The Future of Atmospheric Chemistry Research Remembering Yesterday, Understanding Today, Anticipating Tomorrow (2016) / Chapter Skim
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3 Understanding Today
3 Understanding Today
Pages 41-76
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Highly reactive chemicals like ozone and the hydroxyl radical partici pate in many chemical reactions, so their sources, sinks, and concentrations, as well as their spatial and temporal variability, are required to understand reac tions that drive atmospheric chemistry.
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3.1 HUMAN ACTIVITIES AND NATURAL PROCESSES GOVERN EMISSIONS THAT DETERMINE THE CHEMICAL COMPOSITION OF THE ATMOSPHERE Overview An overarching goal of atmospheric chemistry research is to understand humaninduced changes in the atmosphere so that accurate predictive capabilities can be developed to assess future scenarios and aid in the development of effective policies to minimize risk. This goal requires accurate data on the emissions to the atmosphere, including gases and particles from all sources, anthropogenic as well as natural.
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Emissions include a very large suite of organic and inorganic compounds, including trace metals such as mercury. Quantification of these emissions in an inventory provides data needed for a multitude of atmospheric chemistry applications, such as inputs to chemistry and climate models, evaluation of trends, and explanation of in situ observations.
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These various techniques not only enable the quantification of species emitted to the atmosphere, but also evaluate the conditions that control the emissions. It is important to continue application and development of new approaches to verify and predict emissions and to evaluate their impact on atmospheric chemistry and climate (Bond et al., 2013; Tong et al., 2012)
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. Emissions from Specific Sources The relative magnitude of emissions from biogenic or anthropogenic sources can play a key role in atmospheric chemistry and the chemical pathway of emissions.
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. Models can replicate the measurements only after the emission inventories are revised and ethane emissions, predominantly from oil and gas extraction and production activities in North America, are increased.
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In former decades, ozone mitigation strategies have been informed and validated by combined model/measurement programs. In more recent decades, the field of atmospheric chemistry has been tasked with gaining insight into the difficult problem of urban and anthropogenic aerosol particles (Heald et al., 2005; Odum et al., 1997; Volkamer et al., 2006)
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. Transformations are at the core of atmospheric chemistry.
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. Products with different vapor pressures and solubility behaviors, combined with direct emissions of VOCs of varying molecular mass, lead to a continuum of organic species that have been designated intermediate, semi-, low, and extremely low volatility organic compounds (Donahue et al., 2012)
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. Sulfur dioxide emitted during combustion of sulfur-containing fossil fuels is converted by oxidation in both the gas and condensed phases to form sulfuric acid as well as organosulfates.
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and their ability to act as cloud condensation nuclei and ice nuclei (Schill and Tolbert, 2013)
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Evidence for SOA formation is provided by laboratory experiments demonstrating that the aqueous chemistry of small water-soluble compounds forms organic acids/salts (e.g., oxalate) as well as high molecular weight compounds that are also found in atmospheric aerosol particles.
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. Gas uptake in/on snow and subsequent chemistry and photochemistry in the snow contribute to changing chemical composition above or in the snowpack, oxidant formation, and ozone depletion, showing the important role of surface atmospheric chemistry within the cryosphere component of the Earth system (Abbatt et al., 2012)
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. Better agreement has been attained with improved emission inventories and identification of additional precursors and semivolatile and intermediate volatility species, as well as updated chemical mechanisms and chemistry in the condensed phase (Hayes et al., 2015; Heald et al., 2011; Shrivastava et al., 2011)
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. Obtaining molecular structures of complex organic species in particles remains challenging using online particle mass spectrometry techniques.
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In more complex environments -- particularly those with more VOCs or heterogeneous reaction pathways that compete with gas-phase reactions, and in the condensed phase -- the understanding is less complete. Stratospheric Ozone Depletion As described in Chapter 2, one success story in atmospheric chemistry is discovery of the processes that cause stratospheric ozone depletion (Newman et al., 2009)
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and the abundance and impact of the full range of atmospheric oxidants on atmospheric composition are still uncertain. Oxidants seem to be best understood in cleaner environments, such as much of the tropical-free troposphere where oxidation chemistry involves primarily OH in the gas-phase and hydrogen peroxide in the aqueous phase.
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. Gas-Phase Oxidants in Globally Dispersed Forested Environments Forests cover about one-third of the Earth's land mass and their emissions of reduced gases strongly influence the global atmospheric oxidation potential.
