Organic, elemental and water-soluble organic carbon in size segregated aerosols, in the marine boundary layer of the Eastern Mediterranean

Atmospheric Environment 64 (2013) 251e262

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Organic, elemental and water-soluble organic carbon in size segregated aerosols, in the marine boundary layer of the Eastern Mediterranean A. Bougiatioti a, P. Zarmpas a, E. Koulouri a, M. Antoniou a, C. Theodosi a, G. Kouvarakis a, S. Saarikoski b, T. Mäkelä b, R. Hillamo b, N. Mihalopoulos a, * a b

Environmental Chemical Processes Laboratory, Department of Chemistry, University of Crete, P.O. Box 2208, 71003 Heraklion, Greece Finnish Meteorological Institute, Air Quality Research, Erik Palmenin aukio 1, 00101 Helsinki, Finland

h i g h l i g h t s < Carbonaceous aerosol constitutes a significant part of submicron mass fraction. < Size distribution of OC, EC, WSOC and ions was performed during a 3 year period. < OC, EC and WSOC exhibit bimodal distribution with maximum in the accumulation mode. < 83.5
11% of the PM10 OC is secondary. < 70% of the fine mode OC is water-soluble with high summer and low winter values.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 8 April 2012 Received in revised form 4 September 2012 Accepted 28 September 2012

To assess the origin and transformation of carbonaceous material in the marine boundary layer of the Eastern Mediterranean, a total of 111 size segregated aerosol samples have been collected using a 12stage Small-Deposit-area low-volume-Impactor (SDI) covering an almost 3 year period. The samples have been analyzed for organic (OC), elemental (EC), water-soluble organic carbon (WSOC) and ionic components. Maxima of OC, EC and WSOC mass size distributions were found in the accumulation mode (0.449 mm) with occasionally a minor, secondary peak in the coarse mode (2.68 mm). OC and WSOC concentrations peak during summertime due to photochemistry, while EC during autumn, and spring. In general, almost 2/3 of OC and EC concentrations are found in the PM1 fraction of the aerosol with OC being mostly secondary and therefore highly oxidized and water-soluble to a great extent (w70%). Using the EC-tracer method, it was found that 83
11% of the PM10 organic carbon is secondary, with the percentage reaching w70% for the PM1 fraction, a value in very good accordance to WSOC/OC ratio. Ammonium sulfate accounts for 75.5
21.7% and 9.3
1.9% of the aerosol mass in the fine and coarse fraction respectively, exhibiting maximum concentrations also in the accumulation mode. It was estimated that, on average, sea salt and mineral dust account for 33% and 45% of the coarse inorganic mass fraction, respectively. Ó 2012 Elsevier Ltd. All rights reserved.

Keywords: Size distribution Organic carbon Water-soluble organic carbon Chemical composition Marine boundary layer Eastern Mediterranean

1. Introduction Atmospheric aerosol particles are produced either directly by anthropogenic and natural sources (dust, sea salt, soot, biological particles, etc.), or they are formed in the atmosphere by condensation of low-volatility compounds (e.g., sulfuric acid or oxidized organic compounds) (Andreae and Rosenfeld, 2008). These particles have profound impacts on the thermodynamic and radiative budgets of the Earth, affecting climate directly through the * Corresponding author. E-mail address: [email protected] (N. Mihalopoulos). 1352-2310/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.atmosenv.2012.09.071

scattering and absorption of radiation, or indirectly through their role as cloud condensation nuclei (IPCC, 2007). Atmospheric aerosol consists either of solid or liquid particles suspended in the air, with diameters varying between 0.002 and 100 mm, with the composition of each particle range being strongly dependent on their sources and their formation mechanism (Seinfeld and Pandis, 1998). A large part (w50%) of this submicrometer aerosol mass in the troposphere consists of organic material (Kanakidou et al., 2000; De Gouw and Jimenez, 2009). For rural background sites, total carbonaceous material accounts for 30
9% of PM10, of which 27% can be attributed to organic matter (OM) and 3.4% to elemental carbon (EC) (Yttri et al., 2007).

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The carbonaceous fraction of atmospheric particulate matter consists of elemental carbon (otherwise called black carbon) and a variety of organic compounds (organic carbon). Elemental carbon (EC) is produced only by combustion processes and is therefore always primary material. Organic carbon (OC) can be directly emitted into the atmosphere in particulate form or it can be formed in the atmosphere by gas-to-particle conversion (Seinfeld and Pandis, 1998). EC lies generally in the submicrometer range whereas OC exhibits a wider size distribution, which remains poorly documented (Pio et al., 2007). There are several indirect methods employed to estimate the amount of primary and secondary organic components. Among these, the EC-tracer method (Gray et al., 1986; Huntzicker et al., 1986; Turpin et al., 1991; Strader et al., 1999; Cabada et al., 2004; Yu et al., 2004; Chu, 2005) uses EC as a tracer for primary OC, thus evaluating the primary organic aerosol (POA) and secondary organic aerosol (SOA) fraction of the organic matter (OM), with SOA simply appearing as an increase in the OC/EC ratio relative to that of the primary OC/EC ratio (Gelencsér et al., 2007). OC/EC ratios exceeding 2.0 have been used to identify the presence of secondary organic aerosols (Chow et al., 1996; Viana et al., 2007). Secondary organic aerosols are also associated with high levels of water-soluble organic carbon (WSOC), consisting of oxidized and more soluble organic species induced by photochemical reactions, especially during summer (Jaffrezo et al., 2005; Kumagai et al., 2009). Aerosol chemical composition, and especially the inorganic fraction, has been the subject of several studies which have taken place in the Eastern Mediterranean area (Kouvarakis et al., 2002; Koulouri et al., 2008; Sciare et al., 2005, 2008). It has been established that the total particulate matter mass is almost equally divided between the coarse (56
3%) and the fine fraction. According to Koulouri et al. (2008) minerals from earth’s crust (e.g. Fe, Ti, Mn) are principally present in the coarse fraction (84%), with the respective value in the fine fraction not surpassing 12%. A similar behavior is also found for some water-soluble ions (e.g. Naþ, Cl, Mg2þ, Ca2þ and NO 3 ) which are mainly present in the coarse fraction (86%) due to their sources (mineral dust and sea salt) as well as formation mechanisms. In the fine fraction, the majority of the mass (up to 65%) can be attributed to the water2 soluble ions NHþ 4 and SO4 , while a significant component is also organic carbon (23%). The respective percentage of particulate organic matter in the coarse fraction ranges from 8% to 14% during winter and summer (Koulouri et al., 2008) and is comparable to the value found for total carbon (TC) values (13%) in rural PM10 in Southern Europe (Putaud et al., 2010). Similar values have also resulted from another study in the area, according to which the contribution of sulfate and organic carbon in the fine fractions is 50% and 26%, respectively (Sciare et al., 2008). What is missing in the area is a complete study of the aerosol organic component with simultaneous determination of the water-soluble fraction of it. This study will complement the findings of the studies conducted by Gerasopoulos et al. (2007) and Koulouri et al. (2008) in terms of mass size distribution, ionic composition in the fine and coarse fraction, adding the parameter of mass size distribution of the carbonaceous fraction of aerosols in the area. Such a study is even more imperative if one considers the fact that the greater organic carbon concentrations are present in the fine fraction, and fine and ultra-fine particles contribute to several physicochemical properties of the aerosol in the area (optical properties, cloud condensation nuclei (CCN) activity, droplet formation) as well as global climate (Kanakidou et al., 2005). In the current study, we address the issue of organic, elemental and water-soluble organic carbon size-distribution in ambient aerosol during a period of almost three years, in the marine boundary layer of the Eastern Mediterranean.

