Environmental Pollution 158 (2010) 3385e3391
Contents lists available at ScienceDirect
Environmental Pollution
journal homepage: www.elsevier.com/locate/envpol
Implications of high altitude desert dust transport from Western
Sahara 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, USA
Center of Excellence in Earth Observing, Chapman University, Orange, CA 92866, USA
c
Department of Environmental Sciences, Faculty of Science, Alexandria University, Moharem Bek, Alexandria 21522, Egypt
d
National Authority for Remote Sensing and Space Science (NARSS), Cairo, Egypt
b
New evidence of desert dust transport from Western Sahara to Nile Delta during black cloud season and its significance
for regional aerosols, dust models, and potential impact on the regional climate.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 29 April 2010
Received in revised form
20 July 2010
Accepted 24 July 2010
The air over major cities and rural regions of the Nile Delta is highly polluted during autumn which is the
biomass burning season, locally known as black cloud. Previous studies have attributed the increased
pollution levels during the black cloud season to the biomass or open burning of agricultural
waste, vehicular, industrial emissions, and secondary aerosols. However, new multi-sensor observations
(column and vertical profiles) from satellites, dust transport models and associated meteorology present
a different picture of the autumn pollution. Here we show, for the first time, the evidence of long range
transport of dust at high altitude (2.5e6 km) from Western Sahara and its deposition over the Nile Delta
region unlike current Models. The desert dust is found to be a major contributor to the local air quality
which was previously considered to be due to pollution from biomass burning enhanced by the
dominant northerly winds coming from Europe.
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Dust transport
Sahara desert
Dust model
Black cloud
Biomass burning
1. Introduction
Understanding the origin (natural versus anthropogenic) and
movement of aerosols in various parts of the Earth will provide
more knowledge on the impacts of aerosols at regional levels
and therefore regional climate and its ultimate connections to the
Earth’s global climate system (Ginoux et al., 2010). The published
literature to date over Nile Delta attributes the increased pollution
levels during the black cloud season to the emissions from biomass
or open burning of agricultural waste, vehicles, industries, and
formation of secondary aerosols (Zakey et al., 2004; El-Metwally
et al., 2008; Abu-Allaban et al., 2002, 2007; Favez et al., 2008;
Mahmoud et al., 2008; Alfaro and Wahab, 2006). Abu-Allaban
et al. (2007) noted that the major contributors to PM10 over Cairo
includes geological material, mobile source emissions, and open
burning while the PM2.5 tended to be dominated by mobile source
emissions, open burning, and secondary species. Such episodes
of pollution over densely populated cities such as Cairo severely
* Corresponding author.
E-mail address: [email protected] (A.K. Prasad).
0269-7491/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.envpol.2010.07.035
impact public health (Hossny et al., 2001). Clearly identifying the
correct sources of aerosols is important not only for public health
reasons but also for regional climate change.
The seasonal characteristics and source of aerosols over the Nile
Delta have been discussed by several workers using ground sunphotometer data such as AErosol RObotic NETwork (AERONET) and
MICROTOPS (Sabbah et al., 2001; El-Askary and Kafatos, 2008;
El-Askary et al., 2009; El-Metwally et al., 2008). Based on the
AERONET (Holben et al., 1998), the aerosols over Cairo can be
classified into three groups: background pollution, dust-like, and
pollution-like (El-Metwally et al., 2008). The background pollution
refers to aerosols produced by local urban activities. The dust-like
refers to the desert dust from dust storms, which are the dominant
source of aerosols during the spring (Zakey et al., 2004;
El-Metwally et al., 2008). On the other hand, the biomass burning
aerosols are regarded as the dominant source of pollution during
episodes of high aerosol loading in autumn, classified as pollutionlike (El-Metwally et al., 2008; Favez et al., 2008). Based on the
seasonal characteristics of aerosols and black cloud episodes using
MICROTOPS data, the air quality over Alexandria was classified
as dust, pollution, mixed and clear during spring, fall, summer, and
winter seasons, respectively (El-Askary et al., 2009). An unusual
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A.K. Prasad et al. / Environmental Pollution 158 (2010) 3385e3391
dust event over the Mediterranean Sea during the first week of
September, 2000 has been studied using Aerosol Index (AI) and
brightness temperature data (El-Askary et al., 2003).
