Mesoscale convective systems observed during AMMA and their impact on the NO<sub>x</sub> and O<sub>3</sub> budget over West Africa

Abstract

Abstract. During the "African Monsoon MultidisciplinaryAnalysis" (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, O 3 , NO, NO x , NO y , 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 lightningproduced 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 O 3 mixing ratios in the range of 35-40 nmol mol −1 were Correspondence to: H. Huntrieser (heidi.huntrieser@dlr.de) observed. Further downwind, O 3 mixing ratios in the outflow rapidly increased with distance, due to mixing with the ambient O 3 -rich air. After 2-3 h, O 3 mixing ratios in the range of ∼65 nmol mol −1 were observed in the aged outflow. Within the fresh MCS outflow, mean NO x (=NO + NO 2 ) 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 NO x (LNO x ) and NO x transported upward from the BL contributed about equally to this enhancement. On the basis of Falcon measurements, the mass flux of LNO x 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 LNO x were considered). The LNO x mass flux and the stroke rate were combined to estimate the LNO x production rate. For a better comparison with other published results, LNO x estimates per LINET stroke were scaled to Lightning Imaging Sensor (LIS) flashes. The LNO x 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 LNO x production rate was estimated to be ∼1.4 and 3.5 Tg(N) a −1 .

Cloud-resolving chemistry simulation of a Hector thunderstorm

“…Analyses of aircraft NO x observations and flash data from other tropical regions (e.g., Brazil, West Africa) suggest smaller NO production per flash in the tropics than in midlatitudes (Huntrieser et al, 2008;Huntrieser et al, 2011). Aircraft observations of another Hector storm from SCOUT-O3/ACTIVE also showed larger NO x production per flash than in other tropical events.…”

Emission sources contributing to tropospheric ozone over Equatorial Africa during the summer monsoon

“…This may explain lower CO concentrations measured in the UT in the CONV case. Furthermore, Huntrieser et al (2011) showed higher CO, measured by the DLR aircraft, in outflow of MCS observed over southern forests than over the Sahel region.…”

“…Based on analysis of DLR measurements, Huntrieser et al (2011) showed that convective uplift of local emissions over West Africa can have a significant impact on UT chemical composition thereby contributing to observed CO enhancements. As demonstrated by several studies (Stohl et al, 2002;Lawrence et al, 2003;Barret et al, 2008), westward transport of Asian pollution by the TEJ may also impact the composition of the African UT.…”

“…Uplift of such emissions may also contribute to the observed NO increases in the CONV case. Furthermore, Huntrieser et al (2011) showed that NO x measured by the DLR in MCS outflow is a mixture of LiNOx and NO x transported upward from the LT. The modelled profiles underestimate the maximum of NO in the UT, particularly in the CONV case, although KE AMMA has higher NO (150-200 pptv) compared to TI AMMA (50-100 pptv).…”

Sensitivity of tropical tropospheric composition to lightning NOx production as determined by replay simulations with GEOS‐5

“…Therefore, the factor-of-four uncertainty currently associated with the global LNO x source strength stems primarily from the uncertainty in the global mean NO x production per flash. Several methods have been used to estimate this quantity: theoretical estimates [Price et al, 1997;Koshak et al, 2014], laboratory experiments ], cloud-resolving model simulations constrained by lightning flash observations and anvil NO x measurements [DeCaria et al, 2005;Ott et al, 2007Ott et al, , 2010Cummings et al, 2013], analyses of aircraft data [Huntrieser et al, 2008[Huntrieser et al, , 2009[Huntrieser et al, , 2011, and analyses of satellite observations [Beirle et al, 2004[Beirle et al, , 2006[Beirle et al, , 2010Bucsela et al, 2010]. NO x production estimates from these studies vary from 30 to 1100 mol flash À1 , though most estimates fall between 100 and 500 mol flash…”