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Regional and long-range transport of aerosols at Mt. Aitana, Southeastern Spain
STOTEN-21821; No of Pages 8 Science of the Total Environment xxx (2017) xxx–xxx
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Regional and long-range transport of aerosols at Mt. Aitana, Southeastern Spain Nuria Galindo ⁎, Eduardo Yubero, Jose F. Nicolás, Javier Crespo, Montse Varea, Juan Gil-Moltó Atmospheric Pollution Laboratory (LCA), Department of Applied Physics, Miguel Hernández University, Avenida de la Universidad S/N, 03202 Elche, Spain
H I G H L I G H T S
G R A P H I C A L
A B S T R A C T
• PM1 was mainly composed of organic carbon and ammonium sulfate • Concentrations of PM10 and crustal elements were highly correlated • The lowest PM levels were associated to air masses coming from the Atlantic • PM10, Ti and Fe concentrations were significantly enhanced during Saharan events • Ca/Ti and Ca/Fe ratios can be used as sensitive indicators of Saharan events
a r t i c l e
i n f o
Article history: Received 12 December 2016 Received in revised form 12 January 2017 Accepted 16 January 2017 Available online xxxx Editor: D. Barcelo Keywords: PM1 PM10 High mountain Western Mediterranean Chemical composition
a b s t r a c t More than 150 particulate matter (PM) samples with aerodynamic diameters smaller than 1 and 10 μm (PM1 and PM10, respectively) were collected during an 18-month sampling campaign at Mt. Aitana (1558 m a.s.l.), located in the western Mediterranean basin. PM samples were analyzed for water-soluble ions, carbonaceous species and trace metals using standard procedures. Average mass concentrations of PM1 and PM10 were, respectively, 5.0 and 13.3 μg m−3. PM1 was composed mostly of organic carbon and ammonium sulfate, while nitrate and crustal elements were major components of the PM10 fraction. A significant positive correlation was determined between PM10 and mineral elements such as Ca or Fe. The study of the influence of air mass origin upon PM mass concentrations and composition showed that Saharan dust outbreaks were associated with the highest PM10 levels (24.9 μg m−3 average during African events). Nitrate and crustal components were also considerably increased during these episodes, especially Ti and Fe (~190% higher compared with the average value for the whole study period). The results indicate that Ca/Ti and Ca/Fe ratios can be considered reliable indicators of Saharan dust intrusions. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Aerosols can be released into the atmosphere by a wide variety of local sources, both natural and anthropogenic, or come from remote
areas transported over long distances by strong winds. Alternatively, some inorganic and organic gaseous pollutants can be converted into secondary aerosols by chemical reactions induced by sun light. All these processes determine the physico-chemical characteristics of
⁎ Corresponding author. E-mail address: [email protected] (N. Galindo).
http://dx.doi.org/10.1016/j.scitotenv.2017.01.108 0048-9697/© 2017 Elsevier B.V. All rights reserved.
Please cite this article as: Galindo, N., et al., Regional and long-range transport of aerosols at Mt. Aitana, Southeastern Spain, Sci Total Environ (2017), http://dx.doi.org/10.1016/j.scitotenv.2017.01.108
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N. Galindo et al. / Science of the Total Environment xxx (2017) xxx–xxx
aerosols and therefore their impacts on human health and the climate system (Raes et al., 2000). Among the natural sources of aerosols, mineral dust from the Sahara desert is one of the most important on a global scale. Dust plumes often enter the atmosphere and can travel up to thousands of kilometers before settling back to earth (Engelstaedter et al., 2006). During transport, dust particles tend to adsorb acidic gases such as SO2 or NOx, facilitating their transformation into sulfates and nitrates (Abdelkader et al., 2015). The western Mediterranean basin is especially sensitive to Saharan dust events due to its proximity to Northern Africa (Galindo et al., 2008; De la Paz et al., 2013). Additionally, the region's typical climate, characterized by low precipitations and summer droughts, favors soil dust resuspension and contributes to increase background levels of mineral matter. The relative input of local versus Saharan dust is rather difficult to estimate due to a similar chemical composition (Nicolás et al., 2008). Other important processes associated with high aerosol levels in this area are the typical recirculation patterns of air masses and the intense solar radiation that promote the formation of secondary compounds through photochemical reactions (Millán et al., 2002; Rodríguez et al., 2002). Stations located at high mountain sites are particularly suitable to study regional processes and long range transport of pollutants since the interference of anthropogenic local sources is often extremely low (Ripoll et al., 2014; Moroni et al., 2015). With this aim, at the end of 2010 we established a monitoring station at Mt. Aitana (1558 m), the highest peak of the Betic Cordillera located in southeastern Spain. The present study is focused on the influence of air mass origins on the levels and composition of PM1 and PM10 at this site, with special attention to Saharan dust outbreaks. 