Stephane C. Alfaro

Modeling mineral aerosol production by wind erosion: Emission intensities and aerosol size distributions in source areas

A dust production model (DPM) is obtained by combining preexisting models of saltation and sandblasting, the two processes that lead to mineral aerosol release in arid areas. From a description of the soil characteristics and wind conditions, the DPM allows computation of the amounts of aerosol released and of their size distributions. Semiquantitative comparisons of the model outputs with the few field data available in the literature validate its main implications. The first one is that the aptitude of a soil to release particles smaller than 20 μm depends on (1) the dry size distribution of aggregates constituting its loose fraction, (2) its roughness length, and (3) the wind velocity. The second implication is that the size distribution of aerosols released in source areas also strongly depends on these parameters.

Modeling the size distribution of a soil aerosol produced by sandblasting

In order to develop a model providing the mass size distribution of the dust raised from the ground by the sandblasting process, mechanical characteristics of 240 μm saltating sand grains meant to be used as projectiles in wind tunnel sandblasting experiments were carefully determined. It was found that for values of the measured friction velocity less than about 55 cm/s, the constraint of the relatively small dimensions of the wind tunnel test section did not prevent saltation from developing freely. The kinetic energy of the sand grains was also determined. Aerosols were then produced in the wind tunnel by bombarding a clay target with the saltating 240 μm quartz grains. The size distributions of these aerosols were measured for three wind speeds with an optical particle analyzer. For the lowest wind speed the size distribution of the aerosol was similar to that of the 8.6 μm aggregates originally constituting the agglomerates of clay, but disaggregation into smaller particles became more important when wind speed increased. A theory of sandblasting was then developed that gave theoretical results agreeing with the experimental ones. A consequence of this theory was that submicron particles could be released from aggregates for high wind speeds. Experiments meant to check this implication were carried out and confirmed it. Cohesion energies of kaolin particles of three different sizes, 8.6, 2.8, and approximately 0.5 μm, were calculated and found to be a decreasing exponential function of the particle size. This explains (1) why the soil-derived aerosol size distributions present a common mode in the 1–10 μm size range, these particles being readily released even in not particularly energetic conditions, and (2) why observation of a submicron mode in natural aerosols requires a higher friction velocity than for the common 1–10 μm mode.

Mineral aerosol production by wind erosion: Aerosol particle sizes and binding energies

Size distributions of mineral aerosols released by wind-erosion from arid areas are of primary importance to model their transport patterns as well as their effect on climate. Wind tunnel experiments meant to test the influence of wind strength and soil characteristics are carried out with two natural soils differing both in texture and in mineral composition. In all cases, the aerosols can be considered as mixtures, in proportions depending on wind speed, of the same three lognormally distributed particle populations. By using appropriate binding energies for these populations, a sandblasting model previously developed allows retrieval of the size distributions of the aerosols produced by the two soils at any wind speed. A major implication of this study is that the size distributions of mineral aerosols greatly depend on the wind conditions prevailing during their generation. Aerosols optical properties being size dependent, this should clearly be taken into account in climate modeling.

Soil crusting on sandy soils and its influence on wind erosion

Abstract Under rainfall, a crust forms at the surface of most soils. For soils rich enough in clay (clay content >5%), it is well known that a physical crust is responsible for a supply limitation of particles available for wind erosion. Sandy soils are very prone to crusting as well as to wind erosion. Indeed, structural ‘sieving’ crusts develop on sandy soils even after light rainfalls. This kind of crust shows a loose sand layer at the surface overlying a thin layer where fine particles are concentrated. The main objectives of this study were to determine whether such a crust formation limits the availability of (1) sand grains for entrainment into saltation and (2) fine particles for uptake into a vertical wind erosion flux (suspension in atmosphere of particles

Large-scale advection of continental aerosols during INDOEX

In this paper, we present passive and active remote sensing measurements of atmospheric aerosols over the North Indian Ocean (NIO) during the Intensive Field Phase (IFP, January to March 1999) of the Indian Ocean Experiment. The variability of the aerosol load over NIO is discussed based on three-dimentional numerical simulations made at a local scale by use of Regional Atmospheric Modeling System (RAMS) and at a regional scale using the zoomed Laboratoire de Meteorologie Dynamique global circulation model (LMD-Z version 3.3). Ground-based measurements of the columnar aerosol optical thickness (AOT) and of surface black carbon (BC) concentration were carried out at two different sites in India: Goa University on the NIO coast and Dharwar 150 km inland. Local-scale investigations point out that the trend in BC concentration at the ground is not correlated with AOT. Lidar profiles obtained both from the surface at Goa and in the NIO from the Mystere-20 research aircraft indicate that a significant contribution to the total AOT (more than 50%) is due to a turbid monsoon layer located between 1 and 3 km height. RAMS simulation shows that the advection of aerosols in the monsoon layer is due to the conjunction of land-sea breeze and topography. We present the regional-scale extent of the aerosol plume in terms of AOT derived from the visible channel of Meteosat-5. During March, most of the Bay of Bengal is overcast by a haze with a monthly average AOT of 0.61±0.18, and a spatially well-defined aerosol plume is spreading from the Indian west coast to the Intertropical Convergence Zone with an average AOT of 0.49±0.08. Those values are bigger than in February with AOT at 0.35±0.18 and 0.37±0.09 for the Bay of Bengal and the Arabian Sea, respectively. One of the principal findings of this paper is that a significant contribution to the aerosol load over the NIO is due to the advection of continental aerosols from India in a well-identified monsoon layer above the marine boundary layer. Moreover, it is suggested that the increase in biomass burning plays a significant role in the increasing trend in AOT during the winter dry monsoon season.

