Physicochemical characteristics of aerosols measured in the spring

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Atmospheric Environment 54 (2012) 545e556

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Atmospheric Environment

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Physicochemical characteristics of aerosols measured in the spring time in theMediterranean coastal zone

J. Piazzola a,*, K. Sellegri b, L. Bourcier c, M. Mallet d, G. Tedeschi a, T. Missamou a

aAix-Marseille Univ, Mediterranean Institute of Oceanography, Marseille, Franceb LAMP, Blaise Pascal University, Francec Institute for Reference Materials and Measurements, JRC, B-2440 Geel, BelgiumdUniversité de Toulouse, UPS, LA, 31400 Toulouse, France

a r t i c l e i n f o

Article history:Received 15 October 2011Received in revised form27 January 2012Accepted 13 February 2012

Keywords:Coastal aerosolsAnthropogenic compounds

* Corresponding author. Tel.: þ33 (0) 494142082; fE-mail address: [email protected] (J. Piazzola).

1352-2310/$ e see front matter � 2012 Elsevier Ltd.doi:10.1016/j.atmosenv.2012.02.057

a b s t r a c t

Aerosol particles in coastal areas result from a complex mixing between sea-spray aerosols locallygenerated at the sea surface by breaking waves and a continental component arising from natural and/oranthropogenic sources. This paper presents physicochemical characterization of aerosols observedduring meteorological conditions characteristics of coastal areas. In particular, we study the influence ofsea-breezes and land-breezes as well as the fetch variation, which superpose on larger synopticconditions, on aerosol properties. This was achieved using a physical, chemical and optical analysis of theaerosol data acquired in May 2007 on the French Mediterranean coast. The aerosol distributions weremeasured using a TSI SMPS 3081 model and the chemical characterization was made using an IonChromatography analysis (IC) and a thermo-optical technique. In addition, aerosol optical characteristicswere provided by aethalometer (absorption) and nephelometer (scattering) measurements. For low windspeeds, we detect high aerosol number concentrations as well as high NO�

3 and carbonaceouscompounds contributions, which are observed even when the aerosol is sampled in pure maritime airmasses. These results indicate that air masses are strongly impacted by pollution transported over theMediterranean. In addition, the combination of low wind speeds and land/sea-breezes lead to theproduction of new ultrafine particle formation events that seem to take place over the sea before beingtransported back to the coast. Under higher wind speed conditions, aerosol number and mass concen-trations of smaller sizes are significantly lowered due to the dispersion of anthropogenic pollutants.Optical measurements reveal that mean scattering and absorbing coefficients are about 15.2 Mm�1 and3.6 Mm�1, respectively. Associated mean aerosol single scattering albedo is found to be about 0.87 and0.94 (at 520 nm) for continental and maritime influences.

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1. Introduction

The role of atmospheric aerosols in climate change is well-recognized (Intergovernmental Panel on Climate Change (IPCC),2007), but the estimation of their impact remains an importantscientific challenge. In particular, the uncertainties related toaerosol radiative forcings remain very large (IPCC, 2007). This isdue to the heterogeneous spatial and temporal distribution oftropospheric aerosol particles, their different origins (natural oranthropogenic), and their physical and chemical transformation inthe free troposphere. In marine areas, the sea-sprays generatedat the airesea interface by wave breaking represent a major

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component of the natural aerosol mass (Jaenicke, 1984; Andreae,1995; Yoon et al., 2007; Piazzola et al., 2009) and therefore areimportant in the Earth radiative budget (Laskin et al., 2003; Malletet al., 2003; Mulcahy et al., 2008). In addition, they have a signif-icant influence on the coastal urban air quality (Knipping andDabdub, 2003) through their ability to have chemical and phys-ical interactions with other aerosol species and gases. In addition,sea-spray aerosols transport a large variety of organic matter toecosystems of estuaries (Paerl et al., 2000). The size of sea-sprayscan vary over a wide range (0.02e50 mm diameter). Moreover,they are also very soluble and hygroscopic, and increase to twicetheir size for a relative humidity varying from 50 to 100% (Tang,1996; Fitzgerald, 1989). Sea-spray is made primarily of sodiumchloride (NaCl) and small amounts of sulphate, calcium andpotassium, but they can also contain significant amounts oforganic carbon. On the basis of measurements carried out over

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Fig. 1. Detailed view of the study area. The black square shows the location of theexperimental station. The grey part of the circle shows the wind directions whichcorrespond to fetch limited conditions, while the empty portion of the circle shows thewind direction interval denoting infinite fetch conditions.

J. Piazzola et al. / Atmospheric Environment 54 (2012) 545e556546

the North Atlantic coastal area, recent studies have evidenceda strong seasonal variation in the chemical aerosol propertiesconnected to biological oceanic activity (Cavalli et al., 2004;O’Dowd et al., 2004). In particular, a dominant inorganic sea-saltcontribution to sub-micron aerosol particle mass was observedduring winter, while, during periods of high biological activity, theinorganic sea-salt signal was progressively replaced by organicmatter and nss-sulphate. The organic component was mainlyattributed to bubble bursting processes, due to the predominantlyinsoluble and surface active character of organic carbon in themarine aerosol particles (Ceburnis et al., 2008). However, thecontribution of secondary sources to organic matter is still underdebate.

