INSIGHTS | P E R S P E C T I V E S
ATMOSPHERIC CHEMISTRY
Atmospheric radical chemistry revisited Sunlight may directly drive previously unknown organic reactions at environmental surfaces
Oxidation with a difference Rossignol et al. report evidence for direct light-driven oxidation of an organic acid. Such oxidation processes may occur in the natural environment at water surfaces that are reached by solar radiation, generating gas-phase functionalized molecules and macromolecular condensed-phase products. These products affect aerosol formation and properties, influencing climate, air quality, and health. Photolysis of organic compounds
Condensedphase products
Sea-surface microlayer
Organic surfactants
Soluble organics
Aerosols Sea spray
Air-water interface
Gas-phase products
Bulk ocean Chemical processes
that radical reactions initiated by absorption of sunlight can follow mechanisms previously unknown in Earth’s atmosphere. Current models assume that carboxylic acids and fatty alcohols are not directly affected by sunlight in Earth’s atmosphere and that they are instead processed by reaction with hydroxyl radicals. The reason Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309, USA. Email: [email protected]
650
ate gas-phase and aqueous-phase functionalized complex organic species. The latter can form secondary organic aerosols in Earth’s atmosphere (12). Organic radical recombination reactions can lead to the formation of oligomers, although competitive reactions involving oxygen addition may occur. At the water surface, however, where organic material is found in high concentration, radical initiation and recombination is expected to occur effectively, resulting in oligomers, polymers, and aggregates that in turn affect aerosol properties with consequences to climate. Complex organic molecules and aggregates are known to form at the sea surface from biological processes (7). Recent findings (2, 3, 10–12), including those reported by Rossignol et al. (1), show that sunlight-initiated photochemistry contributes complex nonbiological molecules at the sea surface (1, 3, 10, 11). These findings suggest that sunlightinitiated photochemical reactions at water surfaces have important consequences in the natural environment. However, questions remain about the generality of direct photolysis of organic compounds in the natural environment, calling for further studies of organic reaction mechanisms. The yields of direct and indirect organic photochemistry and the factors affecting their magnitude must be determined quantitatively before these reactions can be included in chemical models of the atmosphere. Nevertheless, the information now available suggests that sunlight-driven organic chemistry at the surface of water can produce high-molecular-weight products and aggregates. These products will affect secondary organic aerosol mass, composition, and optical properties, in turn defining the particle’s overall effect on climate, air quality, and health. j
Downloaded from http://science.sciencemag.org/ on August 11, 2016
S
unlight is the largest energy source for Earth and therefore determines many aspects of our planet’s chemistry and climate. For example, light-driven splitting (photolysis) of ozone at high altitude leads to the formation of hydroxyl radicals, which are involved in most oxidative processes in the environment. On page 699 of this issue, Rossignol et al. (1) report on an alternative process. They show that direct photolysis of a fatty acid at an air-water interface leads to the formation of oxidized products in the gas phase and of macromolecular products in water. This example, along with recently reported indirect photolysis of organic molecules (2, 3), shows
for Rossignol et al.’s observation of direct photolysis of nonanoic acid (1) is that the reaction occurs not in the gas phase but at a water-air interface. Such interfaces are found at the surfaces of oceans, lakes, cloud and fog droplets, and atmospheric aerosol particles (see the figure) (4–6). Hydrophobic organic molecules, such as the fatty acids and alcohols that are abundant in sea spray (7), concentrate at these interfaces (5, 6, 8, 9), where their properties are modified compared with the gas phase (4, 5, 10). Rossignol et al. show that when nonanoic acid is present in coatings at a water-air interface, it weakly absorbs ultraviolet light at wavelengths present in sunlight at Earth’s surface. Gas- and condensed-phase organic photoproducts are observed as a result.
REFERENCES
Organic photochemical reactions of environmental interest in water are expected to proceed through the triplet state either by direct excitation, as is the case in nonanoic acid (1), or indirectly by either intersystem crossing from a strongly absorbing singlet state (2, 10) or energy transfer from a sensitizer (3, 11). After excitation of the triplet state, reactive organic radicals are produced. Not unlike the hydroxyl radical, these organic radicals then react with stable organic molecules to gener-
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.