AMMA dust experiment: An overview of measurements performed during the dry season special observation period (SOP0) at the Banizoumbou (Niger) supersite

In the frame of the African Monsoon Multidisciplinary Analysis-Special Observation Period 0 (AMMA SOP0) field campaign, a sophisticated instrumental setup specially designed for documenting the characteristics of aerosols in the surface boundary layer has been operated for 1 month (13 January to 13 February 2006) at the Banizoumbou supersite in Niger. The analysis of the long-term routine sunphotometer measurements shows that the intensive measurements were performed during a transition phase between a period when biomass burning aerosols are present in the atmospheric column and another when mineral dust is clearly predominant. On the basis of ground-level observations, three types of events can be defined. (1) Type B events are those corresponding to advection of Biomass burning plumes from southern Sahel. They represent only 3% of the measurement period. (2) Type D events are those during which mineral dust concentrations are larger than 200 mu g m(-3). They represent 35% of the measuring time, and coincide with advection of mineral dust from distant sources to the measurement site. The composition of the dust differs significantly from one episode to the other, depending on the trajectory of air mass that transported it. (3) Type L events are those that can be explained by occurrence of local erosion and that are of short duration. During them, the aerosol size distribution is dominated by a coarser mode (9 mu m) than during the events of the D type dominated by a finer mode (4.5 mu m). Elemental analysis also shows that mineral dust is relatively rich in Fe and poor in Ca, which is typical of the Sahelian aerosols.

Validation of a dust production model from measurements performed in semi-arid agricultural areas of Spain and Niger

Abstract Models of two processes (saltation and sandblasting) that lead to fine dust release in arid areas have recently been combined to form a dust production model (DPM), the physical bases of which are summarized. In order to validate DPM at field scale, its predictions in terms of horizontal and vertical mass fluxes are compared to direct measurements made in natural conditions on a silt loam soil in north–east Spain during Wind Erosion and Loss of SOil Nutrients in semi-arid Spain (WELSONS) experiment and a sandy soil in Niger during the PROgramme Soil and Erosion (PROSE) experiment. In the Spanish case, due to the formation of a coalescing crust after rainfall, a limited supply of loose particles on the soil surface restricts the availability of soil aggregates for saltation. Once this supply limitation is taken into account in the saltation submodel, the vertical fluxes, predicted by DPM with the binding energies previously determined in a wind tunnel experiment, agree well with the measured ones. In the Niger case, the agreement is found to be directly satisfactory for unlimited saltation, even in crusted conditions. However, in order to retrieve measured vertical fluxes, the aggregate binding energies that constitute key factors involved in the sandblasting computation must be divided by 3. These results validate the sandblasting part of DPM but show that saltation has to be adequately tuned when a coalescing crust forms on loamy soils. These results also show that the potential of soil for fine-dust production does not increase with its clay content which rather tends to slightly reduce sandblasting efficiencies. A high clay content favours formation of a coalescing crust that efficiently traps formerly loose soil aggregates. The supply limitation resulting from this crust formation controls the importance of saltation, and hence, of fine-dust production. Finally, saltation clearly appears as the limiting factor for fine-dust production.

Validation of the Surface Downwelling Solar Irradiance Estimates of the HelioClim-3 Database in Egypt

HelioClim-3 (HC3) is a database providing time series of the surface downwelling solar irradiance that are computed from images of the Meteosat satellites. This paper presents the validation results of the hourly global horizontal irradiance (GHI) and direct normal irradiance (DNI), i.e., beam irradiance at normal incidence, of versions four and five of HC3 at seven Egyptian sites. The validation is performed for all-sky conditions, as well as cloud-free conditions. Both versions of HC3 provide similar performances whatever the conditions. Another comparison is made with the estimates provided by the McClear database that is restricted to cloud-free conditions. All databases capture well the temporal variability of the GHI in all conditions, McClear being superior for cloud-free cases. In cloud-free conditions for the GHI, the relative root mean square error (RMSE) are fairly similar, ranging from 6% to 15%; both HC3 databases exhibit a smaller bias than McClear. McClear offers an overall better performance for the cloud-free DNI estimates. For all-sky conditions, the relative RMSE for GHI ranges from 10% to 22%, except one station, while, for the DNI, the results are not so good for the two stations with DNI measurements.

