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Atmospheric Environment 113 (2015) 223e235

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Atmospheric Environment

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An outstanding Saharan dust event at Mt. Cimone (2165 m a.s.l., Italy)in March 2004

Erika Brattich a, *, Angelo Riccio b, Laura Tositti a, Paolo Cristofanelli c, Paolo Bonasoni c

a Dept. of Chemistry “G. Ciamician”, Alma Mater Studiorum University of Bologna, Bologna (BO), Italyb Dept. of Applied Science, University of Napoli “Parthenope”, Napoli (NA), Italyc ISAC-CNR, Via Piero Gobetti, Bologna (BO), Italy

h i g h l i g h t s

� Saharan dust event raises PM10 concentration to 80 mg m�3 above 2000 m asl.� Mt. Cimone, WMO-GAW station representative of the Southern Europe free troposphere.� Bulk aerosol experimental data in connection with highly time resolved particle countings.� Synoptic analysis accompanied by time/space characterizations of the event.� Integrated assessment of OPC and AERONET data for the event.

a r t i c l e i n f o

Article history:Received 15 October 2014Received in revised form6 May 2015Accepted 11 May 2015Available online 14 May 2015

Keywords:Saharan dustPM10

Ambient aerosolAtmospheric radiotracers7Be210PbAERONET

* Corresponding author. Environ. Chemistry andChemistry “G. Ciamician”, Alma Mater Studiorum Un2, 40126 Bologna (BO), Italy.

E-mail address: erika.brattich@unibo.it (E. Brattich

http://dx.doi.org/10.1016/j.atmosenv.2015.05.0171352-2310/© 2015 Elsevier Ltd. All rights reserved.

a b s t r a c t

A severe PM10 episode was observed at the high elevation observatory of Mt. Cimone (2165 m a.s.l.) in theperiod of 13th-15th March 2004; during the event PM10 reached the maximum concentration (80 mg m�3

against an average of 8.8 ± 8.0 mg m�3) between 1998 and 2011. Meteo-synoptical analysis allowed toascribe this event to a long lasting and highly coherent Saharan dust outbreak, starting at the beginningof March. The peculiar synoptic configuration causing this massive transport of dust was characterized bya steep gradient between an upper level trough extending to low latitudes with a minimum centred overthe North-Western Algerian coast and a Saharan high extending all over the Mediterranean Sea with anelongated north-eastward tongue, whose synergic effect led to a peculiar funnel-shaped dust plume.During the period Mt. Cimone was located exactly along its main axis. The event was first analysed inassociation with simultaneous more or less conventional compositional parameters such as 7Be, 210Pb,and ozone. Subsequently, it was characterized in details both in terms of time and space evolution. Theformer aspect was investigated using number densities of fine and coarse particles obtained through anOptical Particle Counter which allowed to follow the event evolution at the sub-daily time scale whilePM10 membrane gravimetric analysis was limited by the 48-h sampling schedule suggesting the value of80 mg m�3 recorded is even potentially smoothed down by sampling duration. Besides precise timing,optical counting enabled to detect the inception and development of the event through a steep andsimultaneous increase of both coarse and fine particle number densities. Although the former increasewas much more relevant, the latter occurrence is much less frequently documented for Saharan Dustevents: a clear increase of particles in all the diameter ranges from 0.3 mm (lower limit of an OPC) up to5.0 mmwas observed during the event. The spatial extension of the event was also examined by means ofthe analysis of the AERONET ground-based sun photometer data from the Venice station for the event.Results confirmed a relevant increase of coarse particles over a distance of more than 150 km. Inter-estingly AERONET data indicates a more significant variation in the scattering properties of the aerosolrather than in the absorbing ones in connection with the arrival of the Saharan dust, an observation thatwithin the intrinsic limitations of inverse methods to derive aerosol's optical properties is in agreement

Radioactivity Lab., Dept. ofiversity of Bologna, Via Selmi

).

E. Brattich et al. / Atmospheric Environment 113 (2015) 223e235224

with some previous observations showing that dust in the Saharan desert region is much less absorbingthan previously measured.

© 2015 Elsevier Ltd. All rights reserved.

1. Introduction

Mineral dust from desert regions is one of the major constitu-ents of airborne particles on the global scale. Dust plumesfrequently enter the atmosphere and can travel up to tens ofthousands of kilometres downwind before settling back to thesurface (Engelbrecht and Derbyshire, 2010), being one of the mostprominent and commonly visible features in satellite imagery.

Dust is presently recognized as one of the main uncertainties inclimate change assessment (IPCC, 2013), owing to thewide range inthe estimates of global dust emissions spanning a factor of aboutfive (Huneeus et al., 2012). Even if the effects of dust on visibilityand human health (e.g., are fairly well known (e.g., Middleton et al.,2008; Zauli Sajani et al., 2011), the impact on ecosystems and oncloud processing has been focused only recently (see for exampleOkin et al., 2004; Bangert et al., 2012; Smoydzin et al., 2012).Mineral dust particles deeply affect climate, acting both directly (byscattering and absorbing radiation) and indirectly (through themodification of the optical properties of clouds and cryospheresuch as albedo) on the Earth's radiation balance (IPCC, 2013).Complex feedbacks between dust, ecosystems and climate exist: forinstance, dust can modify regional patterns of rainfall, disruptingecosystems. The associated changes in vegetation might close theloop in this feedback mechanism being possibly able to create newdust sources (Gass�o et al., 2010). Moreover climate change itselfmight induce variations on dust loadings, with a wide range ofresults reflecting different responses of the atmosphere and vege-tation cover to climate change forcings (Tegen et al., 2004;Woodward et al., 2005; Mahowald et al., 2006; Jacobson andStreets, 2009; Liao et al., 2009). Iron- and phosphorus-rich parti-cles are able to affect the primary phytoplankton production andthe carbon cycle through biogeochemical interactions acting asfertilising agents (Ridgwell, 2002; Maher et al., 2010), while dustdeposited in marine areas could trigger algal blooms (Guerzoniet al., 1999).

