Bodele Research Articles

The Role of Meteorological Variables and Aerosols in the Transmission of COVID-19 During Harmattan Season.

The role of atmospheric parameters and aerosols in the transmission of COVID‐19 within tropical Africa, especially during the harmattan season, has been under‐investigated in published papers. The harmattan season within the West African region is associated with significant dust incursion from the Bodele depression and biomass burning. In this study, the correlation between atmospheric parameters (temperature and humidity) and aerosols with COVID‐19 cases and fatalities within seven locations in tropical Nigeria during the harmattan period was investigated. COVID‐19 infection cases were found to be significantly positively correlated with atmospheric parameters (temperature and humidity) in the southern part of the country while the number of fatalities showed weaker significant correlation with particulate matters only in three locations. The significant correlation values were found to be between 0.22 and 0.48 for particulate matter and −0.19 to −0.32 for atmospheric parameters. Although, temperature and humidity showed negative correlations in some locations, the impact is smaller compared to particulate matter. In December, COVID‐19 cases in all locations showed strong correlation with particulate matter except in Kano State. It is suggested that a reduction in atmospheric particulate matter can be used as a control measure for the spread of COVID‐19.

Assessment of Aeolian Activity in the Bodélé Depression, Chad: A Dense Spatiotemporal Time Series From Landsat-8 and Sentinel-2 Data

There are several hotspots of dust production in the central Sahara, the Bodélé Depression (BD) in northern Chad is considered the largest source of aerosol dust worldwide, with the fastest Barchan dunes that migrate southwesterly. Less is known about the complex patterns of dune movement in the BD, especially on a short time scale. Time-series inversion of optical image cross-correlation (TSI-OICC) proved to be a valuable method for monitoring historical movements with low uncertainties, high spatial coverage, and dense temporal coverage. We leveraged ∼8 years of Landsat-8 and ∼6 years of Sentinel-2 data to capture the dune migration patterns at BD. We used TSI-OICC, creating four independent networks of offset maps from Landsat-8 and Sentinel-2 images, and forming three networks by fusing data from the two sensors. We depended on the multi spatial coherence estimated from Sentinel-1 interferograms to automatically discriminate between the active and stagnant regions, which is important for the postprocessing steps. We combined the data from the two sensors in areas of overlap to assess the performance of the fusion between two sensors in increasing the temporal scale of the observations. Our results suggest that dune migration at BD is subject to seasonal and multiyear variations that differed spatially across the dune field. Seasonal variations were observed with migration slowing during the summer months. We estimated the median for velocities belonging to the same season and calculated the seasonal sliding coefficient (SSC) representing the ratio between seasonal velocities. The median SSC reached a maximum value of ∼2 for winter/summer, while the ratios were ∼1.10 and ∼1.35 for winter/spring and winter/autumn, respectively. The seasonal variability of the temporal patterns was strongly supported by the wind observations. Between (1984–1998 and 1998–2007) and (1998–2007 and 2013–2021), decelerations in dune velocities were observed with percentages of ∼4 and ∼28%, respectively, and these decelerations were supported by a deceleration in wind velocities. Inversion of time series provides dense spatiotemporal monitoring of the dune activity. The fusion between two sensors allows condensing the temporal sampling up to a weekly scale especially for locations exposed to contamination of high cloud cover or dust.

Inferring iron oxides species content in atmospheric mineral dust from DSCOVR EPIC observations

