Enhancement and depletion of lower/middle tropospheric ozone in Senegal during pre-monsoon and monsoon periods of summer 2008: observations and model results

Abstract. During the summer (8 June through 3 September) of 2008, nine ozonesondes are launched from Dakar, Senegal (14.75° N, 17.49° W) to investigate the impact of the Saharan Dust Layer (SAL) on ozone (O₃) concentrations in the lower troposphere. Results during June (pre-monsoon period) show a reduction in O₃, especially in the 850–700 hPa layer with SAL events. However, O₃ concentrations are increased in the 950–900 hPa layer where the peak of the inversion is found and presumably the highest dust concentrations. We use the WRF-CHEM model to explore the causes of elevated O₃ concentrations that appear to have a stratospheric contribution. During July and August (monsoon period), with the exception of one SAL outbreak, vertical profiles of O₃ are well mixed with concentrations not exceeding 55 ppb between the surface and 550 hPa. In the transition period between 26 June and 2 July lower tropospheric (925–600 hPa) O₃ concentrations are likely enhanced by enhanced biogenic NO_x emissions from the Saharan desert and Sahelian soils following several rain events on 28 June and 1 July.