Over the Mediterranean basin, the atmospheric aerosolrepresents a mixing between particles produced by naturalprocesses (of both continental and marine origin) and particles ofanthropogenic origin released in dense urban areas. As a conse-quence, the sea-spray aerosol is often strongly impacted byanthropogenic emissions. In the Mediterranean Sea, the amountof aqueous phase pollutants are expected to be much higher thanin the Atlantic due to important inputs from major continentalrivers such as the Rhône (France) or Pô (Italy). Pollution in coastalwaters combined with the high intensity of solar radiation in theMediterranean area can cause an excess in nutrients, as phosphateand nitrate compounds resulting in eutrophication processesthroughout the year (Jamet et al., 2001). The size distribution ofmarine aerosol particles has also been proved to experiencesignificant seasonal variations (Yoon et al., 2007), which could alsobe due to the biological activity. During laboratory measurements,surfactants present in artificial sea water also proved to modifythe aerosol size distribution (Sellegri et al., 2005). However, theatmospheric aerosol composition over the western part of theMediterranean Sea has rarely been characterized away from largeurban areas.

In addition, coastal areas are characterized by the occurrenceof local meteorological processes, specifically induced by thelandesea transition, as the land breeze, sea breeze and the fetchoccurring for offshore wind conditions. A better knowledge of theinfluence of these processes on the aerosol properties is of greatimportance for improvement of the models dealing with theaerosol transport and transformation in the atmosphere. The mainobjectives of the present paper are to (1) determine the extent towhich marine aerosol is impacted by anthropogenic activities inpure maritime air masses representative and (2) to study the roleof the local meteorological conditions typical of coastal zone, inparticular the sea breeze and land-breeze conditions on the aerosolproperties. In this latter case, a better knowledge of the mixingbetween sea-sprays produced for short fetch and continentalsources is provided. The study area is located on the FrenchMediterranean coast, which represents a densely populated zonewhere large amounts of aerosol particles of anthropogenic originare expected. If a strong contribution of sea-spray aerosols wasnoted even for offshore winds at short fetches (Piazzola andDespiau, 1997a; Piazzola et al., 2002), pollutants are much higher,even for long fetches, than in the Atlantic atmosphere. Previouswork conducted by Sellegri et al. (2001) then showed theconcomitant presence of sea-sprays and high levels of anthropo-genic species such as nitrate.

This paper presents an analysis of aerosols measured in theMediterranean coast in May 2007 on the island of Porquerolles,which is located at about 5 km from the coastline. The results dealswith the physicochemical and optical properties of Mediterraneanaerosols for sea breeze and land breeze as well as offshore windconditions. The data are then compared with aerosol concentra-tions measured in other coastal locations.

2. Field site and experiments

The experiments took place from the 5th May to the 29th May2007. The present paper deals more particularly with the aerosoldata recorded from the 23rd to the 28th of May. The study area isthe Toulon-Hyères bay (Fig. 1) located on the French Riviera,between 6.15 and 6.25� east longitude and at 43� north latitude.Measurements of aerosol particle size distributions and meteoro-logical parameters were performed at the extreme west point ofthe island of Porquerolles (Fig. 1). Fig. 1 shows that the sea statedepends on the wind’s trajectory over water, i.e., the fetch. Thestation is exposed to air masses from the open sea, which corre-sponds to infinite fetches as defined by the criterion applied forfully developed sea conditions (Hsu, 1986), as well as to air massesoriginating over the European mainland, with a very short fetch(about 5 km), that represent continentally polluted conditions.Fetch limited conditions occur for local wind directions varyingbetween 290 and 30�, whereas the 160e220� wind directioninterval generally corresponds to open waters (Fig. 1). In Fig. 2, thewind vectors recorded during the sampling period are reported.

2.1. Sampling and analytical procedures

Meteorological datameasured at the sitewerewind speed, winddirection, air and sea temperature and relative humidity. Sizedistributions of particles in the 10e430 nm diameter range weremonitored using an SMPS-TSI model 3081. The aerosol countersand the meteorological sensors were located at heights of 22 and30 m, respectively, above the sea surface. We did not monitor therelative humidity in the SMPS. However, although aerosol was notdried before measurement for cascade impactor samplings, it waspresumably dry for SMPS samples since data counting was per-formed at higher temperatures than ambient temperature. Thiswas shown to dry the aerosol to humidity lower than 50% even invery humid environments, such as cloudy environments as it wasreported at the Puy de Dôme station (Venzac et al., 2009).

For chemical characterization, aerosols were sampled directlyin the atmosphere with two low pressure cascade impactors(Dekati) from the 5th of May to the 10th of May 2007. The firstimpactor was a 30 lpm 13-stages low pressure cascade impactorwhich cut-off aerodynamic diameters were 0.03, 0.06, 0.108, 0.17,0.26, 0.4, 0.65, 1, 1.6, 2.5, 4.4, 6.8 and 9.97 mm, which was dedicatedto the organic carbon (OC) and elemental carbon (EC) analysiswhile the second impactor operated at 20 lpmwith approximatelythe same size cut diameters for subsequent analysis with Ion

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Fig. 2. (a) Wind direction and strength from the 23rd to the 25th of May, i.e., episode 1as referred in the text. (b) Wind direction and strength from the 25th to the28th of May, i.e., episode 2 as referred in the text. (c) Wind direction and strength the28/05/07, i.e., episode 3 as referred in the text.

