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Environmental Pollution 158 (2010) 3385e3391

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Environmental Pollution

journal homepage: www.elsevier .com/locate/envpol

Implications of high altitude desert dust transport from WesternSahara to Nile Delta during biomass burning season

Anup K. Prasad a,b,*, Hesham El-Askary a,b,c,d, Menas Kafatos a,b

a School of Earth and Environmental Sciences, Schmid College of Science, Chapman University, Orange, CA 92866, USAbCenter of Excellence in Earth Observing, Chapman University, Orange, CA 92866, USAcDepartment of Environmental Sciences, Faculty of Science, Alexandria University, Moharem Bek, Alexandria 21522, EgyptdNational Authority for Remote Sensing and Space Science (NARSS), Cairo, Egypt

New evidence of desert dust transport from Western Sahara to Nile Dfor regional aerosols, dust models, and potential impact on the region

elta during black cloud season and its significanceal climate.

a r t i c l e i n f o

Article history:Received 29 April 2010Received in revised form20 July 2010Accepted 24 July 2010

Keywords:Dust transportSahara desertDust modelBlack cloudBiomass burning

* Corresponding author.E-mail address: aprasad@chapman.edu (A.K. Prasa

0269-7491/$ e see front matter � 2010 Elsevier Ltd.doi:10.1016/j.envpol.2010.07.035

a b s t r a c t

The air over major cities and rural regions of the Nile Delta is highly polluted during autumnwhich is thebiomass burning season, locally known as black cloud. Previous studies have attributed the increasedpollution levels during the black cloud season to the biomass or open burning of agriculturalwaste, vehicular, industrial emissions, and secondary aerosols. However, new multi-sensor observations(column and vertical profiles) from satellites, dust transport models and associated meteorology presenta different picture of the autumn pollution. Here we show, for the first time, the evidence of long rangetransport of dust at high altitude (2.5e6 km) from Western Sahara and its deposition over the Nile Deltaregion unlike current Models. The desert dust is found to be a major contributor to the local air qualitywhich was previously considered to be due to pollution from biomass burning enhanced by thedominant northerly winds coming from Europe.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Understanding the origin (natural versus anthropogenic) andmovement of aerosols in various parts of the Earth will providemore knowledge on the impacts of aerosols at regional levelsand therefore regional climate and its ultimate connections to theEarth’s global climate system (Ginoux et al., 2010). The publishedliterature to date over Nile Delta attributes the increased pollutionlevels during the black cloud season to the emissions from biomassor open burning of agricultural waste, vehicles, industries, andformation of secondary aerosols (Zakey et al., 2004; El-Metwallyet al., 2008; Abu-Allaban et al., 2002, 2007; Favez et al., 2008;Mahmoud et al., 2008; Alfaro and Wahab, 2006). Abu-Allabanet al. (2007) noted that the major contributors to PM10 over Cairoincludes geological material, mobile source emissions, and openburning while the PM2.5 tended to be dominated by mobile sourceemissions, open burning, and secondary species. Such episodesof pollution over densely populated cities such as Cairo severely

d).

All rights reserved.

impact public health (Hossny et al., 2001). Clearly identifying thecorrect sources of aerosols is important not only for public healthreasons but also for regional climate change.

The seasonal characteristics and source of aerosols over the NileDelta have been discussed by several workers using ground sun-photometer data such as AErosol RObotic NETwork (AERONET) andMICROTOPS (Sabbah et al., 2001; El-Askary and Kafatos, 2008;El-Askary et al., 2009; El-Metwally et al., 2008). Based on theAERONET (Holben et al., 1998), the aerosols over Cairo can beclassified into three groups: background pollution, dust-like, andpollution-like (El-Metwally et al., 2008). The background pollutionrefers to aerosols produced by local urban activities. The dust-likerefers to the desert dust from dust storms, which are the dominantsource of aerosols during the spring (Zakey et al., 2004;El-Metwally et al., 2008). On the other hand, the biomass burningaerosols are regarded as the dominant source of pollution duringepisodes of high aerosol loading in autumn, classified as pollution-like (El-Metwally et al., 2008; Favez et al., 2008). Based on theseasonal characteristics of aerosols and black cloud episodes usingMICROTOPS data, the air quality over Alexandria was classifiedas dust, pollution, mixed and clear during spring, fall, summer, andwinter seasons, respectively (El-Askary et al., 2009). An unusual

