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| Titel |
Mesoscale convective systems observed during AMMA and their impact on the NOx and O3 budget over West Africa |
| VerfasserIn |
H. Huntrieser, H. Schlager, M. Lichtenstern, P. Stock, T. Hamburger, H. Höller, K. Schmidt, H.-D. Betz, A. Ulanovsky, F. Ravegnani |
| 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 ; 11, no. 6 ; Nr. 11, no. 6 (2011-03-17), S.2503-2536 |
| Datensatznummer |
250009504
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| Publikation (Nr.) |
 copernicus.org/acp-11-2503-2011.pdf |
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| Zusammenfassung |
| 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, ~10° 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, O3, 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 O3 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 (~10–12 km) and distances from the convective core (<500 km). Trace gas
signatures similar to the conditions in the MCS inflow region were observed in the outflow
close to the convective core, due to efficient vertical transport. In the fresh MCS outflow,
low O3 mixing ratios in the range of 35–40 nmol mol−1 were observed. Further
downwind, O3 mixing ratios in the outflow rapidly increased with distance, due to
mixing with the ambient O3-rich air. After 2–3 h, O3 mixing ratios in the
range of ~65 nmol mol−1 were observed in the aged outflow. Within the fresh MCS
outflow, mean NOx (=NO+NO2) mixing ratios were in the range of
~0.3–0.4 nmol mol−1 (peaks ~1 nmol mol−1) and only slightly
enhanced compared to the background. Both lightning-produced NOx (LNOx) and
NOx transported upward from the BL contributed about equally to this enhancement. On
the basis of Falcon measurements, the mass flux of LNOx in the investigated MCS was
estimated to be ~100 g(N) s−1. The average stroke rate of the probed
thunderstorms was 0.04–0.07 strokes s−1 (here only strokes with peak currents ≥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 ~44 s−1), the annual global LNOx
production rate was estimated to be ~1.4 and 3.5 Tg(N) a−1. |
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