Saharan dust particle properties over the central Mediterranean

Atmospheric Research 81 (2006) 67–93

www.elsevier.com/locate/atmos

Saharan dust particle properties over the

central Mediterranean

A.M. Tafuro a, F. Barnaba b,1, F. De Tomasi a,

M.R. Perrone a,*, G.P. Gobbi b

a Dipartimento di Fisica, Universita di Lecce, Lecce, Italyb Istituto di Scienze dell’Atmosfera e del Clima (ISAC-CNR), Roma, Italy

Received 23 May 2005; received in revised form 10 November 2005; accepted 10 November 2005

Abstract

AERONET sun photometer measurements performed at five different sites of the Central Mediterranean

during strong Saharan dust outbreaks are used to characterize optical and physical properties of dust particles.

Sun photometer retrievals are combined at two of the five sites with lidar observations. It is shown that at low

aerosol optical depths (AODs) the dust particle properties are quite dependent on dust load andmonitoring site

location. Differences on retrieved particle properties reducewith increasing dust load. AERONETretrievals at

AOD (440 nm)z0.6 are then used in this paper to characterize dust particles over the central Mediterranean

basin leading to columnar averaged values of the real refractive index bnN=1.5F0.1, the imaginary

refractive index bkN=0.004F0.002, the single scattering albedo bSSAN=0.89F0.03, and the Angstrom

exponent bAN=0.2F0.1. It is shown that A represents the best marker to trace the temporal evolution of dust

events. Volume size distributions show a dominant coarse mode peaking at 1.7–3 Am. In particular, the

average coarse mode that is centred ati2.2 Am at Lampedusa, which is ~200 km away from the northwest

Africa coast, gets peaked at i1.7 Am at Lecce, which is ~800 km away. Lidar retrievals are used to

characterize the vertical distribution of dust particles by the vertical profiles of the backscatter and extinction

coefficients, the lidar ratio, and the depolarization ratio. Lidar retrievals show that over the Mediterranean

basin, dust layers generally extend from 1 up to 6 km and that their vertical distribution can significantly

change within a few hours. It is also shown that at high values of the AODs dust particles are characterized by

lidar ratios spanning the 50–70 sr range and depolarization ratios larger than 30%.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Sun-photometer; Saharan dust; Aerosol optical and microphysical properties; Lidar

0169-8095/$ -

doi:10.1016/j.a

* Correspond

E-mail add1 Now at En

see front matter D 2005 Elsevier B.V. All rights reserved.

tmosres.2005.11.008

ing author.

ress: [email protected] (M.R. Perrone).

te Nazionale Nuove Tecnologie Energia ed Ambiente (ENEA), Frascati, Roma, Italy.

A.M. Tafuro et al. / Atmospheric Research 81 (2006) 67–9368

1. Introduction

Mineral dust lifted from arid regions of the Earth exerts a large influence over radiative

transfer processes: directly through scattering and absorbing processes and indirectly by

affecting microphysical properties and lifetime of clouds (e.g., Andreae, 2001). Model studies

suggest that the direct radiative forcing of dust on regional as well as on a global scale may be

comparable to or even exceed the forcing by anthropogenic aerosols (Tegen and Fung, 1995;

Li et al., 1998; Sokolik and Toon, 1996). Nevertheless, net radiative forcing and climate

impact of dust particles are still largely uncertain (IPCC, 2001). This indetermination arises

because of a still limited knowledge of production sources, transport patterns, particle

properties and evolution and changes during the particles’ lifetime. To infer microphysical

properties of dust particles from different sources, it is then important to monitor them at

various locations as well as to highlight changes occurring during the particles’ lifetime. On a

global scale, the dominant sources of mineral dust are all located in the Northern Hemisphere,

mainly in North Africa, the Middle East, Central Asia, and the Indian subcontinent (Prospero

et al., 2002). North-west Africa is the most important source of mineral aerosols over the

Mediterranean basin (e.g. Moulin et al., 1997; De Tomasi et al., 2003; Perrone et al., 2004).

Large-scale atmospheric circulation spreads Saharan dust particles both across the Mediter-

ranean, reaching up to Northern Europe (Ansmann et al., 2003) and across the Atlantic,

reaching the Caribbean and the Amazon basin (Husar et al., 1997). Several dedicated

campaigns employing correlated ground, airborne, and space-based observations have been set

up to reduce uncertainties in modelling and forecasting mineral aerosol effects on climate. The

European Mediterranean Dust Experiment—MEDUSE (e.g. Soderman and Dulac, 1998;

Hamonou et al., 1999), the second Aerosol Characterization Experiment—ACE 2 (e.g. Raes et

al., 2000), the Saharan Dust Experiment—SHADE (e.g. Tanre et al., 2003) are just a few

examples.

The establishment of the Aerosol Robotic Network (AERONET), which is a federated

international network funded in 1993 and coordinated by the NASA Goddard Space Flight

Center that maintains more than 200 automatic sun/sky radiometers worldwide (Holben et al.,

1998, 2001), has significantly contributed in the last years to obtain a global coverage and good

sampling of aerosol properties (http://aeronet.gsfc.nasa.gov/). The goal of AERONET is to

assess aerosol optical properties and validate satellite retrievals of aerosol optical parameters

worldwide. To this end, the network imposes standardization of instruments, calibration, and

data processing.

In this study, AERONET sun photometer measurements performed at five sites of the

central Mediterranean are used to characterize dust particles advected from north-west Africa

and to investigate the effects of the dust event intensity and monitoring site location on

retrieved properties. In particular, the main goal of the paper is to define average values of

the main optical and physical parameters that characterize dust particles over the central

Mediterranean. We believe that these average parameters can allow a better representation of

dust particle properties in global circulation and/or chemical transport models. Since model

spatial resolutions are generally larger than 300�300 km (e.g. Kinne et al., 2003), average

parameters characterizing aerosols over a broader area can be of larger interest than aerosol

data referring to a single location. To this aim, we have considered AERONET retrievals of

two strong Saharan dust outbreaks occurring in the second half of July 2003, which have

spread large tongues of dust particles all over the central Mediterranean. In particular,

AERONET sun photometer measurements performed at Lecce, Lampedusa, Oristano, Rome

http://www.aeronet.gsfc.nasa.gov/

A.M. Tafuro et al. / Atmospheric Research 81 (2006) 67–93 69

and Etna have been used to spatially and temporally investigate the aerosol optical properties

during the 15–18 and 22–25 July dust events. For both the Etna and the Lecce AERONET

sites, lidar measurements of aerosols were also available in the period addressed in this

study. Lidars are important remote sensing tools that, besides providing a clear view of the

temporal evolution of dust outbreaks, allow characterizing the vertical distribution of dust

particles by the vertical profiles of the backscatter and extinction coefficient, the lidar ratio,

and the depolarization ratio, thus adding information complementary to that provided by

sunphotometers. Satellite images have also been used to infer the geographical extent of

dust-plumes.