The Sahara desert is the world's largest source of aeolian desertdust (Swap et al., 1996; Middleton and Goudie, 2001). Hugeamounts of dust are transported every year from the Sahara towardthe American and European continents (Chiapello et al., 1997;Collaud Coen et al., 2004; Kaufman et al., 2005). It was estimatedthat Saharan dust events may induce up to 20 exceedances of thePM10 standard per year in southern Europe (Rodríguez et al., 2001).Mineral dust originates from well-defined source regions mainlyactive during the summermonths, with the exception of the Bodeledepression which is active all year-round (Prospero, 1996; Ginouxet al., 2001; Prospero et al., 2002; Washington et al., 2003;Barkan et al., 2004). Significant interannual and seasonal varia-tions in the atmospheric dust input exist (Dulac et al., 1996; Moulinet al., 1997; Barkan et al., 2004). The seasonal trend of Saharan dusttransport shows a peak during spring and summer, while thewinter and autumn seasons are usually characterized by a mini-mum activity. The more intense dust activity during the warmperiod is due to several factors, among which high wind speeds,cloud free conditions, high air temperatures (thus thermalconvective activity) which promote the dust lifting and transport(Dulac et al., 1996; Varga et al., 2014). All of these factors ultimatelydepend on the synoptic situation and possibly on teleconnections

leading to relevant interannual variations including precipitation,another factor largely affecting the intensity of Saharan dusttransport on the synoptical scale (Middleton, 1985; Dulac et al.,1996; Prospero, 1996; Bonasoni et al., 2004; Collaud Coen et al.,2004). Despite the changing nature of large scale atmospheric os-cillations, circulation patterns and drought periods in generalcannot unequivocally explain the frequency of dust outbreaks(Varga et al., 2014). In particular, the dust level may be influencedby variations in the activity of low pressure disturbances from yearto year (Pederzoli et al., 2010). Dust transported across the Medi-terranean and into Europe mainly originates from the northern-most African dust sources mainly located in Tunisia, Algeria, andLibya (Prodi and Fea, 1979; Avila et al., 1996; Bonelli andMarcazzan,1996).

The transportation of Saharan dust towards Europe through theMediterranean Sea is associated with different synoptic meteoro-logical situations (Barkan et al., 2005; Engelstaedter et al., 2006;Stuut et al., 2009; Barkan and Alpert, 2010; Israelevich et al.,2012; Varga et al., 2013, 2014), usually caused by intense cyclonespassing the Northern African coast fromwest to east (Barkan et al.,2005; Meloni et al., 2008) or convective injection of dust in northAfrica coupled with anticyclonic conditions at upper atmosphericlevels (e.g., Pey et al., 2013). The intensity and frequency of thesedisturbances are strongly affected by the penetration depth of coldair from higher latitudes southward (Dayan et al., 1991; Conte et al.,1996; Moulin et al., 1997; Barkan et al., 2005). At times the presenceof very severe disturbances can add huge amounts of dust affectingdaily and annual mean levels of airborne particulate matter oftenresulting in exceedances of the thresholds indicated by environ-mental regulations on air quality aerosol standards (Alpert andGanor, 1993; Tsidulko et al., 2002; Rodríguez et al., 2001; Kutieland Furman, 2003; Gerasopoulos et al., 2006).

All over the observational activity at the WMO-GAW station ofMt. Cimone (44.18N, 10.7E, 2165 m asl; Italy) Saharan dust eventshave been and still are a relevant field of investigation owing to itsfavourable geographical position in the core of the Mediterraneanregion frequently exposed to African air masses (site descriptionand information available at http://www.isac.cnr.it/cimone/site_location). Results have been widely reported in many scientificpapers (e.g., Balkanski et al., 2003; Bonasoni et al., 2004; Beineet al., 2005; Marenco et al., 2006; Marinoni et al., 2008;Cristofanelli et al., 2009; Zauli Sajani et al., 2011). More recentlyPM10 variability at Mt. Cimone station has been analysed as afunction of various factors, revealing a clear and frequent influenceof Saharan Dust outbreaks at this location in the centre of theMediterranean basin typically leading to aerosol maxima (Marinoniet al., 2008; Riccio et al., 2009; Tositti et al., 2013). Other authorshave so far published papers based on the mid-March 2004Saharan Dust event though in a marginal way. In particular, Beineet al. (2005) associated the relationship between the mineral dustdeposited on the snow surface and the release of HONO from it,while Mangold et al. (2011) used the event as a case study toevaluate the performance of an aerosol modelling system coupledto a numerical weather prediction model with data assimilation. Inthis paper instead the focus is the thorough characterization of theevent in itself. The reason for such a specific interest emerged froma deep and still on-going analysis of our own dataset. In the specific