2. Observational data set 2.1. Measurement site The measurements were conducted at the Finokalia research station (35 320 N, 25 670 E; http://finokalia.chemistry.uoc.gr). The station is considered as a remote marine background site, established and operated by the University of Crete and the nearest major city is Heraklion located 70 km west of the station with a population of about 176,000 inhabitants. A detailed description of the site and prevailing meteorology can be found in Mihalopoulos et al. (1997) and Sciare et al. (2003). Finokalia is located at a unique “crossroad” of aged aerosol types, which can originate from the marine boundary layer, Saharan desert, European mainland, and biomass burning events during the summer period. 2.2. Sample collection Size-resolved measurements presented in this study cover a time period of almost three years, starting on July 2004 until February 2007. During this period a number of 111 samples have been collected, equally distributed throughout the seasons (22% in winter, 20% in spring, 27% in summer and 32% in autumn). It is obvious that the collected samples represent a wide range of air masses, containing “polluted” air masses from Continental Europe, the former Soviet Union and Asia Minor, but also “cleaner” air masses from the arid areas of Northern Africa and the Mediterranean. Aerosol samples were collected using two Small-Deposit-area low-volume-Impactors (SDI; Maenhaut et al., 1996), running in parallel. The inlet preceding the SDI has a cut-off size of 10 mm and 12 collecting stages over the particle size range 0.041e10 mm with nominal aerodynamic diameter cut-offs at 0.041, 0.085, 0.138, 0.225, 0.346, 0.585, 0.762, 1.06, 1.66, 2.68, 4.08 and 8.39 mm (stages 1e12 respectively). More technical details for the operation and the set up of the SDI can be found in Teinilä et al. (2000). The sampling time varied from 1 to 3 days, with a mean collection time of 2 days. The PM10 values were obtained by summing up values obtained for all impactor stages, while the PM1 values were extracted by summing up the corresponding stages. Therefore, it should be noted that, strictly speaking, the SDI actually provides PM1.06. Aerosol particulate matter was collected by the parallel deployment of two identical SDIs; in the first SDI particles were collected on greased aluminum with polycarbonate filters and on the second one on pre-combusted quartz fiber filters (Whatman QMA). The sampling flow rate in both SDIs was set at 11 L min1. In addition to aerosol particles, quartz material collects an unknown fraction of gaseous organic compounds (Turpin and Lim, 2001). To reduce this positive artifact, three multi-annular denuders were used (URG-2000, 30  242 mm, Chapel Hill, NC), to remove gas-phase organic compounds from the air stream before the quartz-SDI. The denuders were coated with XAD-4 (polystyreneedivinylbenzene) adsorbent according to Gundel et al. (1995). After the sampling the denuders were regenerated by extracting the adsorbent with acetonitrile and hexane. According to Saarikoski et al. (2008) the use of quartz fiber filters in one of the SDIs changes significantly the nominal cut-off diameters, which consequently has an effect on the size distributions derived from these measurements. The new cut-off diameters for the 12-stage SDI using quartz fiber filters become 0.009 (estimated), 0.037, 0.063, 0.145, 0.277, 0.449, 0.608, 0.878, 1.66, 2.68, 4.08 and 8.39 mm (stages 1e12 respectively) and these new cut-offs are the diameters used for the mass size distributions of OC and EC.