A short overview of the AERONET sun photometers and monitoring sites is given in Section

2. An overview of the dust events as observed in satellite images and lidar profiles is given in

Section 3. Results and discussion are presented in Sections 4 and 5, respectively.

2. Instruments and experimental sites

The Cimel sun/sky radiometers used within AERONET are automatic, robotically operated

instruments. Two detectors are used for the measurements of direct sun and sky radiance.

Spectral observations of sun radiance are generally made at seven spectral channels: 340,

380, 440, 500, 675, 870, and 1020 nm, while measurements of sky radiance are made at

440, 675, 870, and 1020 nm. Holben et al. (1998, 2001) give detailed descriptions of the

instrument and data acquisition procedures. A flexible inversion algorithm, developed by

Dubovik and King (2000) is used to retrieve columnar aerosol volume size distributions, real

and imaginary refractive indices (n and k), and SSAs from direct-sun and diffuse-sky

radiance measurements. A brief discussion on the accuracy of individual retrievals is reported

in Dubovik et al. (2002a). Cloud-screened retrievals (level 1.5) are used in this study

(Smirnov et al., 2000).

Fig. 1 shows the geographical location of the Mediterranean AERONET sites selected for

this study. Lampedusa is a small Italian island located in the middle of the Sicilian Channel,

about 130 km east of the coasts of Tunisia. The sun photometer is located in the area of the

Military Base LORAM (35.528N, 12.638E, 45 m asl). Thanks to its proximity to the African

continent, the instrument is particularly appropriate to detect the frequent dust outbreaks

from the Sahara desert. A minor impact of anthropogenic pollution is expected at this site.

The Oristano site is situated on the west coast of Sardinia, at about 400 km north from the

African coasts. In Oristano, the sunphotometer is located on the roof of the International

Marine Center (IMC, 39.918N, 8.58E, 10 m asl). The AERONET site on the Etna Volcano

is located in Nicolosi (37.618N, 15.028E, 736 m asl), a small village on the Southern flank

of Mt. Etna, and 400 km far from the African coasts. The sunphotometer is situated on the

roof of the building of the Italian Institute of Geophysics and Volcanology (INGV). The

Cimel radiometer in Rome–Tor Vergata is placed on the roof of the ISAC-CNR building

(41.848N, 12.658E, 130 m asl). The Rome–Tor Vergata site, in the southern suburbs of

Rome, is at about 15 km from the city centre, 30 km from the Tyrrhenian Sea, and 600 km

from the northern African coasts. Both anthropogenic particles (of local origin or

transported) and desert dust from the Sahara dominate the aerosol load over this site

(e.g. Gobbi et al., 2004a). The sun photometer in Lecce is located on the roof of the

Physics Department of Lecce University (40.338N, 18.108E, 27 m asl). The site is in a rural

area, 6 km away from the city centre, 16 km from the Adriatic Sea, 23 km from the Ionian

Sea, and about 800 km from the North African coasts. Thanks to its location, far from

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Fig. 1. Selected sunphotometer sites.


anthropogenic pollution, the Lecce instrument is appropriate to detect the frequent dust

outbreaks from the Sahara desert.

As mentioned, lidar measurements of aerosols are also available at the locations of Etna and

Lecce. An elastic-Raman lidar employing a XeF excimer laser is routinely used at the Physics

Department of Lecce’s University for the aerosol vertical monitoring. The XeF laser emits light

pulses of 30 ns duration at 351 nm, with a maximum energy and repetition rate of 250 mJ and 100

Hz, respectively. The XeF-based Raman lidar has been designed to derive vertical profiles of

aerosol extinction and backscatter coefficients (aaer and baer) and hence of lidar ratios (aaer /baer)

during night time operations, and to get backscatter coefficient profiles during day time

measurements. Each profile is obtained by averaging signals from about 2�105 laser shots.

Details on experimental apparatus and data analysis are reported in De Tomasi and Perrone

(2003). The vehicle-mounted lidar system (VELIS, Gobbi et al., 2000) was operating at Mount

Etna in July 2003. The VELIS system, specifically designed to be easily employed in field

campaigns, was deployed at Milo (37.738N, 15.128E, 815 m asl), approximately 11 km east of

the Mt. Etna summit and 15 km northeast of the AERONET sunphotometer. VELIS is an

elastic, polarization-sensitive lidar that employs a Nd:YAG laser generating 15 Hz, 100 mJ

pulses at 532 nm. VELIS provides tropospheric aerosol depolarization and backscatter

coefficient profiles (at 532 nm) in both daylight and night time conditions. Each profile is

obtained by averaging signals from about 6000 laser shots. As mentioned, the system is

polarization-sensitive, i.e., it emits linearly polarized light and collects the signal backscattered

on both the parallel and cross-polarized planes. Since only non-spherical particles can produce

a change in the polarization plane of the backscattered light, this set-up is particularly efficient

in revealing presence of non-spherical particles as desert dust, volcanic ash, cirrus clouds (e.g.,

Gobbi et al., 2004a).

Fig. 2. SeaWiFS true-colour images of (a) 15 (11 :57 UTC), (b) 16 (12 :39 UTC), (c) 17 (11 :40), (d) 18 (12 :21 UTC), (e)

19 (11 :23 UTC), (f) 20 (12 :04 UTC), (g) 21 (11 :07 UTC), (h) 22 (11 :48 UTC), (i) 23 (12 :29 UTC), (l) 24 (11 :31

UTC), (m) 25 (12 :11 UTC), and (n) 26 (11 :14 UTC), July 2003.



3. Dust events overview by satellite images and lidar profiles

In order to provide an overview of the July 2003 Saharan dust outbreaks to the Mediterranean

region, some indication of both the horizontal (i.e., geographical) and the vertical displacement

of the dust plumes based on satellite images and lidar data is given first. In Section 4, we shall

use the picture emerging from such satellite and lidar data as a support to the interpretation of the

photometric observations.

Fig. 2 shows the true-colour images obtained between July 15 and July 26, 2003, from the

Sea Wide Field-of-view Sensor (SeaWiFS, http://seawifs.gsfc.nasa.gov) on board the NASA

SeaStar spacecraft. One picture per day is shown for the whole Mediterranean region, as a

composite of the data collected between 11 and 13 UTC along the spacecraft polar orbit (data

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Fig. 3. Contour plots of the (a) backscatter and (b) depolarization ratio at 532 nm retrieved at Etna from 15 to 27 July,

2003.

http://www.seawifs.gsfc.nasa.gov


acquisition and processing by the Satellite Observation Group, GOS, of CNR ISAC—Rome,

http://www.gos.ifa.rm.cnr.it). The overview of the vertically-resolved evolution of the aerosol

layers (and dust plumes) over the Etna site is given in Fig. 3, in which both backscatter (Fig.

