CHAPTER FIVE

Anticipating Tomorrow: Research Priorities in Atmospheric Chemistry

As discussed in the preceding chapters, atmospheric chemistry has become a robust scientific discipline. The fundamental science of understanding atmospheric chemistry can contribute to both core knowledge of the way the planet works as well as to many issues that directly relate to societal challenges. Thus, in Chapter 4, the Committee identified several issues central to human health and welfare where atmospheric chemistry research can make important contributions in the coming decade. The goal of such research is improved predictive capability to be able to inform policy decisions about these issues.

In addressing its task, the Committee identified atmospheric chemistry research priorities that need intensive attention in the coming decade, as well as actions that the National Science Foundation (NSF) could take to address key scientific gaps related to these priorities. The next section (see Chapter 5.1) describes the Committee’s process for developing these priorities, and the following section (see Chapter 5.2) lists five Priority Science Areas identified by the Committee.

5.1 DEVELOPING PRIORITIES

As described in Appendix B, the Committee engaged in an expansive process for gathering input from the atmospheric chemistry community. Using this community input as its underlying basis, the Committee deliberated extensively and ultimately chose five Priority Science Areas that it believes will drive atmospheric chemistry research over the next decade. In choosing these areas, the Committee prioritized potential research areas based on two criteria—scientific imperative and societal relevance. Scientific imperative describes research that is central to enabling substantial progress toward addressing important fundamental questions in atmospheric chemistry. This includes laboratory experiments, theory, field and modeling research that advances frontier knowledge in atmospheric chemistry. This criterion incorporates important issues within atmospheric chemistry that have already been identified but where substantial further research is needed.

Much of the current research in atmospheric chemistry relates in various ways to the health and well-being of human society and the natural world, including understanding impacts on ecosystems, agriculture, human health, and the economy. In an increasingly globalized world, research that can address current and project future societal and ecological problems is increasingly valued as an essential component of the development of effective public policies by a broad segment of society, including Congress and the public. The Committee therefore considered societal relevance to be an important criterion in choosing their Priority Science Areas.

The Committee has given priority to closing key science gaps that are impeding the development of atmospheric chemistry predictive capability and to building collaborations with other disciplines to solve urgent societal and environmental issues. The emphasis is on the science of atmospheric chemistry, not the engineering applications of the science. While the Committee recognizes the breadth of science that is important in addressing these issues, we focus on just five Priority Science Areas. Within each Priority Science Area, the Committee identified key scientific gaps, chosen to be at a level of effort that could be addressed with funding through the NSF proposal process. These key scientific gaps were chosen based on their necessity for addressing the Priority Science Area under which they fall and on their transformative potential, which is a central tenet of research supported by the NSF. By transformative potential, the Committee refers to higher-risk and higher-reward research areas that have the potential to dramatically change the understanding of atmospheric processes. There is, by the nature of the problems, some overlap of key scientific gaps from one Priority Science Area to another, particularly between the foundational and societally relevant areas.

5.2 PRIORITY SCIENCE AREAS

Following the process described in the preceding section (see Chapter 5.1), the Committee identified five priority areas for scientific research in atmospheric chemistry. The first two Priority Science Areas are necessary for building the foundation of atmospheric chemistry, aimed at providing further growth in understanding of how the atmosphere works. The next three Priority Science Areas directly address major challenges facing society, for which advances in atmospheric chemistry are required to make progress.

In total, these Priority Science Areas cover a broad range of research questions. More specific areas that represent key scientific gaps are described within each area, along

with examples of actions needed to address those gaps. All examples are intended to be illustrative, and not exclusionary or indicative of priorities within each area.

Priority Science Area 1

Advance the fundamental atmospheric chemistry knowledge that enables predictive capability for the distribution, reactions, and lifetimes of gases and particles.

Motivation

Predictive capability starts with a fundamental understanding of the atmospheric chemistry occurring now. While some predictions can be made with confidence using current understanding, there remain major gaps and inconsistencies in the understanding of fundamental atmospheric chemical processes. These major gaps need to be closed and inconsistencies resolved.