A.K. Prasad et al. / Environmental Pollution 158 (2010) 3385e33913386

dust event over the Mediterranean Sea during the first week ofSeptember, 2000 has been studied using Aerosol Index (AI) andbrightness temperature data (El-Askary et al., 2003).

The model products for dust generation and transport (namelythe Navy Aerosol Analysis and Prediction System (NAAPS) GlobalAerosolModel, and theDust REgional AtmosphericModel (DREAM))forecasts dust outbreaks over Sahara (Nickovic et al., 2001; Kalloset al., 1998, 2007). The dust module and its parameterizationsin the SKIRON/Eta modeling system was described in Nickovicet al. (2001). Saharan dust outbreak over Athens, Greece is fairlywell captured by Barcelona Supercomputing Center (BSC)e DREAMsimulations during the summer season (Amiridis et al., 2009).The CALIPSO data show presence of dust in the free troposphere(4e5 km) during this event (Amiridis et al., 2009). However, thepublished literature based on ground data ormodels over the Sahararegion, do not report long range transport of desert dust and itsimpact during the biomass burning season (Kallos et al.,1998, 2007).In contrast, the dust transport over the Mediterranean region isknown to decrease strongly during autumn and winter (Moulinet al., 1998). However, the cross-Atlantic transport during October2007 is visible inMODIS images (Source: NASA/Jeff Schmaltz,MODISRapid Response Team, NASA-Goddard Space Flight Center, http://www.boston.com/bigpicture/2009/01/earth_observed.html). Trans-port of air masses for all seasons over the Euro-Mediterranean-Sahara region does not show the west-to-east transport of dustor anthropogenic pollutants (Kallos et al., 2007). The dust model(DREAM) has been found to capture Saharan dust events that arewell corroborated by satellite and ground observations (Perez et al.,2006). Systematic lidar observations of Saharan dust over Europe(southern and south-eastern Europe) and Greece (Athens) showpresence of high altitude dust with variable thickness 300e7500and 680e4800 m, respectively. These European Aerosol ResearchLidar Network (EARLINET) observations over Europe show verticalcharacteristics of Saharan dust during late spring, summer, and earlyautumn periods (Papayannis et al., 2008, 2009).

Though the daily optical properties of aerosols from AERONETare capable of providing important information for the nature ofaerosols (desert dust and biomass burning aerosols) during theblack cloud season, the mixing of desert dust with local pollutants isnot known during autumn. The absence of an AERONET stationin the Nile Delta providing continuous coverage warrants analysisof aerosol data available from remote sensing. Our results providea new outlook on the cause of atmospheric pollution duringenhanced black carbon season (Mahmoud et al., 2008; El-Askaryet al., 2009) and would have global implications on thecomplexity of natural and anthropogenic aerosol loadings (Ginoux

Table 1The characteristics and source of data used in this study

Data Name Product Level

MODIS Terra -AOD MOD04 2MODIS Aqua AOD (ocean) MYD08_D3 3MODIS Aqua-Deep

Blue AOD (land)MYD08_D3 3

CALIPSO Vertical profile 2OMI AURA -UV AI OMAEROe 3DREAM Model Dust Optical Depth forecast

Dust Loading forecastNAAPS Model Total Optical Depth forecast

Surface Concentration forecastNAAPS-AERONET Time series Optical Depth (Total, coarse, fine),

Dust, Sulphate, Smoke, Total1.5

HYSPLIT Model Trajectories (forward and backward) e

NCEP/NCAR Reanalysis 1 Temperature, wind (u, v, and omega) e

et al., 2010). Similarly, in another part of the world, the source ofhigh aerosol optical depth (AOD) and black carbon during thewinterseason was primarily attributed to biomass burning over the Indo-Gangetic plains (Venkataraman et al., 2005). It was recently foundthat emissions from thermal power plants are instead the majorsource of anthropogenic pollution in that region of India (Prasadet al., 2006). These two cases illustrate that understanding suchloadings is important to provide a better knowledge of pollution aswell as regional climate and ultimately the global climate itself.

