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| Titel |
Air mass origins influencing TTL chemical composition over West Africa during 2006 summer monsoon |
| VerfasserIn |
K. S. Law, F. Fierli, F. Cairo, H. Schlager, S. Borrmann, M. Streibel, E. Real, D. Kunkel, C. Schiller, F. Ravegnani, A. Ulanovsky, F. D'Amato, S. Viciani, C. M. Volk |
| 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. 22 ; Nr. 10, no. 22 (2010-11-17), S.10753-10770 |
| Datensatznummer |
250008893
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| Publikation (Nr.) |
 copernicus.org/acp-10-10753-2010.pdf |
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| Zusammenfassung |
| Trace gas and aerosol data collected in the tropical tropopause layer (TTL)
between 12–18.5 km by the M55 Geophysica aircraft as part of the SCOUT-AMMA
campaign over West Africa during the summer monsoon in August 2006 have been
analysed in terms of their air mass origins. Analysis of domain filling back
trajectories arriving over West Africa, and in the specific region of the
flights, showed that the M55 flights were generally representative of air
masses arriving over West Africa during the first 2 weeks of August, 2006.
Air originating from the mid-latitude lower stratosphere was under-sampled
(in the mid-upper TTL) whilst air masses uplifted from central Africa (into
the lower TTL) were over-sampled in the latter part of the campaign.
Signatures of recent (previous 10 days) origins were superimposed on the
large-scale westward flow over West Africa. In the lower TTL, air masses
were impacted by recent local deep convection over Africa at the level of
main convective outflow (350 K, 200 hPa) and on certain days up to 370 K
(100 hPa). Estimates of the fraction of air masses influenced by local
convection vary from 10 to 50% depending on the method applied and from
day to day during the campaign. The analysis shows that flights on 7, 8 and
11 August were more influenced by local convection than on 4 and 13 August
allowing separation of trace gas and aerosol measurements into
"convective" and "non-convective" flights. Strong signatures,
particularly in species with short lifetimes (relative to CO2) like CO,
NO and fine-mode aerosols were seen during flights most influenced by
convection up to 350–365 K. Observed profiles were also constantly perturbed
by uplift (as high as 39%) of air masses from the mid to lower
troposphere over Asia, India, and oceanic regions resulting in
import of clean oceanic (e.g. O3-poor) or polluted air masses from Asia (high
O3, CO, CO2) into West Africa. Thus, recent uplift of CO2 over Asia may contribute to the observed positive CO2 gradients in
the TTL over West Africa. This suggests a more significant fraction of
younger air masses in the TTL and needs to taken into consideration in
derivations of mean age of air. Transport of air masses from the mid-latitude
lower stratosphere had an impact from the mid-TTL upwards (20–40% above
370 K) during the campaign period importing air masses with high O3 and
NOy. Ozone profiles show a less pronounced lower TTL minimum than
observed previously by regular ozonesondes at other tropical locations.
Concentrations are less than 100 ppbv in the lower TTL and vertical
gradients less steep than in the upper TTL. The air mass origin analysis and
simulations of in-situ net photochemical O3 production, initialised
with observations, suggest that the lower TTL is significantly impacted by
uplift of O3 precursors (over Africa and Asia) leading to positive
production rates (up to 2 ppbv per day) in the lower and mid TTL even at
moderate NOx levels. Photochemical O3 production increases with
higher NOx and H2O in air masses with O3 less than 150 ppbv. |
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