Bernadette Chatenet

Origins of African dust transported over the northeastern tropical Atlantic

On the basis of 3 years of daily measurements of mineral dust concentrations at Sal Island (Cape Verde), this study focuses on the origin of the dust transported over the northeastern tropical Atlantic during the dusty season (October to April). Using backward trajectories, we have identified three different sectors of origin of the transported dust, covering the Sahara (north/west and central/south) and the Sahel regions. Time variations of the dust transport from these different areas can be explained by the seasonal shift of the subtropical high-pressure center. A geochemical approach highlights that the calcium amount and the Si/Al ratio of the transported dust differ according to these source regions, in relation to their respective soil compositions.

Saharan dust: Clay ratio as a relevant tracer to assess the origin of soil-derived aerosols

The aim of this study is to find a tracer allowing retrieval of the regional origin of mineral dust for Saharan aerosols transported over the North Atlantic Ocean. Because of physical and chemical fractionation processes occurring at the soil-atmosphere interface and during the atmospheric transport of dust, clay mineral species seem to be the best candidate. This study shows that the ratio between relative abundance of illite and kaolinite (I/K ratio) is the parameter that is the most sensitive to the regional origin of Saharan dust collected on Sal Island (Cape Verde). By comparing the I/K ratio measured in dust emitted from the same Saharan source and collected along its transport both on Sal Island and Barbados (Caribbean Sea), we show that this ratio seems to remain unchanged after long-range transport.

Modeling the radiative impact of mineral dust during the Saharan Dust Experiment (SHADE) campaign

[1]xa0The Oslo chemical transport model (Oslo CTM2) is driven by meteorological data to model mineral dust during the Saharan Dust Experiment (SHADE) campaign in September 2000. Model calculations of the optical properties and radiative transfer codes are used to assess the direct radiative impact in the solar and terrestrial regions of the spectrum. The model calculations are compared to a wide range of measurements (satellite, ground-based, and aircraft) during the campaign. The model reproduces the main features during the SHADE campaign, including a large mineral dust storm. The optical properties and the vertical profiles are in reasonable agreement with the measurements. There is a very good agreement between the modeled radiative impact and observations. The strongest local solar radiative impact we model is around −115 Wm−2. On a global scale the radiative effect of mineral dust from Sahara exerts a significant negative net radiative effect.

Contribution of the different aerosol species to the aerosol mass load and optical depth over the northeastern tropical Atlantic

Simultaneous ground-based measurements of chemical composition, size distribution, and column optical thickness at 670 nm of atmospheric aerosols have been performed at Sal Island (Cape Verde) during the winter season when African dust is transported in the lower troposphere. Mineral dust and, occasionally, sea salt dominate the aerosol mass load, whereas the excess sulfates plus the carbonaceous aerosol (particulate organic matter and black carbon) contributions to the mass load remain lower than 5% on average. We compute the total aerosol optical depth (AOD) by combining optical properties derived from measured size distributions and vertical concentration profile of each aerosol type estimated from surface elemental concentrations and meteorological observations. Results are very consistent with direct Sun photometer measurements. This allows us to derive the chemical apportionment of AOD in this region: mineral dust from Africa controls the total AOD and generally dominates AOD even in the absence of dust outbreak; on average, sea salt, excess sulfate, and carbonaceous aerosols all together only contribute to an averaged background AOD of 0.04 at 670 nm.

Profiling of a Saharan dust outbreak based on a synergy between active and passive remote sensing

[1]xa0Spaceborne lidars will give a new view of the vertical distribution of atmospheric aerosols and clouds. CALIPSO will be launched in the fall of 2004 and will provide, for the first time, a global picture of the profile of atmospheric scattering layers using an onboard lidar radiated at 0.532 and 1.064 μm. CALIPSO will fly in an orbital formation with passive radiometers, such as the Moderate resolution Imaging Spectrometer (MODIS) and the Polarization and Directionality of Earths Reflectance (POLDER) instruments, that monitor Earths atmosphere. The purpose of this paper is to analyze the improvement in retrieval capabilities of profiles of aerosol optical properties using a synergy between passive and active (lidar) remote sensing techniques. Aerosol properties derived from the MODIS spectroradiometer are used to constrain the inversion of the lidar signal in terms of aerosol optical thickness and effective radius. We use the lidar spectral backscattering coefficient between 0.532 and 1.064 μm to determine the profile of backscatter-to-extinction ratio. The effective radius of an assumed bimodal aerosol size distribution is then retrieved as a function of the altitude. A sensitivity analysis demonstrates the robustness of the inversion procedure in case of noise detection and calibration error. The algorithm has been tested during the Saharan dust experiment, which took place in the northeastern tropical Atlantic in September 2000. The vertical profile of extinction compares well with in situ measurements of the aerosol extinction. Profiles derived from lidar measurements on 25 September highlight the presence of the Saharan air layer located between 2.2 and 4.5 km with particle effective radii of 1.19 ± 0.6 μm. Another dust layer within the sub-Saharan transition layer over the marine boundary layer is also observed, with particle radii significantly smaller than within the Saharan air layer.

ANALYSIS OF COLUMN-INTEGRATED AEROSOL OPTICAL THICKNESS IN BEIJING FROM AERONET OBSERVATIONS

Aerosol Optical Thickness (AOT), water vapor content and derived Angstrom exponent acquired by a CIMEL sun photometer in Beijing are analyzed. Monthly means computed from quality-assured daily means, seasonal trends and inter-annual variations are presented and discussed. Summer has the highest seasonal average AOT at 440 nm (τa440), Angstrom exponent (α440–870) and water vapor content with the values 0.93, 1.34 and 3.0 cm, respectively. The second highest seasonal average τa440 appears in spring with the largest variation of α440–870 and minimum α440–870 0.99 due to the impact of coarse particles. The minimum seasonal average τa440 (0.44) and water vapor content (0.4 cm) appear in winter. The annual average τa440, α440–870 and water vapor content for about 4-year observation period are 0.70, 1.19 and 1.4 cm, respectively. All monthly average Angstrom exponents are within 0.8–1.4, which indicates aerosol in Beijing is a very complex mixture of both fine- and coarse-mode particles (from anthropogenic influence and natural mineral dust).

An estimate of the dry deposition of trace elements onto the coastal margin of the mediterranean sea from the Fos/mer industrial area

Abstract From recently estimated atmospheric emissions data of the Fos/mer industrial complex, we attempted to estimate the impact of such a source of heavy metals on the northern coastal margin of the Mediterranean Sea. The contribution of these emissions to the particulate dry deposition onto this region is predicted with the help of a semi‐box semi‐gaussian transport and deposition model. The model predictions suggest that the dry deposition flux from this single source may be a major fraction of the total atmospheric flux over the first hundred kilometers from the coast line.

The use of a simple predictive model to quantify trace element emissions from an industrial complex (Fos/Mer, France)

Abstract Bulk and size‐separated aerosol samples were, collected at three sites within the industrial area of Fos/Mer close to the French Mediterranean shore. Using a simple gaussian atmospheric dispersion model which was developed and validated by comparing the SO2 concentrations measured in‐situ with those calculated by the model from the known SO2 emission rate, industrial emissions of trace elements (Cu, Cd, Ni, Pb, Zn, Mn, P) from this industrial area were calculated. The results show that these emissions represent between 2 and 10% of the total emission of these elements from industrial, activity in France.