2. Data used

The MODIS Terra and Aqua provide aerosols related parametersfor the entire globe since 2000 and 2002, respectively. The MODISTerra derived aerosol optical depth (AOD) (level-2, collection 5) at10 km spatial resolution was acquired from NASA-developed EarthObserving System (EOS) Clearinghouse (ECHO). The MODISAqua AOD and deep blue AOD (level 3, collection 5) at 1� spatialresolution were also obtained from NASA EOS ECHO. The expectederrors in MODIS derived AODs are �(0.05 þ 0.15 � AOD) and�(0.03 þ 0.05 � AOD) over land and ocean respectively(Remer et al., 2005, 2008). The vertical structure of the atmosphere(clouds and aerosols layers) is provided by CloudeAerosol Lidar andInfrared Pathfinder Satellite Observation (CALIPSO) since 2004(Winker et al., 2006). We have obtained the CALIPSO data fromNASA Langley Research Center (LARC). The ultra-Violet (UV)Aerosol Index (AI) data at 0.25� spatial resolution was obtainedfrom NASA DISC. We have used OMI AURA product OMAEROe(version 3) to extract UV AI over Africa region (Levelt et al., 2006).The DREAM model derived dust AOD (550 nm) and dust loading(g/m2) have been obtained from BSC-DREAM. The NAAPS (NRL/Monterey Aerosol Modeling) model derived total optical depth,and surface concentration of dust (mg/m3) has been obtained fromNRL Monterey website. The HYSPLIT model derived backward andforward trajectories were produced using the software providedby NOAA Air Resources Laboratory (ARL). The 6 hourly meteoro-logical data used in this study have been obtained fromNCEP/NCARReanalysis 1. The source and characteristics of data used in thisstudy is summarized in Table 1.

3. Satellite and current Model observations

Three consecutive day aerosol composite loadings from theModerate Resolution Imaging Spectroradiometer (MODIS) Terrashow very high concentration over the Mediterranean Sea espe-cially over the Nile Delta region on October 2 and 3, 2008. The high

Spatialresolution

Temporalresolution

Web link

10 km 1 day http://www.echo.nasa.gov/1� 1 day http://www.echo.nasa.gov/1� 1 day http://www.echo.nasa.gov/

e e http://www-calipso.larc.nasa.gov/0.25� 1 day http://disc.sci.gsfc.nasa.gove 6 hourly http://www.bsc.es/projects/earthscience/DREAMe 6 hourly http://www.bsc.es/projects/earthscience/DREAMe 6 hourly http://www.nrlmry.navy.mil/aerosole 6 hourly http://www.nrlmry.navy.mil/aerosole daily http://www.nrlmry.navy.mil/aerosol

e hourly http://ready.arl.noaa.gov/HYSPLIT.php2.5� 6 hourly http://www.cdc.noaa.gov/data/gridded