Extreme Variability of Aerosol Optical Properties: The Cairo Aerosol Characterization Experiment Case Study

Because they scatter and absorb solar and terrestrial radiation, aerosol plumes can easily be detected on satellite images. Thus, the time-dependent spatial extension of the aerosol clouds can be derived from space-borne observations. However, using remote observations for estimating particle concentrations, let alone for apportioning aerosol loads between all the potential sources of particulate matter, is less straightforward. Indeed, this apportionment would require perfect knowledge of the scattering and absorbing potential of particles of different origins as well as the spectral dependence of these potentials. Contrary to what can be done with most atmospheric gases, it is usually impossible to reproduce in the laboratory the complexity of natural atmospheric aerosols. As a consequence, measuring their wavelength-dependent optical properties can only be done during specially designed experiments performed in natural conditions. This is the case of the Cairo Aerosol CHaracterization Experiment (CACHE) that was performed in the Egyptian capital from the end of October 2004 to mid April 2005. During this period a wide variety of aerosol conditions have been sampled, but this work is focused on the spring intensive observation period during which several occurrences of mineral dust transport to Cairo were observed. We detail the modifications of optical properties resulting from these inputs of mineral particles into the background ‘urban aerosol’ and show that scattering and absorption, as well as their spectral dependence are extremely sensitive to the proportions of the “urban pollution/mineral dust” mixtures that form over Cairo during the dust events. Unfortunately, this precludes the use of predefined aerosol models supposed to represent particularly simple aerosol types (e.g., urban pollution, mineral dust,...) for inverting satellite observations over areas where aerosol mixing is known to be the rule rather than the exception (e.g., over the eastern Mediterranean in spring, over or downwind of continental China during the dust season, over west Africa during the biomass burning period, ...). In these cases, sophisticated parameterizations of the optical properties must be used for assessing the impact of aerosol mixtures on radiative transfer.

Tackling the mortality from long-term exposure to outdoor air pollution in megacities: Lessons from the Greater Cairo case study

Objective The poor outdoor air quality in megacities of the developing world and its impact on health is a matter of concern for both the local populations and the decision‐makers. The objective of this work is to quantify the mortality attributable to long‐term exposure to PM2.5, NO2, and O3 in Greater Cairo (Egypt). Methods We analyze the temporal and spatial variability of the three pollutants concentrations measured at 18 stations of the area. Then, we apply the method recommended by the WHO to estimate the excess mortality. In this assessment, three different shapes (log‐linear, linear, and log‐log) of the concentration‐response functions (CRF) are used. Results With PM2.5 concentrations varying from 50 to more than 100 &mgr;g/m3 in the different sectors of the megacity, the spatial variability of this pollutant is found to be one important cause of uncertainty on the excess mortality associated with it. Also important is the choice of the CRF. With the average (75 &mgr;g/m3) PM2.5 concentration and the most favorable log‐log shape of the CRF, 11% (CI, 9–14%) of the non‐accidental mortality in the population older than 30 years can still be attributed to PM2.5, which corresponds to 12520 (CI, 10240–15930) yearly premature deaths. Should the Egyptian legal 70 &mgr;g/m3 PM10 limit (corresponding to approximately 37.5 &mgr;g/m3 for PM2.5) be met, this number would be reduced to 7970, meaning that 4550 premature deaths could be avoided each year. Except around some industrial or traffic hot spots, NO2 concentration is found to be below the 40 &mgr;g/m3 air quality guideline of the WHO. However, the average concentration (34 &mgr;g/m3) of this gas exceeds the stricter 10 &mgr;g/m3 recommendation of the HRAPIE project and it is thus estimated that from 7850 to 10470 yearly deaths can be attributed to NO2. Finally, with the ozone concentration measured at one station only, it is found that, depending on the choice of the CRF, between 2.4% and 8.8% of the mortality due to respiratory diseases can be attributed to this gas. Conclusion In Greater Cairo, PM2.5 and NO2 constitute major health risks. The best estimate is that in the population older than 30 years, 11% and 8% of the non‐accidental mortality can be attributed to these two pollutants, respectively. HighlightsMortality due to long‐term exposure to air pollution in Greater Cairo is estimated.Air‐suspended matter (PM2.5) is the most severe problem.At least from 10240 to 15930 people die each year of exposure to PM2.5.From 7850 to 10470 die of exposure to NO2.