Abstract. The iron-oxide content of dust in the atmosphere and most notably its apportionment between hematite (α-Fe2O3) and goethite (α-FeOOH) are key determinants in quantifying dust's light absorption, its top of atmosphere UV radiances used for dust monitoring, and ultimately shortwave dust direct radiative effects (DRE). Hematite and goethite column mass concentrations and iron-oxide mass fractions of total dust mass concentration were retrieved from the Deep Space Climate Observatory (DSCOVR) Earth Polychromatic Imaging Camera (EPIC) measurements in the ultraviolet–visible (UV–Vis) channels. The retrievals were performed for dust-identified aerosol plumes using aerosol optical depth (AOD) and spectral imaginary refractive index provided by the Multi-Angle Implementation of Atmospheric Correction (MAIAC) algorithm over six continental regions (North America, North Africa, West Asia, Central Asia, East Asia, and Australia). The dust particles are represented as an internal mixture of non-absorbing host and absorbing hematite and goethite. We use the Maxwell–Garnett effective medium approximation with carefully selected complex refractive indices of hematite and goethite that produce mass fractions of iron oxides species consistent with in situ values found in the literature to derive the hematite and goethite volumetric/mass concentrations from MAIAC EPIC products. We compared the retrieved hematite and goethite concentrations with in situ dust aerosol mineralogical content measurements, as well as with published data. Our data display variations within the published range of hematite, goethite, and iron-oxide mass fractions for pure mineral dust cases. A specific analysis is presented for 15 sites over the main dust source regions. Sites in the central Sahara, Sahel, and Middle East exhibit a greater temporal variability of iron oxides relative to other sites. Niger site (13.52° N, 2.63° E) is dominated by goethite over Harmattan season with median of ~2 weight percentage (wt.%) of iron-oxide. Saudi Arabia site (27.49° N, 41.98° E) over Middle East also exhibited surge of goethite content with the beginning of Shamal season. The Sahel dust is richer in iron-oxide than Saharan and northern China dust except in Summer. The Bodele Depression area shows a distinctively lower iron-oxide concentration (~1 wt. %) throughout the year. Finally, we show that EPIC data allow to constrain the hematite refractive index. Specifically, we select 5 out of 13 different number of hematite refractive indices widely variable in published laboratory studies by constraining the iron-oxide mass ratio to the known measured values. Provided climatology of hematite and goethite mass fractions across main dust regions of the Earth will be useful for dust shortwave DRE studies and climate modeling.

Natural atmospheric deposition of molybdenum: a global model and implications for tropical forests

Molybdenum (Mo) is an essential trace metal that plays a central role in biological nitrogen fixation (BNF) as the cofactor in the conventional form of the nitrogenase enzyme. The low availability of Mo in soils often constrains BNF in many terrestrial ecosystems. Atmospheric sources may supply a critical source of exogenous Mo to regions with highly weathered soils likely low in Mo, particularly in tropical forests where BNF is thought to be high. Here, we present results of a global model of Mo deposition that considers the principal natural sources of atmospheric Mo—windborne mineral dust, sea-salt aerosols, and volcanic sources—which operate over geologic time. The largest source of mineral dust globally is from North Africa. We quantified Mo concentrations in dust and sediments from the Bodele Depression, a large source within North Africa, to constrain our model. Because the Mo concentration of seawater is relatively high for a trace element, we also hypothesized that sea-salt aerosols would contribute atmospheric Mo. Our model predicts higher Mo deposition to terrestrial ecosystems along coasts downstream in trade winds, near active volcanoes, and in areas that receive dust deposition from North Africa, such as the northern Amazon Basin, the Caribbean, and Central America. Regions with higher Mo deposition tend to be areas where BNF has previously been measured. The lowest Mo deposition occurs in the high latitudes, northern parts of North America, Western Australia, Southern Africa, and much of central South America. Atmospheric transport of Mo likely plays an important role in supplying Mo to ecosystems across geologic time, particularly in regions with highly weathered soils.

Dust Source, Vertical Profile and Climate Impact by RegCM3 Regional Climate Model over West Africa during 2006

This study aims to evaluate dust impact on climate parameters over the Sahel region by RegCM3 regional model during 2006. Indeed, aerosols are one of the main uncertainties in climate models. The aerosol optical depth (AOD) derived from RegCM3 model has been validated with various observed datasets. The aerosol sources are identified over North Algeria and East of Sahel (Bodele depression). Discrepancies are noted when considering dust temporal and spatial distribution. Dust season extends between March and October, with two peaks of AOD recorded in March (spring) and June (summer). The dust vertical distribution showed that the mineral aerosol layer is located between 850 hPa and 300 hPa (1.5 km to 7 km). The RegCM3 model simulates fairly well the transport in the upper layers, especially in the Saharan Air Layer (SAL) during the summer. However, RegCM3 simulates poorly the transport and sedimentation of particles in the lower layers (below 2 km). The investigation of dust radiative impact shows a general cooling. The maximum of radiative forcing is located around 18°N - 20°N, with values of about -80 W/m2 in June - August (JJA) and -40 W/m2 at the surface during March - May (MAM). This study also showed the indirect effect of dust with a decrease in precipitation about -0.7 mm/day around 15 - 20°N during the rainy season.