/data.ncep.reanalysis.htm

A.K. Prasad et al. / Environmental Pollution 158 (2010) 3385e3391 3387

AOD zone was observed over the Libya coast on October 1, west ofthe Nile Delta on October 2, and over the Nile Delta and rivervalley on October 3, 2008, indicating an eastward movement of theaerosols (Fig. 1). The observed aerosol layer extends between2.5‑6 km height above the surface as shown in the Lidar profilefrom CALIPSO (Fig. 1). The large increase in AOD (>0.9), witha corresponding drop in the Angstrom Exponent (0.8e1 to 0.4e0.6)suggests that the particles were relatively coarser on October 2e3,compared to September 28. Nearby areas such as Cyprus andTurkey also exhibit similar patterns of change in the AODand Angstrom Exponent (September 28 and October 3). The resultsimply dust-like characteristics of these aerosols compared toaerosols produced by biomass burning (Prasad and Singh, 2007).Terra MODIS has algorithm limitations, which makes it hard toobtain information over land due to high surface reflectance(Remer et al., 2005; Ichoku et al., 2002; Hsu et al., 2004). However,combination of aerosol loading information available from othersensors and algorithms such as Deep Blue AOD from MODIS Aqua(over land) and AOD from MODIS Aqua (over ocean) clearly showscontinuity of high AOD from the arid regions of Mauritania, Algeria,extending into Libya, Egypt and the Mediterranean Sea (Fig. 2a, b)(Hsu et al., 2004). The three day composite of UV e AI from OzoneMonitoring Instrument (OMI) AURA (Levelt et al., 2006) corrobo-rate the findings (Fig. 2b). Similar high aerosol events over the NileDelta have been found for other years (2002e2007) during Sep-tembereOctober. Overlay of wind field at 600 mb (e4.4 km from thesurface) shows the presence of favorable wind pattern for trans-portation of soil dust from the presumed source (western Sahara)to the sink (Egypt, north-eastern Sahara). Satellite derived AODfrom other sources (Polarization and Anisotropy of Reflectances forAtmospheric Sciences coupled with Observations from a Lidar(PARASOL) AOD, daily Multiangle Imaging SpectroRadiometer(MISR) AOD) also shows similar aerosol distribution.

However, such transport pathways deduced from aerosol or dustdistribution during this season are not visible in dust transportmodels (DREAM) over northern Africa and surrounding region(Fig. 2c, d). DREAM only shows high AOD over the NigereChad area

Fig. 1. The high aerosol loading over Nile Delta and surrounding region as seen in the three1e3, 2008. The vertical profile of feature mask obtained from CALIPSO on October 2, 2008

while it miss the other major source (MauritianaeAlgeriaeMali)(Fig. 2c). The Bodele depression, northeast of Lake Chad (17 �N, 18�E), is known to contributemost of the dust (transport over Atlantic)to the Amazon forest (Koren et al., 2006). Gerasopoulos et al. (2009)found that the BSC-DREAM model, during identified dust events,under-predicts AOD by 10e15% and 30e40% for Angstrom expo-nents in the range of 0e1 and 1‑2, respectively. Mahler et al. (2006)found a positive correlation (r ¼ 0.59) between measured (DeepblueMODIS Aqua) and predicted (DREAM) AOD during a dust eventover western Texas. However, in the present case, a comparison ofMODIS and DREAM AOD (at 550 nm) over Nile Delta and Nile Fanregion show AOD to be >0.8 and <0.15, respectively (Fig. 2a, b).Further, the satellite and DREAM model estimates of AOD showstrong disagreement over Western Sahara (marked as brokenellipse, Fig. 2a,b). The NAAPS show high dust AOD (>0.8) duringSeptember 29e30 over Algeria (Tamanrasset station) and October1e2 over the Mediterranean Sea (Forth station, Crete). Such path-ways which are apparent in high frequency (daily) data, are also notvisible inmonthlymean fromMODIS Terra andAquaAOD, andMISRAOD due to various limitations of temporalespatial coverage andlimitations of the algorithm itself (Remer et al., 2005; Ichoku et al.,2002; Hsu et al., 2004). Thus, the detection of dust transport duringthe biomass burning season presently remains undetected inmodel runs and monthly composites from satellite observations.Our present analysis from multi-satellite data coupled to meteo-rological data, reveals the origin of high aerosol loading duringbiomass burning season over the Nile Delta region.

