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| Titel |
Variability of aerosol vertical distribution in the Sahel |
| VerfasserIn |
O. Cavalieri, F. Cairo, F. Fierli, G. Donfrancesco, M. Snels, M. Viterbini, F. Cardillo, B. Chatenet, P. Formenti, B. Marticorena, J. L. Rajot |
| 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. 24 ; Nr. 10, no. 24 (2010-12-16), S.12005-12023 |
| Datensatznummer |
250008966
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| Publikation (Nr.) |
 copernicus.org/acp-10-12005-2010.pdf |
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| Zusammenfassung |
In this work, we have studied the seasonal and inter-annual variability of
the aerosol vertical distribution over Sahelian Africa for the years 2006, 2007
and 2008, characterizing the different kind of aerosols present in the
atmosphere in terms of their optical properties observed by ground-based and
satellite instruments, and their sources searched for by using trajectory
analysis. This study combines data acquired by three ground-based micro lidar
systems located in Banizoumbou (Niger), Cinzana (Mali) and M'Bour (Senegal)
in the framework of the African Monsoon Multidisciplinary Analysis (AMMA), by
the AEROsol RObotic NETwork (AERONET) sun-photometers and by the space-based
Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) onboard the CALIPSO
satellite (Cloud-Aerosol Lidar and Infrared Pathfinder Observations).
During winter, the lower levels air masses arriving in the Sahelian region
come mainly from North, North-West and from the Atlantic area, while in the
upper troposphere air flow generally originates from West Africa, crossing a
region characterized by the presence of large biomass burning sources. The
sites of Cinzana, Banizoumbou and M'Bour, along a transect of aerosol
transport from East to West, are in fact under the influence of tropical
biomass burning aerosol emission during the dry season, as revealed by the
seasonal pattern of the aerosol optical properties, and by back-trajectory
studies.
Aerosol produced by biomass burning are observed mainly during the dry season
and are confined in the upper layers of the atmosphere. This is particularly
evident for 2006, which was characterized by a large presence of biomass
burning aerosols in all the three sites.
Biomass burning aerosol is also observed during spring when air masses
originating from North and East Africa pass over sparse biomass burning
sources, and during summer when biomass burning aerosol is transported from
the southern part of the continent by the monsoon flow.
During summer months, the entire Sahelian region is under the influence of
Saharan dust aerosols: the air masses in low levels arrive from West
Africa crossing the Sahara desert or from the Southern Hemisphere crossing
the Guinea Gulf while in the upper layers air masses still originate from
North, North-East. The maximum of the desert dust activity is observed in
this period which is characterized by large AOD (above 0.2) and
backscattering values. It also corresponds to a maximum in the extension of
the aerosol vertical distribution (up to 6 km of altitude). In
correspondence, a progressive cleaning up of the lowermost layers of the
atmosphere is occurring, especially evident in the Banizoumbou and Cinzana
sites.
Summer is in fact characterized by extensive and fast convective phenomena.
Lidar profiles show at times large dust events loading the atmosphere with
aerosol from the ground up to 6 km of altitude. These events are
characterized by large total attenuated backscattering values, and alternate
with very clear profiles, sometimes separated by only a few hours, indicative
of fast removal processes occurring, likely due to intense convective and
rain activity.
The inter-annual variability in the three year monitoring period is not very
significant. An analysis of the aerosol transport pathways, aiming at
detecting the main source regions, revealed that air originated from the
Saharan desert is present all year long and it is observed in the lower
levels of the atmosphere at the beginning and at the end of the year. In the
central part of the year it extends upward and the lower levels are less
affected by air masses from Saharan desert when the monsoon flow carries air
from the Guinea Gulf and the Southern Hemisphere inland. |
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