S. Rossignol et al., Science 353, 699 (2016). A. E. Reed Harris et al., J. Phys. Chem. A 118, 8505 (2014). H. Fu et al., J. Am. Chem. Soc. 137, 8348 (2015). D. J. Donaldson, V. Vaida, Chem. Rev. 106, 1445 (2006). C. George, M. Ammann, B. D’Anna, D. J. Donaldson, S. A. Nizkorodov, Chem. Rev. 115, 4218 (2015). E. C. Griffith, A. F. Tuck, V. Vaida, Acc. Chem. Res. 45, 2106 (2012). X. Wang et al., ACS Cent. Sci. 1, 124 (2015). M. T. C. Martins-Costa, J. M. Anglada, J. S. Francisco, M. F. Ruiz-Lopez, J. Am. Chem. Soc. 134, 11821 (2012). R. Vacha, P. Slavicek, M. Mucha, B. J. Finlayson-Pitts, P. Jungwirth, J. Phys. Chem. A 108, 11573 (2004). E. C. Griffith, R. J. Rapf, R. K. Shoemaker, B. K. Carpenter, V. Vaida, J. Am. Chem. Soc. 136, 3784 (2014). R. Ciuraru et al., Sci. Rep. 5, 12741 (2015). P. Renard et al., J. Phys. Chem. C 118, 29421 (2014). 10.1126/science.aah4111
sciencemag.org SCIENCE
12 AUGUST 2016 • VOL 353 ISSUE 6300
Published by AAAS
GRAPHIC: K. SUTLIFF/SCIENCE
By Veronica Vaida
Atmospheric radical chemistry revisited Veronica Vaida (August 11, 2016) Science 353 (6300), 650. [doi: 10.1126/science.aah4111]
This copy is for your personal, non-commercial use only.
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Science (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by the American Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. Copyright 2016 by the American Association for the Advancement of Science; all rights reserved. The title Science is a registered trademark of AAAS.
Downloaded from http://science.sciencemag.org/ on August 11, 2016
Editor's Summary
ATMOSPHERIC CHEMISTRY
Atmospheric radical chemistry revisited Sunlight may directly drive previously unknown organic reactions at environmental surfaces
Oxidation with a difference Rossignol et al. report evidence for direct light-driven oxidation of an organic acid. Such oxidation processes may occur in the natural environment at water surfaces that are reached by solar radiation, generating gas-phase functionalized molecules and macromolecular condensed-phase products. These products affect aerosol formation and properties, influencing climate, air quality, and health. Photolysis of organic compounds
Condensedphase products
Sea-surface microlayer
Organic surfactants
Soluble organics
Aerosols Sea spray
Air-water interface
Gas-phase products
Bulk ocean Chemical processes
that radical reactions initiated by absorption of sunlight can follow mechanisms previously unknown in Earth’s atmosphere. Current models assume that carboxylic acids and fatty alcohols are not directly affected by sunlight in Earth’s atmosphere and that they are instead processed by reaction with hydroxyl radicals. The reason Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309, USA. Email: [email protected]
650
ate gas-phase and aqueous-phase functionalized complex organic species. The latter can form secondary organic aerosols in Earth’s atmosphere (12). Organic radical recombination reactions can lead to the formation of oligomers, although competitive reactions involving oxygen addition may occur. At the water surface, however, where organic material is found in high concentration, radical initiation and recombination is expected to occur effectively, resulting in oligomers, polymers, and aggregates that in turn affect aerosol properties with consequences to climate. Complex organic molecules and aggregates are known to form at the sea surface from biological processes (7). Recent findings (2, 3, 10–12), including those reported by Rossignol et al. (1), show that sunlight-initiated photochemistry contributes complex nonbiological molecules at the sea surface (1, 3, 10, 11). These findings suggest that sunlightinitiated photochemical reactions at water surfaces have important consequences in the natural environment. However, questions remain about the generality of direct photolysis of organic compounds in the natural environment, calling for further studies of organic reaction mechanisms. The yields of direct and indirect organic photochemistry and the factors affecting their magnitude must be determined quantitatively before these reactions can be included in chemical models of the atmosphere. Nevertheless, the information now available suggests that sunlight-driven organic chemistry at the surface of water can produce high-molecular-weight products and aggregates. These products will affect secondary organic aerosol mass, composition, and optical properties, in turn defining the particle’s overall effect on climate, air quality, and health. j
