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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. H??ller, K. Schmidt, H. D. Betz, A. Ulanovsky, F. Ravegnani show all authors
Atmospheric Chemistry and Physics ()


During the ``African Monsoon Multidisciplinary Analysis{''} (AMMA) field\nphase in August 2006, a variety of measurements focusing on deep\nconvection were performed over West Africa. The German research aircraft\nFalcon based in Ouagadougou (Burkina Faso) investigated the chemical\ncomposition in the outflow of large mesoscale convective systems (MCS).\nHere we analyse two different types of MCS originating north and south\nof the intertropical convergence zone (ITCZ, similar to 10 degrees N),\nrespectively. In addition to the airborne trace gas measurements, stroke\nmeasurements from the Lightning Location Network (LINET), set up in\nNorthern Benin, are analysed. The main focus of the present study is (1)\nto analyse the trace gas composition (CO, O-3, NO, NOx, NOy, and HCHO)\nin the convective outflow as a function of distance from the convective\ncore, (2) to investigate how different trace gas compositions in the\nboundary layer (BL) and ambient air may influence the O-3 concentration\nin the convective outflow, and (3) to estimate the rate of\nlightning-produced nitrogen oxides per flash in selected thunderstorms\nand compare it to our previous results for the tropics. The MCS outflow\nwas probed at different altitudes (similar to 10-12 km) and distances\nfrom the convective core (<500 km). Trace gas signatures similar to the\nconditions in the MCS inflow region were observed in the outflow close\nto the convective core, due to efficient vertical transport. In the\nfresh MCS outflow, low O-3 mixing ratios in the range of 35-40 nmol\nmol(-1) were observed. Further downwind, O-3 mixing ratios in the\noutflow rapidly increased with distance, due to mixing with the ambient\nO-3-rich air. After 2-3 h, O-3 mixing ratios in the range of similar to\n65 nmol mol(-1) were observed in the aged outflow. Within the fresh MCS\noutflow, mean NOx (=NO + NO2) mixing ratios were in the range of similar\nto 0.3-0.4 nmol mol(-1) (peaks similar to 1 nmol mol(-1)) and only\nslightly enhanced compared to the background. Both lightning-produced\nNOx (LNOx) and NOx transported upward from the BL contributed about\nequally to this enhancement. On the basis of Falcon measurements, the\nmass flux of LNOx in the investigated MCS was estimated to be similar to\n100 g(N) s(-1). The average stroke rate of the probed thunderstorms was\n0.04-0.07 strokes s(-1) (here only strokes with peak currents >= 10 kA\ncontributing to LNOx were considered). The LNOx mass flux and the stroke\nrate were combined to estimate the LNOx production rate. For a better\ncomparison with other published results, LNOx estimates per LINET stroke\nwere scaled to Lightning Imaging Sensor (LIS) flashes. The LNOx\nproduction rate per LIS flash was estimated to 1.0 and 2.5 kg(N) for the\nMCS located south and north of the ITCZ, respectively. If we assume,\nthat these different types of MCS are typical thunderstorms occurring\nglobally (LIS flash rate similar to 44 s(-1)), the annual global LNOx\nproduction rate was estimated to be similar to 1.4 and 3.5 Tg(N) a(-1).

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