J. Piazzola et al. / Atmospheric Environment 54 (2012) 545e556 547

Chromatography. Sampling duration varied from 4 to 24 h. Thecollection plates were custom-made out of aluminium foilfor Ionic Chromatography analysis and quartz Whatman for theOC/EC analysis. Blank levels were performed on a sample-to-sample basis. Collection plates were handled and extractedunder a laminar flow hood. Before sampling, the aluminium foilwere washed with MeOH and rinsed with Milli-Q water. Thenthe collection plates were dried in a class 1000 clean room(T ¼ 22 �C � 1; DP ¼ þ60 Pa; RH ¼ 50% � 10; Clermont Ferrand,France). Before and after sampling, the collection plates wereweighted, using a microbalance UMT2 Metler Toledo, after 24 h in

the clean room in order to reach the equilibrium temperature andrelative humidity. Impaction plates were stored at �4 �C beforeanalysis.

As we will discuss in Section 4, the measurement of NO�3 is

delicate and may be affected by large uncertainties (Chang et al.,2001). A possible artefact can occur by the adsorption of gaseousnitric acid onto the filter material, which will be then accounted asnitrate (Ten Brink et al., 2009). On the other hand, nitrate can beevaporated in the form of semi volatile ammonium nitrate andthere is some nitrate losses during impactor sampling due to lowpressures in the instrument. However, these losses are generallylower than those observed with filter samplers (Zhang andMcMurry, 1992; Wang and John, 1988). Wang and John (1988)estimated evaporation losses up to 7%. For impactor sampling, thesame authors determined wall losses of nitrate of 1.1% for particleslarger than 2 mm, of 3.0% for particles between 0.5 and 2 mm and of8.1% for particles smaller than 0.5 mm. Thus, in the present results,nitrate data has been corrected only from the filter blank.

The soluble inorganic compounds were analysed by IC. Thecollection plates were extracted in their storage bottle for fewminutes using 10mL of Milli-Q water. Cations (Naþ, NHþ

4 , Kþ, Mg2þ

and Ca2þ) were analysed with a Dionex ICS-1500 chromatograph,using a CS16 column, a CG16 guard column and chemical regen-eration was made with a CSRS ULTRA II autosuppressor anda 0.20% MSA eluent. Concentrations of major water-soluble anions(Cl�, NO�

3 , SO2�4 and C2O

2�4 ) were determined with a Dionex IC25

chromatograph, using an AS11 column, an AG11 guard columnand an ASRS ULTRA II autosuppressor. Injection was performeda KOH gradient and an EG40 eluant generator. Anions and cationsare injected in parallel with an AS40 automated sampler with theinjection loop of 750 mL. The collection plate blank arithmeticaverages were subtracted from sample concentrations. Thecontribution of blank collection plates varied according to thechemical species, the highest contribution observed for the lessconcentration compounds such as chloride and sodium was 3.6and 4.6% respectively. For samples under the detection limit, weused the average blank value to rise up their detection levels.The carbonaceous fraction (OC and EC) was determined bya thermo-optical technique with correction of pyrolysis by lasertransmission (the TOT method) on a Sunset Lab analyser (Birchand Cary, 1996). The temperature ramp uses four steps up to870 �C under pure helium for the quantification of OC (OrganicCarbon), and four steps up to 900 �Cwith a mixture of 98% Heþ 2%O2 for EC (elemental carbon) (Aymoz et al., 2007).

For optical characterization, a Cimel sun photometer wasinstalled near the station which provided the aerosol opticaldepth (AOD) at four wavelengths (440, 670, 870 and 1020 nm) andthe associated Angstrom coefficient (spectral dependence of AOD).The absorption coefficient (babs) has been obtained using a multi-channel Aethalometer (Model AE-31, Magee Scientific) at sevenwavelengths (370, 470, 520, 590, 660, 880 and 950 nm) using theraw absorbance data following Weingartner et al. (2003). Themethodology used to estimate the aerosol absorbing coefficient isdetailed in Saha et al. (2008). Scattering coefficient of aerosols(bscat) has been measured using a Nephelometer (Model M9004,Ecotech) at 520 nm. Nephelometer draws the ambient air throughan inlet tube, which is then illuminated by a flash lamp and thescattered light intensity is measured by a photo-multiplier tube.Temperature and pressure measurements made in the scatteringchamber are used to calculate the scattering by air molecules,which are then subtracted from the total scattering to get thescattering due to aerosols. The instrument was calibrated regularlyusing standard procedure, inwhich the nephelometer is adjusted toread zero by passing particle free air and the span calibration wasdone in the laboratory using CO2 gas. The scattering data were


corrected for truncation errors following the work of Heintzenberget al. (2006) for Ecotech Nephelometer.