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2.3. Sample treatment The polycarbonate filters were pre- and post-weighed using a 6digit microbalance (ATI-CAHN/CA27). The readability of the balance is 10 mg with a precision of
40 mg corresponding to mass concentration uncertainty of
0.86 and
0.77 mg m3, for fine and coarse fraction, respectively. The PM10 masses from the SDI were also checked against PM10 from a b-attenuation particulate monitor operating at Finokalia station on a continuous basis, with PM10 form the monitor being averaged to correspond to the sampling duration of each SDI set. The comparison between the two values reveal a very good correlation (R2 ¼ 0.95) with a slope of 1.08, showing a slight under estimation of the PM10 masses from the SDI (Gerasopoulos et al., 2007). After weighing, the samples were stored in a freezer in petrislides (Millipore Inc.) and later analyzed for water-soluble ions and water-soluble organic carbon. For these analyses it was decided to combine certain SDI stages as follows: stages 1e3 (A), stage 4 (B), stage 5 (C), stages 6e8 (D), stages 9e10 (E) and stages 11e12 (F). Samples were extracted in ultrasonic bath using 12 ml of nanopure water and then filtered using syringe filters (PALL IC AcrodiscÒ (PES), 0.45 mm, 13 mm) to remove any non-soluble species and subsequently analyzed by ion chromatography and a carbon analyzer. Carbonate carbon is derived from the used method as Inorganic Carbon (IC) by acidification and transformation into carbon dioxide. The WSOC blank value for the polycarbonate filter used for the SDI was of 0.11
0.02 ppm. For the smaller particles (stages 1e3) this value corresponds up to 23
8% of the WSOC samples’ concentration, while for stages 4 and 5, the percentage is somewhat lower (19
10%). For stages 6e8 the contribution of the blank WSOC is the lowest (17
9) and finally for the coarse fraction particles is equal to 22
10% of the WSOC samples’ concentration. All reported WSOC results are blankcorrected. The remaining solutions were analyzed by ion chromatography for the determination of the main ionic species. Before sampling, quartz filters were pre-combusted at 550  C overnight. After sampling the filters were also stored in a freezer in Petrislides (Millipore Inc.) and later analyzed for organic carbon and elemental carbon. The blank values for OC and EC for the filters used were 0.565
0.100 mg cm2 and 0.024
0.013 mg cm2, respectively. For the smaller particles (stages 1e2) the blank OC and EC concentrations may account for 27
12% and 25
14% of the measured OC and EC levels respectively. For stage 3, the respective percentages drop to 15
9% and 11
5% for OC and EC, while for the remaining stages until PM1 (stages 4e8) do not exceed 10% (OC 10
5% and EC 5
4%). For the larger, coarse fraction particle ranges (stages 9e10), OC blank concentrations may account for 14
8% and EC 10
8% of the measured concentrations. Finally, for stages 11e12, the blank concentrations exceed 29
12% and 26
24% of the samples’ OC and EC concentrations, respectively. For the quartz filters as well, all OC and EC reported results were blank-corrected. 2.4. Chemical analyses Samples were analyzed for organic carbon and elemental carbon, with the thermaleoptical transmission method, using a Sunset Laboratory Inc. (Oregon) carbon analyzer. In short, 1 cm2 punch from a quartz filter sample is placed in a quartz boat and positioned in the path of a red light diode laser, which is used to monitor transmittance of the filter (used to determine the OCeEC split time) during analysis. A thermocouple at the end of the boat is used to monitor sample temperature during analysis. All carbon species evolved from the filter are converted to carbon dioxide in an oxidation oven immediately downstream from the primary oven, and the carbon dioxide is reduced to methane before passing into

253

a flame ionization detector (FID). The method used was modified from the method developed by the National Institute for Occupational Safety and Health (NIOSH), as at the time of the analysis, there was no established standard procedure for the determination of carbonaceous aerosol fraction in Europe. During the first phase (He-mode) the sample is kept in helium atmosphere and heated in four steps: 310, 480, 610 and 850  C, while the second phase (He/O2 mode) has five consecutive temperature steps: 600, 675, 750, 825 and 890  C. Nevertheless, it should be kept in mind that according to Cavalli et al. (2010) in such protocols with high temperatures in the He-mode, incorrectly accounting for pyrolytic carbon (PC) formation can very significantly bias the discrimination between OC and EC and premature evolution of light-absorbing-carbon species (LAC) containing native EC may shift the split point. The solutions obtained after the extraction in ultrasonic bath of the polycarbonate filters of the SDI were analyzed firstly for WSOC, using an organic carbon analyzer (TOC-VCSH, Shimadzu) and the remaining solutions were analyzed by ion chromatography (IC) for 2 2 þ þ þ anions (Cl, Br, NO 3 , SO4 , C2 O4 ) and cations (Na , NH4 , K , Mg2þ and Ca2þ) using the method of Bardouki et al. (2003).

3. Results and discussion 3.1. Organic carbon, elemental carbon and water-soluble organic carbon mass size distributions Adding up the corresponding stages of the SDI, it was established that 74
11% of the total (PM10) organic carbon concentrations are attributed to the PM1.06 fraction (y ¼ 0.735x, R2 ¼ 0.94 when PM1.06 versus total PM10 is plotted). For the elemental carbon, by using the SDI concentrations, it occurred that 79
12% of the total (PM10) elemental carbon concentrations are attributed to the PM1.06 fraction (R2 ¼ 0.86). Therefore, the grand majority (more than 2/3) of both carbon constituents is found in the PM1.06 fraction of the aerosol. Similar results were presented at Koulouri et al. (2008) by using a Virtual impactor (VI). Taking into account the cut-off from the filters of each collection stage, the geometrical mean of each pair of values was determined and according to these values, the mass size distributions of OC and EC concentrations were calculated (dm/dlog (Da)). Following the procedure reported in Section 2.3 concerning the determination of the WSOC concentrations in the polycarbonate filters of the samples, and applying the same calculations for the corresponding geometrical mean, the WSOC mass size distributions were also calculated. All observed mass size distributions of OC, EC and WSOC can be divided into two types: unimodal and bimodal. The unimodal distributions represent the grand majority of the samples (65%) and were observed mainly during the transport of air masses coming from the Northern sector (N, NE, NW) while the bimodal distributions represent 23% of the samples and were observed mainly when mineral dust-laden air masses arrive at the site from the Southern Sector (S, SW, W). Fig. 1 shows the average mass distributions of OC, EC and WSOC for the two main shapes, with vertical error bars representing the standard deviation between the observed values. All mass size distributions show a pronounced maximum for OC, EC and WSOC at 0.449 mm (aerodynamic diameter) both for the unimodal as well as for the bimodal distributions. For the bimodal distributions there is a secondary peak in the coarse mode, at 2.68 mm (aerodynamic diameter) which is apparent in all three carbonaceous components. It has been demonstrated that the thermaleoptical transmission method depends on the chemical composition of the aerosol sample under investigation and part of these bimodal distributions are associated with mineral dust-laden air masses.