2. Experimental 2.1. Sampling site The sampling station is located at Mt. Aitana (38°38′56.8″N 0°15′ 55.2″W; 1558 m a.s.l.), in southeastern Spain. It is situated 16 km from the nearest Mediterranean coast and b40 km from the city of Alicante (Fig. 1). Since the surrounding area is sparsely populated, pollutant emissions from human activities are very low. Vegetation is scarce and large areas of soil are exposed to wind erosion. Average temperatures vary between 3 °C in winter and 20 °C in summer. Prevailing winds blow mainly from the coast during summer and from the NNW direction during the winter season. Annual precipitation at Mt. Aitana usually ranges from 600 to 800 mm, while coastal areas receive a rainfall of b300 mm. More details of this site can be found in Nicolás et al. (2015) and Galindo et al. (2016). 2.2. Sampling and analysis The sampling campaign was carried out between 17th March 2014 and 4th September 2015. Twenty-four-hour PM1 and PM10 samples were collected with an approximate frequency of three times a week starting at 00:00 UTC each day. The collection of PM1 samples was simultaneously performed by means of high-volume (MCV, 720 m3 day−1) and low-volume samplers (LVS 3.1, 55 m3 day− 1) using quartz fiber (Whatman QM-H, 150 mm) and Teflon (Whatman 2 μm PTFE 46.2 mm PP ring supported) filters, respectively, as substrates. PM10 particles were sampled onto quartz fiber filters (150 mm) using a Digitel high-volume sampler (820 m3 day−1). During the measurement period, about 180 and 160 PM1 samples were collected with the low- volume and high-volume samplers, respectively. In the case of PM10, a total of 162 valid samples were collected and analyzed. It is important to mention that the fraction of samples gathered during the summer season was significantly higher than in winter since (1) the measurement period does not cover two complete years and (2) samplers failure is more frequent during winter due to adverse ambient conditions.
Fig. 1. Location of the sampling site in the southeast of the Iberian Peninsula. EU: European, MED: Mediterranean, NAF: North Africa, ASW: Atlantic South West, ANW: Atlantic North West, AN: Atlantic North, REG: Regional.
All filters were conditioned for at least 24 h at a relative humidity of 50 ± 5% and temperature of 20 ± 1 °C and weighted using electronic balances (Ohaus AP250D and Mettler-Toledo XP105) with 10-μg sensitivity. Concentrations were then calculated by dividing PM masses by the sampled air volume. After weighting, the filters were stored in the fridge at 4 °C until chemical analysis. PM1 samples on Teflon filters and a punch of 47 mm diameter of each PM10 sample were extracted ultrasonically with 15 mL of ultrapure water and heated at 60 °C for about 6 h. The aqueous extracts were then analyzed by ion chromatography (IC) for the determination 2− 2− + + + of ion concentrations (Cl−, NO− 3 , SO4 , C2O4 , Na , NH4 , K , Mg2+ and Ca2+). Anions were analyzed by means of a Dionex DX-120 ion chromatograph with an IonPac AS11-HC separation column using KOH 15 mM as eluent. The analysis of cations was performed using a Dionex ICS-1100 ion chromatograph equipped with a CS12A analytical column and 20 mM methane sulfonic acid as eluent. To determine organic and elemental carbon concentrations punches of 1.5 cm2 area from PM1 and PM10 samples collected onto quartz fiber filters were analyzed with a Thermal-Optical Carbon Aerosol Analyser by Sunset Laboratory using the NIOSH 5040 protocol to quantify the elemental and organic carbon fractions (Birch and Cary, 1996). A detailed description of the analytical procedures can be found in Yubero et al. (2015). The elemental composition of PM was determined by means of Energy Dispersive X-Ray Fluorescence (ED-XRF) using an ARL Quant'x Spectrometer (Thermo Fisher Scientific, UK). The excitation X-rays were obtained with an X-ray tube with an Rh anode (Imax = 1.98 A, Vmax = 50 kV). The fluorescenced X-ray photons are detected and converted to an electrical signal by means of a Si(Li) detector. The instrument was calibrated using different standards (Micromatter). The accuracy of the quantitative method was checked by analyzing the SRM NIST2783 standard (PM2.5 on polycarbonate membrane). Detection limits ranged from 0.1 to 60 μg cm−2 depending on the element. In the case of PM1, these analyses were performed on samples collected on Teflon filters since the use of quartz fiber filters as substrates only enables the reliable determination of elements from Ca due to the presence of silicon in the filter. 