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| Titel |
Chemical and aerosol characterisation of the troposphere over West Africa during the monsoon period as part of AMMA |
| VerfasserIn |
C. E. Reeves, P. Formenti, C. Afif, G. Ancellet, J.-L. Attié, J. Bechara, A. Borbon, F. Cairo, H. Coe, S. Crumeyrolle, F. Fierli, C. Flamant, L. Gomes, T. Hamburger, C. Jambert, K. S. Law, C. Mari, R. L. Jones, A. Matsuki, M. I. Mead, J. Methven, G. P. Mills, A. Minikin, J. G. Murphy, J. K. Nielsen, D. E. Oram, D. J. Parker, A. Richter, H. Schlager, A. Schwarzenboeck, V. Thouret |
| 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 ; 10, no. 16 ; Nr. 10, no. 16 (2010-08-16), S.7575-7601 |
| Datensatznummer |
250008706
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| Publikation (Nr.) |
 copernicus.org/acp-10-7575-2010.pdf |
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| Zusammenfassung |
During June, July and August 2006 five aircraft took part in a campaign over
West Africa to observe the aerosol content and chemical composition of the
troposphere and lower stratosphere as part of the African Monsoon
Multidisciplinary Analysis (AMMA) project. These are the first such
measurements in this region during the monsoon period. In addition to
providing an overview of the tropospheric composition, this paper provides a
description of the measurement strategy (flights performed, instrumental
payloads, wing-tip to wing-tip comparisons) and points to some of the
important findings discussed in more detail in other papers in this special
issue.
The ozone data exhibits an "S" shaped vertical profile which appears to
result from significant losses in the lower troposphere due to rapid
deposition to forested areas and photochemical destruction in the moist
monsoon air, and convective uplift of ozone-poor air to the upper
troposphere. This profile is disturbed, particularly in the south of the
region, by the intrusions in the lower and middle troposphere of air from
the southern hemisphere impacted by biomass burning. Comparisons with longer
term data sets suggest the impact of these intrusions on West Africa in 2006
was greater than in other recent wet seasons. There is evidence for net
photochemical production of ozone in these biomass burning plumes as well as
in urban plumes, in particular that from Lagos, convective outflow in the
upper troposphere and in boundary layer air affected by nitrogen oxide
emissions from recently wetted soils. This latter effect, along with
enhanced deposition to the forested areas, contributes to a latitudinal
gradient of ozone in the lower troposphere. Biogenic volatile organic
compounds are also important in defining the composition both for the
boundary layer and upper tropospheric convective outflow.
Mineral dust was found to be the most abundant and ubiquitous aerosol type
in the atmosphere over Western Africa. Data collected within AMMA indicate
that injection of dust to altitudes favourable for long-range transport
(i.e. in the upper Sahelian planetary boundary layer) can occur behind the leading edge of
mesoscale convective system (MCS) cold-pools. Research within AMMA also
provides the first estimates of secondary organic aerosols across the
West African Sahel and have shown that organic mass loadings vary between 0
and 2 μg m−3 with a median concentration of 1.07 μg m−3.
The vertical distribution of nucleation mode particle concentrations reveals
that significant and fairly strong particle formation events did occur for a
considerable fraction of measurement time above 8 km (and only there). Very
low concentrations were observed in general in the fresh outflow of active
MCSs, likely as the result of efficient wet removal of aerosol particles due
to heavy precipitation inside the convective cells of the MCSs. This wet
removal initially affects all particle size ranges as clearly shown by all
measurements in the vicinity of MCSs. |
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