3a) and depolarization ratios (Fig. 3b) are reported. The backscatter ratio, defined as

R =(baer+bmol) /bmol, where baer (bmol) represents the atmospheric aerosol (molecules)

backscatter coefficients, provides the information on the aerosol load: in a pure molecular

atmosphere R =1, while R increases for increasing aerosol contribution. Conversely, the

depolarization ratio (D =cross-polarized /parallel-polarized signals) provides the information on

the aerosol morphology: spherical particles produce a low depolarization whilst non-spherical

particles are associated to D values larger than 10–15%. Note in Fig. 3 the typical fingerprints

of dust conditions revealed by the high D values (up to 50%) associated with aerosol layers

reaching up to the free troposphere. We believe that the rather high depolarization ratios

retrieved during the first dust event are mainly due either to the fact that Mt. Etna is rather

close to the North Africa coast and to the high intensity of the 15–18 July dust event (e.g.

Gobbi et al., 2003; Gobbi et al., 2004b). On average, lower depolarization ratios have been

retrieved during the 23–25 dust event that is also characterized by lower backscatter ratios

(Fig. 3a). It is worth mentioning that the depolarization ratios of Sahara dust retrieved by

VELIS in several locations of the Mediterranean region and in the Canary Islands, range from

10% to 50% and higher depolarization ratios have on average been retrieved at sites closer to

the dust source. Aged dust layers, as a consequence of internal/external mixings with

hygroscopic aerosols, are expected to lead to lower depolarization ratios (e.g. Gobbi et al.,

2003). It is also worth mentioning that laboratory measurements performed with different

0.0080.0040.000 0.0080.0040.000

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16/07/2003 Lecce 14:40 - 15:40 UTC Etna 15:30 - 15:40 UTC

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Backscatter Coefficient (km*sr)-1

Depolarization Ratio (%)

(a) (b)

Fig. 4. Vertical profiles of the backscatter coefficient retrieved at Lecce (grey line) and Etna (black line), and of the

depolarization ratio (dotted line) retrieved at Etna on July 16, 2003 (a) around midday and (b) in the evening.

http://www.gos.ifa.rm.cnr.it


types of pure mineral aerosol samples report depolarization ratios in the range 46–62% for

Sahara dust and depolarization ratios in the range 24–41% for feldspar, loess, red clay and

quartz particles (Volten et al., 2001).

The first satellite image in Fig. 2 referring to July 15 at 11:57 UTC (Fig. 2a) shows a

dust plume in the western Mediterranean reaching both Sardinia and Sicily. This plume is

therefore likely to affect both the Oristano and Lampedusa sites. Over the Etna site, lidar

measurements confirm the presence of the dust layer between 2.7 and 4.3 km (Fig. 3). On

the following day (July 16), a well-defined and intense dust plume is visible in Fig. 2b over

most of the Mediterranean basin and completely covering the Italian peninsula. A

comparison of the aerosol backscatter-coefficient profiles collected at Lecce (solid grey

line) and Etna (solid black line) on July 16 at different day hours is shown in Fig. 4a and b.

Error bars have not been added on backscatter coefficient profiles to make the figure clearer.

Relative uncertainties are lower than 10% at Lecce for baerz1.5�10�3 km�1 sr�1. In the

Etna backscatter profiles, relative uncertainties below 5 km are less than 10% (7% at night)

and up to 30% (12% at night) in the altitude range 5–8 km. Conversely, relative

uncertainties on D profiles are lower than 10% (5% at night) and up to 50% (15% at night)

in the range 5–8 km.

It is worth observing that the backscatter coefficient profiles are rather similar at the two sites.

In particular, a well-defined 3 km-thick dust layer also revealed at Etna by the high

depolarization ratios D N20% (Fig. 4, dotted line), is visible between 2 and 5 km altitude,

with increasing intensity from 14:40–15:40 UTC (Fig. 4a) to 18–19 UTC (Fig. 4b). Given that

the two-lidar sites are 400 km apart, we can therefore suggest that a rather uniform dust layer

0.60.40.20.0

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Fig. 5. Vertical profiles of the extinction coefficient (grey symbols) and of the lidar ratio (black symbols) retrieved at

Lecce on July 16, 2003 from measurements performed between 18:00 and 19:00 UTC.


was present in the central Mediterranean atmosphere on the afternoon of July 16. The full

evolution of the dust layer over the Etna site on July 16 can be found in Fig. 3. This also reveals

some mixing of dust with the local, boundary-layer aerosols occurring below 2 km (high D

values with respect to no-dust days).

Fig. 5 shows the vertical profiles at 351 nm of the extinction coefficient (grey dots) and of

the extinction to backscatter coefficient ratio (lidar ratio, black dots) obtained from nighttime

(18:00–19:00 UTC) lidar measurements at Lecce. Lidar ratio values allow inferring the

vertical distribution of predominant aerosol types (Ackermann, 1998). Fig. 5 shows that lidar

ratios span the range 50–70 sr between 2 and 5 km. In accordance with literature data, these

values are representative of non-spherical dust particles (e.g. Barnaba et al., 2004; Perrone et

al., 2004; Mattis et al., 2002). Fig. 2c shows that on July 17 at 11:40 UTC, the dust plume

covers the western and central Mediterranean, reaching up to Corsica. The plume shape

indicates a southwesterly flow from the northwestern African coasts. It is worth observing

that the horizontal pattern of the dust plume (brighter and darker regions) indicates that the

dust layer over the Mediterranean is rather non-uniform. In fact, lidar measurements

performed at Lecce from 13:00 to 14:00 UTC and at Etna around 14:05 UTC show a

significantly different vertical displacement of the desert dust load (Fig. 6). The backscatter

coefficient (black line) and depolarization ratio (dotted line) vertical profiles of Fig. 6 reveal

the presence of a non-uniform dust layer between 2.5 and 6.5 km at the Etna site (see also

Fig. 3), while at Lecce, the lidar (grey line) detects a 2 km-thick dust layer between 3.5 and

5.5 km. Over both sites, however, the dust load significantly reduces during the day as is

clearly visible in Figs. 3 and 7, the latter figure showing (a) the backscatter coefficient and

0.0080.0040.000

Backsc. Coeff. (km*sr)-1

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Fig. 6. Vertical profiles of the backscatter coefficient retrieved at Lecce (grey line) and Etna(black line), and of the

depolarization ratio (dotted line) retrieved at Etna on July 17, 2003.

0.60.40.20.0

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100806040200

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(a) (b)

Fig. 7. Vertical profiles (a) of the backscatter coefficient and (b) of the extinction coefficient (grey symbols) and lidar ratio

(black symbols) retrieved at Lecce on July 17, 2003 from measurements performed between 18:10 and 19:10 UTC.


(b) the extinction coefficient (grey dots) and lidar ratio (black dots) profiles retrieved at

Lecce from 18:10 to 19:10 UTC. On July 18 (12:21 UTC), Fig. 2d shows the dust plume

residing over the Southern Mediterranean, thus affecting only two of the five sites considered

(i.e., Etna and Lampedusa). In this respect, note that a transition between dusty and non-

dusty conditions over the Etna site is registered half way through July 18 (Fig. 3). Fig. 2e

and 2f indicate July 19 and 20 as dust-free days over the addressed sites.