Numerical Dust Storm Simulation Using Modified Geographical Domain and Data Assimilation: 3DVAR and 4DVAR (WRF-Chem/WRFDA)

Dust storms have severe environmental, economic, health, weather, and climate change impacts. A severe dust storm that hit Egypt on 22 January 2004 was selected as a case study to establish an accurate numerical model to simulate dust storms over Egypt using the Weather Research Forecast with a chemistry module (WRF-Chem). Two simulation setups using WRF-Chem were conducted using two geographic domains: the first exclusively included dust sources within Egypt, while the second included an external dust source, the Bodele Depression in southwestern Chad. The first simulation was only able to capture the core of the dust plume from the internal dust source Egyptian Qattara Depression, but the second was able to capture the spatial dust distribution from both the Qattara Depression and the Bodele Depression. Moreover, the results from our second simulation model had less errors than comparable results using moderate resolution imaging spectroradiometer (MODIS). We then investigated the impact of meteorological data assimilation methods using both three-dimensional and four-dimensional variational assimilation algorithms to simulate the aerosol optical depth of the dust storm using weather research forecast data assimilation (WRFDA) framework.

WRF 1960-2014 Winter Season Simulations of Particulate Matter in the Sahel: Implications for Air Quality and Respiratory Health.

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.

North African mineral dust across the tropical Atlantic Ocean: Insights from dust particle size, radiogenic Sr-Nd-Hf isotopes and rare earth elements (REE)

Abstract Large amounts of mineral dust are exported from North Africa across the Atlantic Ocean, impacting the atmosphere and ocean during transport and after deposition through biogeochemical processes. In order to characterize the isotopic signature of dust from different seasons and years, in relation to their bulk particle size, and to obtain a general idea of its provenance, Saharan dust was collected using subsurface sediment traps moored in the tropical North Atlantic Ocean in 2012–2013, and by shipboard aerosol collection during three trans-Atlantic research cruises in 2005, 2012 and 2015. The samples were analysed for radiogenic Strontium (Sr), Neodymium (Nd), and Hafnium (Hf) isotopes, rare earth element (REE; La-Lu) abundances and particle size. In addition, soil sediments from Mauritania, a potential source area, were analysed and compared to the Atlantic dust samples. The results indicate no relation between Sr and Nd isotopic compositions and dust particle size. In contrast, Hf isotopic compositions show a strong relation with particle size, associated to the so-called zircon effect. We explored alternative sources of lithogenic particles to the sediment traps such as Amazon River sediments. Our results reveal that the sediment-trap samples bear distinctly different geochemical signatures from sediments from the Amazon Basin and Amazon River tributaries, and confirm that the primary source of lithogenic particles is northern Africa. The collected dust samples show close relations to African dust aerosols collected at Barbados and samples from the Bodele Depression, although differences between seasons are observed, which we relate to differences in source areas.

The role the Saharan Heat Low plays in dust emission and transport during summertime in North Africa

The Saharan Heat Low (SHL) is an important element in the meteorological system in North Africa in summer. However, it is unclear how the SHL affects North African dust emission and transport. Here we investigate the impact of the SHL on dust emission and transport on synoptic time scales in North Africa during the summer over the period 2003–2015 using satellite retrievals of dust optical depth (DOD) and vertical profiles. During the cool phases of the SHL, dust is preferentially emitted from the Bodele depression. Then, as the SHL warms, the dust emitted from the Bodele depression is advected westward due to the intensified African Easterly Jet, resulting in anomalously high DOD values over Mali and Mauritania, where persistent lower troposphere convergence within the SHL sustains the high concentrations of dust. Thus, the persistently high dust DOD from satellite retrievals over Mali and Mauritania in conditions of warm SHL are a combination of dust advection from the east and local emission.