E. Brattich et al. / Atmospheric Environment 113 (2015) 223e235 225

case treated in this paper our standpoint is first of all the relevanceof the monitoring stationwhere our long-term experiment was runand last but not least the exceptionality of the event described. Infact, extensive literature on Saharan dust phenomenology is avail-able at ground level (e.g., Kabatas et al., 2014; Marconi et al., 2014)where dust transport may lead to increases in the particulate massload, but that must be resolved against complex source mixtures.Under these conditions the relative importance of Saharan Dustcontribution is largely a function of distance from the source region,while occurrences like those described in our paper is not at allcommon at more than 2000 m asl especially in the framework ofthe long time series of observations to which this occurrence be-longs. Moreover, similarly extensive literature concerning SaharanDust and vertical profiles of particle densities with height is alsoavailable (e.g., Yang et al., 2012; Tsamalis et al., 2013; Mona et al.,2014), but this data are not time resolved at a sub-hourly timescale. Our study in fact is set at Mt. Cimone, a high altitude WMO-GAW station at over 2000 m a.s.l. representative of the southernEuropean/Mediterranean basin free troposphere, a region which isa well-known hot-spot for air quality and climate change (Monkset al., 2009) and a major “crossroad” for atmospheric compoundtransport processes (Lelieveld et al., 2002). Given the altitude of Mt.Cimone station average mass loadings are typically much lowerthan ground based stations (mean ¼ 8.8 ± 8.0 mg m�3;median ¼ 6.5 mg m�3). During the event herein described a con-centration of 80 mg m�3 in PM10 was recorded, a value well abovethe EU Directive limits of 50 mg m�3 (Directive 2008/50/EC, thoughthe Directive itself allows for the subtraction of the natural aerosolcontributions to the monitored PM10 levels after assessing theirorigin and amount) usually referring to ground level stations forenvironmental protection purposes. This value largely in excess ofatmospheric regulations even at ground level is of even higherimpact for such a high elevation site as an increase of almost 10times over the 12-years mean has been recorded. Moreover aswidely explained in our previous work (Tositti et al., 2013), also theuse of PM10 data is not discounted since this parameter which is astandard metric for air quality monitoring purposes is much lessused for climatological stations providing a further efficient meanof large scale circulation comparison when treating backgroundproblems.

Following data validation the event was investigated in moredetails in order: 1) to understand the circulation pattern which canfavour the occurrence of similar events, 2) to show the capacity ofnon-conventional atmospheric tracers to diagnose the occurrenceof this event and to trace the horizontal and vertical transport of airmasses, and 3) to enrich the observation concerning PM10 with lesscommonly exploited experimental data providing a deeper insightbased both on a highly time resolved and a large scale scenario.

The paper is finally organized as follows: Section 2 describes thesampling techniques and research methods we adopted for thisresearch, while Section 3 describes the results of our analysisintegrating both meteorological as well as experimental informa-tion and Section 4 draws the main conclusions.

2. Material and methods

Mt. Cimone is the highest peak of the Italian northern Apen-nines and hosts a global WMO-GAW station maintained by theMeteorological Service of the Italian Air Force and the Institute ofAtmospheric Sciences and Climate of the Italian National ResearchCouncil (ISAC-CNR “Ottavio Vittori” station). The site is located faraway from large industrialized and urban areas, has a 360� freehorizon experiencing both regional and long range transport of airmasses (Bonasoni et al., 1999, 2000; Cristofanelli et al., 2006, 2009,2013; Cristofanelli and Bonasoni, 2009; Tositti et al., 2013). The

station lies above the planetary boundary layer (PBL) during mostof the year, even if an influence of the innermost layer cannot becompletely ruled out in particular during warm months due to theincreased mixing height and the influence of the mountain/valleybreeze regimes (Fischer et al., 2003; Cristofanelli et al., 2007). Themeasurements of atmospheric compounds and meteorologicalparameters at this site can be considered representative of thesouthern European-Mediterranean basin free troposphere(Bonasoni et al., 2000; Fischer et al., 2003; Cristofanelli et al., 2013).

As a WMO-GAW station, a number of atmospheric compoundsare measured at Mt. Cimone since a long time. Being 7Be and 210Pbtracers of vertical and horizontal transport processes, it is sug-gested that they are routinely monitored at WMO-GAW stations(Lee et al., 2004). In the framework of the research activities of thisWMO-GAW station, the Department of Chemistry of the Universityof Bologna has measured the natural radionuclides 7Be and 210Pband aerosol mass loading in the form of PM10 (airborne particulatematter with amean aerodynamic diameter lower than 10 mm) sincethe early 1990's, even if measurements became regular only since1998 following the acquisition of a Thermo-Environmental PM10high-volume sampler with average flow rate of 1.13 m3 min�1 atSTP. All the details about the experimental techniques used forPM10, 7Be and 210Pb sampling andmeasurements at Mt. Cimone canbe found in previously published papers (Tositti et al., 2012, 2013,2014).

Number density and size distribution of particles have beenmeasured since 2000 by ISAC-CNR by using an Optical ParticleCounter (OPC, Mod. GRIMM 1.108). Data collected was re-organizedand collectively grouped as a finemode (0.3 mm� Dp < 1 mm) and acoarse mode (1 mm � Dp � 20 mm) of particles with a 1-min timeresolution (data available at the Nextdata database http://nextdata.sharegeonetwork.evk2cnr.org/). More details about the measure-ment techniques of aerosol physical parameters at the Mt. Cimonestation, as well as a detailed analysis of the dataset are available forexample in Marinoni et al. (2008). Tropospheric ozone is measuredby ISAC-CNR since 1996, by means of UV-analyser Dasibi 1108 W/GEN, which continuously samples ambient air from a non-heatedTeflon sampling head. Tropospheric ozone measurements at Mt.Cimone have been extensively described in previously publishedpapers (Bonasoni et al., 1999, 2000; Fischer et al., 2003;Cristofanelli et al., 2006, 2013).