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Fig. 1. Average prevailing unimodal mass size distributions of organic carbon (a), elemental carbon (b) and water-soluble organic carbon (c) and with second largest frequency bimodal mass size distributions of organic carbon (d), elemental carbon (e) and water-soluble organic carbon (f), with standard deviations.

The organic carbon, elemental carbon and water-soluble organic carbon average monthly mass size distributions were subsequently plotted all together in contour plots, per diameter and per month, with higher concentrations being represented by darker colors, and the particle size expressed by the logarithm of the mean geometrical diameter (log (Dg)), which are shown in Fig. 2. It can be seen that for the organic carbon, the maximum concentrations are found for July, with the second high occurring in June, September and March, for particles in the 0.449 mm aerodynamic diameter particle range, due to enhanced photochemistry. There are also high OC concentrations in the coarse particle mode (Da 2.68 mm) in April and July, which can be associated with the occurrence of dust events during the transitional periods, originating from Northern Africa and biomass burning events during summer. These higher concentrations can be attributed to organic species (e.g. oxalate, methane sulfonic acid (MSA), malonic acid etc.) which also show higher concentrations during dust (Sullivan and Prather, 2007) and biomass burning events (Sciare et al., 2008; Kundu et al., 2010; Agarwal et al., 2010). The lowest OC concentrations are found during January and February, due to

reduced photochemical activity and increased washout via precipitation. Gerasopoulos et al. (2007) identified seven distinct aerosol modes for the polycarbonate SDI samples; A: Aitken 1 (0.04e0.08 mm), B: Aitken 2 (0.08e0.25 mm), C: Accumulation 1 (0.25e0.55 mm), D: Accumulation 2 (0.55e1 mm), E: Coarse 1 (1e 3 mm), F: Coarse 2 (3e7 mm) and G: Extra Coarse (>7 mm). According to this classification, organic carbon concentrations are prominent in the Accumulation 1 mode (C) as well as the Coarse 1 and 2 modes (E, F). The biggest contribution is from the Accumulation mode particles, which add up to 62
9% of the total organic mass. These particles may result from photochemical processes and secondary organic aerosol formation but also from burning processes and arrive at the site via long-range transport from Central-Eastern Europe. For the elemental carbon, minimum concentrations occur on January and February, which coincide with the organic carbon ones, but also for June. Maximum concentrations occur on November, but high concentrations are also observed during May, July, August and September. During both transitional periods (spring and autumn) there is an occurrence of dust events, when mineral dust-laden air

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255

Fig. 2. Contour plots of monthly averages of organic carbon (a), water-soluble organic carbon (b) and elemental carbon (c) concentrations (mg m3), relatively to different particle diameters.

masses transported from Northern Africa arrive at the site by strong southern winds. Apart from a large amount of crustal components, air masses of this origin seem to also contain large amounts of elemental carbon, which has also been observed in a more recent study (Pikridas et al., 2010). High elemental carbon concentration can also be associated with air masses coming from Central-Eastern Europe, transported by strong northern winds throughout the year (Sciare et al., 2008). According to the different particle modes aforementioned, elemental carbon concentrations are also

prominent in the Accumulation mode (80
4%) and occasionally exhibit a lower peak for the Coarse 1 particle mode. As it is well established, elemental carbon mainly results from burning processes and can be attributed either to urban pollution (traffic/ heating), or biomass burning, especially during summer, from areas around the Black Sea. Water-soluble organic carbon follows the trend of organic carbon, exhibiting maximum concentrations during July and September, in the Accumulation particle mode (aerodynamic

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A. Bougiatioti et al. / Atmospheric Environment 64 (2013) 251e262

diameters 0.225e0.585 mm). Minimum concentrations occur once more in January and February, with concentrations for the lower particle range remaining very low throughout the year. As it is the case for OC, WSOC also exhibits higher concentrations during April, in the Coarse 2 and Extra Coarse particle modes. Having in mind that during spring Saharan dust intrusion occurs in the area, it is possible that this maximum is due to the absorption of WSOC species in the large particles. These supermicrometer concentrations can be associated with organic species (e.g. diacids) attributed to uptake/formation on mineral dust and sea salt particles (Sullivan and Prather, 2007).

influence (e.g. heating), reducing the OC/EC ratio, while the organic carbon concentrations remain relatively low due to absence of photochemistry. Nevertheless, OC/EC ratio always remains higher than the indicative value of 2, therefore concluding that the organic aerosols in the area are mostly secondary. Fig. 3b shows the radar plot of the OC/EC mass concentration ratios relatively to the geographical sectors. As it is anticipated, lower ratios are observed for the N, NE sector which represent 24% of the wind occurrence, when air masses are influenced by highly populated areas such as the former Soviet Union and Istanbul. Low ratios are also observed for the NW sector, with 36% wind occurrence, when air masses arrive from the Central European mainland. The highest ratios are observed for the E, SE sectors (9% occurrence), as well as for the W sector (12.5% occurrence) as they represent “cleaner” environments deprived from strong anthropogenic influence. There are many data available in the literature for fossil fuel primary OC/EC ratios, but care has to be taken not to rely unconditionally on primary ratios reported for urban aerosol as they also may include non-fossil combustion and other sources (e.g. biomass burning and food preparation) (Gelencsér et al., 2007). Kupiainen and Klimont (2005) combined a literature review of OC/EC ratios with details of the European emissions activities and estimated a ratio of 0.71e1 for fossil fuel sources in the “old” 15 EU countries, but a ratio of 1.2 for the new member states. These values are in good agreement with the value of 0.67 reported by Lonati et al. (2007) for a tunnel site in the urban area of Milan, representing a real traffic source. Therefore, using an OC/EC ¼ 1, we estimated the contribution of primary and secondary organic aerosols in the organic carbon concentrations using the following equation:

