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| Titel |
Fractionation of sulfur isotopes during heterogeneous oxidation of SO2 on sea salt aerosol: a new tool to investigate non-sea salt sulfate production in the marine boundary layer |
| VerfasserIn |
E. Harris, B. Sinha, P. Hoppe, S. Foley, S. Borrmann |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 12, no. 10 ; Nr. 12, no. 10 (2012-05-24), S.4619-4631 |
| Datensatznummer |
250011171
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| Publikation (Nr.) |
 copernicus.org/acp-12-4619-2012.pdf |
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| Zusammenfassung |
The oxidation of SO2 to sulfate on sea salt aerosols in the marine
environment is highly important because of its effect on the size
distribution of sulfate and the potential for new particle nucleation from
H2SO4 (g). However, models of the sulfur cycle are not currently able
to account for the complex relationship between particle size, alkalinity,
oxidation pathway and rate – which is critical as SO2 oxidation by O3
and Cl catalysis are limited by aerosol alkalinity, whereas oxidation by
hypohalous acids and transition metal ions can continue at low pH once
alkalinity is titrated. We have measured 34S/32S fractionation
factors for SO2 oxidation in sea salt, pure water and NaOCl aerosol, as
well as the pH dependency of fractionation.
Oxidation of SO2 by NaOCl aerosol was extremely efficient, with a reactive
uptake coefficient of ≈0.5, and produced sulfate that was enriched in
32S with αOCl = 0.9882±0.0036 at 19 °C.
Oxidation on sea salt aerosol was much less efficient than on NaOCl aerosol,
suggesting alkalinity was already exhausted on the short timescale of the
experiments. Measurements at pH = 2.1 and 7.2 were used to calculate
fractionation factors for each step from SO2(g) → multiple
steps → SOOCl2−. Oxidation on sea salt aerosol resulted in a
lower fractionation factor than expected for oxidation of SO32− by
O3 (αseasalt = 1.0124±0.0017 at 19 °C).
Comparison of the lower fractionation during oxidation on sea salt aerosol to
the fractionation factor for high pH oxidation shows HOCl contributed 29%
of S(IV) oxidation on sea salt in the short experimental timescale,
highlighting the potential importance of hypohalous acids in the marine
environment.
The sulfur isotope fractionation factors measured in this study allow
differentiation between the alkalinity-limited pathways – oxidation by O3
and by Cl catalysis (α34 = 1.0163±0.0018 at 19 °C in
pure water or 1.0199±0.0024 at pH = 7.2) – which favour the heavy
isotope, and the alkalinity non-limited pathways – oxidation by transition
metal catalysis (α34 = 0.9905±0.0031 at 19 °C,
Harris et al., 2012a) and by hypohalites (α34 =
0.9882±0.0036 at 19 °C) – which favour the light isotope. In
combination with field measurements of the oxygen and sulfur isotopic
composition of SO2 and sulfate, the fractionation factors presented in
this paper may be capable of constraining the relative importance of
different oxidation pathways in the marine boundary layer. |
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