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Communication incl. Poster: BibTeX citation key:  Bechara
Bechara, J., Borbon, A., Jambert, C. & Perros, P. E. 2009. Direct evidence of deep convective transport on VOC trace gases in the upper tropical troposphere during AMMA-SOP2a2. Work presented at Third International AMMA Conference, July 20—24, at Ouagadougou, Burkina Faso.
Added by: roussot 2009-10-19 18:04:11
Categories: Aerosol and Chemistry, Atmospheric processes
Keywords: Chemistry, Clouds - Convection
Creators: Bechara, Borbon, Jambert, Perros
Publisher: African Monsoon Multidisciplinary Analyses (Ouagadougou, Burkina Faso)
Collection: Third International AMMA Conference

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A large and unprecedented dataset of reactive trace gases was collected for the first time on the two French aircrafts, the ATR-42 and the French Falcon-20 from 0 to 12 km during the special observation period SOP2a2 of AMMA in August 2006. The goal of the study is (i) to evaluate the impact of deep convection on trace gases distribution and chemistry in the tropical upper troposphere (UT) and (ii) to characterize its spatial and temporal extensions. This work mainly focuses on volatile organic compounds (VOC) measurements from C5 to C9.
First, an original set of chemical and dynamical tracers (CO, O3 and Relative Humidity) was used to distinguish background conditions from those perturbed by convection. During perturbed conditions, enhanced levels of VOC are observed in the UT up to 3 times higher than in non convective conditions. They are also found to be comparable to those observed near the surface, even for the most reactive one (e.g. isoprene). As a consequence, total air masses reactivity is increased by a factor of two. Moreover, Mesoscale Convective System outflow composition appears to be strongly dependent on surface coverage. It is then used as a proxy of emissions pointing out that near surface emissions are directly injected into the UT.
Furthermore, statistical tools based on tracer ratios and the photochemical aging equation in non convective vs. convective conditions have been developed and derived from VOC observations to characterize deep convection. On the one hand, the fraction of near surface air present in convective outflow is estimated to be at 40±15% on average regarding all VOC observations. On the other hand, vertical transport timescale derived from VOC observations is estimated to be about 25±10 minutes. Results are slightly variable from an MCS to another and characterize deep convection occurring over West Africa. It is the first time that these tools are adapted and applied for VOC observations. These results, being in good agreement with other studies, attest of the efficiency of the method.
Finally, photochemical box modeling is implemented to evaluate the impact of injected trace gases on photooxidant production in the UT.
Added by: roussot