Some advection of desert dust from the west over Sardinia and Corsica is visible again on

July 21 (Fig. 2g), then reaching the Tyrrhenian coasts of central-northern Italy on July 22 (Fig.

2h). Oristano and then Rome are therefore the first sites reached by dust during this event. The

satellite picture for July 22 (11:48 UTC) shows the dust plume to reach the Tyrrhenian coasts of

Central-Northern Italy (Fig. 2h). Note however, that the arrival of this event over the Etna site

(i.e., over Southern Italy) is also registered on July 22, but this occurs in the evening (dust layer

between 3 and 5 km, Fig. 3). Desert dust is shown to cover almost completely the central

Mediterranean basin on July 23 (Fig. 2i) and to persist over the whole area of interest on July 24

(Fig. 2l). For the latter case, the comparison of the dust plume vertical extent over the Etna and

Lecce sites is shown in Fig. 8. In the evening of July 24, an almost homogeneous dust layer

characterized by depolarization ratios varying in the 15–25 range (dotted line) is detected at Mt.

Etna from about 1 to 6 km altitude, while at Lecce the dust layer extends mainly from 1.5 to 4

km (grey solid line). Extinction (grey dots) and lidar ratio (black dots) profiles retrieved at Lecce

on July 24 from 20:00–21:00 UTC are shown in Fig. 9. We observe that the lidar ratio (black

line) takes values in the 40–60 sr range from about 1.5 to 4 km and reduces below 1.5 km

0.0080.0040.000


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Fig. 8. Vertical profiles of the backscatter coefficient retrieved at Lecce (grey line) and Etna (black line), and of the

depolarization ratio (dotted line) retrieved at Etna on July 24, 2003.

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Lecce 24/07/200320:00 - 21:00 UTC

Fig. 9. Vertical profiles of the extinction coefficient (grey symbols) and of the lidar ratio (black symbols) retrieved a

Lecce on July 24, 2003 from measurements performed between 20:00 and 21:00 UTC.


t


revealing some mixing of dust with local, boundary-layer aerosols. In fact, the lidar ratio

decreases to about 15 sr below 1 km: these values are typical of marine aerosol (Ackermann,

1998; Barnaba et al., 2004). At the Etna location, lidar depolarization shows this dust event to

terminate on the afternoon of July 25 (Fig. 3). This timing is in good agreement with the satellite

records of Fig. 2m and n.

4. AERONET data and discussion

AERONET sunphotometer retrievals at the five selected monitoring sites (level 1.5) from July

13 to July 25, 2003 are considered in this study to characterize typical optical and physical

parameters of the dust particles advected from north-west Africa over the Mediterranean. Results

are presented in terms of the temporal evolution of the aerosol optical depth (AOD), Angstrom

exponent (A), imaginary (k) and real (n) refractive index, and single scattering albedo (SSA).

Volume size distributions and coarse-to-fine volume ratios Cc /Cf are also discussed.

4.1. Aerosol optical depth and Angstrom exponent results

The aerosol optical thickness represents one of the most commonly used parameters in

aerosol studies. Fig. 10 (black symbols) shows the AOD (at 440 nm) temporal evolution as

recorded by the 5 selected AERONET photometers. The AOD accuracy is DAOD=F0.01

(Dubovik et al., 2002a). Vertical dotted lines in Fig. 10 indicate the (approximate) duration of the

investigated dust events. Besides SeaWiFS images, analytical backtrajectories have also been

used to classify dusty and dust-free days at the different monitoring sites. 5-day analytical

backtrajectories are provided by NASA GSFC (http://www.croc.gsfc.nasa.gov/aeronet/

index.html). The trajectories are based on a kinetic trajectory analysis using NASA GMAO

assimilated gridded data. Data are provided for two arrival times (00:00 UTC and 12:00 UTC)

and for four distinct arrival height levels (950, 850, 700, and 500 hPa). Analytical

backtrajectories show that north-west Africa was the source region of the dust events addressed

in this study.

As can be seen from the overview of Section 3, Lampedusa is affected by desert dust from 15

to 18 and from 23 to 25 July (Fig. 10a). Dust overpasses Oristano from 15 to 17 July and from

22 to 24 July (Fig. 10b). Rome is affected by desert dust on 16 and 17 July and from 22 to 24

July (Fig. 10c), as in Lecce except for July 22 (Fig. 10d). Desert dust is observed at the Etna

station from 15 to 18 July and from 23 to 25 July (Fig. 10e).

Fig. 10 shows that, with respect to dust-free days, AODs tend to be larger during dust events

at all sites. Oristano’s AODs span the 0.6–1.3 and 0.3–0.6 range during the first and second July

dust event, respectively, while, AODs lower than 0.4 are detected during dust-free days. We

believe that the quite high AOD monitored at Rome on July 18 (dust-free day) is due to local

anthropogenic emissions. We also observe from Fig. 10 that the 15–18 July dust-event is more

intense than that of the 22–25 July. In fact, AODs larger than 0.9 are detected in Lampedusa,

Oristano, Rome, and Etna during the first dust event, whereas during the second dust event

AODs up to 0.9 are only detected at Etna in the early morning of July 24. Lidar measurements

support this last conclusion. Dust layers leading to backscatter coefficients up to about

5.5�10�3 (Fig. 4b, grey line) and 3�10�3 km�1 sr�1 (Fig. 8, grey line) have been retrieved at

Lecce during the first and second dust event, respectively. The lower intensity of the second dust

event is clearly revealed at Etna by the contour plots of the backscatter and depolarization ratio

of Fig. 3.

http://www.croc.gsfc.nasa.gov/aeronet/index.html

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Rome

13 14 15 16 17 18 19 20 21 22 23 24 25 26

2

1

0

Etna

AO

D (

440

nm

) A

ng

strom

Exp

on

ent (440/870 n

m)

Day of July, 2003

(a)

(c)

(b)

(d)

(e)

Fig. 10. Temporal evolution at the five selected AERONET sites of the AOD at 440 nm (black symbols) and of A

computed from AOD at 440 and 870 nm (grey symbols) between 13 and 25 July, 2003.



The Angstrom exponent A is defined by the spectral dependence of the aerosol optical depth

AOD(k)~k�A. It mainly depends on the aerosol size distribution: typical values range from

A N2.0 for fresh smoke particles, which are dominated by accumulation mode aerosols, to nearly

zero for large dust particles (Dubovik et al., 2000). The temporal evolution of the Angstrom

exponent (computed from AOD values at 440 and 870 nm) is also reported in Fig. 10 (grey

symbols). We clearly observe that AODs and Angstrom exponents are anti-correlated at all sites.