Iron oxide minerals in dust-source sediments from the Bodélé Depression, Chad: Implications for radiative properties and Fe bioavailability of dust plumes from the Sahara

Abstract Atmospheric mineral dust can influence climate and biogeochemical cycles. An important component of mineral dust is ferric oxide minerals (hematite and goethite) which have been shown to influence strongly the optical properties of dust plumes and thus affect the radiative forcing of global dust. Here we report on the iron mineralogy of dust-source samples from the Bodele Depression (Chad, north-central Africa), which is estimated to be Earth’s most prolific dust producer and may be a key contributor to the global radiative budget of the atmosphere as well as to long-range nutrient transport to the Amazon Basin. By using a combination of magnetic property measurements, Mossbauer spectroscopy, reflectance spectroscopy, chemical analysis, and scanning electron microscopy, we document the abundance and relative amounts of goethite, hematite, and magnetite in dust-source samples from the Bodele Depression. The partition between hematite and goethite is important to know to improve models for the radiative effects of ferric oxide minerals in mineral dust aerosols. The combination of methods shows (1) the dominance of goethite over hematite in the source sediments, (2) the abundance and occurrences of their nanosize components, and (3) the ubiquity of magnetite, albeit in small amounts. Dominant goethite and subordinate hematite together compose about 2% of yellow-reddish dust-source sediments from the Bodele Depression and contribute strongly to diminution of reflectance in bulk samples. These observations imply that dust plumes from the Bodele Depression that are derived from goethite-dominated sediments strongly absorb solar radiation. The presence of ubiquitous magnetite (0.002–0.57 wt%) is also noteworthy for its potentially higher solubility relative to ferric oxide and for its small sizes, including PM

Derivation of an observation-based map of North African dust emission

Changes in the emission, transport and deposition of aeolian dust have profound effects on regional climate, so that characterizing the lifecycle of dust in observations and improving the representation of dust in global climate models is necessary. A fundamental aspect of characterizing the dust cycle is quantifying surface dust fluxes, yet no spatially explicit estimates of this flux exist for the World’s major source regions. Here we present a novel technique for creating a map of the annual mean emitted dust flux for North Africa based on retrievals of dust storm frequency from the Meteosat Second Generation Spinning Enhanced Visible and InfraRed Imager (SEVIRI) and the relationship between dust storm frequency and emitted mass flux derived from the output of five models that simulate dust. Our results suggest that 64 (±16)% of all dust emitted from North Africa is from the Bodele depression, and that 13 (±3)% of the North African dust flux is from a depression lying in the lee of the Air and Hoggar Mountains, making this area the second most important region of emission within North Africa.

Variability in Sources and Concentrations of Saharan Dust Phosphorus over the Atlantic Ocean

Saharan dust that is transported over the Atlantic Ocean provides an important input of phosphorus (P) to the oligotrophic waters of ocean and the P-depleted rain forests of America. To establish more firmly the role of Saharan dust events as P suppliers, the dust P sources needs to be identified. From analysis of phosphate oxygen isotopes of all the major Saharan dust events of 2011 over the Cape Verde islands in the northeastern Atlantic, supported by remote-sensing imagery, we infer that the dust P originates from widespread sedimentary sources and magmatic P “hot spots”, in which the latter enrich the dust in bioavailable P. The fraction contributed from each source varied markedly between dust events. We also found that phosphate from the Bodele depression is not evident in Cape Verde. Our results provide new information for global biogeochemical studies and suggest that identification of Saharan dust P sources is required to improve their accuracy.

Estimating the sensitivity of regional dust sources to sea surface temperature patterns

AbstractExploring the impact of sea surface temperature (SST) anomaly patterns on local climate in major dust source regions helps clarify our understanding of variability in the global dust cycle. In contrast to previous work, this research focuses explicitly on the influence of SST anomalies on dust emissions and attempts to explain the mechanisms by which SST anomalies affect seasonal dust emissions. This study investigates the seasonal sensitivity of mineral aerosol emissions to SST anomaly patterns from the Bodele Depression, West Africa, Sahel, Kalahari Desert, Arabian Desert, and Lake Eyre basin. The global teleconnection operator, which relates regional climate responses to SST anomaly patterns, is estimated for relevant variables in an ensemble of the National Center for Atmospheric Research Community Atmosphere Model version 5 forced by randomly perturbed climatological SST fields. Variability in dust emissions from major dust sources is linked to tropical SST anomalies, particularly in the Indian and western Pacific Oceans. Teleconnections excited by remote SST anomalies typically impact dust emissions via changes in near‐surface wind speeds and friction velocity. However, SST‐driven impacts on the threshold friction velocity can be of the same order of magnitude as changes in the friction velocity, suggesting the impact of SST anomalies on precipitation and soil moisture is also significant. Identifying SST anomaly patterns as a component of internal variability in regional dust emissions helps characterize human influences on the dust cycle as well as improve predictions of climate, nutrient cycles, and human environments.

