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| Titel |
Measurements of OH and HO2 at a tropical marine location as part of the SOLAS Seasonal Oxidant Study |
| VerfasserIn |
Stewart Vaughan, Trevor Ingham, Daniel Stone, Lisa Whalley, Mathew Evans, Dwayne Heard |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250040374
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| Zusammenfassung |
| The hydroxyl radical, OH, is the dominant daytime oxidant in the troposphere, particularly in
the tropical marine boundary layer, where the warm, humid conditions and high solar
incidence lend themselves favourably to the formation of OH. Chemical models show that the
majority of atmospheric methane is oxidised in tropical regions so that accurate
measurements of OH in those regions are crucial for our understanding of the global
oxidizing capacity, especially in terms of climate change. However, there are few published
measurements of OH, and also of HO2, which is closely coupled to OH, in such
environments.
Long-term measurements of HOx (OH and HO2) using FAGE (Fluorescence Assay by
Gas Expansion) were made at the Cape Verde Atmospheric Observatory (16.85Ë N
24.87Ë W), on the island of São Vicente, as part of the Seasonal Oxidant Study
(SOS) in the tropical Atlantic during three distinct seasonal periods; SOS1 during
February—March (generally dry with clear sky), in June during SOS2 (clear and dry),
and SOS3 in September (some sustained periods of heavy rainfall). Preliminary
data analysis suggests mean midday concentrations of OH were 3.6, 3.5 and 4.6 Ã
106 cm-3 for SOS1, 2 and 3; the corresponding mean noontime concentrations
of HO2 were 1.8, 2.1 and 1.8 Ã 108 cm-3. Compared to measurements made in
May—June 2007, during the RHAMBLE project the peak concentration of HO2 for
SOS2 seem similar, whereas the concentration of OH in SOS2 appears about 30%
lower.
A chemical box model, based on the Master Chemical Mechanism (MCM) and includes
halogen chemistry and heterogeneous loss of HOxto aerosol surfaces, has been used to
describe the diurnal behaviour of OH and HO2 for each campaign. By assuming that the
concentrations of the halogen oxides BrO and IO, key intermediates in the depletion of
tropospheric ozone – an important precursor for HOx – are similar to those observed at the
observatory in 2007, the model is able to recreate well the relative diurnal trends of both OH
and HO2. A preliminary comparison shows that the model overpredicts the daytime
concentrations of both HOx-species by factors of ~1.9 and ~1.6 for SOS1 and SOS2,
respectively, suggesting an overestimation of HOx-sources and/or an underprediction of
HOx-sinks; modelling analysis of the data for SOS3 will take place in spring 2010. Attempts
to resolve the differences between the model predictions and the measurements,
including a sensitivity analysis with respect to halogen oxides, will be discussed. |
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