In particular, A appears as a good marker to trace the temporal evolution of dust outbreaks due to

its evident and quite rapid variation. At Lampedusa, A that is ~1.5 on July 13, and spans the 0.3–

0.4 range on July 15, reduces to 0.07 on July 16 when the AOD is 0.9. Then, we observe from

Fig. 10 that A increases fast as the dust event ends (A =1.8 on July 20), and again reduces to 0.2

during the second dust event characterized by AODs in the 0.4–0.7 range. Similar results are

found for the other sites with dust-free days characterized by A N1 (typical continental aerosols

with modal radii 60.1 Am show A61–1.5) and A b1 during dusty days. As mentioned in

Section 3, a large tongue of dust from north-western Africa is first advected to the west coasts of

Sardinia and to the south of Sicily on July 15 (Fig. 2a) while, on July 17 dust particles spread all

over the Italian peninsula (Fig. 2c). Accordingly, Fig. 10a and b reveal that Oristano and

Lampedusa are the first sites affected by this dust event. A different path characterizes the

second dust event (see Section 3 and Fig. 2), so that the presence of dust is first detected over

Oristano and Rome on July 22. The whole central Mediterranean is loaded with dust on July 23

and A values lower than 0.5 are retrieved at the five sites (Fig. 10). In agreement with the

satellite view (Fig. 2) the Rome site shows a transition from dusty to non-dusty conditions on

July 24.

The discussion above has clearly highlighted the A usefulness to trace the temporal evolution

of dust outbreaks. Nonetheless, it is worth observing from Fig. 10 that A values are quite

dependent on the dust load intensity: on average lower A values have been retrieved at all sites

during the July 15–18 dust event that is characterized by larger AODs. We report A versus

AODs on Fig. 11. Only data from the five sites obtained during dusty days (Fig. 10) have been

considered. We observe the Angstrom exponent variability range to reduce as the AOD

increases, leading to Ai0.1 for AODs z0.8. The larger A values obtained during low AOD

dusty-days may be ascribed to the more significant contribution of background aerosols typical

1.5

1.0

0.5

0.0

An

gst

rom

Exp

on

ent

1.21.00.80.60.40.20.0

AOD (440 nm)

Lampedusa Oristano Rome Lecce Etna

Fig. 11. A values versus AODs at 440 nm. A values have been retrieved at the five selected AERONET sites during dusty

days.

Table 1

Mean values of the Angstrom exponent (bAN) computed from AOD values at 440 and 870 nm, the imaginary (bkN) and

real (bnN) part of the refractive index, and the single scattering albedo (bSSAN) retrieved at each site by AERONET

sunphotometer measurements at AOD (440 nm)z0.6

Sites bANFr bkNFr bnNFr bSSANFr

Lampedusa 0.16F0.06 0.004F0.001 1.53F0.09 0.88F0.03

Lecce 0.3F0.2 0.003F0.001 1.5F0.1 0.91F0.02

Etna 0.3F0.1 0.004F0.003 1.51F0.08 0.90F0.03

Oristano 0.18F0.09 0.004F0.001 1.5F0.1 0.88F0.02

Rome 0.17F0.09 0.005F0.001 1.50F0.09 0.87F0.02


of the monitoring site. In fact, the largest A values are recorded at the urban location of Rome

and at Lecce, where fine particles from combustion and/or long-range transported can strongly

increase the Angstrom parameter if their AOD is comparable to that of dust.

In Table 1 we report for each site the Angstrom exponents’ mean value, bAN (F1 standard

deviation), determined by averaging only A values corresponding to AODsz0.6: the Angstrom

exponent variability range reduces more than 50% at AODsz0.6. Table 1 shows that the

Angstrom exponent mean value reaches the smallest (bAN=0.16F0.06) and largest (bAN=

0.3F0.2) value at Lampedusa and Lecce, respectively, i.e. at the closest and farthest monitoring

site from north-west Africa (the source region of the dust events addressed in this analysis).

Nevertheless, the five bAN values are all within standard deviations. The averaged Angstrom

exponents obtained, including all data at AODz0.6 retrieved at the different sites, is

bAN=0.2F0.1. This value given in Table 2 can be thought as a reference value of the

Angstrom exponent that characterizes dust conditions over the central Mediterranean under

medium-high dust loads (AODz0.6).

The latter results are in good accordance with Saharan dust studies performed by Tanre et al.

(1988a) during April and May 1986 at M’bour, Senegal. They also found that smaller mean A

values corresponded to larger AODs. In particular, Tanre et al. (1988a) observed AODs (450 nm)

spanning the 0.4–2 range and noted that A values (computed from AODs at 450 and 650 nm)

were slightly negative at AOD (450 nm)N1.5. Our results are also in good accordance with

AERONET sunphometer measurements (Dubovik et al., 2002a) performed during dust

outbreaks from 1993 to 2000 at Cape Verde (Africa), which have provided aerosol optical

depths at 1020 nm spanning the 0.1–2.0 range and Angstrom exponents in the 0.1–0.7 range (A

values were computed from AODs at 440 and 870 nm). Recent AERONET sunphotometer

measurements performed at Leipzig, Germany during a Saharan dust outbreak reaching central

Europe from 12 to 15 October 2001, have also revealed that A reduces as the AOD increases. In

particular, in that case A computed from AODs at 380 and 1020 nm, took the value i0.45 at

AOD (380 nm)i0.3 and spanned the 0.1–0.15 range for 0.6VAODV0.7 (Muller et al., 2003).

Finally, it is worth mentioning that sunphotometer measurements performed within a dust layer

at 2400 m (Izana, Tenerife) during the second Mineral Dust and Tropospheric Chemistry

Table 2

Averaged values of the Angstrom exponent (bAN), the imaginary (bkN) and real (bnN) part of the refractive index, and

the single scattering albedo (bSSAN) calculated including all dusty-day values retrieved at AOD (440 nm)z0.6

bANFr bkNFr bnNFr bSSANFr

0.2F0.1 0.004F0.002 1.5F0.1 0.89F0.03


(MINATROC) campaign (July 15–August 15, 2002) have provided Ai0.20 for AODs (500

nm) in the 0.3–0.9 range (A values computed from AODs at 500 and 1020 nm). Due to the

altitude of the site, no contamination by other types of aerosols in the atmospheric column was

expected in that case (Gobbi et al., 2004b).

