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| Titel |
Missing SO2 oxidant in the coastal atmosphere? – observations from high-resolution measurements of OH and atmospheric sulfur compounds |
| VerfasserIn |
H. Berresheim, M. Adam, C. Monahan, C. O'Dowd, J. M. C. Plane, B. Bohn, F. Rohrer |
| 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. 22 ; Nr. 14, no. 22 (2014-11-20), S.12209-12223 |
| Datensatznummer |
250119175
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| Publikation (Nr.) |
 copernicus.org/acp-14-12209-2014.pdf |
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| Zusammenfassung |
| Diurnal and seasonal variations of gaseous sulfuric acid (H2SO4)
and methane sulfonic acid (MSA) were measured in NE Atlantic air at the
Mace Head atmospheric research station during the years 2010 and 2011. The
measurements utilized selected-ion chemical ionization mass spectrometry
(SI/CIMS) with a detection limit for both compounds of 4.3 × 104 cm−3
at 5 min signal integration. The H2SO4 and MSA
gas-phase concentrations were analyzed in conjunction with the
condensational sink for both compounds derived from 3 nm to 10 μm
(aerodynamic diameter) aerosol size distributions. Accommodation
coefficients of 1.0 for H2SO4 and 0.12 for MSA were assumed,
leading to estimated atmospheric lifetimes on the order of 7 and 25 min,
respectively. With the SI/CIMS instrument in OH measurement mode alternating
between OH signal and background (non-OH) signal, evidence was obtained for
the presence of one or more unknown oxidants of SO2 in addition to OH.
Depending on the nature of the oxidant(s), its ambient concentration may be
enhanced in the CIMS inlet system by additional production. The apparent
unknown SO2 oxidant was additionally confirmed by direct measurements
of SO2 in conjunction with calculated H2SO4 concentrations.
The calculated H2SO4 concentrations were consistently lower than
the measured concentrations by a factor of 4.7 ± 2.4 when considering the
oxidation of SO2 by OH as the only source of H2SO4. Both the
OH and the background signal were also observed to increase significantly
during daytime aerosol nucleation events, independent of the ozone
photolysis frequency, J(O1D), and were followed by peaks in both
H2SO4 and MSA concentrations. This suggests a strong relation
between the unknown oxidant(s), OH chemistry, and the atmospheric photolysis
and photooxidation of biogenic iodine compounds. As to the identity of the
atmospheric SO2 oxidant(s), we have been able to exclude ClO, BrO, IO,
and OIO as possible candidates based on {ab initio} calculations. Never\-theless, IO
could contribute significantly to the observed CIMS background signal. A
detailed analysis of this CIMS background signal in context with recently
published kinetic data currently suggests that Criegee intermediates (CIs)
produced from ozonolysis of alkenes play no significant role for SO2
oxidation in the marine atmosphere at Mace Head. On the other hand, SO2
oxidation by small CIs such as CH2OO produced photolytically or
possibly in the photochemical degradation of methane is consistent with our
observations. In addition, H2SO4 formation from dimethyl sulfide
oxidation via SO3 as an intermediate instead of SO2 also appears to
be a viable explanation. Both pathways need to be further explored. |
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