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Determining the mechanisms of the reactions between organic surfactants and
atmospheric oxidants.
Recent field measurements have shown that a significant
fraction of the mass of atmospheric
aerosols is organic. Laboratory experiments have
shown that organic compounds will preferentially
partition to the surface of the aerosols and will
be extremely susceptible to oxidation by OH, O3,
halogen atoms, and NO3. Oxidation reactions
with hydrocarbons have two possible consequences
for the structure of the molecules: (1)
reaction can lead to the fission of any carbon-carbon
bond, resulting in shorter-chain molecules or
(2) reaction forms oxygenated compounds. Either
pathway results in changes to the molecules at the
aerosols' interface which will affect the ability of
the aerosols to act as cloud condensation nuclei.
This has direct implications towards modeling
and predicting climate change.
In this work, thin films and pure organic aerosols
were investigated as proxies for surface alkane
and alkene organic compounds. We studied the
processing of 2-octenoic acid, 10-undecenoic acid,
hexadecane and 1-dodecene by O3 and OH. We
probed the gas-phase and condensed-phase products
to elucidate the mechanisms of these reactions
via a combination of infrared spectroscopy
and mass spectrometry. 1-Dodecene was used to
investigate the competition between O3 and OH
for the carbon-carbon double bond.
Ozonolysis of surface compounds proceeds by a
similar mechanism as in the gas-phase, and produces
oxidized, hydrophilic and more volatile
compounds than the corresponding parent molecule.
The dominant pathway for the reaction of
the hydroxyl radical with a thin film was different
than this reaction in the gas-phase. Our experiments
have shown that a hydrogen is initially
abstracted, then oxygen adds to the unstable radical.
This RO2 radical will either form a ketone of
the initial compound or decompose to shortchain
aldehydes and alkyl radicals. In the presence
of a carbon-carbon double bond, gas-phase
OH reactions yield approximately 50 / 50 mixtures
of products arising from the addition of
OH across the double bond and from hydrogen
abstraction by the OH. However, in a thin film,
the dominant products seen in our experiments
include those from the ozonolysis mechanism
and then the products from OH reacting with
these products.
Our results show that ozone will be a powerful
oxidant for unsaturated compounds on atmospheric
surfaces due to the consistently higher
concentration. Ozonolysis reactions are important
in the atmosphere because the mechanism
involves the shredding of the initial compound,
releasing volatile organics, which can be further
oxidized to produce HOX. However, OH will
still be the dominant atmospheric oxidizer due to
its reactivity towards any organic compound and
the fast rate of hydrogen abstraction.
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CIRES Research Theme
Planetary Metabolism
Project Personnel
T. L. Eliason, J. B. Gilman and V. Vaida
Funding Source(s)
CIRES, NSF
Publications
2003. Processing of unsaturated organic acid films and aerosols by ozone. Atmos. Environ. 37: 2207-2219.
Oxidation of organic films relevant to atmospheric aerosols. Submitted to Atmos. Environ.
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