3.2. Correlations and estimation of sources As already stated in the introduction, the ratio between organic carbon and elemental carbon may be a first indication of the nature of the aerosols, namely if they are primary or secondary. Ratios greater than 2.0 may indicate the presence of secondary, therefore “aged” aerosols, namely aerosols which have stayed long enough in the atmosphere and have been subject to all possible chemical evolution. Organic carbon concentrations are expected to be higher in such aerosols because of secondary aerosol formation via condensation of lower-volatility organic compounds onto particles as a result of photochemical reactions and oxidation, rendering the organic species less volatile and enabling their partitioning in the particulate phase (Donahue et al., 2006). Having in mind that studies in the area have already established the “aged” nature of aerosols sampled (Bougiatioti et al., 2009, 2011), we would expect a high OC/EC ratio, higher than the one found in urban areas, where the elemental carbon concentrations are higher, reducing the ratio. This is indeed the case, where the OC/EC ratio for the majority of the samples for all 12 stages of the SDI (n ¼ 1024) is 5.1 (R2 ¼ 0.44). The same ratio, but for the stages where maximum concentrations of both components are observed (0.277 < Da < 0.608 mm) is higher (5.9, R2 ¼ 0.64), which shows that in the fine fraction, there are common sources of organic and elemental carbon. This value is very close to the filter-based value of 5.4, found for the PM1 fraction during the Finokalia Aerosol Measurements Experiment 2008 (FAME08) (Pikridas et al., 2010). Fig. 3a shows the monthly average OC/EC mass concentration ratios for particles of aerodynamic diameter of 0.277 < Da < 0.608 mm where it can be seen than the lowest ratios are found during wintertime as well as during the transitional periods when we have the occurrence of dust events. On the contrary, the highest ratios are observed during summertime. This is more or less expected, as during winter, the elemental carbon concentrations are the highest due to additional urban

OC/EC mass concentration ratio

a

OCtot ¼


   OC *EC þ OCsec EC prim prim

(1)

Based on this rough estimate and with the use of the majority of the samples containing the 12 SDI stages, it is found that for PM10 83.5
10.8% of the organic carbon is secondary (86.1
7.9% for PM1.06). Having that in mind, and knowing that physicochemical processes (e.g. photochemical oxidation) that lead to secondary organic aerosol formation augments the oxygen content of the organic fraction, adding oxidized functional groups which are hydrophilic and therefore increasing the solubility of organic carbon in water (Saxena and Hildemann, 1996; Jaffrezo et al., 2005; Kumagai et al., 2009), we studied the relationship between organic and water-soluble organic concentrations in the samples.

b

9,0

8,0

8,0

NW 7,0

N

7,0

NE

6,0 5,0

6,0

W

5,0

E

4,0

4,0

SW

3,0

SE S

2,0 Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

Fig. 3. a) Average monthly OC/EC mass concentration ratios for particles of aerodynamic diameter of 0.277 < Da < 0.608 mm with standard deviation, and b) Radar chart of OC/EC mass concentration ratios relatively to the geographical sector.

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In the fine fraction we decided to use stages 4 and 5 (of the polycarbonate SDI, 212 samples in total) which i) were analyzed individually for WSOC, and ii) exhibited high WSOC concentrations, to reduce uncertainty of the measurements. The corresponding stages of the quartz SDI were stages 5 and 6. For particles with an aerodynamic diameter of w0.25 mm (aerodynamic diameters 0.225e0.277 mm) it occurred that 64
19% of the organic carbon was water-soluble (R2 ¼ 0.75), while for particles of 0.4 mm (aerodynamic diameters 0.346e0.449 mm) the percentage was of 70
19% (R2 ¼ 0.7). Bougiatioti et al. (2011) reported similar values (67
7%) for WSOC/OC ratio in the fine fraction. Fig. 4a shows the monthly average WSOC/OC mass concentration ratios for both particle size fractions where it can be seen that, as expected, the highest ratios occur during summertime and the lowest during wintertime. It can also be seen that during April the standard deviation between the observed values is relatively high, which is due to the occurrence of dust-affected samples which have low WSOC concentrations in the fine fraction. If we consider the WSOC/ OC ratios for summer and winter individually, for the 0.25 mm particles the ratios are of 0.75 (R2 ¼ 0.93) and 0.55 (R2 ¼ 0.72) respectively, while for 0.4 mm the same ratios are of 0.88 (R2 ¼ 0.94) and 0.67 (R2 ¼ 0.62) respectively. This increase in both particle ranges during summer may be due to enhanced photochemical oxidation forming oxygen containing functional groups which increase water solubility during summer, as well as to enhanced wet depositional losses of WSOC relative to the insoluble carbon in winter (Viana et al., 2007). Although high, these ratios are consistent with the aged nature of the aerosol (Jaffrezo et al., 2005 and references within) and consistent with PM1 WSOC reported for the area (Bougiatioti et al., 2009, 2011). If we take into account the total SDI samples that add up to PM1.06, this percentage drops to 40
13% and for the total SDI samples is 41
14%. This may be due to the larger uncertainties and errors for lower concentrations and to effects caused by extracting/analyzing together more than one filters in the same sample, as well as to the difference in the actual cut-off for the SDI using quartz filters, for the determination of the OC concentrations. Finally Fig. 4b shows the radar plot of the WSOC/OC mass concentration ratios for both particle size fractions relatively to the geographical sectors. As expected, the highest ratios are observed for the northern sector (NW, N, NE) as air masses contain larger amounts of organic carbon and also of organic components that via photochemical transformation/ oxidation during transport lead to secondary organic aerosol formation adding more functional hydrophilic groups and augmenting the solubility in water. Finally, the lowest ratios are observed for the southern sector and can be attributed to the presence of substantial amounts of mineral dust.