Fortunately, current advances in atmospheric chemistry are enabling atmospheric chemists to identify and begin to narrow the gaps and to resolve discrepancies in understanding the basis of today’s atmospheric composition. This progress is being driven by a combination of laboratory experiments, theory, modeling, and observational capabilities. As climate and other conditions continue to change, developing new and quantitative understanding of the fundamental underlying chemistry will be essential for proactively exploring and defining future chemical and dynamical regimes.

Actions to Address Key Scientific Gaps

1. Quantify reaction rates and understand detailed chemical mechanisms in multipollutant and multiphase environments that cover the chemical and dynamical regimes from polluted urban to natural remote regions.

   There are many important problems in atmospheric chemistry that fall under this category, the background for which is summarized in Chapter 3. A few illustrative (but not exclusive) examples out of many important needs include quantum chemistry studies of reactions of organic radicals that may not be possible to address with experimental methods (see Chapter 3.2); chemical species and factors controlling the formation, growth, and aging of aerosol

2. particles (see Chapters 3.5, 3.6, 4.1, 4.2, and 4.3); and the role of water in mediating chemistry in the gas phase, in particles and on surfaces (see Chapter 3.2).

3. Identify and quantify important atmospheric oxidants or other reactants that lead to transformation and removal of chemical species from the atmosphere across broad spatial and temporal scales.

   Examples include quantification of the gas phase oxidative capacity across broad spatial and temporal scales (see Chapter 3.3); developing improved understanding of what controls OH reactivity in different environments (see Chapters 3.2 and 3.3); understanding instrumental biases in measurements of oxidants (see Chapter 3.3); and identifying and quantifying important individual oxidants in the condensed phase and their role in determining the composition and chemistry of particles, fogs, and clouds (see Chapters 3.3 and 4.2).

4. Develop a stronger understanding of the influences that heterogeneous chemistry exerts on tropospheric composition.

   Examples include identifying and quantifying changes in the spectroscopy and photochemistry of species (organic and inorganic as well as neutral species and ions) in surface films compared to the gas or bulk phases; elucidating reaction mechanisms and products of organic species in thin films under different conditions and mixtures of co-pollutants, including trace metals; and quantifying the partitioning and exchange of species between the gas phase and surface films due to physical and chemical processes.

5. Understand and quantify the influence of the coupling between chemical and meteorological processes on the distribution of trace constituents in the troposphere.

   As discussed in Chapter 3.4, atmospheric composition and chemistry is the result of chemical, physical, and meteorological processes whose coupling needs to be understood to develop predictive capabilities. Examples include obtaining a better understanding of the transformation and transport of chemical constituents in convective systems; quantifying the role of meteorology on the frequency of air pollution stagnation events; and quantifying how different forms of convection affect lightning-generated NO_x production and subsequent ozone production.

5. Understand and quantify the influence of the coupling between chemical, dynamical, and radiative processes involving stratospheric chemistry.

   As discussed in Chapters 2.1 and 3.4, coupling between the troposphere and stratosphere has consequences for atmospheric chemistry and composition in both regions of the atmosphere. Examples include understanding how changes in the Brewer-Dobson circulation, driven by changes in ozone and the well mixed greenhouse gases, will affect the distribution of stratospheric species; quantifying the impact of deep convective transport of tropospheric water vapor and other tropospheric constituents on the budgets of chemically and radiatively important species in the stratosphere; and quantifying how transport of stratospheric ozone will impact tropospheric ozone in a changing climate.

Approaches and Support Needed

• Develop the next generation of accurate, sensitive, and specific measurement capabilities for atmospheric constituents in single-phase and multiphase environments and on surfaces. This includes development and interpretation of observations from miniaturized, low-power sensors for deployment in networks and on a variety of platforms for sampling a wide range of chemical and dynamical regimes. An important part of this effort is instrument and technique validation.
• Enhance the integration of laboratory, computational chemistry, field and chamber studies, and photochemical and multiphase modeling.
• Mine existing high-quality atmospheric chemistry field and environmental chamber datasets to probe chemical mechanisms, to examine trends in time, and to compare the chemistry across different chemical and dynamical regimes.
• Accelerate the trend of developing and including more realistic atmospheric gas-phase, aqueous-phase, and multiphase chemistry in next-generation models.
• Explore the interconnection of stratosphere-troposphere atmospheric chemistry using a combination of in situ and remote sensing techniques in combination with models.