2.2. Campaign overview

The present study focuses on three particular episodes, whichare characteristics of the general meteorology of the FrenchMediterranean coast. In addition to the local wind speed and

Fig. 3. Calculated air mass trajectories for (a) e

direction as measured during the sampling period (Fig. 2aec),numerical calculations of the air mass trajectory were performedusing the hysplit model (Draxler and Rolph, 2003), as shown inFig. 3aec. Furthermore, to help the data analysis (see Section 3), theMixed Layer height, as calculated using the hysplit model, wasreported in Fig. 4.

First, the so-called “episode 1” covering the period 23/05/07 to25/05/07, 08:00 LT deals with typical coastal conditions with air

pisode 1 (b) episode 2 and (c) episode 3.

Fig. 4. The mixed layer height calculated using the Hysplit model for the period covering the Porquerolles experiments.

Fig. 5. Daily variation of the aerosol size distribution recorded using the SMPS systemunder light coastal winds typical of episode 1.


masses coming from the north of Italy. This low wind speed periodis characterized by the occurrence of sea-breezes and land-breezeswhich are superimposed on the synoptic conditions (Fig. 2a). Inparticular, both the 23rd and 24th of May correspond to a very cleardiurnal variation with land breeze from 00:00 LT to 08:30 LT anda sea breeze from 10:00 LT to 19:00 LT. Under these conditions, weexpect a strong continental background aerosol mixed with parti-cles of marine origin.

Episode 2 covers the period from the 25 May 08:00 LT to the 27May 2007 at 12:00 LT. This episode corresponds to pure marine airmass conditions with winds coming from the Mediterranean Sea,as shown in Fig. 3b. This corresponds to a local wind directionvarying from 220 to 270� in the study area. The fetch for these winddirections can range from 100 km to unlimited fetch (open waterconditions). The 26th of May, Fig. 2b shows a local wind directionshift from land to sea breeze also in the mid-morning (around09:00 LT). Again, sea breeze and land breeze add to the synoptic airmass path (Fig. 2b).

Episode 3 took place the 28th of May 2007 and deals with airmasses originate over the Rhône valley, northwest of the studyarea, and locally correspond to a short 25 km fetch on the island ofPorquerolles (Fig. 2c). These conditions are typical of the study area,and defined as “Mistral” conditions. They are associated to highwind speed of northwestern origin (Fig. 3c). For episode 3, localwinds shift from northwest (00:00 LT to 12:00 LT) to southwest(12:00 LT to 23:00 LT) around noon.

3. The aerosol size distributions

This section deals with the aerosol size distributions measuredusing the SMPS system for the three episodes as described inSection 2. First of all, Fig. 5 shows daily variation of the aerosol sizedistribution recorded during periods of low wind speed typical ofepisode 1 (coastal conditions). During sea breeze or land breeze,day or night, fine particle bursts were observed, correspondingprobably to coastal anthropogenic activities which take placenearby on the urbanized coast. These local anthropogenic activitiesare more frequent during the night/land breeze. The end of anaerosol formation event, which is characterized by the growth ofultrafine particles, might be visible in the afternoon. This could bedue to grown particles nucleating over the sea, which stronglyimpact the daytime aerosol size spectrum. The mixed layer depth

show little variations during the whole period of episode 1, andmore particularly the 24th May (Fig. 3a). Such variation has prob-ably a minor influence on the aerosol concentration.

Fig. 6 shows averaged size distributions measured during landand sea breeze conditions, respectively. Each aerosol size spectrumcorresponds to average of about one hundred distributions. Inaddition, the aerosol size spectra were fitted using lognormaldistributions and their parameters are listed in Table 1. We cannote that the accumulation mode mean diameter and integratedconcentration are rather constant from one local wind condition tothe other at around 100 nm and 4500 cm�3. The differences in theland/sea breeze size distributions occur mainly in the particle burstmode (less than 10 nm) which are more numerous during night-time and on the grown nucleation mode (around 50 nm) which areobserved during the sea breeze period. As a consequence, the totalnumber concentrations do not vary substantially from night to day,i.e., 10,150 and 9285 cm�3, respectively.

A typical daily variation of aerosol size distributions recordedduring episode 2 (pure marine air mass conditions at low wind) isshown in Fig. 7. Local contaminations are not observed in theMediterranean Sea sector air masses. Fig. 7 shows larger aerosolconcentrations during nighttime, which corresponds to periodsof local land-breezes, than during daytime which correspond to

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Fig. 6. Average size distributions measured during land and sea breeze conditionsoccurring during episode 1.

Fig. 7. Daily variation of the aerosol size distribution recorded using the SMPS systemfor pure marine air mass conditions of episode 2.

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cleaner southern sea breeze conditions. Indeed, total concentra-tions decrease from 8050 cm�3 during the night to 3930 cm�3

during the day. Such a variation in the aerosol concentration isclearly related to the change in local wind direction from North to

Table 1Characteristics of lognormal distributions fitted on the experimental aerosol spectraas recorded using the SMPS-TSI for each mode diameter Dp (in nanometre). Thenumber concentration, N (in cm�3) and the standard deviation of the lognormalfunction, s are reported.