WSOC/OC mass concentration ratio

a

257

3.3. Chemical composition and distribution of major inorganic species According to Gerasopoulos et al. (2007) the overall PM10 average mass concentrations from the SDI is 31 mg m3 and for PM1.06 it is 10 mg m3 (about 40% of PM10). The reported values are in very good agreement with the values reported for Crete during a 5-year period (Gerasopoulos et al., 2006) and close to the values of the VI for the same sampling period reported by Koulouri et al. (2008) of 36 and 12 mg m3 for PM10 and PM1, respectively. PM10 masses present a peak in spring, due to the increased frequency of dust transport from Northern Africa (Gerasopoulos et al., 2007). Average concentrations of water-soluble ions of the SDI samples are presented in Table 1, according to their geometrical mean diameter. Ammonium sulfate accounted for 53.4
38.1% of the total inorganic mass fraction (75.5
21.7% in the fine and 9.3
1.9% in the coarse fraction), with 84.4
12.8% of the total sulfate being in the fine fraction (PM1.06). Koulouri et al. (2008) found that ammonium sulfate accounts for 10.3
4.7% of the total coarse inorganic mass and 80.9
20.9% of the total fine inorganic mass. For the majority of the samples analyzed by ion chromatography, 2 þ PM1.06 SO2 4 and NH4 correlated well with each other (R ¼ 0.77) 2 =SO molar indicating a common origin, with an average NHþ 4 4 mass ratio equal to 1.75, which is equal to the molar mass ratio found by Koulouri et al. (2008) for the fine fraction (R2 ¼ 0.85) and close to the value of 1.61 derived from long-term measurements (July 2007eDecember 2009) of the VI fine fraction (R2 ¼ 0.93) in the area (unpublished data). This indicates an incomplete neutralization of ammonium due to the concurrent presence of ammonium bisulfate. On average, about 82.9
8.5% of particulate nitrate ðNO 3Þ is associated with coarse particles, likely formed from the reaction of gaseous nitric acid or some other nitrogen compounds with sea salt and mineral dust particles (Metzger et al., 2006). The remaining ionic mass consists mostly of Cl, Naþ, Kþ and Ca2þ, and on occasion small amounts of Mg2þ and oxalate. During the study Naþ and Cl in the coarse fraction samples E and F (SDI stages 9e12) correlated well (R2 ¼ 0.72) with an average Cl/Naþ molar mass ratio equal to 0.52, which is lower to that reported for seawater (0.77) and indicates a significant deficit in Cl. When we examine the E and F sets of samples separately, it occurs that the Cl/Naþ ratio is significantly lower for the larger particles (sample F, stages 11e12) having minimum values during spring and autumn (0.43 R2 ¼ 0.94 and R2 ¼ 0.88, respectively) and a summer value of 0.52 (R2 ¼ 0.78). The respective values of sample E (stages 10e11) for spring, autumn and summer are 0.52 (R2 ¼ 0.79), 0.57 (R2 ¼ 0.68) and 0.62 (R2 ¼ 0.56). During winter the correlation of Cl and Naþ concentrations is very low for both sets of samples. This phenomenon of Cl deficit has

b

0,9 0,8

0,8 NW

0,7

N NE

0,6 0,4

0,6

W

E

0,2

0,5 0,4

SW

0,3 Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

SE S

Fig. 4. a) Average monthly WSOC/OC mass concentration ratios for particles of aerodynamic diameter of 0.277 < Da < 0.608 mm with standard deviation, and b) Radar chart of WSOC/OC mass concentration ratios relatively to the geographical sector.

258

A. Bougiatioti et al. / Atmospheric Environment 64 (2013) 251e262

Table 1 Average values of the main ions measured in the SDI fractions of aerosols collected at Finokalia, grouped by their geometrical mean diameter.

Oxalate Cl NO 3 SO2 4 NHþ 4 Naþ þ K Mg2þ Ca2þ

0.036 mm (mg m3)

0.200 mm (mg m3)

0.353 mm (mg m3)

0.863 mm (mg m3)

2.600 mm (mg m3)

7.450 mm (mg m3)

0.01 0.17 0.05 0.41 0.22 0.08 0.06 0.02 0.03

0.02 0.11 0.03 1.17 0.39 0.07 0.05 0.01 0.02

0.02 0.13 0.03 1.43 0.47 0.11 0.07 0.010 0.04

0.02 0.26 0.18 1.03 0.34 0.24 0.08 0.038 0.11

0.02 0.79 0.69 0.37 0.12 0.58 0.05 0.082 0.24

0.02 1.15 0.75 0.42 0.11 0.73 0.06 0.11 0.35

(
0.01) (
0.13) (
0.04) (
0.25) (
0.26) (
0.12) (
0.06) (
0.01) (
0.07)

(
0.01) (
0.18) (
0.02) (
0.57) (
0.23) (
0.13) (
0.04) (
0.01) (
0.06)

(
0.02) (
0.20) (
0.03) (
0.91) (
0.29) (
0.15) (
0.07) (
0.007) (
0.09)

also been signalized in other studies in the area (Koulouri et al., 2008; Pikridas et al., 2010) and is mostly attributed to the reactions of sodium chloride with acidic species, such as sulfuric acid (H2SO4) and nitric acid (HNO3), present in the Mediterranean atmosphere in relatively high levels (Bardouki et al., 2003b; Metzger et al., 2006), leading to the formation of gaseous hydrochloric acid (Seinfeld and Pandis, 1998), which through reaction with OH radicals produces Cl atoms in the gas phase (Arsene et al.,

(
0.02) (
0.18) (
0.16) (
0.82) (
0.28) (
0.26) (
0.08) (
0.030) (
0.17)

(
0.02) (
0.56) (
0.35) (
0.35) (
0.16) (
0.37) (
0.04) (
0.04) (
0.26)

(
0.01) (
0.70) (
0.34) (
0.38) (
0.08) (
0.43) (
0.07) (
0.11) (
0.28)

2007). Finally, PM10 Naþ and Mg2þ correlated well with each other (Mg2þ/Naþ ¼ 0.118, R2 ¼ 0.67) indicating a common source, as both constituents are mainly attributed to sea salt (ratio in seawater 0.12) in the area (Kouvarakis et al., 2002; Bardouki et al., 2003; Pikridas et al., 2010). Fig. 5 shows the seasonal variability of the median mass size distribution for the major ionic species OC and WSOC. The average mass size distribution of the lumped set of samples was also

Fig. 5. Seasonal variability of the median mass size distribution of the major ionic species, OC and WSOC: (a) Sulfate, (b) Ammonium, (c) Nitrate, (d) Potassium, (e) OC and (f) WSOC.