2.3. Meteorological data and back-trajectory analysis Temperature, solar radiation, relative humidity, rainfall, wind speed and wind direction were continuously monitored by a meteorological station located at the sampling site. Back trajectories of air masses arriving at Mt. Aitana were calculated using the HYSPLIT model developed by the National Oceanic and Atmospheric Administration (NOAA) (Draxler and Rolph, 2013). Ninety-six hour backward trajectories (for 12 a.m. modelling vertical velocity and for 3 different heights, 500, 1500 and 2500 m above ground level,
Please cite this article as: Galindo, N., et al., Regional and long-range transport of aerosols at Mt. Aitana, Southeastern Spain, Sci Total Environ (2017), http://dx.doi.org/10.1016/j.scitotenv.2017.01.108
N. Galindo et al. / Science of the Total Environment xxx (2017) xxx–xxx
a.g.l.) were computed on each day of measurements. Air mass origins were divided into seven geographical sectors (see Fig. 1) as described in Ripoll et al. (2014). 3. Results and discussion 3.1. Concentrations and chemical composition of PM1 and PM10 The concentrations and chemical species of PM1 and PM10 averaged for the whole study period are shown in Table 1. In order to facilitate comparisons with previous studies, a table with PM values registered at other high altitude sites has also been included (Table 2). Since our previous works concern the PM1 fraction (Nicolás et al., 2015; Galindo et al., 2016), comparisons in the present study are mainly focused on PM10. PM1 and PM10 mean concentrations were similar to the values measured at the Montsec station, which is located at a similar altitude approximately 400 km north of Mt. Aitana. However, PM10 levels were considerably higher than those reported for puy de Dôme, in central France, although it is also situated at a comparable altitude. This can be explained by the high insolation degree that account for the greater concentrations of secondary inorganic ions at Mt. Aitana. The contribution of crustal elements to the levels of PM10 was also higher at Mt. Aitana since it is located in a semi-arid area closer than puy de Dôme to the African continent. Compared to the values measured at high mountain sites in Italy, the PM10 mean concentration at Mt. Aitana was slightly lower than the value reported for Mt. Martano and higher than that observed at Mt. Cimone, which is located in the Po valley, one of the most populated and industrialized areas in Europe. When interpreting this result, it is important to keep in mind that the average levels calculated in this work are more representative of the summer season, the period with maximum PM concentrations at high altitude sites (Galindo et al., 2016; Moroni et al., 2015). In fact, the average PM10 concentration at Mt. Aitana was very similar to the summer value obtained at Mt. Cimone (13.7 μg m−3; Tositti et al., 2013). These values were significantly higher than those reported for sites located at higher altitudes, such as the Jungfraujoch high alpine station (Bukowiecki et al., 2016) and the Nepal Climate Observatory-Pyramid (NCO-P), in the Southern Himalaya (Decesari et al., 2010). Organic carbon and sulfate were the main aerosol components at Mt. Aitana, being the joint contribution to the average mass of PM1 and PM10 around 60% and 25%, respectively. Crustal elements, such as calcium, and nitrate also accounted for a significant fraction of the PM10 mass. Due to the close proximity of the measurement location to the Mediterranean Sea, marine ions in PM10 showed concentrations notably higher than those measured at sites located farther from the coast, like Mt. Martano (Moroni et al., 2015) or puy de Dôme (Bourcier et al.,
Table 1 Average chemical composition (±standard deviation) of PM1 and PM10 at Mt. Aitana between March 2014 and September 2015. Concentrations are given in ng m−3, except for PM1 and PM10 (μg m−3).
Total OC EC SO2− 4 NO− 3 − Cl C2O2− 4 NH+ 4 + Na K+ Ca2+ Mg2+ S Si
PM10
PM1
13.3 ± 12.1 1927 ± 695 70 ± 41 1504 ± 895 817 ± 553 127 ± 165 156 ± 95 388 ± 268 250 ± 197 65 ± 57 467 ± 665 52 ± 39 270 ± 162
5.0 ± 2.8 2138 ± 727 92 ± 44 1013 ± 709 50 ± 77 67 ± 108 37 ± 36 373 ± 277 62 ± 98 45 ± 37 62 ± 86 6±8 400 ± 247 122 ± 221
Pb Al Zn Ti V Mn Fe Ni Cu K Ca Sr Ba Br
PM10
PM1
1±2
1±1 49 ± 84 2±2 4±6 2±1 1±2 24 ± 42 1±1
6 ± 11 20 ± 40 4±6 6 ± 10 173 ± 329 3±3 1±1 106 ± 187 413 ± 668 3±4 8 ± 21 3±2
55 ± 80 1±1 1±1 2±1
3
Table 2 Average concentrations of PM1 and PM10 measured at high mountain sites (μg m−3).