In this work we used version 2.0 i.e., pre and post field cali-brated, automatically cloud cleared and manually inspected, spec-tral Aerosol Optical Depth (AOD) data from AERONET (http://aeronet.gsfc.nasa.gov/). The network provides a long-term,continuous and readily accessible public domain database ofaerosol optical, microphysical and radiative properties for aerosolresearch and characterization. A standardization of instruments,calibration, processing and distribution and measurement pro-tocols is guaranteed by the network. Measured AOD uncertainty isrelatively small in the AERONET database, about 0.01 in the visibleand near-infrared wavelengths and 0.02 for wavelengths in the UV(Eck et al., 1999). AERONET Sun Direct Algorithm (SDA) retrievals(fine and coarse mode AOD as well as fine mode fraction) are basedon SDA Version 4.1 (O'Neill et al., 2003, 2005). In addition to thedirect solar irradiance measurements, the radiometer measures thesky radiance in four spectral bands (440, 670, 870 and 1020 nm)along the solar principal plane (i.e., at constant azimuth angle,changing the scattering angle) and along the solar almucantar (i.e.,at constant elevation angle, varying the azimuth angle). Aureoleand sky radiances are acquired through a large range of scatteringangles from the sun through a constant aerosol profile to retrievesize distribution, phase function, aerosol optical depth, and singlescattering albedo (u0). For all sky radiance wavelengths, the un-certainty of the retrieved single scattering albedo is expected to be

E. Brattich et al. / Atmospheric Environment 113 (2015) 223e235226

±0.03 based on Version 1 almucantar retrieval computations whenAOD(440 nm) > 0.4 (Holben et al., 1998; Eck et al., 1999; Duboviket al., 2000, 2002). Aerosol properties of size distribution andphase function over the particle size range 0.1e5 mm are derivedfrom the Dubovik and Nakajima inversions (Nakajima et al., 1983,1996; Dubovik and King, 2000; Dubovik et al., 2000). The inver-sion assumes aerosol particles partitioned into two components:spherical and non-spherical. The spherical component is modelledby an ensemble of polydisperse, homogeneous spheres (complexindex of refraction is the same for all sizes). The non-sphericalcomponent is a mixture of randomly oriented spheroids (e.g.,Mishchenko et al., 1997). Recent field measurements reported thatthe measured optical properties are comparable to the AERONETretrievals (Petzold et al., 2009; McConnell et al., 2010).

3. Results and discussion

As described in the Introduction, Saharan Dust outbreaks arefairly frequent throughout the year, exhibiting a seasonal peakfrequency towards the Atlantic Ocean in spring, and in late spring/summer (May, June, July) towards the Mediterranean Sea. Notice-ably, winter and especially autumn events are less frequent, butusually very intense (Prospero et al., 2002; Baldacci, 2005; Barkanet al., 2005).

In 2004, from 13 to 15 March, the most severe dust outbreakepisode was observed in over 12 years of monitoring activity(1998e2011) at Mt. Cimone. The event reported in the presentpaper represents the absolute maximum value of PM10 recorded inover twelve years of observations, exceeding 80 mg m�3, a con-centration which immediately drove the attentions of the authorsdue to the extremely large exceedance as compared to the averageconcentrations usually encountered (mean ± standarddeviation ¼ 8.8 ± 8.0 mg m�3; median ¼ 6.5 mg m�3; geometricmean ¼ 6.0 mg m�3) at this high altitude station. This value isalmost ten folds higher than the local mean reaching a concen-tration level absolutely unprecedented for a station located above2000 m a.s.l. This value is also considerable since it is largely inexcess of the EU air quality metrics for PM10 of 50 mg m�3 mostlyreferring to ground level stations. Owing to this strong singularity,the event was analysed in deeper detail starting with a series ofsimultaneous parameters related to airborne particulate availableat this site and already treated in a more general way in our pre-vious papers (Tositti et al., 2013, 2014).

During the event the abrupt increase of PM10 reported abovewas also accompanied by a remarkable increase of fine and coarsenumber densities, 210Pb, and 7Be (Fig. 1aee). Substantially incomparisonwith March 2004 monthly mean (this and all followingaverages calculated including the event), not only PM10 increasedof þ540%, but the parameters mentioned above increased respec-tively of þ54%, þ360%, þ73%, and þ32%, whereas the comparisonwith the whole 2004 dataset shows that PM10 increase amountedtoþ820%,þ33% for 210Pb,þ42% for fine particles,þ1257% for coarseparticles. The variations are also significant if compared to theaverage March average concentrations calculated for the period1998e2011 at the site: þ812% PM10, þ61% 210Pb, þ4% 7Be, þ9% fineparticles, þ94% coarse particles. It is noticeable though expectedboth the variation in PM10 and in the coarse fraction concentrationclearly indicating the close relationship between mass loading andcontribution of large sized particles. In the SupplementaryInformation (herafter SI) I the box-and-whiskers plots of thehigher resolution values of fine, coarse particles number densities(30 min averaged) and ozone (1 h averaged).for the event againstyear 2004 and month March 2004 (Figure SI1).