Discriminating dusts and dusts sources using magnetic properties and hematite:Goethite ratios of surface materials and dust from North Africa, the Atlantic and Barbados

Magnetic measurements and Diffuse Reflectance Spectroscopy are used in an attempt to differentiate dusts and dust sources in North Africa, over the Atlantic and in Barbados. Special attention is paid to dusts and to lacustrine clay and diatomite samples from the Bodele Depression, in view of its alleged importance in trans-Atlantic and global dust generation. The results indicate that dusts from the Bodele Depression can be distinguished from other dusts and potential sources in Niger, Chad, Burkina and Mali on the basis of their magnetic properties, notably their low magnetic concentrations, negligible frequency dependent magnetic susceptibility and distinctive IRM demagnetization characteristics. Dust from over the Atlantic and from Barbados, obtained from meshes in the 1960s and ’70s have high frequency dependent susceptibility values, are quite distinctive from the Bodele Depression samples and are more closely comparable to samples from elsewhere in the Sahara and especially the Sahel. The Diffuse Reflectance Spectroscopy data, though of limited value here, are not inconsistent with the inferences based on the magnetic measurements. Overall, the results obtained point to a wide range of sources for dusts both over North Africa itself and across the Atlantic. They do not offer support to the view that dusts from the Bodele Depression have dominated supply across the Atlantic over the last five decades.

Loess and Bee-Eaters II: The ‘loess’ of North Africa and the nesting behaviour of the Northern Carmine Bee-Eater (Merops nubicus Gmelin 1788)

The Northern Carmine Bee-Eater (Merops nubicus) lives and breeds in a well demarcated region stretching across Africa close to the 15°N line of latitude. The Bee-Eater zone appears to be associated with a band of loess, defined by Scheidig on his 1934 map as second-order loess. Bee-eaters are known to favour loess for nesting tunnels and it appears that the 15°N material is sufficiently loess-like. Obvious sources for particulate materials for the 15°N band are the Fonta-Djalon highlands which supply sedimentary material to the River Niger; the Bodele Depression, the deepest part of Lake Megachad, source of dust for the World; the Ethiopian highlands at the eastern end of 15°N which supply silt to the Nile system and particulates to the 15°N region. In soil moisture terms the region is ustic, which is possibly a necessary condition for bee-eater nests. The clastic material requires an ustic environment. The River Niger can be seen as a loess river; in some senses a mirror-image of a major loess river like the Danube; but where a restricted range of particle inputs leads to a restricted range of loess deposit outputs. Nevertheless loess river considerations can be applied. The Niger delivers second-order loess and an important loessic admixture to the landscape. Enough loess for selective nesters like the Carmine Bee-Eaters to build their nest tunnels in it. It seems likely that climate change will cause a change in bee-eater distribution; it seems unlikely that they will abandon their nesting regions, the living and wintering zones may shift.

A three-year time series of mineral dust deposits on the West African margin: Sedimentological and geochemical signatures and implications for interpretation of marine paleo-dust records