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Gas-Phase Oxidants in Urban Regions The atmospheric oxidation chemistry of urban areas and regions characterized by copious emissions of VOCs, NOx, and particles has been studied for decades and is thought to be understood. However, measured ozone generally exceeds modeled concentrations at high ozone abundances (Appel et al., 2007; Im et al., 2015)
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As discussed in Chapter 4.2, ROS are thought to play a key role in inflammation, pulmonary oxidative stress, vascular dysfunction, atherosclerosis and lung cancer, so there are active research programs studying the health effects of ROS. In the atmospheric chemistry community, mechanistic studies of ROS in particles and on surfaces are in their infancy.
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Climate-related changes in these processes will drive changes in the distribution of the trace constituents, which can feed back on the dynamics, with implications for weather, air quality, and climate. In the troposphere, extratropical cyclones, which travel along the storm tracks, play an important role in the export of pollution from the continental planetary boundary layer to the global atmosphere.
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. A positive trend in stratospheric water vapor that exceeded the increase expected from the oxidation of atmospheric methane has been suggested (Rosenlof et al., 2001; Rosenlof and Reid, 2008)
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It is unclear how changes in storm tracks in the Southern Hemisphere, combined with increasing emissions from countries in South America and Africa, for example, affect the composition and chemistry of the remote atmosphere in the Southern Hemisphere. Aerosol particles can have an important impact on the radiation budget and dynamics of the atmosphere.
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Aerosol particles are the nuclei upon which clouds form and variations thereof can modulate cloud properties, the radiation budget, and the hydrological cycle. Most CCN grow from particles formed by condensation of molecules produced by atmospheric chemistry, but that are initially too small to act as CCN.
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. Recent Advances Atmospheric aerosol particles are emitted from a wide variety of sources including soil and deserts, the ocean, volcanoes, biogenic activity, biomass burning, burning of fossil fuels, and numerous other anthropogenic activities.
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. The unperturbed cloud contains larger cloud drops as only natural aerosol particles are available as cloud condensation nuclei, while the perturbed cloud contains a greater number of smaller cloud drops as both natural and anthropogenic aerosol particles are available as cloud condensation nuclei (CCN)
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. The highly variable relative humidity across the scales, and the nonlinear response of aerosol water uptake to it, introduces aerosol direct radiative forcing uncertainty.
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may also deplete water vapor availability in the early stages of cloud formation, strongly affecting the sensitivity of cloud droplets to aerosol particle variations (e.g., Ghan et al., 1998; Morales Betancourt and Nenes, 2014)
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, the lack of an accepted mechanistic description for ice nucleation processes severely limits predictive capability in this area. Compared to pure liquid and pure ice clouds, the effects of aerosol particles on mixedphase clouds are more complex, owing to thermodynamic and microphysical interactions between cloud particle types throughout the cloud (Cheng et al., 2010; Fan et al., 2013; Lance et al., 2011; Lebo and Morrison, 2014; Lebo and Seinfeld, 2011; Rosenfeld et al., 2008; Saleeby et al., 2011; Storer et al., 2010; Van den Heever et al., 2006)
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Further research is needed to confirm aerosol particle influence on cloud-scale dynamics and to embed these influences in multiscale models that can ultimately evaluate changes in radiative balance and precipitation on regional and global scales. 3.6 ATMOSPHERIC TRACE GASES AND AEROSOL PARTICLES COUPLE TO GLOBAL BIOGEOCHEMICAL CYCLES Overview Atmospheric chemistry plays a central role in the biogeochemical cycling of elements (e.g., carbon, nitrogen, sulfur)
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Recent Advances New approaches and tools have largely enabled advances in connecting atmospheric chemistry processes with biogeochemical cycles. The advent of Earth system models, which dynamically couple reservoirs in the Earth system, has initiated new avenues of retrospective and predictive atmospheric chemistry modeling connected to biogeochemistry (e.g., investigation of the role of ozone on the land carbon sink [Sitch et al., 2007]
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. Recent work suggests nitrogen deposition will decline worldwide as increases in ammonia emissions will be offset by large decreases in nitrogen oxide emissions (Lamarque et al., 2013)
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. These interactions of metals with atmospheric acidity, whether from sulfate, nitrate, or organic acids, lie at the intersection of atmospheric biogeochemistry and more traditional atmospheric chemistry.
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However, while network measurements regularly report the wet deposition flux of inorganic acids throughout much of North America and Europe (e.g., the National Atmospheric Deposition Program in the United States) , the wet removal of organic species (including organic nitrogen and organic acids)
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. 3.7 CONCLUSION The topics described in this chapter span the discipline of atmospheric chemistry research.
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