Aitana Montsec Puy de Dôme Martano Cimone Jungfraujoch NCO-P
Location
Altitude (m)
PM10
PM1
References
Spain Spain France Italy Italy Swiss Alps Nepal Himalaya
1558 1570 1465 1100 2165 3580 5079
13.3 12 5.6 14.6 8.8 b7 6
5.0 5 3.9
This work Ripoll et al., 2014 Bourcier et al., 2012 Moroni et al., 2015 Tositti et al., 2013 Bukowiecki et al., 2016 Decesari et al., 2010
2012). Actually, the concentrations of PM10 major components at Mt. Aitana were comparable to the values found at Mt. Martano, except for sodium and chloride. As expected, anthropogenic elements like Pb or Cu exhibited concentrations much lower than the values found at urban areas (Nicolás et al., 2011; Padoan et al., 2016) and were frequently under the detection limit. OC and EC average concentrations in PM1 were slightly higher than in PM10 because not all the samples of both fractions were concurrently collected. From the standard deviations shown in Table 1, it can be inferred that PM10 concentrations were more variable than those of PM1. This can be explained by the large variations in daily concentrations of crustal components (represented by Ca, Fe, Ti, Mn and Sr), which are much more abundant in PM10. One of the reasons for such variations is the important and episodic contribution of long-range transported mineral dust coming from the Sahara desert, mostly observed in spring and summer. Table 3 presents the correlation coefficients calculated between specific components of the PM10 fraction. The correlation obtained between calcium concentrations measured by X-ray fluorescence and ion chromatography (slope = 0.84) indicates that, in the study area, calcium is in the form of water-soluble calcium salts, such as CaCO3 and CaSO4. In contrast, soluble potassium did not correlate with total potassium, suggesting different source emissions for soluble and insoluble potassium. Water-soluble potassium is primarily released by biomass burning (Decesari et al., 2010; Lee et al., 2011) or sea spray (Almeida et al., 2006), while insoluble potassium has a mineral origin. The significant positive correlations between total potassium and crustal elements confirm the geological origin of insoluble potassium at the measurement site. Although the concentration of mineral matter was considerably lower in PM1 than in PM10, good correlation coefficients between the most abundant crustal elements were also obtained in PM1, particularly between Fe, Si and Al (r N 0.99). Sulfate showed an excellent positive correlation with sulfur, both in PM10 and PM1 (r = 0.97 and 0.94, respectively), suggesting that sulfur is mainly in the form of sulfates. This was confirmed by the slope of the linear regression between both species (2.6), close to the expected to S mass ratio of 3.0. SO2− 4 3.2. Influence of air mass origin on aerosol levels and composition Average concentrations of PM1, PM10 and selected chemical species were calculated from daily values classified according to the origin of air masses as indicated by HYSPLIT. The results are shown in Fig. 2. Regional Table 3 Pearson correlation coefficients between individual PM10 components. Only significant values are shown.
Ca K Fe Ti Sr SO2− 4
Ca2+
K
Fe
Ti
Sr
Mn
0.99
0.89
0.81 0.97
0.72 0.94 0.99
0.77 0.84 0.89 0.86
0.86 0.95 0.99 0.98 0.84
S
0.97
Please cite this article as: Galindo, N., et al., Regional and long-range transport of aerosols at Mt. Aitana, Southeastern Spain, Sci Total Environ (2017), http://dx.doi.org/10.1016/j.scitotenv.2017.01.108
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episodes were divided into summer and winter episodes (REG-S and REG-W, respectively). During the study period, air masses mainly came from the Atlantic North West (ANW, 29%) and North Africa (NAF, 26%). Summer regional episodes (REG-S) occurred on 18% of the sampling days, while winter regional (REG-W) prevailed for 4% of the days. Air masses coming from the Mediterranean (MED), European (EU), Atlantic North (AN) and Atlantic South West (ASW) sectors occurred for b 10% of the sampling days. As expected, the highest PM10 concentrations (24.9 μg m− 3) occurred under Saharan dust outbreaks, together with those of crustal elements. The percentage increase in the levels of these components with respect to the average value for the whole study period was around 190% for Ti and Fe, 170% for K, and 130% for Ca. NO− 3 concentrations also showed a remarkable increase, most likely due to the formation of coarse mode nitrate on mineral dust particles (Galindo et al., 2008; Karydis et al., 2016). Interestingly, a concurrent increase in K+ concentrations was not observed, indicating again a different origin of soluble and insoluble potassium. Regional episodes during summer were also characterized by relatively high PM10 values (13.3 μg m− 3) mostly caused by the production of secondary aerosols, both inorganic and organic, under high insolation conditions (Millán et al., 2002). The enhancement in SO24 −, NH+ 4 and OC concentrations in the submicron fraction is consistent with this statement. A similar effect was observed under the influence of Mediterranean air masses, although their occurrence was much lower (b 5%). It is worth mentioning that during summer regional episodes relatively high levels of crustal elements were also recorded, possibly because these events often alternate with Saharan dust intrusions during the summer season. The lowest PM levels were observed when air masses came from the Atlantic and during winter regional episodes. Winter events were characterized by elevated concentrations of EC, K+ and NO− 3 , particularly in the submicron fraction. EC and soluble potassium could be emitted by biomass burning in the surroundings of the sampling site. Regarding nitrate, previous