A. Bougiatioti et al. / Atmospheric Environment 64 (2013) 251e262

compared to the average mass size distribution of 18 SDI impactors from which each of the 12 stages has been individually analyzed by ion chromatography. A good agreement in the shape and values for the two averages for most of the major ionic species was found (not þ shown). SO2 4 and NH4 exhibit a pronounced maximum in the fine fraction for particles in the range of 0.28e0.67 mm (Da 0.225e 0.585 mm) and clear seasonal variability having maximum concentrations during summer and minimum during winter. NO 3, Naþ and Cl, on the other hand, show a maximum in their mass size distributions for particles in the coarse fraction, at 1.66e4.08 mm. NO 3 exhibits its maximum values in the coarse fraction (Da 1.66e 2.68 mm) with concentrations not varying much between the different seasons, nevertheless being clearly lower during winter. Kþ shows high concentrations in both fine and coarse fraction particles, with w60% of the Kþ concentrations being in the fine fraction. In the coarse fraction, 46
2.7% of the total Kþ concentrations are found to be non-sea salt (nss-Kþ). Potassium is present in biogenic, marine and mineral dust aerosol particles, with amounts depending on the particle range: Kþ in the fine fraction is almost entirely associated with particles of anthropogenic origin (combustion, biomass burning), 1.0 mm size range Kþ is mainly associated with mineral dust, for the 2.0 mm size range it can be attributed to biogenic and mineral sources, as well as from marine aerosol (ss-Kþ), while for larger particles K is exclusively of a biogenic component (Artaxo and Hansson, 1995; Mateu et al., 1995). For particles with aerodynamic diameters of 0.225 < Da < 0.277 mm Kþ concentrations show a statistically significant correlation with EC concentrations (R2 ¼ 0.54) and with OC concentrations (R2 ¼ 0.44) as well as with WSOC concentrations (R2 ¼ 0.41) demonstrating the common sources of these constituents. A similar behavior is also observed for particles of aerodynamic diameters of 0.346 < Da < 0.449 mm (with respective correlation coefficients of 0.35, 0.67 and 0.65). In the coarse fraction, Kþ concentrations show a significant correlation only with WSOC concentrations (R2 ¼ 0.66) and no correlation neither with OC nor with EC, probably due to the fact that in this particle range the sources of potassium are different than combustion. In the seasonal variability (Fig. 5f) the increased potassium concentrations in the Accumulation mode particles which occur during summer and autumn can be attributed to aerosols with biomass burning loading, arriving from the northern-northeaster sector and areas around the Black Sea (Sciare et al., 2008) while during the transitional seasons (spring and autumn) increased concentrations in the coarse fraction may be associated with dust events. Both carbonaceous components (OC and WSOC) have a clear seasonal cycle with winter minimum and concentrations building up to a maximum during summer and then diminishing again. Maximum concentrations are observed once more for particles

259

with Da 0.225e0.585 mm (as for ammonium and sulfate) but with a minor peak in the coarse particle fraction (Da 1.66e2.68 mm) which is observed during the transitional periods (spring and autumn). We also studied the ratio between nss  SO2 4 =OC concentrations for the corresponding stages, from the detailed analysis of the full 18 impactors. For the small particles (Da < 0.6 mm) the SO2 4 =OC ratio is 0.43
0.10. For particles 0.6 < Da < 1.66 mm the average ratio reaches the value of 5.7
3 and for the coarse fraction particles Da > 2.68 the corresponding ratio is of 1.6
0.3. This remarkable difference in ratios may be due, to a large extent, to the different formation mechanisms: in very small particles sulfate is associated with direct gas-to-particle conversion, while in larger particles, it can be produced from cloud processing (heterogeneous reactions) over large scales. The median size-segregated aerosol chemical composition for the whole measurement period is shown in Fig. 6 in terms of mass size distribution. Sea salt concentrations are calculated from the following equation (Sciare et al., 2005; Pio et al., 2007):

i  i       h  h sea salt ¼ Naþ þ Cl þ Mg2þ þ ssKþ þ ssCa2þ h i þ ssSO2 4

(2)

Based on seawater composition, sea-salt sulfate ½ssSO2 4  is calculated as total [Naþ] times 0.252, sea-salt calcium [ssCa2þ] as total [Naþ] times 0.038 and sea-salt potassium [ssKþ] as total [Naþ] times 0.036. The rough mass fraction of dust aerosols is estimated using the non-sea-salt calcium concentrations determined by IC [nssCa2þ; Sciare et al., 2005, 2008)]. Finally, particulate organic matter (OM) is estimated by multiplying OC by a conversion factor, which is the ratio of the average molecular mass to the carbon mass for the organic aerosol. Taking into account the remote location of Crete Island and the aged nature of the aerosol sampled, for our mass reconstruction a conversion factor of 2.1 was used based on Sciare et al. (2005) and references within. Shown as “other” are the average concentrations of remaining ionic species such as nss-Kþ and oxalate. It can be seen that the Accumulation fraction particles consist mainly of ammonium sulfate and carbonaceous matter and Coarse and Extra Coarse fraction particles are dominated by the presence of sea salt, mineral dust and nitrates. The average weighted mass for each fraction is also given (solid line), with the positive error bars representing the standard deviation between the sums of the combined SDI stages. It can be seen that for the fine fraction the mass closure is very satisfactory. When all main aerosol components are considered, on average, they account for w67% of the PM10 and 81% of the PM1 mass. The empirical relation for dust calculation can account for the deficit in the calculated PM10 mass.

Fig. 6. Average mass size segregated aerosol chemical composition observed at Finokalia compared to the measured mass in terms of size distribution.