Mode1 Mode2 Mode3

Dp (nm) Episode 1 Land breeze 7 25 100Sea breeze 7 50 110

Episode 2 Land breeze 7 71 120Sea breeze 7 50 105


N (cm�3) Episode 1 Land breeze 11,000 2500 4300Sea breeze 4000 2800 4800



s Episode 1 Land breeze 1.65 1.75 1.65Sea breeze 1.65 1.3 1.6

Episode 2 Land breeze 1.65 1.3 1.6Sea breeze 1.65 1.5 1.6

Episode 3 Land breeze 1.65 1.5 1.5Sea breeze 1.5 1.5 1.5

South at 10:00 LT to stay South until 18:30 LT. Average size distri-butions were reported in Fig. 8 for both the land and the sea breezeperiods. Again, the accumulation mode particles concentrations donot vary much from night to daytime although their geometricmean diameter is larger during nighttime (120 nm) compared todaytime (105 nm). The Aitken mode particles concentrationsincrease from 700 to 2700 cm�3 between the daytime (sea breeze)and nighttime (land breeze) conditions, and their geometric meandiameter are also larger during nighttime (75 nm instead of 50 nm).

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Fig. 8. Average size distributions measured during land and sea breeze conditionsoccurring during episode 2.

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Fig. 10. Average size distributions measured during land and sea breeze conditionsoccurring, after episode 3.


A nucleation mode is observed in the nighttime average aerosolsize spectra but Fig. 7 shows that a new particle formation eventmight have occurred during the morning local wind shift, i.e., atabout 9:00 LT.

Fig. 9 shows the variation of aerosol size distributions recordedduring episode 3, i.e., during high wind speeds from the northwestsector, which correspond to a 25 km fetch. At 12:00 am, the windshifts from a northwest to a southwest direction. This inducesa sudden decrease of aerosol concentrations from 2450 to 320 par-ticles cm�3. Average size distributions are reported in Fig. 10 for landand sea breeze conditions. The shift from land breeze to sea breeze ismore marked than in episode 1. During the land breeze, the aerosolsize distribution is bimodal with a first mode which mean diameteris 22 nm and the second one at 65 nm. During the sea breezeconditions, average modal diameters are found at 27 nm and 65 nm,with an additional accumulationmode at 170 nm. The differences inthe aerosol size distributions reported in Fig. 10 are probably alsodue to the variation of the Mixed Layer height between the nightand the daytime (Fig. 4). Indeed, we can see in Fig. 4 a stronggradient of the ML height between the night and the day.

The particle size distributions measured during sea breezeconditions of episode 1 and episode 2 can be compared to aerosoldata recorded at the coastal site of Finokalia located at the EasternMediterranean region. and on board of the R/V “Aegaeon” byEleftheriadis et al. (2006) during the SUB-AERO experiment. Thesemeasurements conducted during these studies showed the occur-rence of monomodal particle size distributions centred on a meandiameter ranging from 98 to 144 nm with a standard deviationvarying from 1.56 to 1.9. A large standard deviation on a singlemode could mask one or two modes, which would be more inagreement with our results.

In term of size, the aerosols sampled on the island of Porquer-olles during the Mistral conditions (episode 3) seems to besubstantially smaller than the aerosol sampled on board of the R/V“Aegaeon”, but they showed similar mean diameters during theMediterranean conditions.

A comparison has been made with aerosol size spectrameasured at the coastal site of Mace Head, Ireland, which can bedesignated as “clean marine” in character. In this case, particles inthe Aitken mode have modal median diameters at 31 nm and49 nm during winter and summer respectively, in the accumulationmode at 103 nm and 177 nm during winter and summer respec-tively. Hence, the aerosol sampled during the Mediterranean airmasses and sea breeze conditions also show similar geometricmean diameters to the aerosols sampled in the Atlantic coastal areaduring summer.

Fig. 9. Daily variation of the aerosol size distribution under high northwest windconditions, i.e., Mistral episode (episode 3).

The aerosol size distributions measured during the Porquerollescampaign show that the number concentrations sampled duringepisodes 1 and 2 are of the same order of magnitude measured inthe same study area by Piazzola and Despiau (1997a). During the(cleaner) episode 3, total number concentrations are found to be ofthe same order of magnitude than at Mace Head during the seabreeze conditions. At Mace Head, total concentrations varied from253 cm�3 during winter to 469 cm�3 during summer (Yoon et al.,2007). However, number concentrations are one order of magni-tude higher than in the Atlantic when the synoptic conditions areMediterranean (episode 2), and 20 times higher when air massespass over the French and Italian coast at low wind speed beforereaching the study area (episode 1). These high concentrationsare even higher than the previous measurements performed byEleftheriadis et al. (2006) during SUB-AERO experiment, who foundaverage concentrations of 1340 cm�3 when air mass back trajectorypaths mainly remained over theMediterranean Sea, and 3470 cm�3

over the Aegean Sea when air mass back trajectories passed overthe southern European coastal zone. Ourmeasurements are locatedcloser to the coast for both air mass trajectory type, compared tothose made during the SUB-AERO experiment. In this latter case,the continental pollution is inclined to be less diluted and thiswould explain the higher concentrations that we observed. Inaddition, the high concentrations measured during episodes 1 and2 are largely due to the nucleation mode particles, which werenot reported in the Eleftheriadis et al. (2006) study. Spatial andtemporal inhomogeneity between the two rather short data sets(this study and the one of Eleftheriadis et al. (2006)) can also


explain the differences. However, the concentrations found in theMediterranean originating air masses under sea breeze conditionsshould indicate a rather homogeneous background concentrationover the Mediterranean Sea at this period of the year, also found byEleftheriadis et al. (2006). This was already noted in the resultspublished on the basis of data recorded in the same study area(Piazzola and Despiau, 1997a, 1997b).