The model products for dust generation and transport (namely
the Navy Aerosol Analysis and Prediction System (NAAPS) Global
Aerosol Model, and the Dust REgional Atmospheric Model (DREAM))
forecasts dust outbreaks over Sahara (Nickovic et al., 2001; Kallos
et al., 1998, 2007). The dust module and its parameterizations
in the SKIRON/Eta modeling system was described in Nickovic
et al. (2001). Saharan dust outbreak over Athens, Greece is fairly
well captured by Barcelona Supercomputing Center (BSC) e DREAM
simulations 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, the
published literature based on ground data or models over the Sahara
region, do not report long range transport of desert dust and its
impact during the biomass burning season (Kallos et al., 1998, 2007).
In contrast, the dust transport over the Mediterranean region is
known to decrease strongly during autumn and winter (Moulin
et al., 1998). However, the cross-Atlantic transport during October
2007 is visible in MODIS images (Source: NASA/Jeff Schmaltz, MODIS
Rapid Response Team, NASA-Goddard Space Flight Center, http://
www.boston.com/bigpicture/2009/01/earth_observed.html). Transport of air masses for all seasons over the Euro-MediterraneanSahara region does not show the west-to-east transport of dust
or anthropogenic pollutants (Kallos et al., 2007). The dust model
(DREAM) has been found to capture Saharan dust events that are
well 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) show
presence of high altitude dust with variable thickness 300e7500
and 680e4800 m, respectively. These European Aerosol Research
Lidar Network (EARLINET) observations over Europe show vertical
characteristics of Saharan dust during late spring, summer, and early
autumn periods (Papayannis et al., 2008, 2009).
Though the daily optical properties of aerosols from AERONET
are capable of providing important information for the nature of
aerosols (desert dust and biomass burning aerosols) during the
black cloud season, the mixing of desert dust with local pollutants is
not known during autumn. The absence of an AERONET station
in the Nile Delta providing continuous coverage warrants analysis
of aerosol data available from remote sensing. Our results provide
a new outlook on the cause of atmospheric pollution during
enhanced black carbon season (Mahmoud et al., 2008; El-Askary
et al., 2009) and would have global implications on the
complexity of natural and anthropogenic aerosol loadings (Ginoux
et al., 2010). Similarly, in another part of the world, the source of
high aerosol optical depth (AOD) and black carbon during the winter
season was primarily attributed to biomass burning over the IndoGangetic plains (Venkataraman et al., 2005). It was recently found
that emissions from thermal power plants are instead the major
source of anthropogenic pollution in that region of India (Prasad
et al., 2006). These two cases illustrate that understanding such
loadings is important to provide a better knowledge of pollution as
well as regional climate and ultimately the global climate itself.
2. Data used
The MODIS Terra and Aqua provide aerosols related parameters
for the entire globe since 2000 and 2002, respectively. The MODIS
Terra derived aerosol optical depth (AOD) (level-2, collection 5) at
10 km spatial resolution was acquired from NASA-developed Earth
Observing System (EOS) Clearinghouse (ECHO). The MODIS
Aqua AOD and deep blue AOD (level 3, collection 5) at 1 spatial
resolution were also obtained from NASA EOS ECHO. The expected
errors 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 and
Infrared Pathfinder Satellite Observation (CALIPSO) since 2004
(Winker et al., 2006). We have obtained the CALIPSO data from
NASA Langley Research Center (LARC). The ultra-Violet (UV)
Aerosol Index (AI) data at 0.25 spatial resolution was obtained
from 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 from
NRL Monterey website. The HYSPLIT model derived backward and
forward trajectories were produced using the software provided
by NOAA Air Resources Laboratory (ARL). The 6 hourly meteorological data used in this study have been obtained from NCEP/NCAR
Reanalysis 1. The source and characteristics of data used in this
study is summarized in Table 1.
