of new techniques that will
allow for more representative determination of wet as well as dry
deposition fluxes, perhaps from a low-flying airborne platform,
must therefore also be considered a high priority.
In some cases, such as high-altitude forests and foggy regions,
the deposition of cloud droplets may be the primary avenue by which
toxics and nutrients are delivered to the Earth's surface (Vong et
al., 1991). It is extremely difficult to measure such fluxes,
because the droplets are so transient that their flux is easily
altered by the presence of measuring devices. Thus, new
methodologies should be developed to assess the importance of
droplet deposition and allow reliable flux measurements.
Design and Implement Ecosystem
Exposure Monitoring Networks
In the recent past, deposition monitoring networks have proved
useful for assessing the ecological impacts of atmospheric
deposition (e.g., Cooperative
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Programme for the Monitoring and Evaluation of Long Range Air
Pollutants in Europe, National Crop Loss Assessment Network).
However, these networks have been largely limited to monitoring the
deposition of a specific chemical or class of compounds (e.g., acid
deposition, ozone). For this reason, they have provided very
limited information on the full suite of stresses and benefits
experienced by an ecosystem from atmospheric deposition and, thus,
on the long-term effects of this deposition. With the development
of new deposition measurement techniques, it should be possible to
design more comprehensive atmospheric deposition and exposure
monitoring networks. Implementation of these networks for key
ecosystems and biomes (e.g., at Long-Term Ecological Research
sites) would provide a long-term record of atmospheric deposition;
with co-located ecological monitoring, this record would no doubt
prove useful in establishing causal relationships between
atmospheric deposition and ecosystem vitality and succession.
Carry out Process-Oriented Field
Studies for Algorithm Development and Evaluation
Even with reliable and fully evaluated deposition measurement
techniques, it will never be possible to measure dry and wet fluxes
for all species of interest over all ecosystems of interest, over
all time. For this reason, process-oriented field studies,
involving observations of fluxes under a carefully selected range
of conditions, have to be undertaken in order to identify the
factors that control such fluxes. With these factors identified,
algorithms and parameterizations describing deposition fluxes can
be developed, tested by further observations, and incorporated into
regional and global atmospheric chemistry models, as well as
integrated atmospheric-biospheric response models.
Representative terms from entire chapter:
stratospheric ozone
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