A study of the trace gas columns of O3, NO2 and HCHO over Africa in September 1997.

Retrievals of trace gas columns from the measurements of backscattered radiation by GOME (Global Ozone Monitoring Experiment) show that enhanced tropospheric columns of ozone (O3), nitrogen dioxide (NO2) and formaldehyde (HCHO), over the African continent occur frequently. This study focuses on the behaviour of trace gases over Africa in September 1997, a period impacted by the strongest known El Niño phase of the ENSO. It investigates our qualitative and quantitative understanding of the retrieved tropospheric trace gas column densities. The emissions of NOx and volatile organic compounds (VOC) from biomass burning, biogenic sources and lightning and their photochemical transformation have been investigated. By performing a trajectory analysis, the transport of air masses from the different emission regions was analysed and the potential atmospheric spatial distribution determined. BRemen's Atmospheric PHOtochemical model (BRAPHO) was applied to compute the chemistry along a large number of trajectories. From these results, tropospheric column amounts of O3, NO2 and HCHO were derived. Tropospheric trace gas columns retrieved from GOME measurements and those calculated are in reasonable agreement. Their general spatial extent was similar in the lower troposphere but the modeled trace gas columns in the upper troposphere were located south of the retrieved columns. We attribute this behaviour to uncertainties in the ERA-40 meteorological data in the upper troposphere. The significance of biomass burning and of biogenic emissions with respect to HCHO columns over Africa was investigated. The analysis reveals that the total amounts of HCHO generated over Africa during September 1997 as a result of biomass burning and biogenic emissions are similar. However the HCHO from biogenic sources has the highest specific columns and these are located close to their source. In comparison the HCHO from biomass burning is predicted to be produced and transported over a much wider area. Overall all the emission processes mix together to produce the plume of O3.