Priority Science Area 2

Quantify emissions and deposition of gases and particles in a changing Earth system.

Motivation

Emission and deposition processes govern concentrations and spatial distributions of gases and particles in the atmosphere. A quantitative understanding of these distributions is key for assessing the impacts of atmospheric processes on human and ecosystem health, weather, and climate. Although recent advances have reduced uncertainties, current shortcomings in predicting emissions from both natural and anthropogenic sources remain an issue for developing a predictive capability for atmospheric chemistry. Research is needed to both reduce these uncertainties for known sources and constrain emissions of poorly understood constituents (e.g., bioparticles) (Burrows et al., 2009a; Despres et al., 2012). Similarly, deposition processes for many species are not well understood or quantified, which is important not only for understanding atmospheric composition but also for elucidating atmospheric impacts on ecosystems. The net flux of species to or from the atmosphere is the combination of emissions and deposition. To develop a truly predictive understanding of the net flux, the factors controlling emissions and deposition must both be known.

The global environment is rapidly changing. Sources of atmospheric constituents change annually as humans make new decisions about technology, energy systems, pollution control, and land use. The global geographical distribution of human-derived emissions is also changing, with increases in the developing world and decreases in response to regulatory policies in developed nations. Natural sources respond to meteorological conditions and longer-term changes in climate. Processes that remove atmospheric constituents are also subject to such changes with time. Understanding emission and removal processes that determine atmospheric composition is a key component of modeling not only today’s Earth system, but also a future system that reflects the outcome of policy choices. Quantification of these processes and the feedbacks that occur, for example in response to changing climate, is thus central to the development of a predictive capability.

Actions to Address Key Scientific Gaps

1. Better determine emissions from both anthropogenic and natural sources and their spatial and temporal variations and trends.

2. As discussed in Chapters 3.1 and 3.5, there are many important issues that need to be addressed to better define various aspects of emissions into the atmosphere. Examples include determining the impacts of pollutants on emissions from the terrestrial and marine biosphere (see Chapter 3.5); quantifying trends in emissions from different regions of the world and their impacts on atmospheric composition and chemistry over scales from local to global (see Chapter 1.1); and developing reliable approaches to integrating “top down” and “bottom up” results for emission inventories (see Chapter 3.1).

3. Identify mechanisms and measure rates by which wet and dry deposition removes aerosol particles and trace gases from the atmosphere.

   Examples include determining deposition mechanisms and rates for species where data are sparse but where deposition could have a significant impact on atmospheric composition and ecosystems (see Chapters 3.6 and 4.3); determining synergistic interactions that can change deposition processes for a species from that measured for the single compound alone, and the mechanisms involved (see Chapters 3.6 and 4.3); measuring and characterizing how trace gases are taken up by vegetation, including canopy scale processes; and characterizing how hydrometeors scavenge aerosol particles.

4. Determine the role of meteorology, including temperature, precipitation, and extreme events, on emissions and removal of atmospheric species.

   As discussed in Chapter 3.4, there are close couplings and synergies between atmospheric composition and chemistry, and atmospheric dynamics and meteorology that impact human health and welfare (see Chapters 4.1 and 4.2). Examples in this area include evaluating how changes in precipitation rates and locations alter the removal of species from the atmosphere (see Chapters 3.4 and 3.5); determining how dry deposition is affected by changing wind speed, temperature, and humidity in a changing climate (see Chapter 3.6); and measuring how natural emissions such as bioparticles, BVOC, soil NO_x, and methane are impacted by changing meteorology.

5. Determine the role of global change and societal choices (including changes in climate, energy choices, and land use) on the emissions and removal of atmospheric species.

   As discussed in Chapters 1.1, 3.1, and 4.4, atmospheric composition and chemistry is inextricably intertwined with choices made by society and by global change. Examples include understanding how agricultural activities impact emissions and removal of atmospheric constituents following conversion of

Approaches and Support Needed

• Develop instrumentation and measurement strategies to quantify fluxes.
• Support long-term measurements, especially those of biosphere–atmosphere exchange, including over the oceans, which remain relatively under-studied compared to land.
• Develop integrated strategies that employ both models and observations to attribute atmospheric concentrations to particular sources.
• Data mine measurements (e.g., concentration ratios) that can help constrain emissions and trends. Connected to this is the need to further develop data archiving resources.
• Develop and test model parameterizations for deposition processes.
• Coordinate integrated assessment with agricultural and ecological communities.

