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| Titel |
Re-evaluating the reactive uptake of HOBr in the troposphere with implications for the marine boundary layer and volcanic plumes |
| VerfasserIn |
T. J. Roberts, L. Jourdain, P. T. Griffiths, M. Pirre |
| 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 ; 14, no. 20 ; Nr. 14, no. 20 (2014-10-23), S.11185-11199 |
| Datensatznummer |
250119117
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| Publikation (Nr.) |
 copernicus.org/acp-14-11185-2014.pdf |
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| Zusammenfassung |
The reactive uptake of HOBr onto halogen-rich aerosols promotes conversion
of Br−(aq) into gaseous reactive bromine (incl. BrO) with impacts
on tropospheric oxidants and mercury deposition. However, experimental data
quantifying HOBr reactive uptake on tropospheric aerosols is limited, and
reported values vary in magnitude. This study introduces a new evaluation of
HOBr reactive uptake coefficients in the context of the general acid-assisted mechanism. We emphasise that the termolecular kinetic approach
assumed in numerical model studies of tropospheric reactive bromine
chemistry to date is strictly only valid for a specific pH range and,
according to the general acid-assisted mechanism for HOBr, the reaction
kinetics becomes bimolecular and independent of pH at high acidity.
This study reconciles for the first time the different reactive uptake
coefficients reported from laboratory experiments. The re-evaluation confirms
HOBr reactive uptake is rapid on moderately acidified sea-salt aerosol (and
slow on alkaline aerosol), but predicts very low reactive uptake coefficients
on highly acidified submicron particles. This is due to acid-saturated
kinetics combined with low halide concentrations induced by both
acid-displacement reactions and the dilution effects of
H2SO4(aq). A mechanism is thereby proposed for reported
Br enhancement (relative to Na) in H2SO4-rich submicron particles
in the marine environment. Further, the fact that HOBr reactive uptake on
H2SO4-acidified supra-micron particles is driven by HOBr+Br−
(rather than HOBr+Cl−) indicates self-limitation via
decreasing γHOBr once aerosol Br- is converted into
reactive bromine.
First predictions of HOBr reactive uptake on sulfate particles in
halogen-rich volcanic plumes are also presented. High (accommodation
limited) HOBr+Br- uptake coefficient in concentrated
(> 1 μmol mol−1 SO2) plume environments supports potential for rapid BrO
formation in plumes throughout the troposphere. However, reduced HOBr
reactive uptake may reduce the rate of BrO cycling in dilute plumes in the
lower troposphere.
In summary, our re-evaluation of HOBr kinetics provides a new framework for the
interpretation of experimental data and suggests that the reactive uptake of HOBr
on H2SO4-acidified particles is substantially overestimated in current
numerical models of BrO chemistry in the troposphere. |
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