4.2. Particle size distributions and volume concentrations

In accordance with the above reported discussion, the driving of the optical properties by

large particles is the principal feature of desert dust aerosols. Nonetheless, the non-sphericity of

dust particles also deserves some attention. Such non-sphericity is responsible for the high

depolarization ratios measured at Etna during dusty days (Fig. 3). Numerous studies have

indicated the need to account for particle non-sphericity in modelling the optical properties of

dustlike aerosols. In fact, there is sufficient experimental evidence (Volten et al., 2001; Gobbi et

al., 2002) that the non-sphericity of desert dust particles can lead to scattering properties

significantly different from those predicted by the standard Mie theory (Mishchenko et al.,

2000). A model that considers dustlike aerosol particles as polydisperse, randomly oriented

spheroids has been implemented in the AERONET retrieval by Dubovik et al. (2002b) to

improve the quality of dust particle properties derived from photometric measurements. Fig. 12a

shows the mean volume size distributions obtained at Lampedusa (open grey dots), Oristano

(full black dots), and Rome (full triangles) by averaging, respectively, 33, 11, and 32 different

volume distribution profiles retrieved during dusty days. Vertical error bars represent F1

standard deviation from the average value and indicate the size distribution variability. Fig. 12b

shows the mean volume size distributions obtained at Lecce (open black boxes) and Etna (full

grey boxes) by averaging 17 and 45 different volume distribution profiles, respectively. Several

studies by Dubovik et al. (2002a,b) have shown that for AOD(440 nm)z0.05 and particle size

range 0.1b r b7 Am, the retrieval errors do not exceed 10% in the maxima of the size

distribution. The domination of coarse-mode particles (rN0.6 Am) in desert aerosols is clearly

pointed-out by Fig. 12: particle radius peak values span the 1.7–3 Am range. In particular, the

average coarse mode distribution peaking at i2.2 Am at Lampedusa (Fig. 12a), which is ~200

km away from the African coast, shifts toi1.7 Am at Lecce (Fig. 12b), which is ~800 km away

from the African coast. We believe that latter results are mainly due to sedimentation effects of

large size particles as dust clouds move away from source regions and less to retrieval errors. To

this end, it is worth noting that 64% of the 33 volume distributions that have been averaged to

get the Lampedusa’s profile of Fig. 12a, have the coarse mode distribution peaking at i2.2 Amand 21% have the coarse mode distribution peaking at i2.9 Am. On the contrary, 50% and 31%

of the 17 volume distributions that have been averaged to get the Lecce’s profile of Fig. 12b,

have the coarse mode distribution peaking at i1.7 Am and at i1.3 Am, respectively. The

temporal evolution of the Cc /Cf ratio at Lampedusa, Oristano, Rome, Lecce, and Etna,

respectively (Fig. 13a–d, black symbols), also appears to decrease as dust clouds move away

from source regions. Cc and Cf denote the coarse and fine particle volume concentrations

respectively and accordingly to Dubovik et al. (2002a), all particles with radius smaller than 0.6

Am are considered as belonging to the fine mode and all particles with radius larger than 0.6 Amas belonging to the coarse mode. Cc /Cf ratios up to about 15 are retrieved at Lampedusa (Fig.

13a) and Oristano (Fig. 13b) during the mid-July dust event. Conversely, Cc /Cf V10 are

detected at Lecce (Fig. 13d). It is worth mentioning that scanning electron microscopy analyses

of several dust samples from rainfall residues collected at Lecce during dust storms occurring

from April to June 2002, have provided size distributions with median-radius values between

0.6

0.4

0.2

0.0

5 6 7 8

0.1

2 3 4 5 6 7 8

1

2 3 4 5 6 7 8

10

Radius (µm)

Lecce (17) Etna (45)

0.6

0.4

0.2

0.0

Lampedusa (33) Oristano (11) Rome (32)

(a)

(b)

dV

(r)/

dln

r

Fig. 12. Volume size distributions obtained at (a) Lampedusa, Oristano, and Rome, and (b) Lecce and Etna, by averaging

all available volume distributions retrieved at each site during dusty days.


0.85–1.2 Am (Blanco et al., 2003). AERONET sunphometer measurements performed from

1993 to 2000 at Cape Verde (Africa) during dust outbreaks have shown that the median radius of

the coarse mode was 1.9F0.03 Am (Dubovik et al., 2002b). Remote sensing observations of

dust properties from space and ground have provided dust size distributions, showing a

dominant coarse mode at 1–5 Am and a secondary mode around 0.5 Am effective radius (Tanre et

al., 2001). Measurements by Levin et al. (1980) for a dust storm over the Israeli desert, and by

Patterson et al. (1977) over Texas, show the dust surface area distribution to reach a maximum

around 2 Am: the median radius of the coarse dust particle mode used in several models (Koepke

et al., 1997; Tegen and Lacis, 1996).

Fig. 13 also reveals that lower Cc /Cf ratios have been retrieved at all sites during the 22–25,

July dust event that was less intense than the first one (on average, lower AODs were detected at

10

0

13 14 15 16 17 18 19 20 21 22 23 24 25 26

Day of July, 2003

Etna

10

0

Lecce

10

0

Oristano

10

0

0.04

0.02

0.00

0.04

0.02

0.00

0.04

0.02

0.00

0.04

0.02

0.00

0.04

0.02

0.00

Lampedusa

Cc

/Cf

(a)

(d)

(e)

(c)

(b)

10

0

Rome

Imag

inary R

efractive Ind

ex (440 nm

)

Fig. 13. Temporal evolution at the five selected AERONET sites of the coarse to fine particle volume concentration ratios

(black symbols) and of the k values at 440 nm (grey symbols) between 13 and 25 July, 2003.



all sites). The decreasing relative contribution of the coarse mode to the total aerosol mass for

decreasing dust loads may be responsible for the latter results. The rather low Cc /Cf values that

at all sites characterize dust-free days support this last conclusion. It is worth mentioning that

Cc /Cf ~50 have been observed at Cape Verde, Africa by Dubovik et al. (2002a) at high dust

loads (AODi2 at 1020 nm): besides the differences in dust load, sedimentation effects that

reduce the relative contribution of large particles may also be responsible of the difference

between the Cc /Cf ratios of Cape Verde and those observed over the central Mediterranean.

4.3. Imaginary and real parts of the refractive index

Besides scattering that depends on particle shape, size, and composition, the absorption of

solar radiation by mineral dust also affects the sign of the radiative forcing generated by this

aerosol. Fig. 13 (grey symbols) shows the temporal plots of the refractive index imaginary part k

retrieved at 440 nm at the 5 selected AERONET sites. Analyses by Dubovik et al. (2000 and

2002b) have shown that k values are retrieved with errors on the order of 30–50% for AOD (440

nm)z0.5. While, for AOD (440 nm)V0.2 the accuracy levels drop down to 80–100% for the

imaginary part of the refractive index. It is worth observing from Fig. 13 that on average

significantly larger imaginary refractive indices are retrieved during dust-free-days at

Lampedusa (Fig. 13a), Oristano (Fig. 13b), and Rome (Fig. 13c), while k values at Lecce

(Fig. 13d) appear less dependent on dust outbreaks. Fig. 14a showing dusty-day k values of all

sites versus AODs, reveals that the k variability range reduces as the AOD increases. We believe

that the higher variability of k values at lower AODs may be due either to the lower accuracy

levels that characterize k values retrieved for AOD (440 nm)b0.4 and to the larger variability of

the aerosol properties at lower dust loads. The mean imaginary refractive index bkN that has

been determined by considering k values retrieved at each site at AOD (440 nm)z0.6, is given

in Table 1 with the corresponding standard deviation. Table 1 reveals that the imaginary

refractive index mean value reaches the smallest (bkN=0.003F0.001) and largest

(bkN=0.005F0.001) value at Lecce and Rome, respectively. It is worth noting that at all

sites standard deviations are on the order of k retrieval errors. We believe that the larger bkN

value observed at Rome is mainly due to the contamination of dust particles by background

absorbing aerosols than to retrieval uncertainties. High k values have been retrieved at Rome on

dust-free days, while at Lecce the imaginary refractive indices of dust-free days are on average

smaller than that of dusty days. Hence, we believe that even at medium-high dust loads (AOD

(440 nm)z0.6) averaged dust particle properties may be affected by background aerosols

typical of the monitoring site. This may be due to the fact that dust-free days are on average

characterized by AOD (440 nm) ~0.3 at the sites selected in this study. All k values retrieved at

AODz0.6 have also been used to calculate bkN=0.004F0.002, which is given in Table 2 and

may represent the averaged imaginary refractive index at 440 nm of dust particles over the

central Mediterranean under medium-high dust loads. It is worth noting that the bkN standard

deviation is equal to the highest retrieval error expected for the imaginary part of the refractive

index for high aerosol loading AOD (440 nm)z0.5.