Priority Science Area 3

Advance the integration of atmospheric chemistry within weather and climate models to improve forecasting in a changing Earth system.

Motivation

As described in Chapter 4.1, all greenhouse gases and atmospheric particles impact the Earth’s radiation budget and dynamics of the atmosphere, affecting weather in such ways as changing precipitation patterns and monsoon circulations. Aerosol particles, in particular, play a critical role through their influence on the growth and formation of clouds and precipitation. In global climate models, the effect of an increase in atmospheric aerosol particle concentrations on radiation and the distribution and radiative properties of the Earth’s clouds is the most uncertain component of the overall global radiative forcing. Changes in atmospheric dynamics and circulation that are linked to changes in atmospheric composition (e.g., precipitation patterns, monsoon circulations) are even more uncertain than radiative forcing. During the past decade of intensive aerosol–cloud–climate research, some scientific gaps have been closed, and additional processes have been identified that still elude quantification. As with many

complex systems in intermediate stages of understanding, this progress has not yet reduced the overall magnitude of uncertainty, leaving major deficiencies in the ability to project future climate (Seinfeld et al., 2016).

The chemical reactions involving aerosol particles and gases determine not only particle formation but also the processes by which climate-relevant trace species are removed from the atmosphere. Atmospheric chemistry, therefore, remains the crucial component that allows estimates of the atmospheric lifetimes of many species, and consequently the ability of pollution to accumulate in the atmosphere and thus influence climate and weather.

Some studies of regional climate change (seasonal-to-interannual or longer-term projections) or weather forecasting have included heterogeneously distributed aerosol particles or ozone in their models, but many have not. Many climate models have adopted a cloud–aerosol particle microphysical model without using the proper aerosol mixing state or chemical processes that create and alter aerosol particles. Most anthropogenic aerosol particles are initiated with gas-phase emissions followed by thermal and photochemical reactions in either the gas phase or in clouds or aerosol particles (Bauer et al., 2013; Seinfeld et al., 2016).

The fundamental understanding of atmospheric chemistry impacts on climate need to be improved to allow predictions to be made of those aspects of the changing climate structure that most urgently threaten human health, security, economic opportunity, social stability, and confidence in the future. Thus the atmospheric chemistry community needs to continue to work with the climate and weather research community in several major areas so that knowledge of the many roles that atmospheric composition plays in climate and weather can be built into climate models.

Actions to Address Key Scientific Gaps

1. Determine the global distributions and variability of atmospheric trace gases and aerosol particles, and better understand their climate-relevant properties.

   Some examples of the many important research areas to be explored to better understand how greenhouse gases and aerosol particles impact climate include quantifying how aerosol particle composition, size, mixing state, and morphology determine their radiative properties (see Chapter 3.5); and determining the temporal, regional, and vertical distribution of short-lived climate forcers (see Chapter 1.2).

2. Understand the role of aerosol particles as a modulator of cloud microphysics and precipitation efficiency in natural and anthropogenically-perturbed environments.

   As discussed in Chapter 3.5, there are numerous aspects of aerosol–cloud interactions which need to be better understood for constraining the impacts of aerosol particles on weather and on climate via their impacts on clouds. Examples include determining which aerosol particle sources most effectively seed clouds and lead to enhanced precipitation; and determining how the vertical structure of different aerosol particle sources influences clouds and precipitation processes.

3. Develop accurate descriptions of the complex chemical and physical evolution of atmospheric constituents that can be implemented in models for robust prediction of the impact of the chemical state of the atmosphere on climate and weather.

   As discussed throughout this report, developing a predictive understanding of atmospheric chemistry impacts on climate and weather will rely on developing accurate model descriptions of observed processes. Examples include further developing methods to chemically describe a population of aerosol particles and predict their influence on cloud microphysics (see Chapter 3.5); quantifying how uncertainty in aerosol particle formation processes and properties impacts estimates of climate forcing (see Chapter 3.5); and characterizing the pre-industrial baseline atmospheric chemistry of gases and particles relevant for climate forcing.