Downloaded from http://science.sciencemag.org/ on August 11, 2016
S
unlight is the largest energy source for Earth and therefore determines many aspects of our planet’s chemistry and climate. For example, light-driven splitting (photolysis) of ozone at high altitude leads to the formation of hydroxyl radicals, which are involved in most oxidative processes in the environment. On page 699 of this issue, Rossignol et al. (1) report on an alternative process. They show that direct photolysis of a fatty acid at an air-water interface leads to the formation of oxidized products in the gas phase and of macromolecular products in water. This example, along with recently reported indirect photolysis of organic molecules (2, 3), shows
for Rossignol et al.’s observation of direct photolysis of nonanoic acid (1) is that the reaction occurs not in the gas phase but at a water-air interface. Such interfaces are found at the surfaces of oceans, lakes, cloud and fog droplets, and atmospheric aerosol particles (see the figure) (4–6). Hydrophobic organic molecules, such as the fatty acids and alcohols that are abundant in sea spray (7), concentrate at these interfaces (5, 6, 8, 9), where their properties are modified compared with the gas phase (4, 5, 10). Rossignol et al. show that when nonanoic acid is present in coatings at a water-air interface, it weakly absorbs ultraviolet light at wavelengths present in sunlight at Earth’s surface. Gas- and condensed-phase organic photoproducts are observed as a result.
REFERENCES
Organic photochemical reactions of environmental interest in water are expected to proceed through the triplet state either by direct excitation, as is the case in nonanoic acid (1), or indirectly by either intersystem crossing from a strongly absorbing singlet state (2, 10) or energy transfer from a sensitizer (3, 11). After excitation of the triplet state, reactive organic radicals are produced. Not unlike the hydroxyl radical, these organic radicals then react with stable organic molecules to gener-
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.
S. Rossignol et al., Science 353, 699 (2016). A. E. Reed Harris et al., J. Phys. Chem. A 118, 8505 (2014). H. Fu et al., J. Am. Chem. Soc. 137, 8348 (2015). D. J. Donaldson, V. Vaida, Chem. Rev. 106, 1445 (2006). C. George, M. Ammann, B. D’Anna, D. J. Donaldson, S. A. Nizkorodov, Chem. Rev. 115, 4218 (2015). E. C. Griffith, A. F. Tuck, V. Vaida, Acc. Chem. Res. 45, 2106 (2012). X. Wang et al., ACS Cent. Sci. 1, 124 (2015). M. T. C. Martins-Costa, J. M. Anglada, J. S. Francisco, M. F. Ruiz-Lopez, J. Am. Chem. Soc. 134, 11821 (2012). R. Vacha, P. Slavicek, M. Mucha, B. J. Finlayson-Pitts, P. Jungwirth, J. Phys. Chem. A 108, 11573 (2004). E. C. Griffith, R. J. Rapf, R. K. Shoemaker, B. K. Carpenter, V. Vaida, J. Am. Chem. Soc. 136, 3784 (2014). R. Ciuraru et al., Sci. Rep. 5, 12741 (2015). P. Renard et al., J. Phys. Chem. C 118, 29421 (2014). 10.1126/science.aah4111
sciencemag.org SCIENCE
12 AUGUST 2016 • VOL 353 ISSUE 6300
Published by AAAS
GRAPHIC: K. SUTLIFF/SCIENCE
By Veronica Vaida
Atmospheric radical chemistry revisited Veronica Vaida (August 11, 2016) Science 353 (6300), 650. [doi: 10.1126/science.aah4111]
This copy is for your personal, non-commercial use only.
Article Tools
Permissions
Visit the online version of this article to access the personalization and article tools: http://science.sciencemag.org/content/353/6300/650 Obtain information about reproducing this article: http://www.sciencemag.org/about/permissions.dtl
Science (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by the American Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. Copyright 2016 by the American Association for the Advancement of Science; all rights reserved. The title Science is a registered trademark of AAAS.
Downloaded from http://science.sciencemag.org/ on August 11, 2016
Editor's Summary