These results are in good accordance with those reported by Sinyuk et al. (2003). They have

used combined satellite and surface observations to retrieve the imaginary part of the refractive

index of Saharan dust and have shown for different geographical locations that averaged k

values at 440 nm vary between 0.002 and 0.004. Moreover, satellite images acquired by the

Thematic Mapper during a field experiment in Senegal in 1987 (Tanre et al., 1988b) have

provided k =0.003F0.0003 at 470 nm. These values that are in good agreement with the results

0.03

0.02

0.01

0.00

k (4

40 n

m)

Lampedusa Oristano Rome Lecce Etna

(a)

(b)

1.0

0.9

0.8

0.7

SS

A (

440

nm

)

1.21.00.80.60.40.20.0

AOD (440 nm)

1.6

1.5

1.4

1.3

n (

440

nm

)

(c)

Fig. 14. Plot of the (a) imaginary and (b) real refractive indices, and (c) of the single scattering albedo versus AODs at

440 nm. k, n and SSA values at 440 nm have been retrieved at the five selected AERONET sites during dusty days.



of several recent investigations (Colarco et al., 2002; Dubovik et al., 2002a; Perrone et al.,

2004), and are significantly lower than previously reported values (Patterson et al., 1977; World

Climate Program, 1986). It is also worth mentioning, that spectroscopic characterizations of

water-insoluble components of Saharan dust samples from rainfall residuals collected at Lecce,

have revealed that dust particles with a high content of illite are mainly advected over the

Mediterranean basin during dust storms from north-west of Sahara (Blanco et al., 2003).

According to Sokolik and Toon (1999), illite is characterized by k ~0.001 at 440 nm.

The AERONET inversion model that considers dustlike particles as polydisperse, randomly

oriented spheroids (Dubovik et al., 2002b), has been used to retrieve the refractive index real part

n at 440 nm during dusty days. Numerical tests have shown that the presence of non-spherical

particles may cause an unrealistically strong decrease of the real part of the refractive index with

decreasing wavelength when the model based on the standard Mie theory is used. Dusty-day n

values versus AODs retrieved at Lampedusa, Oristano, Rome, Etna, and Lecce are shown in Fig.

14b. Dubovik et al. (2002b) have shown that n values are retrieved with accuracy to the level of

0.04 for AOD (440 nm)z0.5. While, for AOD (440 nm)V0.2 the accuracy level drops down to

0.05 for the real part of the refractive index. We observe that n values vary at all sites within the

variability range (1.33–1.6) set by the inversion algorithm developed by Dubovik and King

(2000) and do not reveal any significant dependence on AOD. The mean real part of the

refractive index bnN determined at each site by considering the n values retrieved at AODz0.6

is given in Table 1 with the corresponding standard deviation. Table 1 shows that the averaged

real refractive indices retrieved at the different sites are in accordance within one standard

deviation. All n values retrieved at AODz0.6 have then been used to calculate bnN=1.5F0.1,

which is given in Table 2 and may represent the averaged real refractive index at 440 nm of dust

particles over the central Mediterranean basin. It is worth noting that the bnN standard deviation

is significantly larger than the retrieval error (0.04) for the real part of the refractive index for

high aerosol loading AOD (440 nm)z0.5. Several models suggest that the real part of the

refractive index of dust is 1.53 for the visible spectral region (Shettle and Fenn, 1979; Koepke et

al., 1997).

4.4. Single scattering albedos

The single scattering albedo, SSA depends on scattering and absorption properties of

atmospheric particles and represents an important parameter to evaluate the aerosol radiative

impact. The presence in the atmosphere of desert dust layers can lead to either a cooling or a

warming effect, depending on properties such as single-scattering albedo and altitude of the layer

(e.g. Sokolik and Toon, 1999; Tegen and Fung, 1995; Liao and Seinfeld, 1998). In particular,

Hansen et al. (1997) showed that when cloud effects are included in the radiative forcing

analysis, aerosol SSAs smaller than 0.91 lead to warming and higher values to a cooling of the

climate system.

Fig. 15 shows the 440 nm-SSA temporal plots retrieved at the 5 selected AERONET sites

from 13 to 25 July. Dubovik et al. (2002b) have shown that SSA values are retrieved with

accuracy to the level of 0.03 for AOD (440 nm)z0.5. While, for AOD (440 nm)V0.2 the SSA

accuracy level drops down to 0.05–0.07. Fig. 15 reveals that on average SSA values are quite

affected by dust events. In addition, SSA values and variability ranges are also quite dependent

on monitoring site location. Lecce is the site characterized by larger SSA values that span the

0.91–0.99 and 0.84–0.92 range during dust-free- and dusty-days, respectively (Fig. 15d).

Oristano and Rome are the sites on average characterized by smaller SSA values. At Oristano

13 14 15 16 17 18 19 20 21 22 23 24 25 26

Day of July, 2003

Etna

Lecce

Rome

Oristano

1.0

0.9

0.8

0.7

1.0

0.9

0.8

0.7

1.0

0.9

0.8

0.7

1.0

0.9

0.8

0.7

1.0

0.9

0.8

0.7

Lampedusa

SS

A (

440

nm

)

(a)

(e)

(d)

(c)

(b)

Fig. 15. Temporal evolution at the five selected AERONET sites of the single scattering albedo at 440 nm between 13 and

25 July, 2003.



(Fig. 15b) SSAs span the 0.82–0.92 and 0.78–0.91 range during dusty and dust-free-days,

respectively, while at Rome SSAs vary from 0.79 to 0.98 on dusty-days and from 0.75 to 0.98 on

dust-free-days (Fig. 15c). We believe that the lower SSAs retrieved both at Oristano and Rome

are mainly due to the larger contamination of measurements by background absorbing aerosols

and less to retrieval errors. In fact, dust-free-day aerosols are on average characterized by larger k

values (Fig. 13, grey symbols) at both sites. Fig. 14c shows the SSA values retrieved at all sites

during dusty-days versus the corresponding AODs and reveals that the SSA variability range

reduces as the AOD increases. We believe that the higher variability of SSA values retrieved at

lower AODs is due to the lower accuracy of SSA values retrieved at lower AODs and to the

higher variability of the aerosol properties: the contribution of background aerosols is expected

to be more significant at low dust AODs. The average single-scattering-albedo bSSAN that has

been determined at each site by considering the SSA values retrieved at AODz0.6, is given in

Table 1 with the corresponding standard deviation that is of the order of SSA retrieval errors. We

observe that the largest and smallest bSSAN values are obtained at Lecce and Rome,

respectively. Nevertheless, bSSAN values retrieved at the different sites are in good accord

within one standard deviation. It is worth observing from Table 1 that standard deviations are

rather close to the accuracy level of 0.03 typical of SSAs retrieved for AOD (440 nm)z0.5. All

SSAs retrieved at AODz0.6 have also been used to calculate bSSAN=0.89F0.03, which is

given in Table 2 and may represent the average SSA of dust particles over the central

Mediterranean basin.