works performed at urban and suburban areas in the study region (Galindo et al., 2011; Yubero et al., 2015) have reported high levels of fine NH4NO3 in winter under stagnant conditions, which are associated with trajectories with short pathways. 3.3. Saharan dust outbreaks at Mt. Aitana It is evident from the results shown in the previous section that the transport of dust from the Sahara desert has an outstanding influence on aerosol levels and composition at Mt. Aitana. The relative impact of such events is considerably higher in PM10 than in PM1 (Fig. 3). As already reported (Galindo et al., 2016), PM1 concentrations exhibited the seasonal pattern characteristic of high altitude sites with maximum values in summer and minimum during winter time. Although PM10 levels were also highest in summer, the seasonal cycle is less marked than that of PM1 and peak concentrations can be recorded during winter on days affected by African dust events. The temporal variations of crustal elements in both fractions were clearly modulated by the occurrence of Saharan events (Fig. 3c, d, e). It is obvious that, differently from the PM1 fraction, the variability in total PM10 mass concentrations followed quite well those of crustal elements (the correlation coefficients of PM10 with Ca and Fe were around 0.85). Nevertheless, relative increases during these episodes were higher for elements such as Fe and Ti than for Ca, as pointed out in the previous section. This can be attributed to the high calcium background levels in the study area due to local dust resuspension, suggesting that in this region Ca is not the best tracer of Saharan dust events. In fact, the calcium to titanium and iron mass ratios could help to identify African dust intrusions. During these episodes the Ca/Ti and Ca/Fe ratios experienced a significant decrease with respect to non-intrusion days (Fig. 4), indicating that these ratios can be used as sensitive indicators of Saharan dust outbreaks. Similar results were reported in a previous work performed at an urban site in the study area (Nicolás et al., 2008).
30
Concentration (µg m-3)
25
PM10
PM1
20 15 10 5 0 EU
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AN
3000
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Concentration (ng m -3)
Concentration (ng m -3)
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Ca
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OC
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400
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40
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0 EU
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ANW
AN
ASW
NAF
MED
EU
REG-S
REG-W
ANW
AN
MED
Fig. 2. Concentrations and chemical composition of aerosols at Mt. Aitana as a function of the air mass origin.
Please cite this article as: Galindo, N., et al., Regional and long-range transport of aerosols at Mt. Aitana, Southeastern Spain, Sci Total Environ (2017), http://dx.doi.org/10.1016/j.scitotenv.2017.01.108
N. Galindo et al. / Science of the Total Environment xxx (2017) xxx–xxx
5
100
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200 100
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(d)
PM1
Cocentration (ng m -3)
1200 1000
Si Al
800 600 400 200
0 31/08/2015
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17/03/2014'
Fig. 3. Time series of PM mass concentrations and crustal elements at Mt. Aitana during the study period. The most important Saharan events are marked with yellow shadows on Panel a. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
In order to better understand the origin (natural or anthropogenic) of the elements present in atmospheric aerosols, enrichment factors (EFs) have been widely used in the literature (Clements et al., 2014; Nava et al., 2015; Fernández-Olmo et al., 2016). EFx ¼
Xsample =Rsample Xcrust =Rcrust
where X and R are the element (X) and reference element (R) mass concentrations in the sample and the upper continental crust, respectively. A recent work has used titanium as a reference element to distinguish between mineral dust originating from the southern U.S. and North Africa (Bozlaker et al., 2013). In this work, EFs in PM10 samples were also estimated referencing Ti in the upper continental crust (Rudnick and Gao, 2003). The results are presented in Fig. 5.
