Feature pubs.acs.org/est
Aqueous Organic Chemistry in the Atmosphere: Sources and Chemical Processing of Organic Aerosols V. Faye McNeill* Department of Chemical Engineering, Columbia University, New York, New York 10027, United States opening the possibility of partitioning to cloudwater or aerosol water, and aqueous-phase processing (i.e., chemical reactions) to form low-volatility products that remain in the condensed phase.6,7 This family of processes, representing thousands of chemical reactions, products, and intermediates and including chemistry in both atmospheric gas and aqueous phases, is collectively known as aqueous SOA (aqSOA) formation. Driven by the water solubility of (S)VOCs and the availability of condensed-phase water, aqueous processing allows SOA formation from highly volatile species, including oxidation Over the past decade, it has become clear that aqueous products of isoprene, such as glyoxal and isoprene epoxydiols chemical processes occurring in cloud droplets and wet (IEPOX). aqSOA is also typically highly oxidized,6,8 likely atmospheric particles are an important source of organic contributing to the highest observed oxidation states of atmospheric particulate matter. Reactions of water-soluble atmospheric OA.4,9 Although aqSOA modeling is in the volatile (or semivolatile) organic gases (VOCs or SVOCs) in development stages, incorporating specific cloudwater aqSOA these aqueous media lead to the formation of highly oxidized pathways in atmospheric chemistry and air quality models has organic particulate matter (secondary organic aerosol; SOA) led to increased predicted SOA production and improved and key tracer species, such as organosulfates. These processes agreement with observations.10−13 are often driven by a combination of anthropogenic and Because SOA formation, and aqSOA formation in particular, biogenic emissions, and therefore their accurate representation often involves the cooperative effects of biogenic and in models is important for effective air quality management. Despite considerable progress, mechanistic understanding of anthropogenic emissions to create aerosol mass, the extent to some key aqueous processes is still lacking, and these pathways which PM2.5 is controllable or “natural”14 is central to are incompletely represented in 3D atmospheric chemistry and important air quality and climate policy issues, including the air quality models. In this article, the concepts, historical regional and seasonal variability of PM2.5. Tracer studies suggest context, and current state of the science of aqueous pathways of that the aqueous processing of IEPOX to form aqSOA is SOA formation are discussed. significant in the southeast U.S. but not in California.15 Carlton and Turpin analyzed regional variations in the abundance of INTRODUCTION water-soluble VOCs and its correlation with aerosol liquid Atmospheric particulate matter, also known as atmospheric water content in the Community Multiscale Air Quality model 1 aerosol, has serious negative impacts on human health and (CMAQ).16 They found that the potential for organic 2 large, but uncertain, effects on climate. In order to accurately compounds to partition to aerosol water is highest in Houston predict the responses of air quality and climate to changes in and the eastern U.S., with little contribution in the western U.S. emissions, it is necessary to understand and quantify the global They determined that SOA formation in aerosol water sources of atmospheric aerosols. These particles have a wide (aaSOA) in the U.S. is limited by aerosol liquid water, rather variety of sources, and this is reflected in their highly varying than by the availability of water-soluble organics. chemical composition. However, observations made around the Aerosol liquid water can be considered anthropogenic in world show that organic material consistently makes up a major most continental regions, since it is driven by anthropogenic 3,4 fraction of fine aerosol mass. This organic aerosol material secondary inorganic aerosol components (sulfate, ammonium, (OA) is often highly oxidized, especially outside of urban nitrate) which are quite hygroscopic.16,17 Current projections 3 regions. Atmospheric oxidation of volatile organic compounds suggest that future changes in climate as well as SO2, NOx, and (VOCs), followed by partitioning to the condensed phase, is NH3 emissions, will lead to increases in aerosol nitrate,18,19 the predominant source of oxidized OA.5 OA formed in this water vapor,20 and aerosol water21 concentrations globally, manner is referred to as secondary organic aerosol (SOA). providing ideal conditions for enhanced production of SOA in 3D atmospheric chemistry models are largely unsuccessful at aerosol and cloudwater. The water-soluble VOCs which matching observations of the quantity, degree of oxidation, and participate in aqSOA formation may have biogenic or spatial distribution of OA in the atmosphere, suggesting a lack 5 anthropogenic precursors, and NOx levels may influence the of mechanistic understanding of its sources. Gas-phase gas-phase oxidation pathways and therefore the SOA yields of oxidation of VOCs may result in functionalized products with the precursors (for example, the dominant oxidation products lower volatility than their parent compounds, leading to thermodynamic partitioning to the condensed phase. Alternatively, many VOC oxidation products are water-soluble,
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© XXXX American Chemical Society
A
DOI: 10.1021/es5043707 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Feature
Environmental Science & Technology Table 1. Forms of Atmospheric Liquid Watera
a
medium
size
LWC (gH2O m−3)
atmospheric time scale