Mineral dust deposits in the Northeastern Tropical Atlantic Ocean (NETAO) are an important contribution for reconstructing paleoenvironments and paleoclimates of West Africa. However, the interpretation of the changes in the sedimentological and geochemical characteristics of the dust deposits recorded in the marine sediments from the NETAO remains incomplete. In order to improve our understanding of dust proxies, in particular its mineralogical and geochemical provenance tracers, present-day dust deposition has been monitored at Mbour (∼80km south of Dakar) on the Senegalese margin. Here we report a multi-proxy investigation of a unique three-year continuous time series of mineral dust deposits collected at a weekly (or better) temporal resolution over the March 2006–March 2009 period. Mass deposition flux and mean modal grain size display marked but reverse seasonal features, with higher flux during the winter/spring seasons and coarser grain size mode in summer when flux is minimal, reflecting contrasting transport patterns throughout the year. Similarly, clay mineralogy, the illite/kaolinite ratio in particular, shows seasonal fluctuations, manifesting the latitudinal displacement of the contributing domains of provenance in response to the seasonal migration of the ITCZ position and the associated wind systems. Our three-year record also reveals the occurrence of major deposition events superimposed on the seasonal pattern, generally during the winter/spring dry seasons and most frequently during the month of March. Our study shows that these major events, which contribute a large fraction of the total annual deposition flux, all originate from the western Sahara–Sahel (a major area of emission in the region beside the Bodele Depression, stretching from the Mauritanian and Western Saharan coasts to the Hoggar Mountains). Combined with air mass tri-dimensional back-trajectories and satellite images, the mineralogical and Sr–Nd isotopic compositions of these large dust events enable us to identify several mineralogically and geochemically distinct provenance sectors within this vast western area. Unlike the background dust deposits, the 87Sr/86Sr and the εNd isotopic signatures of the major dry events closely match that of the NETAO Late Holocene sediments, supporting the hypothesis that these events account for most of the aeolian terrigenous supply reaching the ocean floor. Although this database needs to be expanded, our results already provide useful constraints for the interpretation of the dust proxies' variations in marine sedimentary archives off Mauritania/Senegal in terms of changes in wind regimes and aridity over West Africa.

Intra-Seasonal Variability of Aerosols and Their Radiative Impacts on Sahel Climate during the Period 2000-2010 Using AERONET Data

This paper presents a study using in situ measurements (AERONET data) to characterize the intra-seasonal variability of aerosols and their shortwave radiative impacts on Sahel (11N - 18N and 20W - 15E) climate using a long time series (2000 to 2010) from AERONET data. We first used the MISR aerosol optical depth (AOD) averaged between 2000 and 2010 to identify dust maxima regions. They are mainly located over Eastern Mauritania-Northern Mali and Eastern Niger-Western Tchad (near the Bodele depression). Moreover the aerosol loading is stronger during the spring (MAM) and particularly during the summer (JJA). The analysis of AERONET data at 4 Sahelian stations (Dakar, Agoufou, Banizoumbou and Ouagadougou) shows that Sahel is under dust influence all year long. The dust season is mainly located between March and October, with two peaks of AOD recorded in March and June; while the maximum of biomass burning activity is recorded in winter (December to February). The radiative forcing of aerosols on the Sahelian climate results mainly in a cooling as well as at the surface than at the top of the atmosphere (TOA) but the forcing is stronger at the surface and particularly during the summer (JJA) and spring (MAM) periods. Although there is not a sharp difference between DJF and SON periods from a station to another one when considering the aerosol loading (AOD), the radiative forcing is stronger at the surface and weaker at the TOA during DJF for most stations. The analysis of the force effectiveness (ratio between AOD and the radiative forcing) suggests that the BOA radiative forcing is mainly influenced by the nature of aerosols.

Dust and deflation from the Bodélé depression in Chad

Dust and deflation from the Bodélé depression in Chad

Swept from Africa to the Amazon

The article discusses what the journey of a handful of dust reveals about the fragile nature of planet Earth. The author notes that scientists have neglected the interrelatedness of natural dust and the atmosphere despite their devotion to the study of air pollution. He suggests that an increasing number of scientists are beginning to appreciate the influence of dust on climate and cloud formation, including its impact on ocean fertilization and rain forests. Topics include the challenges of studying the effect of dust on the atmosphere, such as the complexity of the relationship, and research attempting to follow the migration of a handful of dust originating from the Bodele depression in Africa's Sahara desert to the Amazon in South America. INSET: The Rise and Fall of Dust.

Solitary wave behavior in sand dunes observed from space

[1] Although the dynamics of individual barchan dunes are well understood, their interactions are the subject of ongoing scientific interest and debate. Numerical and analog model predictions of shape-preserving binary dune collisions have been hard to test due to the long timescales over which such processes typically occur. This paper documents ten binary dune collisions in a 45-year time sequence of satellite images from the Bodele Depression in Chad. The observations confirm that when two barchan dunes collide, a transfer of mass occurs so that one dune appears to travel through the other unscathed, like a solitary wave.