3. Satellite and current Model observations
Three consecutive day aerosol composite loadings from the
Moderate Resolution Imaging Spectroradiometer (MODIS) Terra
show very high concentration over the Mediterranean Sea especially over the Nile Delta region on October 2 and 3, 2008. The high
Table 1
The characteristics and source of data used in this study
Data Name
Product
Level
Spatial
resolution
Temporal
resolution
Web link
MODIS Terra -AOD
MODIS Aqua AOD (ocean)
MODIS Aqua-Deep
Blue AOD (land)
CALIPSO
OMI AURA -UV AI
DREAM Model
MOD04
MYD08_D3
MYD08_D3
2
3
3
10 km
1
1
1 day
1 day
1 day
http://www.echo.nasa.gov/
http://www.echo.nasa.gov/
http://www.echo.nasa.gov/
Vertical profile
OMAEROe
Dust Optical Depth
Dust Loading
Total Optical Depth
Surface Concentration
Optical Depth (Total, coarse, fine),
Dust, Sulphate, Smoke, Total
Trajectories (forward and backward)
Temperature, wind (u, v, and omega)
2
3
forecast
forecast
forecast
forecast
1.5
e
0.25
e
e
e
e
e
e
1 day
6 hourly
6 hourly
6 hourly
6 hourly
daily
http://www-calipso.larc.nasa.gov/
http://disc.sci.gsfc.nasa.gov
http://www.bsc.es/projects/earthscience/DREAM
http://www.bsc.es/projects/earthscience/DREAM
http://www.nrlmry.navy.mil/aerosol
http://www.nrlmry.navy.mil/aerosol
http://www.nrlmry.navy.mil/aerosol
e
e
e
2.5
hourly
6 hourly
http://ready.arl.noaa.gov/HYSPLIT.php
http://www.cdc.noaa.gov/data/gridded
/data.ncep.reanalysis.htm
NAAPS Model
NAAPS-AERONET Time series
HYSPLIT Model
NCEP/NCAR Reanalysis 1
A.K. Prasad et al. / Environmental Pollution 158 (2010) 3385e3391
AOD zone was observed over the Libya coast on October 1, west of
the Nile Delta on October 2, and over the Nile Delta and river
valley on October 3, 2008, indicating an eastward movement of the
aerosols (Fig. 1). The observed aerosol layer extends between
2.5‑6 km height above the surface as shown in the Lidar profile
from CALIPSO (Fig. 1). The large increase in AOD (>0.9), with
a 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 and
Turkey also exhibit similar patterns of change in the AOD
and Angstrom Exponent (September 28 and October 3). The results
imply dust-like characteristics of these aerosols compared to
aerosols produced by biomass burning (Prasad and Singh, 2007).
Terra MODIS has algorithm limitations, which makes it hard to
obtain 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 other
sensors and algorithms such as Deep Blue AOD from MODIS Aqua
(over land) and AOD from MODIS Aqua (over ocean) clearly shows
continuity 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 Ozone
Monitoring Instrument (OMI) AURA (Levelt et al., 2006) corroborate the findings (Fig. 2b). Similar high aerosol events over the Nile
Delta have been found for other years (2002e2007) during SeptembereOctober. Overlay of wind field at 600 mb (e4.4 km from the
surface) shows the presence of favorable wind pattern for transportation of soil dust from the presumed source (western Sahara)
to the sink (Egypt, north-eastern Sahara). Satellite derived AOD
from other sources (Polarization and Anisotropy of Reflectances for
Atmospheric 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 dust
distribution during this season are not visible in dust transport
models (DREAM) over northern Africa and surrounding region
(Fig. 2c, d). DREAM only shows high AOD over the NigereChad area
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while it miss the other major source (MauritianaeAlgeriaeMali)
(Fig. 2c). The Bodele depression, northeast of Lake Chad (17 N, 18
E), is known to contribute most 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 exponents in the range of 0e1 and 1‑2, respectively. Mahler et al. (2006)
found a positive correlation (r ¼ 0.59) between measured (Deep
blue MODIS Aqua) and predicted (DREAM) AOD during a dust event
over western Texas. However, in the present case, a comparison of
MODIS and DREAM AOD (at 550 nm) over Nile Delta and Nile Fan
region show AOD to be >0.8 and <0.15, respectively (Fig. 2a, b).
