By Veronica Vaida
Sunlight 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
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 af-
fected by sunlight in Earth’s atmosphere
and that they are instead processed by re-
action with hydroxyl radicals. The reason
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). Hydropho-
bic organic molecules, such as the fatty ac-
ids 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.
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-
ate gas-phase and aqueous-phase functional-
ized complex organic species. The latter can
form secondary organic aerosols in Earth’s
atmosphere (12). Organic radical recombina-
tion reactions can lead to the formation of
oligomers, although competitive reactions
involving oxygen addition may occur. At the
water surface, however, where organic mate-
rial is found in high concentration, radical
initiation and recombination is expected to
occur effectively, resulting in oligomers, poly-
mers, and aggregates that in turn affect aero-
sol 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 sunlight-initiated 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 sun-light-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
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5. C. George, M. Ammann, B. D’Anna, D. J. Donaldson, S. A.
Nizkorodov, Chem. Rev. 115, 4218 (2015).
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7. X. Wang et al., ACS Cent. Sci. 1,124(2015).
8. M. T. C. Martins-Costa, J. M. Anglada, J. S. Francisco, M. F.
Ruiz-Lopez, J. Am. Chem. Soc. 134, 11821 (2012).
9. R. Vacha, P. Slavicek, M. Mucha, B. J. Finlayson-Pitts, P.
Jungwirth, J. Phys. Chem. A 108, 11573 (2004).
10. E. C. Griffith, R. J. Rapf, R. K. Shoemaker, B. K. Carpenter, V.
Vaida, J. Am. Chem. Soc. 136, 3784 (2014).
11. R. Ciuraru et al., Sci. Rep.5, 12741 (2015).
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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.
Department of Chemistry and Biochemistry, University of
Colorado, Boulder, CO 80309, USA. Email: email@example.com
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