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| Titel |
Factors controlling the distribution of ozone in the West African lower troposphere during the AMMA (African Monsoon Multidisciplinary Analysis) wet season campaign |
| VerfasserIn |
M. Saunois, C. E. Reeves, C. H. Mari, J. G. Murphy, D. J. Stewart, G. P. Mills, D. E. Oram, R. M. Purvis |
| 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 ; 9, no. 16 ; Nr. 9, no. 16 (2009-08-27), S.6135-6155 |
| Datensatznummer |
250007592
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| Publikation (Nr.) |
 copernicus.org/acp-9-6135-2009.pdf |
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| Zusammenfassung |
| Ozone and its precursors were measured on board the Facility for Airborne
Atmospheric Measurements (FAAM) BAe 146 Atmospheric Research Aircraft during
the monsoon season 2006 as part of the African Monsoon Multidisciplinary
Analysis (AMMA) campaign. One of the main features observed in the west
African boundary layer is the increase of the ozone mixing ratios from
25 ppbv over the forested area (south of 12° N) up to 40 ppbv over
the Sahelian area. We employ a two-dimensional (latitudinal versus vertical)
meteorological model coupled with an O3-NOx-VOC chemistry scheme
to simulate the distribution of trace gases over West Africa during the
monsoon season and to analyse the processes involved in the establishment of
such a gradient. Including an additional source of NO over the Sahelian region
to account for NO emitted by soils we simulate a mean NOx
concentration of 0.7 ppbv at 16° N versus 0.3 ppbv over the
vegetated region further south in reasonable agreement with the observations.
As a consequence, ozone is photochemically produced with a rate of
0.25 ppbv h−1 over the vegetated region whilst it reaches up to
0.75 ppbv h−1 at 16° N. We find that the modelled gradient is
due to a combination of enhanced deposition to vegetation, which decreases
the ozone levels by up to 11 pbbv, and the aforementioned enhanced
photochemical production north of 12° N. The peroxy radicals required
for this enhanced production in the north come from the oxidation of
background CO and CH4 as well as from VOCs. Sensitivity studies reveal
that both the background CH4 and partially oxidised VOCs, produced from
the oxidation of isoprene emitted from the vegetation in the south,
contribute around 5–6 ppbv to the ozone gradient. These results suggest that
the northward transport of trace gases by the monsoon flux, especially during
nighttime, can have a significant, though secondary, role in determining the
ozone gradient in the boundary layer. Convection, anthropogenic emissions and
NO produced from lightning do not contribute to the establishment of the
discussed ozone gradient. |
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