Further, the satellite and DREAM model estimates of AOD show
strong disagreement over Western Sahara (marked as broken
ellipse, Fig. 2a,b). The NAAPS show high dust AOD (>0.8) during
September 29e30 over Algeria (Tamanrasset station) and October
1e2 over the Mediterranean Sea (Forth station, Crete). Such pathways which are apparent in high frequency (daily) data, are also not
visible in monthly mean from MODIS Terra and Aqua AOD, and MISR
AOD due to various limitations of temporalespatial coverage and
limitations of the algorithm itself (Remer et al., 2005; Ichoku et al.,
2002; Hsu et al., 2004). Thus, the detection of dust transport during
the biomass burning season presently remains undetected in
model runs and monthly composites from satellite observations.
Our present analysis from multi-satellite data coupled to meteorological data, reveals the origin of high aerosol loading during
biomass burning season over the Nile Delta region.
4. Transport Model (HYSPLIT) observations
The transport pathways and altitude from source to the sink
region are discussed here. The HYSPLIT (Draxler and Hess, 1998)
transport model, back-trajectory for 3 days, shows that the air mass
originated from NNE direction at 100 and 500 m from the surface.
The autumn season (OctobereNovember) is generally known to be
largely dominated by the northern winds (<65%) (Favez et al., 2008).
However, at high altitudes (especially 2.5e6 km or 700e500 mb
Fig. 1. The high aerosol loading over Nile Delta and surrounding region as seen in the three day composite of MODIS Terra level-2 AOD (at 10 km spatial resolution) during October
1e3, 2008. The vertical profile of feature mask obtained from CALIPSO on October 2, 2008 shows aerosols at 2.5e6 km height.
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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. The
vectors represent wind field at 600 mb. (b) The 3 day maximum composite of OMI AURA derived Aerosol Index showing the source of aerosols around MauritaniaeAlgeriaeMali
region. 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/).
pressure level) the air mass is originating from Western Sahara
(Fig. 3a). A 3 day forward trajectory from the source (Western
Sahara) shows that the air mass at high altitude (2.5e6 km) was
moving towards the Nile Delta region from September 30 to October
2, 2008 (Fig. 3b). The transport model shows that a west-to-east
pathway from the source to the sink was evident at 2.5e6 km height
from the surface (Fig. 3a, b). This matches aerosol distribution
observed by the Deep Blue AOD from MODIS Aqua (Fig. 2). It is
notable that another 3 day forward trajectory at a region 10 S (or
1100 km) from the source shows completely different trajectories for
air mass at 1e6 km (Fig. 3c). This can explain the presence of other
aerosol plumes over Western Sahara coast reaching into the Atlantic
Ocean. In other words, the HYSPLIT model shows that there are two
different pathways for transport of dust aerosols from Western
Sahara: one extending westwards into the Atlantic Ocean and the
other eastwards towards NE Africa (Egypt). The eastward arm of the
air mass trajectory can potentially affect the Nile Delta, provided the
dust can be lifted high-up in the air. The Lidar profile obtained
from CALIPSO on October 2 corroborates the height of aerosol
layer (2.5e6 km), in agreement with HYSPLIT model. The model and
observations confirm the vertical structure and high altitude transport height of the aerosols. Furthermore, the wind vectors at the
A.K. Prasad et al. / Environmental Pollution 158 (2010) 3385e3391
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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 mid
troposphere (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 Environmental
Prediction (NCEP) reanalysis data. The meteorological parameters show conspicuous changes during the dust transport event.