cloud fog aerosol (adsorbed water)
10−20 μm ∼10 μm
Aqueous Organic Chemistry in the Atmosphere: Sources and Chemical Processing of Organic Aerosols V. Faye McNeill* Department of Chemical Engineering, Columbia University, New York, New York 10027, United States opening the possibility of partitioning to cloudwater or aerosol water, and aqueous-phase processing (i.e., chemical reactions) to form low-volatility products that remain in the condensed phase.6,7 This family of processes, representing thousands of chemical reactions, products, and intermediates and including chemistry in both atmospheric gas and aqueous phases, is collectively known as aqueous SOA (aqSOA) formation. Driven by the water solubility of (S)VOCs and the availability of condensed-phase water, aqueous processing allows SOA formation from highly volatile species, including oxidation Over the past decade, it has become clear that aqueous products of isoprene, such as glyoxal and isoprene epoxydiols chemical processes occurring in cloud droplets and wet (IEPOX). aqSOA is also typically highly oxidized,6,8 likely atmospheric particles are an important source of organic contributing to the highest observed oxidation states of atmospheric particulate matter. Reactions of water-soluble atmospheric OA.4,9 Although aqSOA modeling is in the volatile (or semivolatile) organic gases (VOCs or SVOCs) in development stages, incorporating specific cloudwater aqSOA these aqueous media lead to the formation of highly oxidized pathways in atmospheric chemistry and air quality models has organic particulate matter (secondary organic aerosol; SOA) led to increased predicted SOA production and improved and key tracer species, such as organosulfates. These processes agreement with observations.10−13 are often driven by a combination of anthropogenic and Because SOA formation, and aqSOA formation in particular, biogenic emissions, and therefore their accurate representation often involves the cooperative effects of biogenic and in models is important for effective air quality management. Despite considerable progress, mechanistic understanding of anthropogenic emissions to create aerosol mass, the extent to some key aqueous processes is still lacking, and these pathways which PM2.5 is controllable or “natural”14 is central to are incompletely represented in 3D atmospheric chemistry and important air quality and climate policy issues, including the air quality models. In this article, the concepts, historical regional and seasonal variability of PM2.5. Tracer studies suggest context, and current state of the science of aqueous pathways of that the aqueous processing of IEPOX to form aqSOA is SOA formation are discussed. significant in the southeast U.S. but not in California.15 Carlton and Turpin analyzed regional variations in the abundance of INTRODUCTION water-soluble VOCs and its correlation with aerosol liquid Atmospheric particulate matter, also known as atmospheric water content in the Community Multiscale Air Quality model 1 aerosol, has serious negative impacts on human health and (CMAQ).16 They found that the potential for organic 2 large, but uncertain, effects on climate. In order to accurately compounds to partition to aerosol water is highest in Houston predict the responses of air quality and climate to changes in and the eastern U.S., with little contribution in the western U.S. emissions, it is necessary to understand and quantify the global They determined that SOA formation in aerosol water sources of atmospheric aerosols. These particles have a wide (aaSOA) in the U.S. is limited by aerosol liquid water, rather variety of sources, and this is reflected in their highly varying than by the availability of water-soluble organics. chemical composition. However, observations made around the Aerosol liquid water can be considered anthropogenic in world show that organic material consistently makes up a major most continental regions, since it is driven by anthropogenic 3,4 fraction of fine aerosol mass. This organic aerosol material secondary inorganic aerosol components (sulfate, ammonium, (OA) is often highly oxidized, especially outside of urban nitrate) which are quite hygroscopic.16,17 Current projections 3 regions. Atmospheric oxidation of volatile organic compounds suggest that future changes in climate as well as SO2, NOx, and (VOCs), followed by partitioning to the condensed phase, is NH3 emissions, will lead to increases in aerosol nitrate,18,19 the predominant source of oxidized OA.5 OA formed in this water vapor,20 and aerosol water21 concentrations globally, manner is referred to as secondary organic aerosol (SOA). providing ideal conditions for enhanced production of SOA in 3D atmospheric chemistry models are largely unsuccessful at aerosol and cloudwater. The water-soluble VOCs which matching observations of the quantity, degree of oxidation, and participate in aqSOA formation may have biogenic or spatial distribution of OA in the atmosphere, suggesting a lack 5 anthropogenic precursors, and NOx levels may influence the of mechanistic understanding of its sources. Gas-phase gas-phase oxidation pathways and therefore the SOA yields of oxidation of VOCs may result in functionalized products with the precursors (for example, the dominant oxidation products lower volatility than their parent compounds, leading to thermodynamic partitioning to the condensed phase. Alternatively, many VOC oxidation products are water-soluble,
■
© XXXX American Chemical Society
A
DOI: 10.1021/es5043707 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Feature
Environmental Science & Technology Table 1. Forms of Atmospheric Liquid Watera
a
medium
size
LWC (gH2O m−3)
atmospheric time scale
cloud fog aerosol (adsorbed water)
10−20 μm ∼10 μm