The increase in 210Pb is associated with the crustal origin ofmineral dust where it occurs as a typical geochemical tracer

deriving both from an intrinsic component in equilibrium with theparent 226Ra and from the indirect form produced by the 222Rnemitted by rocks and soils usually known as excess or unsupported210Pb known for being highly particle-reactive and typically asso-ciated with the aerosol accumulation mode (Papastefanou andIoannidou, 1995; Winkler et al., 1998; Gaffney et al., 2004;Ioannidou et al., 2005).

Less obvious is the behaviour of 7Be, known as typical uppertroposphere-lower stratosphere (UT-LS) tracer (Zanis et al., 2003;Allen et al., 2003; Cristofanelli et al., 2006). In fact, though to alesser extent, also this tracer shows an increase during the SaharanDust event herein described, a circumstance which could appearcontradictory when considering the nature and the origin of thetransport analysed. However, on the basis of an ongoing studyfocussing on the characterization of advection patterns and theirimpacts on atmospheric composition at Mt. Cimone (Brattich et al.,manuscript in preparation), the authors anticipate that some epi-sodes of transport from North Africa appear to be characterized byconcurrent uplift of crustal particles (leading to increases of PM10and 210Pb) and downdrafts from the upper troposphere character-ized by air masses enriched in 7Be, in agreement with Due~nas et al.(2011). The observation of stratospheric-upper tropospheric signalstogether with the arrival of dust from the Sahara desert can betentatively attributed to the fact that the transit of troughs mightfavour both dust advection from the North Africa as well astropopause folding.

A simultaneous limited decrease of O3 (Fig. 1 f) was alsoobserved (�9% with respect to March 2004 monthly mean, �5%with respect to the whole 2004 yearly mean, �4% with respect tothe mean March 1998e2011 value; all the variations thereforesignificant when compared to a 2% combined experimental un-certainty). In the case of Saharan dust, ozone is usually observed todecrease at Mt. Cimone, this behaviour being linked to the reducedsources of pollution in Northern Africa together with reactionsremoving O3 and its precursors (i.e., nitrogen oxides) on the surfaceof mineral particles through catalytic mechanisms due to theirphysico-chemical structure (Usher et al., 2003; Bauer et al., 2004;Bonasoni et al., 2004; De Reus et al., 2005; Karagulian and Rossi,2006).

More interesting is the behaviour of the particle countings fromthe OPC (see Fig. 1ced) which provide a high impact image of theevent inception and evolution. The OPC data in fact is collected at a1 min time resolution, while the PM10 samples are collected on a48-h basis. In this way it is possible to assess not only the intensityof the event, but also its duration which amounts approximately to2 days (event started on 14th March 2004 at 0:45 and ended on16th March 2004 at 3:45). Further on in this section we will showthat the behaviour of OPC data reported matches consistently thedata obtained from the AERONET database with a totally inde-pendent approach and at a different location in Italy affected by thesame Saharan Dust episode.

For sake of completeness, it is interesting to re-consider thescatterplot of fine particles’ number density vs. coarse particleswhich was presented in our previous paper and therefore includingthe overall OPC dataset fromMt. Cimone (Tositti et al., 2013). For aneasier availability to the reader, we have included the related plot inthe SI (Figure SI2). As described in Tositti et al. (2013), this simplebut efficient elaboration shows that at Mt. Cimone aerosol particles>300 nm distribute neatly into three well defined clusters. In thepresent work, each cluster has been fitted by a linear regression inorder to single out the average behaviour. Considering linear fitswith increasing angular coefficients m, it is found that the largestcluster (m ¼ 1.44E-3) is represented by particles peaking in the finemode possibly relatedwith secondary, more or less aged aerosols inagreement with Gobbi et al. (2003), Putaud et al. (2004), Van

Fig. 1. (aef) Time series of: (a) PM10 (mg m�3), (b) 210Pb (mBq m�3), (c) 7Be (mBq m�3), (d) number density of fine particles (N cm�3), (e) number density of coarse particles (N cm�3),and (f) O3 (ppb) acquired at Mt. Cimone in the year 2004. Arrows indicate the observed increases (in the case of PM10, 210Pb, fine and coarse particles number densities, 7Be) anddecrease in the case of O3, connected to the outstanding Saharan dust transport of March 2004.

E. Brattich et al. / Atmospheric Environment 113 (2015) 223e235 227

Dingenen et al. (2005), and Marenco et al. (2006); a cluster asso-ciated with Saharan Dust events with a large degree of datadispersion (m ¼ 4.46E-02) and finally a third, highly linear andcoherent cluster (m¼ 0.17) which pertains almost exclusively to themid-March 2004 event fully highlighting its singularity. During theevent, it is shown that both fine and not exclusively coarse particlessimultaneously and abruptly increased, but even their ratio ischaracteristically and coherently conservative around the slopevalue found, which differently from the other two clusters of data,reveals a very limited degree of dispersion. Removing the datainherent to the mid-March 2004 exceptional dust event we arefocussing on, the cluster of data is still found, but with a verylimited occurrence, suggesting the existence of a distinct thoughmuch less frequent Saharan Dust event typology.

The African desert origin of the event was subsequentlyconfirmed by means of the HYSPLIT (http://ready.arl.noaa.gov/HYSPLIT.php) 96-h back-trajectories calculated for the period andby the results of the DREAM (Dust REgional Atmospheric Model)model (http://www.bsc.es/projects/earthscience/DREAM/) (Fig. 2showings examples of the results), starting at the beginning ofMarch, and first impacting the Atlantic Ocean and then the Medi-terranean area.