4. Chemical analysis

The experimental measurements made in May 2007 on theisland of Porquerolles allowed for a survey of different anthropo-genic and natural compounds of atmospheric aerosols for localmeteorological conditions of the Mediterranean coastal area.Among 11 impactor samples, only those acquired during clear andstable meteorological conditions described in Section 2 wereselected. The episode 1 deals with 3 samples, while episode 2 andepisode 3 correspond to 2 samples each. Total concentrationsrecorded for major species are reported in Table 2. Size-segregatedpercentage contribution of chemical species in aerosol sampledduring each episode (as defined in Section 2) is shown in Fig.11. Thesize distributions are plotted versus the aerodynamic diameter,which is the diameter of a sphere of unit density that has the samegravitational settling velocity. Sodium and chloride concentrationsshow the highest concentrations for high northwest wind episode(episode 3), despite the fact that it corresponds to short fetchconditions, i.e., 25 km. Hence, we can conclude that a 25 km fetch isenough to efficiently load the air mass with sea-sprays up to anequilibrium determined by wind speed. The Naþ and Cl� concen-trations are comparable to those found by Sellegri et al. (2001)during the FETCH experiment under high wind speed conditionsof southern sectors in aerosols sampled on board of a vessel sailingoff the French Mediterranean coast (2.37 < [Naþ] < 4.51 mg m�3)for larger fetch, i.e., approximately 80 km. Similar Naþ concentra-tions were found during summer and winter recordings at aneastern Mediterranean coastal site by Bardouki et al. (2003), i.e.,2.03� 0.28 mgm�3 in summer time and 1.20� 18 mgm�3 for winterexperiments.

Anthropogenic tracers such as NHþ4 , nss-SO

2�4 and NO�

3 alsoshowed concentrations in the range of those measured during theFETCH experiment, with however, slightly lower concentrations forNHþ

4 in the present study. It seems that sulphate concentrationsmeasured at Porquerolles are rather low compared to thoseacquired during eastern Mediterranean measurements in theAegean Sea by Eleftheriadis et al. (2006) and in the coastal site ofFinokalia by Bardouki et al. (2003), i.e., 10.12 � 1.10 mg m�3 and6.88 � 0.96 mg m�3, respectively. While sulphate was the dominantspecies in the sub-micron range during coastal conditions, asexpected from the most polluted air mass back trajectories, nitratewas maximum for the Mediterranean air masses, which wereexpected to be corresponding to the cleanest conditions (highestfetches). This could be due to the fact that the fixation of nitrogenspecies is favoured by the presence of sea-sprays. Indeed, in theliterature, nitrate has often been found in the super micron rangewhen sea-salt is present (Sellegri et al., 2001; Bardouki et al., 2003)and hence suspected to originate from the reaction of NaCl with

Table 2Concentrations of major species (mg m�3).

Cl NO3 SO4 Oxalate Na

Episode 1 0.84 1.52 3.1 0.17 0.79Episode 2 1.84 3.48 2.67 0.37 1.79Episode 3 5.14 0.24 1.18 0.02 2.99

HNO3 on the particles surface. We also found the most importantcontribution of nitrate to be in the super micron range, and espe-cially in the Mediterranean air masses. Under Mistral conditions,we observe the smallest concentration of anthropogenic tracers,which could be due to a strong dispersion of the pollutants typicalof large wind speed episodes (Piazzola and Despiau, 1997b).The very low concentrations of particle mass in the sub-micronmode confirm the SMPS size distribution showing a much cleaneratmosphere. As already noted in Section 3, the measurement ofNO�

3 is delicate and may be affected by large uncertainties (Changet al., 2001). Nitrate can be evaporated in the form of semi vola-tile ammonium nitrate and there is some nitrate losses duringimpactor sampling due to low pressures in the instrument.However, these losses are generally lower than those observedwithfilter samplers (Zhang and McMurry, 1992; Wang and John, 1988)and do not really change the shape of the aerosol size distributionsof the present paper.

The major contribution of the carbonaceous compounds isfound in the sub-micronmode (Fig.11). Organics contribute by 39%,14% and 34% to the particle mass for coastal (episode 1), Mediter-ranean (episode 2) and mistral conditions (episode 3), respectively.The OC contribution in the Mediterranean air mass is similar to theone observed in the Eastern Mediterranean for summer and winterseasons (16% at Finokalia in the submicronic mode, Bardouki et al.,2003). During episodes 1 and 3, the OC contribution was signifi-cantly higher and closer to the ones observed in the sub-micronmode during the phytoplanctonic blooming period in the NorthernAtlantic coastal area (Cavalli et al., 2004), although in our case, OC islikely rather composed of anthropogenic species. EC/OC ratio isranging from 0.02 for Mistral conditions (episode 3) to 0.07e0.08for both coastal and Mediterranean conditions (episode 1 andepisode 2, respectively), which is about three times lower than theratios found by Bardouki et al. (2003). As the EC elements, Potas-sium is also emitted during biomass burning specifically in flamingcombustion (e.g., Leslie, 1981). We can note that Potassiumconcentrations were also quite low and we can conclude thatbiomass burning contributed little to the particulate matter duringthe sampling period. This is rather logical since the forest fires inthe Mediterranean occur in the summer time (between June andSeptember) and can explain why their influence is low in May inthe study area.

