Nighttime oxidation of surfactants at the air–water interface: effects of chain length, head group and saturation
Reactions of the key atmospheric nighttime oxidant NO<sub>3</sub> with organic monolayers at the air–water interface are used as proxies for the ageing of organic-coated aqueous aerosols. The surfactant molecules chosen for this study are oleic acid (OA), palmitoleic acid (POA), meth...
Main Authors: | , , , |
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Format: | Article |
Language: | English |
Published: |
Copernicus Publications
2018-03-01
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Series: | Atmospheric Chemistry and Physics |
Online Access: | https://www.atmos-chem-phys.net/18/3249/2018/acp-18-3249-2018.pdf |
Summary: | Reactions of the key atmospheric nighttime oxidant NO<sub>3</sub> with
organic monolayers at the air–water interface are used as proxies
for the ageing of organic-coated aqueous aerosols. The surfactant
molecules chosen for this study are oleic acid (OA), palmitoleic
acid (POA), methyl oleate (MO) and stearic acid (SA) to investigate
the effects of chain length, head group and degree of unsaturation
on the reaction kinetics and products formed. Fully and partially
deuterated surfactants were studied using neutron reflectometry (NR)
to determine the reaction kinetics of organic monolayers with
NO<sub>3</sub> at the air–water interface for the first time.
Kinetic modelling allowed us to determine the rate coefficients for
the oxidation of OA, POA and MO monolayers to be (2.8±0.7) × 10<sup>−8</sup>, (2.4±0.5) × 10<sup>−8</sup>and (3.3±0.6) × 10<sup>−8</sup> cm<sup>2</sup> molecule<sup>−1</sup> s<sup>−1</sup> for
fitted initial desorption lifetimes of NO<sub>3</sub> at the closely
packed organic monolayers, <i>τ</i><sub><i>d</i>, NO<sub>3</sub>, 1</sub>, of 8.1±4.0, 16±4.0 and 8.1±3.0 ns, respectively. The
approximately doubled desorption lifetime found in the best fit for
POA compared to OA and MO is consistent with a more accessible
double bond associated with the shorter alkyl chain of POA
facilitating initial NO<sub>3</sub> attack at the double bond in
a closely packed monolayer. The corresponding uptake coefficients
for OA, POA and MO were found to be (2.1±0.5) × 10<sup>−3</sup>,
(1.7±0.3) × 10<sup>−3</sup> and (2.1±0.4) × 10<sup>−3</sup>,
respectively. For the much slower NO<sub>3</sub>-initiated oxidation of
the saturated surfactant SA we estimated a loss rate of
approximately (5±1) × 10<sup>−12</sup> cm<sup>2</sup> molecule<sup>−1</sup> s<sup>−1</sup>, which we
consider to be an upper limit for the reactive loss, and estimated
an uptake coefficient of ca. (5±1) × 10<sup>−7</sup>. Our
investigations demonstrate that NO<sub>3</sub> will contribute
substantially to the processing of unsaturated surfactants at the
air–water interface during nighttime given its reactivity is ca.
2 orders of magnitude higher than that of O<sub>3</sub>. Furthermore,
the relative contributions of NO<sub>3</sub> and O<sub>3</sub> to the
oxidative losses vary massively between species that are closely
related in structure: NO<sub>3</sub> reacts ca. 400 times faster than
O<sub>3</sub> with the common model surfactant oleic acid, but only
ca. 60 times faster with its methyl ester MO. It is therefore
necessary to perform a case-by-case assessment of the relative
contributions of the different degradation routes for any specific
surfactant. The overall impact of NO<sub>3</sub> on the fate of
saturated surfactants is slightly less clear given the lack of prior
kinetic data for comparison, but NO<sub>3</sub> is likely to contribute
significantly to the loss of saturated species and dominate their
loss during nighttime. The retention of the organic character at
the air–water interface differs fundamentally between the different
surfactant species: the fatty acids studied (OA and POA) form
products with a yield of ∼ 20 % that are stable at the
interface while NO<sub>3</sub>-initiated oxidation of the methyl ester
MO rapidly and effectively removes the organic character ( ≤ 3 % surface-active products). The film-forming
potential of reaction products in real aerosol is thus likely to
depend on the relative proportions of saturated and unsaturated
surfactants as well as the head group properties. Atmospheric
lifetimes of unsaturated species are much longer than those
determined with respect to their reactions at the air–water
interface, so they must be protected from oxidative attack, for example, by incorporation into a complex aerosol matrix or in mixed
surface films with yet unexplored kinetic behaviour. |
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ISSN: | 1680-7316 1680-7324 |