This event originated from the Bodele depression in northernChad, a remarkable source of dust (Knippertz and Fink, 2006; Korenet al., 2006); the analysis of aerosol optical depth revealed thatdustiness conditions occurred along the entire ITCZ starting fromthe beginning of March 2004. On 5th March 2004 images from thevisible channel of the SeaWIFS satellite (Fig. 3) show a huge, dense,meridionally oriented dust plume off the northwest African coastfrom west of Madeira to Cape Verde, sustained by hazy and pro-longed Harmattan conditions. Harmattan conditions are caused bythe strong heating of the surface, which forms a deep warm lowand a strong converging flow, capable of lifting up dust particles.This plume spread westward and formed an arc for over 5000 kmfrom Guinea to the northern tip of Morocco. The plume crossed theAtlantic Ocean and impacted onto the Caribbean region (Knippertzand Fink, 2006). Fig. 4 shows the aerosol optical depth at thebeginning of the second dust outbreak (on 13th March, 2004) andthe average over the period 10e15 March, 2004. At the end of thisphase, the meteorological situation developed into the followingsequence: 1) the penetration of an upper-level trough to low lati-tudes with a minimum centered over the NWAlgerian coast; and 2)a Saharan high extending all over the Mediterranean Sea with anelongated north-eastward tongue, mobilizing dust to the south of

Fig. 2. (aec) a) Back-trajectories (96-h backward) calculated by Hysplit-4 model using the NCEP/NCAR Reanalysis meteorological archive, ending at Mt. Cimone on 15th March 2004,12 UTC and for three arrival heights: 1400, 2200 and 3000 m asl; b) Dust load simulated by the BSC-DREAM8b (Dust REgional Atmospheric Model) (http://www.bsc.es/projects/earthscience/DREAM/) dust regional model for the day 14th March 2004, 00 UTC.

Fig. 3. RGB MSG SEVIRI image from channels 1 (VIS0.6, blue), 2 (VIS0.8, green) and 3 (NIR1.6, red) on 5th march 2004 12:00, showing a major dust outbreak from Western Africaacross the Atlantic. The massive storm formed a huge arc of thick dust reaching Cape Verde Islands and the shores of Western Europe; during the following days, the dust plumecontinued to spread southwards and westwards. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

E. Brattich et al. / Atmospheric Environment 113 (2015) 223e235228

Fig. 4. Aerosol optical depth at 0.55 mm, daily average on 13th March 2004 (left), and time averaged over the period 10e15 March 2004 (right). Every image is the average over datafrom the MODIS Terra and Aqua satellites (MOD08_D3.051 and MYD08_D3.051 collections). Deep Blue retrievals are included into the average (Image courtesy of MODIS instrumentteam, NASA Goddard Space Flight Center). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

E. Brattich et al. / Atmospheric Environment 113 (2015) 223e235 229

the northern Atlas Mountains in Morocco and western Algeria. Theinception of a steep pressure gradient between a trough and aSaharan high extending from theWestern Sahara up to the westernMediterranean basin is a typical condition promoting efficientmineral dust transport toward the central Mediterranean (Barkanet al., 2005).

The synoptic condition characterizing the mid-March period isrepresented by the NCEP-based map of 700 mbar level (Fig. 5). Thechart shows the relative position of the two geopotential maxima/minima, and the NE high extending toward the western Mediter-ranean basin. Altogether, these images clearly show the severe dustoutbreaks across the Atlantic and the Northern part of Italy: duringthis event the monitoring site of Mt. Cimone was located exactlyalong the main axis of the dust plume trajectory, leading to a PM10record maximum of 80 mg m�3.

In order to test the spatial influence of the Saharan Dust trans-port, and as a possible means to infer optical properties of dust, wealso examined the data from the Venice AERONET site. Fig. 6(aec)depicts the aerosol optical thickness (AOT) data in March 2004 (a),the AOD at 500 nm (total, fine and coarse mode) (b) and Angstromexponent (AE) at 500 nm (c) trends during 2004. Fig. 6a reveals anabrupt and relevant increase in aerosol optical thickness at 555 and500 nm, while Fig. 6b, c highlight a remarkable increase of the AODon 14e15th March 2004 (þ194% over the March 2004 monthlymean and þ285% over the whole 2004 annual mean) and adecrease of the AE (�69% over the monthly mean and �75% overthe annual mean). The decrease in AE was mostly connected to anincrease of the coarse fraction (þ1380% over the monthly mean,and þ502% over the 2004 annual mean, as compared to the þ48%and þ59% respective increases in the fine fraction), in agreementwith the ground-based measurements at Mt. Cimone of the OPCpreviously shown (Fig. 1ced). In fact, it is well known that the AE isa measure of the wavelength dependence of the AOD and is

inversely related to particle size (i.e., the decrease of the Angstromexponent in March 2004 indicates larger particle size) (e.g.,Angstrom, 1929; Junge, 1955; Eck et al., 1999; Reid et al., 1999).