Approaches and Support Needed

• Continuously measure aerosol particle properties, precursors, and climate parameters (to unravel natural versus anthropogenic influences on climate) over a range of representative environments (e.g., marine, urban, remote) as a function of altitude.
• Measure cloud properties, including precipitation efficiencies, as a function of relevant gas and particle sources, compositions, and concentrations.
• Improve the predictive capability of models across scales through the integration of laboratory measurements, in situ measurements, and satellite observations with detailed process evaluation.

• Develop a measurement-based modeling framework for atmospheric chemistry that can be readily implemented in Earth system models to augment and improve forecasting of weather and regional climate.
• Develop and improve chemical data assimilation techniques with weather and climate forecasts to enable more accurate air pollution forecasting capabilities.

Priority Science Area 4

Understand the sources and atmospheric processes controlling the species most deleterious to human health.

Motivation

As discussed in Chapters 1 and 4, air pollution has documented adverse health outcomes, including chronic and acute effects that can lead to increased mortality and impacts on cardiovascular and pulmonary functioning, and possibly on reproductive and neurological systems as well. It is estimated that air pollution is responsible for 1 out of 8 premature deaths (more than 7 million annually) worldwide.

While particulate matter has been identified as a major contributor to human health risks, the specific chemical species in both the gas and particle phases that cause these various effects and potential synergisms among them are not well understood. The fate and transport of persistent organic pollutants, toxic metals such as mercury, allergens, and pathogens in the atmosphere are also central to understanding atmospheric impacts on human health (e.g., Kellogg and Griffin, 2006). Atmospheric chemistry is a vital component that connects emissions to atmospheric composition and ultimately to human health. Advances in atmospheric chemistry provide the data at the core of understanding the identities, sources, and fates of health-related atmospheric gases and particles on individual, local, regional, and/or global scales.

Actions to Address Key Scientific Gaps

1. Develop mechanistic understanding to predict the composition and transformations of atmospheric trace species that contribute to impacts on human health.

   A few examples to improve understanding of how air pollutants impact human health include: identifying the source of toxic components in primary and

2. secondary particles and how they depend on precursors and reactions in the atmosphere (see Chapters 3.1, 3.2, and 4.2); identifying and quantifying the important individual oxidants in the condensed phase that contribute to toxicity (see Chapters 3.3 and 4.2); and understanding how ozone chemistry and the associated health risks will evolve with a changed climate (Chapter 3.2, 3.3, 4.2).

3. Quantify the distribution of atmospheric constituents that impact human health.

   Examples include determining which atmospheric constituents need to be measured and on what scale to quantify human exposure (see Chapter 4.2); improving the characterization of both individual and population exposure to air pollutants (see Chapter 4.2); and characterizing urban-scale air quality where a growing fraction of global populations live and breathe (see Chapters 1.1 and 4.2).

4. Determine what unique sources and chemical reactions occur in indoor environments that have implications for atmospheric chemistry and human health.

   The indoor environment represents a key scientific gap in atmospheric chemistry as discussed in Chapter 4.2. Examples of unresolved problems include: better understanding of the interplay between indoor and outdoor air quality; characterizing how human emissions and human activities influence indoor atmospheric chemistry; and determining the dominant fate of chemical species indoors, including gas-phase reactions, surface reactions, surface partitioning, and ventilation.

Approaches and Support Needed

• Develop tools to characterize particle composition over a broad size range from nanometer to micrometer scales, with the ability to differentiate bulk from the surface composition, which may determine bioavailability.
• Develop measurement techniques that facilitate high temporal and spatial resolution measurements of a wide variety of gases and particles that may either have direct health impacts or be precursors to those that do, including highly oxidized, multifunctional, and toxic species. This should also include the development of reliable, inexpensive sensors that can be widely deployed for air pollution exposure assessment.
• Integrate observational and modeling outputs to provide accurate assessment of pollutant concentrations for exposure estimates on the appropriate spatial

• and temporal scales. These should include sources, transformations, and fates of key precursors.

• Develop methods for effective communication of real time air quality data to nonpractitioners.
• Coordinate research with toxicologists and epidemiologists.

Priority Science Area 5

Understand the feedbacks between atmospheric chemistry and the biogeochemistry of natural and managed ecosystems.

