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Mesoscale convective systems observed during AMMA and their impact on the NOx and O3 budget over West Africa

by H. Huntrieser, H. Schlager, M. Lichtenstern, P. Stock, T. Hamburger, H. Höller, K. Schmidt, H. D. Betz, A. Ulanovsky, F. Ravegnani show all authors
Atmospheric Chemistry and Physics ()

Abstract

During the "African Monsoon Multidisciplinary Analysis" (AMMA) field phase in August 2006, a variety of measurements focusing on deep convection were performed over West Africa. The German research aircraft Falcon based in Ouagadougou (Burkina Faso) investigated the chemical composition in the outflow of large mesoscale convective systems (MCS). Here we analyse two different types of MCS originating north and south of the intertropical convergence zone (ITCZ, similar to 10 degrees N), respectively. In addition to the airborne trace gas measurements, stroke measurements from the Lightning Location Network (LINET), set up in Northern Benin, are analysed. The main focus of the present study is (1) to analyse the trace gas composition (CO, O-3, NO, NOx, NOy, and HCHO) in the convective outflow as a function of distance from the convective core, (2) to investigate how different trace gas compositions in the boundary layer (BL) and ambient air may influence the O-3 concentration in the convective outflow, and (3) to estimate the rate of lightning-produced nitrogen oxides per flash in selected thunderstorms and compare it to our previous results for the tropics. The MCS outflow was probed at different altitudes (similar to 10-12 km) and distances from the convective core (= 10 kA contributing to LNOx were considered). The LNOx mass flux and the stroke rate were combined to estimate the LNOx production rate. For a better comparison with other published results, LNOx estimates per LINET stroke were scaled to Lightning Imaging Sensor (LIS) flashes. The LNOx production rate per LIS flash was estimated to 1.0 and 2.5 kg(N) for the MCS located south and north of the ITCZ, respectively. If we assume, that these different types of MCS are typical thunderstorms occurring globally (LIS flash rate similar to 44 s(-1)), the annual global LNOx production rate was estimated to be similar to 1.4 and 3.5 Tg(N) a(-1).

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