Abstract. We investigate how the representation of the boundary layer in a climate model impacts the representation of the near-surface wind and dust emission, with a focus on the Sahel/Sahara region. We show that the combination of vertical turbulent diffusion with a representation of the thermal cells of the convective boundary layer by a mass flux scheme leads to realistic representation of the diurnal cycle of wind in spring, with a maximum near-surface wind in the morning. This maximum occurs when the thermal plumes reach the low-level jet that forms during the night at a few hundred meters above surface. The horizontal momentum in the jet is transported downward to the surface by compensating subsidence around thermal plumes in typically less than 1 h. This leads to a rapid increase of wind speed at surface and therefore of dust emissions owing to the strong nonlinearity of emission laws. The numerical experiments are performed with a zoomed and nudged configuration of the LMDZ general circulation model coupled to the emission module of the CHIMERE chemistry transport model, in which winds are relaxed toward that of the ERA-Interim reanalyses. The new set of parameterizations leads to a strong improvement of the representation of the diurnal cycle of wind when compared to a previous version of LMDZ as well as to the reanalyses used for nudging themselves. It also generates dust emissions in better agreement with current estimates, but the aerosol optical thickness is still significantly underestimated.

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We present results for the vertical distribution of tropospheric ozone from 40 measurements at Dakar, Senegal (14.59• N, 17.5

We use the Weather Research and Forecast model using the Goddard Global Ozone Chemistry Aerosol Radiation and Transport (GOCART) dust module (WRF‐CHEM) to simulate the particulate matter (PM) variations in the Sahel during the winter seasons (January–March) of 1960–2014. Two simulations are undertaken where the direct aerosol feedback is turned off, and only transport is considered and where the direct aerosol feedback is turned on. We find that simulated Sahelian PM10 and PM2.5 concentrations were lower in the 1960s and after 2003 and higher during the period between 1988 and 2002. Higher Sahelian PM10 concentrations are due to stronger winds between the surface and 925 hPa over the Sahara, which transport dust into the Sahel. Negative PM10 concentration anomalies are found over the Bodele Depression and associated with weaker 925 wind anomalies after 1997 through 2014. Further west, positive PM10 concentration anomalies are found across the Adrar Plateau in the Sahara and responsible for dust transport to the Western Sahel. The North Atlantic Oscillation (NAO) is positively correlated to Sahelian dust concentrations especially during the periods of 1960–1970 and 1988–2002. The temporal/spatial patterns of PM10 concentrations have significant respiratory health implications for inhabitants of the Sahel.

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

<p><strong>Abstract.</strong> During the summer (8 June through 3 September) of 2008, 9 ozonesondes are launched from Dakar, Senegal (14.75&amp;deg; N, 17.49&amp;deg; W) to investigate ozone (O₃) variability in the lower/middle troposphere during the pre-monsoon and monsoon periods. Results during June 2008 (pre-monsoon period) show a reduction in O₃ concentrations, especially in the 850–700 hPa layer with Saharan Air Layer (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 also use the WRF-CHEM model to gain greater insights for observations of elevated/reduced O₃ concentrations during the pre-monsoon/monsoon periods. In the transition period between 26 June and 2 July in the lower troposphere (925–600 hPa), a significant increase in O₃ concentrations occur which we suggest is caused by enhanced biogenic NO_x emissions from Sahelian soils following rain events on 28 June and 1 July. During July and August 2008 (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. The results suggest that during the pre-monsoon period ozone concentrations in the lower troposphere are controlled by the SAL, which destroys ozone through heterogeneous processes. At the base of the SAL we also find elevated levels of ozone, which we attribute to biogenic sources of NO_x from Saharan dust that are released in the presence of moist conditions. Once the monsoon period commences, wet and dry deposition become important sinks of ozone in the Sahel with episodes of ozone poor air that is horizontally transported from low latitudes into the Sahel. These results support aircraft chemical measurements and chemical modeling results from the African Monsoon Multidisciplinary Analysis (AMMA) field campaign.</p>

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