260

A. Bougiatioti et al. / Atmospheric Environment 64 (2013) 251e262

3.4. Comparison with other studies Currently, there are not many studies examining the mass size distribution of the different components of carbonaceous matter in marine background sites. Cavalli et al. (2004) present the concentration distributions of the main aerosol constituents of marine aerosols at Mace Head (Ireland), with clear bimodal distributions for black carbon and WSOC, with the main maxima at 0.25 mm and 2 a second one, less pronounced, at 2 mm. NHþ 4 and nssSO4 also exhibit similar maximum. These results are in good agreement with ours, with the main maxima for EC and WSOC for aerodynamic diameters of 0.225e0.585 mm and the second one for 1.66e4.08 mm particles. Furthermore, sea salt and nitrate are almost exclusively present in particles >1 mm with a pronounced maxima at 2 mm, with our maxima being found for particles >1.66 mm. Yu et al. (2004) also report WSOC distributions from a coastal site in Hong Kong, with a dominant fine mode and a minor coarse mode at 0.7
0.1 and at 4.0
0.3 mm, respectively. Timonen et al. (2008) show WSOC mass size distributions from PM1 samples collected in Helsinki (Finland) during spring. Bimodal size distributions were observed, as well, with the major fraction being the accumulation mode between 0.1 and 1 mm (maximum at w0.4 mm) and an average WSOC/OC ration of 54.5%. Finally, Agarwal et al. (2010) also report maximum WSOC concentrations in the fine fraction particles (Dp < 1.1 mm) in aerosols influenced by both biomass burning and pollution, as well as of marine origin. OC in the same samples exhibits high concentrations also in the coarse fraction (Dp > 7.0 mm) in the case of biomass burning-influenced aerosol, aerosols of marine origin as well as of mixed aerosols. As far as size-segregated composition is concerned, our findings fall in the range of results reported for regional marine background sites, with the fine particle range being dominated by ammonium sulfate and carbonaceous matter, with concentrations peaking in the accumulation mode particles (0.32e1 mm) due to their common origin in gas-to-particle conversion mechanisms (Putaud et al., 2004; Carbone et al., 2010). Present in the fine fraction there is also a small amount of sea salt and mineral dust, similar to the values reported by Sciare et al. (2005) for the same area. Pio et al. (2007) reported comparable concentrations for Aveiro (Portugal, marine site) for PM2.5 aerosol, providing an OC/EC ratio of 5.2 and 5.6 during summer and winter respectively, with a large percentage of the OC found to be water-soluble (mean ratio WSOC/OC w56%). Finally, they also note a depletion of chloride relative to sodium during summer and a large fraction of Ca2þ and Kþ present in excess with respect to sea salt. Carbone et al. (2010) mention somewhat lower contributions of ammonium sulfate to the submicrometer aerosol mass and lower concentrations of sea salt in the coarse fraction for the marine site of Rende (Cosenza), with remarkably higher contribution of water-soluble organic matter during winter, but an increase of WSOC/TC (total carbon) in summertime, linked to photochemical oxidation. Median mass size segregated aerosol chemical composition in the suburbs of Marseille (Putaud et al., 2004) exhibits the same characteristics with our results, with particulate OM mainly in the submicrometer fraction and sea salt with mineral dust for particles >2 mm.

area, the mass size distribution of organic, elemental, and watersoluble organic carbon. All three carbonaceous constituents exhibit a prominent maximum in the Accumulation mode particles (Da 0.449 mm) and a second, less pronounced maximum in the Coarse mode (Da 2.68 mm) which is more obvious in the presence of mineral dustladen air masses. Organic and elemental carbon occasionally show a minor, secondary peak in the lower particle range (0.277e 0.449 mm), especially for anthropogenic emission-laden (traffic/ heating) air masses coming from Central-Eastern Europe. Maximum concentrations for OC and WSOC occur during summer, while for EC in winter and the transitional seasons. All three constituents also exhibit high concentrations during spring (Aprile May) for coarse fraction particles. This April maximum is associated with the occurrence of dust events, when air masses transported from Northern Africa arrive at the site by strong southern winds. These supermicrometer concentrations may be associated with organic species (e.g. diacids) attributed to uptake/formation on mineral dust and sea salt particles. In general, it was found that almost 2/3 of OC and EC concentrations are found in the PM1.06 fraction of the aerosol. Using the majority of samples for all 12 stages of the SDI, an OC/ EC ratio of 5.1 is obtained, this ratio being slightly higher for the PM1.06 fraction. This ratio is similar to other ratios found for marine background sites (e.g. Aveiro-Portugal). Using the EC-tracer method, it was found that 83.5
10.8% of the PM10 organic carbon is secondary, with the percentage reaching w86% for the PM1.06 fraction. Having in mind that high secondary organic aerosol concentrations result principally via photochemical oxidation, adding oxidized functional hydrophilic groups and increasing the solubility of organic carbon in water, we studied the WSOC/OC relationship in the samples. It occurred that a great majority (w70%) of the OC in the fine fraction is water-soluble, with the ratio increasing, as expected, during summer. Finally, we concurrently established the mass size distribution and of major water-soluble inorganic aerosol components. SO2 4 NHþ 4 also exhibit a pronounced maximum in the fine fraction (0.45e0.67 mm), with ammonium sulfate accounting for 53.4
38.1% of the total inorganic mass fraction and 84.4
12.8% of the total sulfate being in the submicrometer fraction. Together with ammonium and organic matter, they constitute the largest part of þ  this aerosol faction. NO 3 , Na and Cl , on the other hand, show a maximum in the coarse fraction (aerodynamic diameters 2.68e 4.08 mm). It was estimated that sea salt and mineral dust account for 33% and 45% of the coarse inorganic mass fraction, respectively. Our results represent a valuable long-term measurement dataset of size-segregated aerosol measurements in the area.

Acknowledgments This research project (PENED) is co-financed by E.U.-European Social Fund (75%) and the Greek Ministry of Development-GSRT (25%). The authors would like to thank the two anonymous reviewers for their comments which helped to improve the submitted version.

4. Summary and conclusions Size-segregated aerosol measurements were carried out at the Finokalia measuring site of the University of Crete during a threeyear period from July 2004 until February 2007, using two 12stage Small-Deposit-area low-volume-Impactors for particles with aerodynamic diameters between 0.01 and 13.6 mm. 2  111 impactor samples were collected (2664 in total) and were subsequently analyzed in order to determine, for the first time in the

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