1UPMC Univ. Paris 06; Université Versailles St-Quentin; CNRS/INSU, UMR 8190, LATMOS-IPSL, France
2ISAC-Institute for Atmospheric Sciences and Climate, National Research Council, Italy
3DLR Institut für Physik der Atmosphäre, Oberpfaffenhofen, Germany
4Université de Toulouse, CNRS/INSU, LA (Laboratoire d'Aérologie, UMR 5560), France
Abstract. During the African Monsoon Multidisciplinary Analyses (AMMA) airborne measurements of ozone, CO and nitrogen oxides by the French and German falcon aircraft took place near three cities in West Africa (Cotonou, Niamey and Ouagadougou). Significant ozone production (O3 increase of 40–50 ppbv) took place during two specific events: one near Cotonou on the coast of the Guinea Gulf, and the other near Niamey in the Sahel region. In both cases a high level of NOx (>3 ppbv) is related to the ozone production. The ozone production is mainly driven by the Lagos-Cotonou anthropogenic emissions in Cotonou. In Niamey the combined effect of advection of VOC emissions from the forest and stagnation over the city area and the poorly vegetated soils recently wetted by convected systems is needed to achieve a similar level of ozone precursors. In Ouagadougou no ozone plume is found because of the absence of a pause in the convective activity and of the larger vegetated area around the city which prevented ozone plume formation during the wet season.
To discuss the ozone increase near Cotonou two different approaches have been implemented: a FLEXPART simulation to quantify the probability of transport from the SH compared to air mass stagnation over the emission area and a simulation of the BOLAM mesoscale model with two different tracers for the anthropogenic emission (RETRO inventory for 2000) and the biomass burning. The BOLAM model shows a good agreement with the meteorological observations of the aircraft and allows to identify the key influence of the anthropogenic emissions in the first 3 km while the biomass burning plume remains above this altitude.
The day to day variability of the ozone and CO in Niamey and Ouagadougou is discussed using FLEXPART simulations of the air mass stagnation in the 12° N–14° N latitude band and northward advection of air masses from the vegetated areas influenced by the biogenic volatile organic compound (VOC) emissions. Both conditions need to be fulfilled to be able to detect ozone increase within the city plume. The first condition is necessary to obtain a significant increase of the NOx concentrations by combining the city emission and the soil emission. It also shows that, contrary to the Niamey conditions, the Ouagadougou air mass transport and its timing respective to the convective activity did not correspond to favourable conditions for O3 formation during the time period of the aircraft data.
Finally to check the magnitude of the ozone production related to the observed CO and NOx observations, a 2-days stationary run of the CittyCAT Lagrangian model was conducted at Cotonou location. The initialisation of the chemical concentrations not measured is done by scaling to the NOx and CO concentrations observed in the polluted plume. The scaling factor is derived from the low altitude observations provided by the DF20 and the BAe-146 aircraft during the AMMA campaign. Under such conditions, the simulation show that 50 ppbv of ozone can be produced in a 2-days period.