Radiative properties of aerosol mixture observed during the dry season 2006 over M'Bour, Senegal (African Monsoon Multidisciplinary Analysis campaign)

Abstract

[1] During the dry season in the Western Africa region the outbreaks of biomass burning aerosols occur on a background of frequent dust storms. In the frame of the African Monsoon Multidisciplinary Analysis (AMMA) campaign we studied optical properties and radiative effects of aerosol mixture and dust observed from January to March 2006 over M'Bour, Senegal. The aerosol burden during the observing period was notably below the average of previous years. However, variability of aerosol sizes and spectral absorption were found as representative. The lidar obtained extinction profiles suggested aerosol transport in two layers (or one elevated layer) for events with two different aerosol types, called aerosol mixture in the paper. Low-altitude transport was observed for dust. The broadband solar radiative fluxes and aerosol radiative effects were analyzed by combining measurements and simulations. The simulations relied on retrieved column aerosol properties and are products of Aerosol Robotic Network data processing. The 24-h average aerosol radiative effect yielded cooling at the bottom and at the top of the atmosphere for all the analyzed events of aerosol mixture and dust. The radiative efficiency (radiative effect scaled by optical thickness) at the bottom of the atmosphere and within the aerosol layer appeared to be stronger in case of mixture than in case of pure dust due to enhanced absorption ability of former. We also evaluated the importance of accounting for dust particles nonsphericity for simulating radiative effects. Our test showed that neglecting of aerosol particle shape can result in pronounced bias, up to 10% overestimation of daily average aerosol radiative effect.

Impact of dust aerosols on the radiative budget, surface heat fluxes, heating rate profiles and convective activity over West Africa during March 2006

“…Two different zones appear, with a first one (16 Slingo et al (2006) who indicated that reflected fluxes at the TOA rose by 100 W/m 2 at Niamey. Our simulation for 10 March over Senegal is also in agreement with the instantaneous TOA forcing calculated by Derimian et al (2008) in M'Bour, which range from about −70 to −90 W/m 2 (at noon). In addition, Milton et al (2008) reported values comprised between −10 and −80 W/m 2 over the Sahel region.…”

“…Milton et al (2008) modelled that dust particles considerably reduce net downward shortwave flux at the surface over a large part of West Africa with maximum of 200 W/m 2 . In addition, in the framework of the AMMA program, Derimian et al (2008) estimated the aerosol surface radiative forcing over M'Bour, Senegal. The estimations were based on the aerosol properties derived from AERONET retrievals and validated against broad-band radiation measurements conducted during AMMA campaign period at M'Bour observational site.…”

Aerosol vertical distribution and optical properties over M'Bour (16.96° W; 14.39° N), Senegal from 2006 to 2008

“…In the frame of the AMMA experiment, all the instruments have been installed at the geophysical station of the Institut de Recherche pour le Développement (IRD center) in M'Bour, 80 km south of Dakar (Derimian et al, 2008). The site is located close to the sea shore and is affected by the sea breeze and the advection of maritime aerosols in the boundary layer.…”

“…Four AMMA super stations have been selected for a comprehensive investigation of aerosol properties, along with radiation measurements (Banizoumbou -Niger, Djougou -Benin, Tamanrasset -Algeria, and M'Bour -Senegal). The M'Bour site is located in an area where the contribution of biomass burning aerosol is expected to be significant, in addition to mineral dust influence Derimian et al, 2008). A special observing period has been established for the dry season (SOP0, January-February 2006), with a focus on aerosol chemical, physical, and optical properties and the associated radiative effects Chou et al, 2008;Derimian et al, 2008;Pelon et al, 2008;Rajot et al, 2008;Heese and Wiegner, 2008).…”

“…The M'Bour site is located in an area where the contribution of biomass burning aerosol is expected to be significant, in addition to mineral dust influence Derimian et al, 2008). A special observing period has been established for the dry season (SOP0, January-February 2006), with a focus on aerosol chemical, physical, and optical properties and the associated radiative effects Chou et al, 2008;Derimian et al, 2008;Pelon et al, 2008;Rajot et al, 2008;Heese and Wiegner, 2008). One of the primary aims of this SOP-0 was to document both mineral dust (mostly natural) and anthropogenic biomass burning aerosol, and their interactions over the North West Africa region.…”