Please cite this article as: Galindo, N., et al., Regional and long-range transport of aerosols at Mt. Aitana, Southeastern Spain, Sci Total Environ (2017), http://dx.doi.org/10.1016/j.scitotenv.2017.01.108
6
N. Galindo et al. / Science of the Total Environment xxx (2017) xxx–xxx
800
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6000 y = 17.04x R² = 0.90
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17/03/2014'
Fig. 4. Relationship between (a) calcium and titanium concentrations and (b) calcium and iron concentrations during Saharan dust events and non-event days. Panel c shows the daily variability of Ca/Ti and Ca/Fe ratios during the study period. Black dots and red crosses denote those days with back-trajectories from North Africa. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Elements with a clear crustal origin (K, Mn, Fe and Sr) showed enrichment factors close to unity. Additionally, EF values for these elements were very similar during intrusion and non-intrusion periods, indicating that they are not emitted by other anthropogenic local sources. Enrichment factors for trace elements associated with traffic (Cu, Zn; Clements et al., 2014) or oil combustion (V, Ni; Becagli et al., 2012) were higher than 10, especially on non-intrusion days. Although these values are considerably lower than those calculated for urban areas (EFs N 100; Fernández-Olmo et al., 2016; Malandrino et al., 2016) the results suggest that these elements are predominantly emitted by anthropogenic sources, in particular when no transport from the Sahara desert occurs and the mineral dust load is low. The reduction in EFs of anthropogenic elements during African intrusions has been reported in previous works (Bozlaker et al., 2013). It is worth mentioning that the concentrations of vanadium and nickel exhibited a significant increase during Saharan dust events (average V and Ni levels were around 4 and 3 times higher, respectively, than on non-intrusion
days). This can be primarily attributed to the transport by Saharan dust plumes of pollutants emitted in the Mediterranean basin and northern Africa, since both elements were still enriched (EF ~ 7 and 11, respectively, for V and Ni) during Saharan outbreaks. The enrichment of Cl could be due to sea spray. Regarding calcium, EFs calculated for this element suggest Ca enrichment in this area given that the average value for non-intrusion days was double that obtained for intrusion periods (~6 and 3, respectively). The Ca/Sr ratio measured in the local soil (~ 200; Gallello et al., 2013) was high compared with the values (~80) for the upper continental crust (Rudnick and Gao, 2003) and Saharan dust (Bozlaker et al., 2013), supporting Ca enrichment in this region. 4. Conclusions Average concentrations of PM1 and PM10 measured at Mt. Aitana between March 2014 and September 2015 (5.0 and 13.3 μg m−3,
Please cite this article as: Galindo, N., et al., Regional and long-range transport of aerosols at Mt. Aitana, Southeastern Spain, Sci Total Environ (2017), http://dx.doi.org/10.1016/j.scitotenv.2017.01.108
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60 Intrusion
50 Enrichment factors
Non-intrusion 40 30
20 10 0 Cl K Ca Ti V Cr Mn Fe Ni Cu Zn Sr
Fig. 5. Enrichment factors of elements in PM10 at Mt. Aitana on Saharan event- and nonevent days.
respectively) were of the same order as those found at other high mountain stations located in northeastern Spain and Italy. The main components of PM1 were organic carbon and ammonium sulfate, representing N 60% of the total mass. In the PM10 fraction, nitrate and mineral elements such as calcium were also major components. Significant day-to day variations in PM concentrations were observed, particularly for the PM10 fraction. This variability was mainly associated with changes in the concentrations of major and trace elements of crustal origin due to the important and episodic contribution of Saharan dust outbreaks. The analysis of the effect of air mass origins on the PM10 fraction revealed that concentrations considerably higher than the average were primarily recorded during African events, which occurred on 26% of the sampling days. During summer, Saharan intrusions often alternate with regional episodes that prevailed on 18% of the days in the study period. These events were also associated to relatively high PM10 levels due to the formation of secondary organic and inorganic species and the persistence of crustal elements. The relative influence of Saharan dust events on the PM1 fraction was much lower. As a result, PM1 concentrations showed a more defined seasonal cycle than those of PM10. For both fractions, the lowest levels were generally observed during Atlantic advections, which is the most frequent origin of air masses arriving at Mt. Aitana (N 30%). Enrichment factors calculated for intrusion and non-intrusion days pointed to Ca enrichment of local soil. In fact, the relative increase in the levels of titanium and iron during Saharan dust intrusions was appreciably higher than that of calcium, implying a significant reduction in Ca/Ti and Ca/Fe mass ratios. These ratios could be used as suitable indicators for the identification of Saharan events. Acknowledgements This work was supported by the Spanish Ministry MINECO CGL2012-39623-C02-2 (PRISMA-AITANA) project. This study was also co-financed by the European Union, through FEDER funds. We would also like to thank the Spanish Defense Ministry (EVA n. 5) for allowing access to its facilities. References Abdelkader, M., Metzger, S., Mamouri, R.E., Astitha, M., Barrie, L., Levin, Z., Lelieveld, J., 2015. Dust–air pollution dynamics over the eastern Mediterranean. Atmos. Chem. Phys. 15, 9173–9189.