5. The optical characteristics

The absorption coefficient (babs) has been obtained usinga multi-channel Aethalometer (Model AE-31, Magee Scientific) atsevenwavelengths (370, 470, 520, 590, 660, 880 and 950 nm) usingthe raw absorbance data following Weingartner et al. (2003).Scattering coefficient of aerosols (bscat) has been measured usinga Nephelometer (Model M9004, Ecotech) at 520 nm. Results on theoptical properties of different aerosol species were published inMallet et al. (2011). Fig. 12 reports the time evolution of the scat-tering coefficient during the experiment. Results display bscat valuescomprised between 2 and 30 Mm�1 (mean of 15.2 Mm�1). Suchvalues are found to be logically lower than those obtained by Sahaet al. (2008) over the city of Toulon with higher bscat ranging

NH4 K Mg Ca OC EC

0.48 0.13 0.13 0.3 4.97 0.330.21 0.16 0.35 1.78 2.07 0.170.08 0.12 0.41 0.1 5.37 0.13

Fig. 11. (a) Size-segregated percentage contribution of chemical species ( mg m�3) in aerosol sampled during coastal air mass conditions (episode 1). Da is the aerodynamic diameter(see Section 4). (b) Size-segregated percentage contribution of chemical species ( mg m�3) in aerosol sampled during episode 2. (c) Size-segregated percentage contribution ofchemical species ( mg m�3) in aerosol sampled during a high wind speed period of northwest direction (episode 3).


from 20 to 120 Mm�1, due to the proximity of pollutedaerosol sources. Similar results are obtained for absorbing coeffi-cients (Fig. 12) with lower values observed at Porquerolles(1 < babs < 10 Mm�1, associated mean of 3.6 Mm�1) compared tothe Toulon site, i.e., 10 < babs < 50 Mm�1 (Saha et al., 2008).

Our results show that scattering coefficients present highervalues (between 10 and 30 Mm�1) during the 23rd to the 26thperiod (episode 1 and beginning of episode 2), followed by a rapiddecrease due to a change in wind direction (South, East/South

wind) for episode 2 which begins the 26th May. Maritime clean airmasses characteristic of episode 2 exhibited scattering coefficients,with values close to 5 Mm�1. Low values of bscat are also observedduring and after the strong “mistral” case (28th), cleaning the lowertroposphere. Fig. 12 shows the associated aerosol single scatteringalbedo (SSA) at 520 nm, our results indicate values comprisedbetween 0.80 and 0.90 (at 550 nm) for the period from the 23rdto the 26th May. Such values reveal an absorbing aerosol in casesof continental air masses generally associated with higher

Fig. 12. Time evolution of (a) the scattering coefficient (b) the absorbing coefficient and (c) the associated aerosol single scattering albedo measured during the whole campaign.


concentration of EC, as shown in Fig.11a in Section 4. For oceanic airmasses, SSA values rapidly increase to reach up to 0.95e0.98, whichis consistent with the absence of continental polluted absorbingaerosols.

6. Discussion and conclusion

The aim of this work was to provide a physicochemical analysisof aerosols measured for meteorological conditions characteristicsof coastal areas. The present data, recorded in May 2007 on theisland of Porquerolles, confirm previous results obtained in thestudy area in the nineties by Piazzola and Despiau (1997a). First ofall, both data sets are roughly in the same order of magnitude interms of aerosol concentration. In addition, in accordance withPiazzola and Despiau (1997a), we noted a substantial contributionof sea-salts in the aerosol concentrations measured during highwind speed periods of Northwest direction (episode 3), whichcorresponds to short fetch conditions. In addition, a rather constantcontribution of anthropogenic particles is observed for low windspeed periods (episode 1 and episode 2). This shows that at highwind speeds, the sea surface production is the dominant mecha-nism while low wind speed periods allow accumulation of highcontinental aerosol concentrations, even for marine air, due toa slower atmospheric dispersion. As a consequence, episode 3 ischaracterized by an absence of continental polluted absorbingaerosols, as shown by the optical analysis reported in Section 5.

The concentrations measured on the island of Porquerolles werealso compared to those recorded in the Atlantic and in otherMediterranean locations. The present aerosol data recorded during

episode 2, i.e., when the synoptic conditions are Mediterranean,exhibit values one order of magnitude larger than those recorded inthe Atlantic and 20 times higher when air masses are transportedabove the French and Italian coasts at low wind speed (episode 1).These concentrations are even larger than the previous measure-ments performed by Eleftheriadis et al. (2006), who found averageconcentrations of 1340 cm�3 when air mass back trajectory pathsmainly remained over the Mediterranean Sea, and 3470 cm�3 overthe Aegean Sea when air mass back trajectories were transportedover the southern European coastal zone. Larger concentrations onthe island of Porquerolles can be explained by the vicinity of theFrench urban coasts, as already noted in Section 3, expected torelease large amount of anthropogenic aerosols, in particular in theform of new particle formation events. These events seem to takeplace away from the coast and after precursors have been trans-ported over the sea by land breezes where they can be photo-chemically transformed to low vapour pressure compounds.Besides these new particle formation events, the concentrationsfound under sea breeze conditions during episode 2 (Mediterra-nean air masses) could indicate a rather homogeneous backgroundconcentration over theMediterranean Sea at this period of the year,since this corresponds to aerosol concentrations in the same orderof magnitude of those found by Eleftheriadis et al. (2006).