Our results confirm the values recently derived by Kaufman et al. (2001), i.e., 0.90F0.02 in

the blue. It is also worth noting that Dubovik et al. (2002a) have obtained at Cape Verde an

averaged SSA value of 0.93F0.01 at 440 nm, while sunphotometer measurements made during

SHADE have provided a mean value of 0.96 at 440 nm (Haywood et al., 2003). The results from

SHADE concern unpolluted dust aerosols (Tanre et al., 2003). The lower averaged SSA values

obtained in this study (bSSAN=0.89F0.03) with respect to those retrieved over North Africa by

Dubovik et al. (2002a) and Haywood et al. (2003), may be due to both changes and/or

contamination occurring during the dust particle transport and to the optical effects of local

aerosols. According to Hansen et al. (1997), small differences in aerosol SSA about the value

~0.90 can be quite significant since they may change the warming to a cooling of the climate

system. In any case, our SSA values further indicate that Saharan dust has significantly less

absorption than that reported in the past by several authors (e.g. d’Almeida et al., 1991). This

difference is possibly due to discrepancy between dust absorption inferred from discrete in situ

measurements and absorption directly inferred from the radiation field in the atmosphere (Tanre

et al., 2001).

5. Summary and conclusion

Two dust outbreaks occurring on the second half of July 2003 over the central Mediterranean

basin have been studied at 5 different AERONET sites to retrieve averaged columnar values of

the main optical and physical parameters that characterize Saharan dust particles. At two sites

(Etna and Lecce), sunphotometer measurements have been combined with lidar observations to

infer the vertical distribution of dust layers and to characterize dust layer properties by the

vertical profiles of the backscatter and extinction coefficient, and of the depolarization and lidar

ratio. The selected sites are differently affected by anthropogenic pollution. Sunphotometer

retrievals have clearly revealed that at low dust loads, dust-particle physical and optical

parameters are characterized by a larger variability range and depend more on monitoring site


location. Latter results are maybe due to larger retrieval errors and to the larger variability of

aerosol parameters retrieved at lower dust loads. The plot of the Angstrom exponent (that mainly

depends on particle’s size) versus AOD reveals that the A variability range reduces more than

50% at AOD (440 nm)z0.6, and that A values i0.1 are retrieved at all sites at AODz0.8.

Accordingly, the Cc /Cf plots reveal the larger contribution of coarse particles under higher dust

loads. The coarse- to fine-particle volume concentration ratios reach larger values at all sites

during the 15–18 July dust event that is characterized by higher AODs. The larger intensity

(higher AODs) of the first dust event revealed by sunphotometer measurements is supported at

both Lecce and Etna by lidar measurements. Dust particle vertical profiles on average

characterized by larger backscatter coefficients have been retrieved at Lecce and Etna during the

15–18, July dust event.

It is also shown that k and SSA values are quite dependent on AODs: at all sites the

variability range of k and SSA reduces as AODs increase. Besides the higher retrieval errors

that characterize k and SSA values retrieved for AOD (440 nm)b0.6, the contamination by

anthropogenic and/or natural background aerosols typical of the monitoring site that gets

more significant at lower AODs is likely responsible of these results. To this end, it is

worth noting that A, Cc /Cf, k, and SSA temporal plots reveal that on average, dust-free-day

values are quite different from dusty-day values. Hence, to reduce the effects either of

retrieval errors or of background aerosols, AERONET data at AODsz0.6 have been used at

all sites to get averaged values of relevant parameters characterizing at 440 nm the optical

and physical properties of dust particles over the central Mediterranean. These are:

bnN=1.5F0.1, bkN=0.004F0.002, bSSAN=0.89F0.03, and bAN=0.2F0.1. Volume size

distributions have shown a dominant coarse mode peaked at 1.7–3 Am. In particular, the

averaged coarse mode that is peaked at i2.2 Am at Lampedusa, which is ~200 km away

from the northwest Africa coast, gets peaked at i1.7 Am at Lecce, which is ~800 km

away.

Lidar retrievals have allowed characterizing dust layer vertical distributions and have

provided a clear view of dust outbreak temporal evolutions mainly at the Etna site where lidar

measurements have been more frequent. Lidar backscatter coefficients have revealed that over

the Mediterranean basin, dust layers are generally located from 1 up to 6 km and that their

vertical distribution can significantly change within a few hours. At Lecce it has also been

shown that dust particles are characterized by lidar ratios spanning the 50–70 sr range under

high dust loads and that lidar ratios reduce with dust load possibly because of the more

significant contamination by spherical background aerosols. Depolarization ratio measurements

performed at Etna and reaching values as high as 50% under high dust loads, support the last

comment.

Besides revealing a satisfactory accordance, the comparison of the results of this paper to

those of recent studies mainly performed over North Africa has furthermore indicated that

Saharan dust is significantly less absorbing than has been reported in the past by several authors.

It has also been suggested that the small differences in averaged parameters characterizing dust

particles at different sites, are mainly due to changes and/or contamination occurring during the

dust particle transport at the monitoring site and less to retrieval errors. To this end it is worth

noting that the sites selected in this study were characterized by AOD (440 nm) ~0.3 during

dust-free days and that averaged dust particle parameters have been retrieved during dusty days

with AODsz0.6.

In any case, we believe that the retrieved average values of the main parameters that

characterize dust particles over the central-east Mediterranean basin under medium-high dust


loads, and the indication of the dust-layer vertical distribution, can be usefully employed in

global circulation models and/or chemical transport models, to represent dust outbreaks over the

central Mediterranean.

Acknowledgements

This work has been supported by Ministero dell’Istruzione dell’Universita e della Ricerca of

Italy (Programma di Ricerca 2004. Prot. 20004023854). Results presented in this paper have been

obtained using data from the Aerosol Robotic Network, AERONET. The authors kindly thank the

Principal Investigators of Oristano, Lampedusa, and Etna sites. Lidar measurements performed at

the Etna site were supported by the Italian Space Agency (ASI), under the contract I/R/157/02.

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Saharan dust particle properties over the central Mediterranean

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