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Contents lists available at ScienceDirect
Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv
Regional and long-range transport of aerosols at Mt. Aitana, Southeastern Spain Nuria Galindo ⁎, Eduardo Yubero, Jose F. Nicolás, Javier Crespo, Montse Varea, Juan Gil-Moltó Atmospheric Pollution Laboratory (LCA), Department of Applied Physics, Miguel Hernández University, Avenida de la Universidad S/N, 03202 Elche, Spain
H I G H L I G H T S
G R A P H I C A L
A B S T R A C T
• PM1 was mainly composed of organic carbon and ammonium sulfate • Concentrations of PM10 and crustal elements were highly correlated • The lowest PM levels were associated to air masses coming from the Atlantic • PM10, Ti and Fe concentrations were significantly enhanced during Saharan events • Ca/Ti and Ca/Fe ratios can be used as sensitive indicators of Saharan events
a r t i c l e
i n f o
Article history: Received 12 December 2016 Received in revised form 12 January 2017 Accepted 16 January 2017 Available online xxxx Editor: D. Barcelo Keywords: PM1 PM10 High mountain Western Mediterranean Chemical composition
a b s t r a c t More than 150 particulate matter (PM) samples with aerodynamic diameters smaller than 1 and 10 μm (PM1 and PM10, respectively) were collected during an 18-month sampling campaign at Mt. Aitana (1558 m a.s.l.), located in the western Mediterranean basin. PM samples were analyzed for water-soluble ions, carbonaceous species and trace metals using standard procedures. Average mass concentrations of PM1 and PM10 were, respectively, 5.0 and 13.3 μg m−3. PM1 was composed mostly of organic carbon and ammonium sulfate, while nitrate and crustal elements were major components of the PM10 fraction. A significant positive correlation was determined between PM10 and mineral elements such as Ca or Fe. The study of the influence of air mass origin upon PM mass concentrations and composition showed that Saharan dust outbreaks were associated with the highest PM10 levels (24.9 μg m−3 average during African events). Nitrate and crustal components were also considerably increased during these episodes, especially Ti and Fe (~190% higher compared with the average value for the whole study period). The results indicate that Ca/Ti and Ca/Fe ratios can be considered reliable indicators of Saharan dust intrusions. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Aerosols can be released into the atmosphere by a wide variety of local sources, both natural and anthropogenic, or come from remote
areas transported over long distances by strong winds. Alternatively, some inorganic and organic gaseous pollutants can be converted into secondary aerosols by chemical reactions induced by sun light. All these processes determine the physico-chemical characteristics of
⁎ Corresponding author. E-mail address: [email protected] (N. Galindo).
http://dx.doi.org/10.1016/j.scitotenv.2017.01.108 0048-9697/© 2017 Elsevier B.V. All rights reserved.
Please cite this article as: Galindo, N., et al., Regional and long-range transport of aerosols at Mt. Aitana, Southeastern Spain, Sci Total Environ (2017), http://dx.doi.org/10.1016/j.scitotenv.2017.01.108
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aerosols and therefore their impacts on human health and the climate system (Raes et al., 2000). Among the natural sources of aerosols, mineral dust from the Sahara desert is one of the most important on a global scale. Dust plumes often enter the atmosphere and can travel up to thousands of kilometers before settling back to earth (Engelstaedter et al., 2006). During transport, dust particles tend to adsorb acidic gases such as SO2 or NOx, facilitating their transformation into sulfates and nitrates (Abdelkader et al., 2015). The western Mediterranean basin is especially sensitive to Saharan dust events due to its proximity to Northern Africa (Galindo et al., 2008; De la Paz et al., 2013). Additionally, the region's typical climate, characterized by low precipitations and summer droughts, favors soil dust resuspension and contributes to increase background levels of mineral matter. The relative input of local versus Saharan dust is rather difficult to estimate due to a similar chemical composition (Nicolás et al., 2008). Other important processes associated with high aerosol levels in this area are the typical recirculation patterns of air masses and the intense solar radiation that promote the formation of secondary compounds through photochemical reactions (Millán et al., 2002; Rodríguez et al., 2002). Stations located at high mountain sites are particularly suitable to study regional processes and long range transport of pollutants since the interference of anthropogenic local sources is often extremely low (Ripoll et al., 2014; Moroni et al., 2015). With this aim, at the end of 2010 we established a monitoring station at Mt. Aitana (1558 m), the highest peak of the Betic Cordillera located in southeastern Spain. The present study is focused on the influence of air mass origins on the levels and composition of PM1 and PM10 at this site, with special attention to Saharan dust outbreaks. 