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| Titel |
Evidence of the impact of deep convection on reactive Volatile Organic Compounds in the upper tropical troposphere during the AMMA experiment in West Africa |
| VerfasserIn |
J. Bechara, A. Borbon, C. Jambert, A. Colomb, P. E. Perros |
| 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. 21 ; Nr. 10, no. 21 (2010-11-04), S.10321-10334 |
| Datensatznummer |
250008869
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| Publikation (Nr.) |
 copernicus.org/acp-10-10321-2010.pdf |
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| Zusammenfassung |
| A large dataset of reactive trace gases was collected for the first time
over West Africa during the African Monsoon Multidisciplinary Analysis
(AMMA) field experiment in August 2006. Volatile Organic Compounds (VOC from
C5–C9) were measured onboard the two French aircrafts the ATR-42 and the
Falcon-20 by a new instrument AMOVOC (Airborne Measurement Of Volatile
Organic Compounds). The goal of this study is (i) to characterize VOC
distribution in the tropical region of West Africa (ii) to determine the
impact of deep convection on VOC distribution and chemistry in the tropical
upper troposphere (UT) and (iii) to characterize its spatial and temporal
extensions. Experimental strategy consisted in sampling at altitudes between
0 and 12 km downwind of Mesoscale Convective Systems (MCS) and at cloud
base. Biogenic and anthropogenic VOC distribution in West Africa is clearly
affected by North to South emission gradient. Isoprene, the most abundant
VOC, is at maximum level over the forest (1.26 ppb) while benzene reaches
its maximum over the urban areas (0.11 ppb). First, a multiple physical and
chemical tracers approach using CO, O3 and relative humidity was
implemented to distinguish between convective and non-convective air masses.
Then, additional tools based on VOC observations (tracer ratios, proxy of
emissions and photochemical clocks) were adapted to characterize deep
convection on a chemical, spatial and temporal basis. VOC vertical profiles
show a "C-shaped" trend indicating that VOC-rich air masses are
transported from the surface to the UT by deep convective systems. VOC
mixing ratios in convective outflow are up to two times higher than
background levels even for reactive and short-lived VOC (e.g. isoprene up to
0.19 ppb at 12 km-altitude) and are dependent on surface emission type. As a
consequence, UT air mass reactivity increases from 0.52 s−1 in
non-convective conditions to 0.95 s−1 in convective conditions.
Fractions of boundary layer air contained in convective outflow are
estimated to be 40 ± 15%. Vertical transport timescale is
calculated to be 25 ± 10 min between 0 to 12 km altitude. These
results characterize deep convection occurring over West Africa and provide
relevant information for tropical convection parameterization in
regional/global models. |
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