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A.K. Prasad et al. / Environmental Pollution 158 (2010) 3385e3391
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 from
September 30 to October 2 (at 0 and 12z) shows uplift conditions near source (MauritaniaeAlgeria) and subsidence conditions over sink (Egypt) region. (For interpretation of the
references to colour in this figure legend, the reader is referred to the web version of this article).
600 mb height are shown to be westerly, matching with other
observations (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 downward
movement of air mass at pressure level 500e700 mb (2.5e6 km
height) on October 2 and 3 (Fig. 4a). This suggests that the dust
transported from Western Sahara at 2.5e6 km height was deposited over the Delta region as conditions were favorable. The horizontal components of wind (U and V wind) also show contrasting
features for this period (Fig. 4b, c). The temperature profile shows
strong change (up to 8.4 K) in air temperature near surface (up to
925 mbar or 775 m height) on October 3‑4, 2008. The warming of
the air (over 300 K) is unique compared to the entire month
(Fig. 4d) and shows contrasting changes over the Nile Delta region
during the dust deposition period.
The regional map of omega at 600 mb on October 2 shows
positive values (subsidence conditions) over the Nile Delta (Fig. 5a).
A west-to-east (source to sink, Fig. 5b, c) vertical profile of omega
shows evolution of the favorable meteorological conditions for this
dust transport. On October 29‑30 (at 0, 12z), strong uplift conditions
are observed over the source (Mauritania, Mali, and Algeria) from
ground up to mid troposphere (Fig. 5c). The sink region shows the
presence of strong downward movement of air mass, especially
over Egypt, leading to deposition of transported dust. The favorable
wind vectors (westerly winds) at mid troposphere facilitate transport of the dust uplifted from source to the sink region (Fig. 2a). The
Deep Blue AOD from MODIS Aqua corroborates that the aerosol
loading is very high over the source during this period, which in
turn can be transported to the sink region far away, if the meteorological and topographic factors are favorable. The presence of high
mountains NW and SE of the source (Fig. 5b) forms a topographic
channel for the transport of dust along this west-to-east axis
(Mauritania-to-Egypt).
A.K. Prasad et al. / Environmental Pollution 158 (2010) 3385e3391
6. Conclusions
In this work, we have discovered for the first time a dust transport
pathway along the MauritaniaeMalieAlgeriaeLibyaeEgypt axis that
significantly affects NE Africa, especially the Nile Delta region, during
the biomass burning season. The increase in aerosol loading
(A0D >0.9) along with corresponding decrease in the Angstrom
Exponent, a typical feature of desert dust, point towards the presence
of desert dust over the Nile Delta region. The high aerosol concentration episodes cannot be solely attributed to biomass burning or
local pollution. Our results show that the long range transport of dust
at high altitudes is a major contributing factor to the black cloud
pollution during the biomass burning season. Our new findings may
create a new outlook to investigate the chemical and physical nature
of air pollution by scientists and informed decisions by policy makers.
The complexity of aerosol transport and different sources of origin is a
most challenging issue, not just for pollution control in densely
populated areas, but also for effects on the overall climate system. The
current models such as DREAM that forecasts dust aerosols require
revision in estimates during the autumn season. The satellite data
such as MODIS, CALIPSO provide useful complementary information
to validate and constrain forecast from dust models. Our results
indicate that hastily assigning origin to aerosols (such as black cloud
which implies anthropogenic pollution), may mask the more
complex origin of aerosol loadings.
Acknowledgements
This work is supported by NSF grant 0922772 and the Science
and Technology for Development Fund (STDF, http://www.stdf.org.
eg/) through the USeEgypt joint agreement. We also would like to
acknowledge the support from Science Systems and Applications,
Inc (SSAI) (No. E201152-5).
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