The variation of the height of the boundary layer between thedaytime and the nighttime could be responsible for the aerosolconcentration variations due to dilution effects. However, forlowwind speed periods encountered during episode 1 and episode2, the calculations of the mixed layer depth using the Hysplitmodel, show little variations. The mixed layer height varies from


approximately 50 m height to a maximum value of about 300 mheight, typically reached at noon as already noted by Nilsson et al.(2001). This has probably a minor influence on the aerosolconcentration variations during episode 1, which, by the way, doesnot vary a lot from night to day (see Section 3).

From the chemical point of view, the results show that the Naþ

and Cl� concentrations are of the same order of magnitude thanwhat it was measured in Mediterranean areas by Sellegri et al.(2001) and Bardouki et al. (2003). Anthropogenic tracers such asNHþ

4 , nss-SO2�4 and NO�

3 also showed concentrations in the range ofthose measured in the Mediterranean by Sellegri et al. (2001), withslightly lower concentrations for NHþ

4 in the present study. Ifsulphate concentrations are rather low at Porquerolles compared tothe eastern Mediterranean measurements, the contribution oforganics are significantly higher than the ones observed in EasternMediterranean for summer and winter seasons (Bardouki et al.,2003) but rather close to those observed in the sub-micron modeduring the phytoplanctonic blooming period in the NorthernAtlantic coastal area (Cavalli et al., 2004). Indeed, the resultsreported in Section 4 show that organics contribute by 39%, 14%and 34% to the particle mass for coastal influence (episode 1),Mediterranean (episode 2) and mistral conditions (episode 3),respectively. Episode 1 then corresponds to the major contributionof the carbonaceous compounds, more particularly found in thesub-micron mode (Fig. 11). The OC contribution in the Mediterra-nean air mass is similar to the one observed in the Eastern Medi-terranean for summer and winter seasons (16% at Finokalia inthe submicronic mode, Bardouki et al., 2003). In the meantimescattering coefficients present higher values (between 10 and30 Mm�1) and the associated aerosol single scattering albedo (SSA)at 520 nm recorded during episode 1 indicate values comprisedbetween 0.80 and 0.90 (at 550 nm) which reveal an absorbingaerosol which is generally associated with higher concentration ofcarbon particles, as confirmed by data reported in Fig. 11.

Although it is suspected that a large fraction of organiccompounds found in the aerosol phase comes from anthropogenicsources, the relatively large concentrations of organics compoundscan also be partly due to larger amounts of organics in the seawater. These organics can be produced either biologically or bealready present in the sea water as primary pollutants. On one side,biological activity is primarily induced by light and furthermoredepends strongly on the input of nutrients into the sea. Highconcentrations of pollutants have been observed in the Mediter-ranean area, in particular from the run-off after a rain which maycause an excess of phosphate and nitrate compounds resulting ineutrophication. Biological studies conducted in the Toulon bay andin the Hyeres bay, have indeed revealed an excess in nutrients andeutrophication processes throughout the year (Jamet et al., 2001).

In addition, during the marine air masses (episode 2), particleshad not travelled over land for 3 days, which indicates that airmasses are strongly impacted by pollution over the whole Medi-terranean basin. A large fraction of the sub-micron mode wascomposed of organics, for aerosols sampled during purely marineair mass episodes, closely followed by sulphate. Both compoundsare presumably of anthropogenic origin, but this hypothesis shouldbe checked in future work. Another indication of anthropogenicinfluence during marine air mass periods is the strong contributionof nitrate in the super micron mode. Sub-micron sulphate andorganics, and super micron nitrate contribute as much to theaerosol mass during pure marine air masses thanwithin air massesoriginating from the densely populated Mediterranean coast andNorthern Italy.

The chemical analysis, combined with the high aerosolconcentrations measured on the island of Porquerolles, make thinkthat air masses are strongly impacted by pollutionwhich is present

over the Mediterranean basin. However, in spite of the largenumber of anthropogenic aerosol sources of such urbanizedcoastal sites with large population densities, as the Mediterraneanstudy area, the anthropogenic compounds seem to be efficientlydispersed within the lower atmosphere resulting in the dominantsignature of sea-salts for Mistral conditions. In this case, theabsence of continental polluted absorbing aerosols is confirmed bythe optical analysis.

Acknowledgements

The experimental campaign on Porquerolles Island wassponsored by the national PEA MIRA program in partnershipwith the Parc National de Port-Cros. The authors wish to expresstheir gratitude to Dr. Alain Barcelo, for his technical and adminis-trative help in the implementation of the experimental campaignin a protected area of the island of Porquerolles.

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