The occurrence of a well-defined coarse mode in the particlesize distribution on 15th March 2004 is also clearly discernible inFig. 7. Even considering the difference between the two sites(Venice being an urban coastal polluted site, as compared to Mt.Cimone being a high-mountain remote site), this indicates a goodagreement between our experimental data collected at Mt. Cimoneand the AERONET data retrieved through an inverse method,providing a rough tool of intercalibration between different ap-proaches and within the limits of the overall uncertainties of both.In fact, the bimodal size distributions retrieved from the AERONETreflect that over Venice dust was almost absent on 9th and 16thMarch, while a shift towards the coarse modewas observed only on15thMarch, in agreement with in-situ observations with the OPC atMt. Cimone. The presence and the height of the aerosol dust layerwas further checked by the visual inspection of the image collectedby the Rayleigh-Mie-Raman lidar of ISAC-CNR Rome (Dionisi et al.,2010; http://lidar.ifa.rm.cnr.it/), approximately 300 km upwind ofMt. Cimone on 15thMarch 2004 at 18 UTC. The images (not shown)of lidar scattering at 532 nm, together with the increase in watervapour (Raman water vapour), highlight the presence of adescending aerosol dust layer at about 3e4 km. Observations fromthe ISAC-CNR Lidar-Ceilometer Vaisala LD-40 (Gobbi et al., 2013),also set in Rome, are also available during the same period, indi-cating a volume of dust compatible with about 100 mg m�3 at 2000m (about the height of Mt. Cimone) for 15th March 2004, while theestimated volume for 16th March at the same height was about60 mg m�3. In Fig. 7 it is easily observed that during the wholeconsidered period the columnar size distributions from AERONETwas largely affected by submicron particles (fine particles). Thisfeature was already observed in a previous study of Reid et al.

Fig. 5. Geopotential height at 700 mbar for 14 March 2004, 12 UTC. Data from the NCEP/DOE AMIP-II Reanalysis project (Image provided by Physical Sciences Division, Earth SystemResearch Laboratory, NOAA, Boulder, Colorado, from their Web site at http://www.esrl.noaa.gov/psd/).

E. Brattich et al. / Atmospheric Environment 113 (2015) 223e235230

(2003), where the Dubovik and King (2000) inversion wascompared with the retrieval of Nakajima et al. (1996). The results ofReid et al. (2003) showed that on average the two inversionsexhibit similar behaviour, but with a most significant difference inthe fine mode. The Nakajima et al. (1996) retrievals showed no sign

Fig. 6. (aec) Time series of: (a) aerosol optical depth (AOD) during the year 2004; (b) Angaerosol optical thickness (AOT) during the month of March 2004. Arrows indicate the observthe outstanding Saharan Dust transport of March 2004.

of a fine mode during the entire study, while the Dubovik and King(2000) inversion typically retrieved an accumulation mode at0.1e0.3 mm. On high AOT days, this finemode from the Dubovik andKing (2000) inversion has an unrealistically small volume mediandiameter < 0.2 mm, due to the asymmetric nature of the particles

strom Exponent (AE) during the year 2004 acquired at the Venice AERONET site; (c)ed increases (in the case of AOT and AOD) and decrease in the case of AE connected to

Fig. 7. Volumetric size distributions retrieved by the inversion algorithm on days prior(9th and 12th March 2004), during (15th March 2004), and after (16th and 17th March2004) the arrival of the Saharan dust at the Venice AERONET site.

E. Brattich et al. / Atmospheric Environment 113 (2015) 223e235 231

(Dubovik et al., 2002). On lower AOT days, when there is less dust,this mode decreases in prominence and the volume mediandiameter increases reaching a value of 0.3 micron. In conclusion,even when restricting the retrievals to low solar zenith angles, thedust asphericity effects can affect the fine mode in the Dubovik and

Fig. 8. (aeb) Extinction AOT a) and single scattering albedo u0 b) wavelength dependence rduring (15th March 2004), and after (2nd and 14th April 2004) the arrival of the Saharan d

King (2000) inversion (Dubovik et al., 2000). Fine particles with amode radius <0.05 mm for the smallest mode and large particleswith a mode radius >10 mm for the largest mode cannot beretrieved with an acceptable accuracy even in error-free conditions.The increase of the errors for both cases of very small and very largeparticles can be explained by the fact that the contribution of theseparticles to the measured optical characteristics is significantlysmaller than for particles of intermediate sizes (0.1 < r < 7 mm).These outlined limitations of AERONET in the retrieved particles’size distribution, however, do not appear to affect its validity incapturing the relevant role of the coarse mode on 15thMarch 2004.

Fig. 8 depicts that the arrival of the Saharan dust in March 2004was connected to an increase in the extinction properties of theaerosol (Fig. 8a), as well as in u0 (Fig. 8b), in agreement with pre-vious observations (u0 of dust in the range 0.8e0.9 (e.g.,Moosmüller et al., 2012; Costabile et al., 2013; Ryder et al., 2013).Altogether, Fig. 8 highlights that the change in the extinctionproperties of the aerosol was linked to a marked change in thescattering as compared to the absorption, confirming once morethe arrival of the Saharan dust. It is in fact known that the ab-sorptivity of dust is smaller compared to anthropogenic aerosols(e.g., Yoshida et al., 2013), whereas the efficiency of scattering byparticles increases when the size parameter is comparable with thewavelength (thus the scattering of small particles is more pro-nounced at the short wavelengths, whereas the scattering of largeparticles (dust) is more pronounced at long ones).

Finally, the change in the retrieved imaginary part of therefractive index (Fig. 9) puts an accent on the variation in thechemical composition of the aerosol (in particular due to iron

etrieved by the inversion algorithm on days prior (17th February and 3rd March 2004),ust of March 2004 at the Venice AERONET site.