4. Transport Model (HYSPLIT) observations

The transport pathways and altitude from source to the sinkregion are discussed here. The HYSPLIT (Draxler and Hess, 1998)transport model, back-trajectory for 3 days, shows that the air massoriginated from NNE direction at 100 and 500 m from the surface.The autumn season (OctobereNovember) is generally known to belargely dominated by the northernwinds (<65%) (Favez et al., 2008).However, at high altitudes (especially 2.5e6 km or 700e500 mb

day composite of MODIS Terra level-2 AOD (at 10 km spatial resolution) during Octobershows aerosols at 2.5e6 km height.

Fig. 2. (a) The distribution of aerosols on October 2, 2008 over the study region using a combination of MODIS Aqua-Deep Blue AOD and AOD over land and ocean, respectively. Thevectors represent wind field at 600 mb. (b) The 3 day maximum composite of OMI AURA derived Aerosol Index showing the source of aerosols around MauritaniaeAlgeriaeMaliregion. DREAM model derived (c) dust AOD (550 nm) and wind field at 3000 m, and (d) dust loading (g/m2) on October 2, 2008 at 12z. NAAPS model derived (e) total optical depth,and (f) surface concentration of dust (mg/m3) on October 2, 2008 at 12z. (Source: DREAM e http://www.bsc.es/projects/earthscience/DREAM/, and NAAPS e http://www.nrlmry.navy.mil/aerosol/).

A.K. Prasad et al. / Environmental Pollution 158 (2010) 3385e33913388

pressure level) the air mass is originating from Western Sahara(Fig. 3a). A 3 day forward trajectory from the source (WesternSahara) shows that the air mass at high altitude (2.5e6 km) wasmoving towards the Nile Delta region from September 30 to October2, 2008 (Fig. 3b). The transport model shows that a west-to-eastpathway from the source to the sink was evident at 2.5e6 kmheightfrom the surface (Fig. 3a, b). This matches aerosol distributionobserved by the Deep Blue AOD from MODIS Aqua (Fig. 2). It isnotable that another 3 day forward trajectory at a region 10 �S (or1100 km) from the source shows completely different trajectories forair mass at 1e6 km (Fig. 3c). This can explain the presence of other

aerosol plumes overWestern Sahara coast reaching into the AtlanticOcean. In other words, the HYSPLIT model shows that there are twodifferent pathways for transport of dust aerosols from WesternSahara: one extending westwards into the Atlantic Ocean and theother eastwards towards NE Africa (Egypt). The eastward arm of theair mass trajectory can potentially affect the Nile Delta, provided thedust can be lifted high-up in the air. The Lidar profile obtainedfrom CALIPSO on October 2 corroborates the height of aerosollayer (2.5e6 km), in agreement with HYSPLIT model. The model andobservations confirm the vertical structure and high altitude trans-port height of the aerosols. Furthermore, the wind vectors at the

Fig. 3. (a) The HYSPLIT model derived (a) 3 day back-trajectory from Sink region (Cairo, Nile Delta), (b) 3 day forward trajectory from Source (MauritaniaeAlgeriaeMali) region, and(c) 3 day forward trajectory from southern Mali showing transport of dust towards the Altantic ocean. The HYSPLIT trajectories are shown at 100, 500 m (near surface) to up to midtroposphere (1, 2, 3, 4, 5, 6 km above surface).

Fig. 4. The vertical profile of meteorological parameters (w-wind or omega, u-wind, v-wind and temperature) over Nile Delta using 6 hourly National Centers for EnvironmentalPrediction (NCEP) reanalysis data. The meteorological parameters show conspicuous changes during the dust transport event.

A.K. Prasad et al. / Environmental Pollution 158 (2010) 3385e3391 3389

Fig. 5. (a) The regional map of omega or vertical velocity (Pa/s) shows presence of strong subsidence conditions (marked as red zone) over Nile Delta on October 2, 2008 at 6z.(b) The map showing west-to-east (source to sink, A to B) transect along which cross section of omega have been extracted. (c) The source to sink cross section of omega fromSeptember 30 to October 2 (at 0 and 12z) shows uplift conditions near source (MauritaniaeAlgeria) and subsidence conditions over sink (Egypt) region. (For interpretation of thereferences to colour in this figure legend, the reader is referred to the web version of this article).