Motivation

Biogeochemical cycles control the elements that are necessary for life and couple chemistry in the atmosphere with oceans, the solid earth, and the terrestrial and marine biospheres. This exchange of compounds supports resource production and maintenance associated with global food security (e.g., agriculture, fisheries) and certain energy sources (e.g., biofuels, wood).

There is a need to understand the spatial and temporal fluxes of elements between the atmosphere and terrestrial and marine ecosystems, as well as how atmospheric chemistry modulates the composition and bioavailability of these elements. These exchange processes are influenced by human activity and global climate, and are directly tied to the societal issue of natural and managed ecosystem health. In addition, biogeochemical cycles and ecosystem health play a central role in climate by regulating carbon uptake by the biosphere and the exchange of greenhouse gases and aerosol particle precursors. A better understanding of how atmospheric chemistry impacts the land and marine carbon sink is key to understand the ability of the Earth to temper the impacts of current and future emissions of CO₂ and other greenhouse gases to the atmosphere. Furthermore, the biogeochemical cycling of toxic constituents (e.g., mercury) directly affects ecosystems, as well as human health. Finally, given the growing pressure on agricultural systems in an era of rising demand for food, it is vitally important to evaluate the impact of atmospheric composition and deposition on crop growth, livestock and seafood stocks, and aquaculture. In addition, these managed ecosystems are also a significant source for a number of important atmospheric species, including methane, nitrous oxide, and ammonia.

Understanding biogeochemical cycles is tightly coupled to fundamentally important atmospheric chemistry questions, including the sources, sinks, and oxidation

processes associated with gas and condensed phase chemistry. However, addressing these inherently interdisciplinary scientific challenges will require the collaboration of atmospheric chemists with scientists from other relevant disciplines. Major scientific goals include understanding the cycling of elements through the various components of the Earth system, the impacts of deposition of atmospheric nutrients and contaminants to natural and managed ecosystems, and the feedbacks of ecosystems onto the atmosphere. New laboratory and field studies are needed to characterize these atmospheric chemistry processes, which are fundamentally important to the atmospheric chemistry community and can be incorporated into predictive models.

Actions to Address Key Scientific Gaps

1. Quantify the full suite of trace gases and particles deposited from the atmosphere and connect these to ecosystem responses.

   As discussed in Chapter 3.6, much work is needed to address this key scientific gap (which is connected with Priority Science Area 2). Examples of such research include determining how the deposition of nitrogen, phosphorus, iron, and other nutrients impact terrestrial and marine carbon uptake; and determining how crop productivity is impacted by ozone and other chemical species.

2. Identify and quantify the chemical composition, transformations, bioavailability, and transport of nutrients and contaminants in the global atmosphere and their interactions with the biosphere.

   As discussed in Chapter 3.6, atmospheric constituents and their transformations and bioavailability can significantly impact ecosystem health. Examples in this area include understanding how atmospheric oxidation processes alter the distribution and form of nitrogen deposited to the biosphere; determining what processes control the bioavailability of iron in the atmosphere and its assimilation into the marine carbon cycle; and quantifying the exchange of organic species between ecosystems and the atmosphere.

3. Identify important feedbacks between atmospheric chemistry and the biosphere under global change.

   As discussed in Chapter 3.6, biosphere-atmosphere interactions respond to global change. Examples of open research questions in this area include: understanding how land use change and urbanization will impact biosphere–atmosphere fluxes; identifying the impact of changing biological processes

Approaches and Support Needed

• Develop a more comprehensive treatment of coupled atmospheric chemistry and natural and managed ecosystems in Earth System Models.
• Develop complementary atmospheric chemistry measurements at existing long-term terrestrial (e.g., NSF Long-Term Ecological Research [LTER], Ameriflux, NEON in the U.S. and international sites) and marine (e.g., Bermuda Institute of Ocean Sciences; Mace Head, Ireland; American Samoa) locations.
• Develop instruments to quantify total reactive carbon and nitrogen.
• Develop measurement techniques that facilitate high time and spatial resolution measurements of a wide variety of gases and particles that impact ecosystems, including highly oxidized, multifunctional, and toxic species.
• Initiate research coordination with the ecology, plant physiology, agriculture, and marine biogeochemistry communities.

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