Fig. 9. Real (a) and imaginary (b) refractive index wavelength dependence retrieved by the inversion algorithm on days prior (17th February and 3rd March 2004), during (15thMarch 2004), and after (2nd and 14th April 2004) the arrival of the Saharan dust of March 2004 at the Venice AERONET site.

E. Brattich et al. / Atmospheric Environment 113 (2015) 223e235232

oxides), and its inherent scattering (real part n in Fig. 9a) and ab-sorption (imaginary part k in Fig. 9b) coefficients. The real refractiveindex presents an uncertainty of 0.04 when the optical depth at440 nm is higher or equal to 0.5, whereas the retrieval of theimaginary refractive index is affected by an error in the angularpointing due to the small value of the AE for dust and the largevalue of optical thickness in dust conditions (Dubovik et al., 2000).On 15th March 2004 the retrieved value of the imaginary refractiveindex at 550 nm was equal to 0.0025, in agreement with recentobservations conducted in the Saharan desert region (Kaufmanet al., 2001; Colarco et al., 2002; Haywood et al., 2005; Petzoldet al., 2009; McConnell et al., 2010; Kim et al., 2011; Costabileet al., 2013), and possibly confirming that there dust is much lessabsorbing (imaginary refractive index in the range0.0001e0.0046 at 550 nm) than previously measured (imaginaryrefractive index in the range 0.0053e0.008 at 550 nm) (Pattersonet al., 1977; Shettle and Fenn, 1979) despite the outlined limita-tions of the AERONET retrievals.

4. Conclusions

An exceptional Saharan dust event was observed at the WMO-GAW station of Mt. Cimone in the period of 13e15 March 2004.The synoptic situation was characterized by a steep gradient be-tween a trough and a Saharan high along the Western Sahara andtheWestern Mediterranean basin, a typical condition during whichdust is transported toward the central Mediterranean. In particular,during this occurrence Mt. Cimone was located exactly along themain axis of the dust plume: the particular funnel-shaped synopticconfiguration lead to a PM10 record concentration of 80 mg m�3

(maximum concentration observed in 12 years observations, withrespect to the mean and median values of 8.0 and 6.5 mg m�3,respectively). A simultaneous relevant increase in the coarse par-ticles number density was observed, while measured increases in210Pb, number density of fine particles, and 7Be were less steep,

even though still remarkable. The results of the back-trajectoryanalysis, together with the DREAM model, as well as satellite im-ages confirmed the origin of the dust plume from the NorthernSaharan desert.

Data from the AERONET site of Venice, also located along themain axis of the dust plume during the dust event, was also ana-lysed to investigate the spatial extension of the event and toinvestigate variations in aerosol's optical properties. A relevantincrease of the AOD and a decrease of the AE, connected to arelevant increase of coarse mode particles, were highlighted. Theappearance of a well-defined coarse mode is clearly discernible inthe retrieved AERONET size distribution, in good agreement withthe ground-based measurements at Mt. Cimone. Within the limi-tations of AERONET products derived from inversion algorithms,data from the AERONET site also indicate a marked change in thescattering properties of the aerosol more than in the absorbingones, in agreement with recent observations showing that mineraldust from the Sahara desert is not as absorbing as previouslythought (imaginary refractive index in the range 0.0001e0.0046 at550 nm with respect to the previously measured range0.0053e0.008). This information, once included in climate models,can be useful to better assess the role of mineral dust transport inaffecting the regional climate in the Mediterranean basin hot-spot.

Acknowledgements

ISAC-CNR is gratefully acknowledged for providing aerosol sizedistribution and ozone data, as well as infrastructural access at theWMO-GAW Global Station Italian Climate Observatory “O. Vittori”at Mt. Cimone. The Italian Climate Observatory “O. Vittori” is sup-ported by MIUR and DTA-CNR throughout the Project of NationalInterest NextData. The authors would like to thank the Italian AirForce (CAMM Monte Cimone) for the valuable co-operation at theCMN station. We acknowledge NOAA (http://www.esrl.noaa.gov/)for providing the HYSPLIT trajectory model (available at http://

E. Brattich et al. / Atmospheric Environment 113 (2015) 223e235 233

ready.arl.noaa.gov/HYSPLIT.php) and the NCEP/NCAR reanalysisdata used in this study. We thank the Barcelona SupercomputingCenter for providing the data from the BSC-DREAM8b (DustREgional Atmospheric Model) model; NASA Goddard Space FlightCenter for providing the SEAWIFS and MODIS Terra and Aqua im-ages. We acknowledge the PI Giuseppe Zibordi (JRC) for his effort inestablishing and maintaining the Venice AERONET site. We thankDr. Gianluigi Liberti and ISAC-CNR Rome for providing images andobservations from the Rayleigh-Mie-Raman lidar of ISAC Rome(http://lidar.ifa.rm.cnr.it/), and Dr. Gian Paolo Gobbi for providingobservations of the Lidar-Ceilometer Vaisala LD-40 from ISAC-CNRRome. We are indebted to Europe Executive Editor, Prof. Dr. AlfredWiedensohler, and two anonymous reviewers for providinginsightful comments and questions which has resulted inimproving the presentation of this work. We also acknowledgeProf. Lucas Alados Arboledas, Dr. Thomas F. Eck, Dr. Ali H. Omar,Prof. Alexander Smirnov, Dr. David M. Giles, and Dr. Edward Now-ottnick for their help and precious information about the AERONETretrievals.

Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.atmosenv.2015.05.017.

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