A.K. Prasad et al. / Environmental Pollution 158 (2010) 3385e33913390

600 mb height are shown to be westerly, matching with otherobservations (HYSPLIT, CALIPSO, and MODIS Aqua) (Fig. 2).

5. Meteorological observations

The meteorological profiles of vertical component of the wind(w-wind or omega or vertical velocity) show strong downwardmovement of air mass at pressure level 500e700 mb (2.5e6 kmheight) on October 2 and 3 (Fig. 4a). This suggests that the dusttransported from Western Sahara at 2.5e6 km height was depos-ited over the Delta region as conditions were favorable. The hori-zontal components of wind (U and V wind) also show contrastingfeatures for this period (Fig. 4b, c). The temperature profile showsstrong change (up to 8.4 K) in air temperature near surface (up to925 mbar or 775 m height) on October 3‑4, 2008. The warming ofthe air (over 300 K) is unique compared to the entire month(Fig. 4d) and shows contrasting changes over the Nile Delta regionduring the dust deposition period.

The regional map of omega at 600 mb on October 2 showspositive values (subsidence conditions) over the Nile Delta (Fig. 5a).A west-to-east (source to sink, Fig. 5b, c) vertical profile of omegashows evolution of the favorable meteorological conditions for thisdust transport. On October 29‑30 (at 0,12z), strong uplift conditionsare observed over the source (Mauritania, Mali, and Algeria) fromground up to mid troposphere (Fig. 5c). The sink region shows thepresence of strong downward movement of air mass, especiallyover Egypt, leading to deposition of transported dust. The favorablewind vectors (westerly winds) at mid troposphere facilitate trans-port of the dust uplifted from source to the sink region (Fig. 2a). TheDeep Blue AOD from MODIS Aqua corroborates that the aerosolloading is very high over the source during this period, which inturn can be transported to the sink region far away, if the meteo-rological and topographic factors are favorable. The presence of highmountains NW and SE of the source (Fig. 5b) forms a topographicchannel for the transport of dust along this west-to-east axis(Mauritania-to-Egypt).

A.K. Prasad et al. / Environmental Pollution 158 (2010) 3385e3391 3391

6. Conclusions

In thiswork,we have discovered for thefirst time a dust transportpathway along theMauritaniaeMalieAlgeriaeLibyaeEgypt axis thatsignificantly affects NE Africa, especially the Nile Delta region, duringthe biomass burning season. The increase in aerosol loading(A0D >0.9) along with corresponding decrease in the AngstromExponent, a typical feature of desert dust, point towards the presenceof desert dust over the Nile Delta region. The high aerosol concen-tration episodes cannot be solely attributed to biomass burning orlocal pollution. Our results show that the long range transport of dustat high altitudes is a major contributing factor to the black cloudpollution during the biomass burning season. Our new findings maycreate a new outlook to investigate the chemical and physical natureof air pollution by scientists and informeddecisions by policymakers.The complexityof aerosol transport anddifferent sources of origin is amost challenging issue, not just for pollution control in denselypopulated areas, but also for effects on the overall climate system. Thecurrent models such as DREAM that forecasts dust aerosols requirerevision in estimates during the autumn season. The satellite datasuch as MODIS, CALIPSO provide useful complementary informationto validate and constrain forecast from dust models. Our resultsindicate that hastily assigning origin to aerosols (such as black cloudwhich implies anthropogenic pollution), may mask the morecomplex origin of aerosol loadings.

Acknowledgements

This work is supported by NSF grant 0922772 and the Scienceand Technology for Development Fund (STDF, http://www.stdf.org.eg/) through the USeEgypt joint agreement. We also would like toacknowledge the support from Science Systems and Applications,Inc (SSAI) (No. E201152-5).

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