2. Experimental 2.1. Sampling site The sampling station is located at Mt. Aitana (38°38′56.8″N 0°15′ 55.2″W; 1558 m a.s.l.), in southeastern Spain. It is situated 16 km from the nearest Mediterranean coast and b40 km from the city of Alicante (Fig. 1). Since the surrounding area is sparsely populated, pollutant emissions from human activities are very low. Vegetation is scarce and large areas of soil are exposed to wind erosion. Average temperatures vary between 3 °C in winter and 20 °C in summer. Prevailing winds blow mainly from the coast during summer and from the NNW direction during the winter season. Annual precipitation at Mt. Aitana usually ranges from 600 to 800 mm, while coastal areas receive a rainfall of b300 mm. More details of this site can be found in Nicolás et al. (2015) and Galindo et al. (2016). 2.2. Sampling and analysis The sampling campaign was carried out between 17th March 2014 and 4th September 2015. Twenty-four-hour PM1 and PM10 samples were collected with an approximate frequency of three times a week starting at 00:00 UTC each day. The collection of PM1 samples was simultaneously performed by means of high-volume (MCV, 720 m3 day−1) and low-volume samplers (LVS 3.1, 55 m3 day− 1) using quartz fiber (Whatman QM-H, 150 mm) and Teflon (Whatman 2 μm PTFE 46.2 mm PP ring supported) filters, respectively, as substrates. PM10 particles were sampled onto quartz fiber filters (150 mm) using a Digitel high-volume sampler (820 m3 day−1). During the measurement period, about 180 and 160 PM1 samples were collected with the low- volume and high-volume samplers, respectively. In the case of PM10, a total of 162 valid samples were collected and analyzed. It is important to mention that the fraction of samples gathered during the summer season was significantly higher than in winter since (1) the measurement period does not cover two complete years and (2) samplers failure is more frequent during winter due to adverse ambient conditions.
Fig. 1. Location of the sampling site in the southeast of the Iberian Peninsula. EU: European, MED: Mediterranean, NAF: North Africa, ASW: Atlantic South West, ANW: Atlantic North West, AN: Atlantic North, REG: Regional.
All filters were conditioned for at least 24 h at a relative humidity of 50 ± 5% and temperature of 20 ± 1 °C and weighted using electronic balances (Ohaus AP250D and Mettler-Toledo XP105) with 10-μg sensitivity. Concentrations were then calculated by dividing PM masses by the sampled air volume. After weighting, the filters were stored in the fridge at 4 °C until chemical analysis. PM1 samples on Teflon filters and a punch of 47 mm diameter of each PM10 sample were extracted ultrasonically with 15 mL of ultrapure water and heated at 60 °C for about 6 h. The aqueous extracts were then analyzed by ion chromatography (IC) for the determination 2− 2− + + + of ion concentrations (Cl−, NO− 3 , SO4 , C2O4 , Na , NH4 , K , Mg2+ and Ca2+). Anions were analyzed by means of a Dionex DX-120 ion chromatograph with an IonPac AS11-HC separation column using KOH 15 mM as eluent. The analysis of cations was performed using a Dionex ICS-1100 ion chromatograph equipped with a CS12A analytical column and 20 mM methane sulfonic acid as eluent. To determine organic and elemental carbon concentrations punches of 1.5 cm2 area from PM1 and PM10 samples collected onto quartz fiber filters were analyzed with a Thermal-Optical Carbon Aerosol Analyser by Sunset Laboratory using the NIOSH 5040 protocol to quantify the elemental and organic carbon fractions (Birch and Cary, 1996). A detailed description of the analytical procedures can be found in Yubero et al. (2015). The elemental composition of PM was determined by means of Energy Dispersive X-Ray Fluorescence (ED-XRF) using an ARL Quant'x Spectrometer (Thermo Fisher Scientific, UK). The excitation X-rays were obtained with an X-ray tube with an Rh anode (Imax = 1.98 A, Vmax = 50 kV). The fluorescenced X-ray photons are detected and converted to an electrical signal by means of a Si(Li) detector. The instrument was calibrated using different standards (Micromatter). The accuracy of the quantitative method was checked by analyzing the SRM NIST2783 standard (PM2.5 on polycarbonate membrane). Detection limits ranged from 0.1 to 60 μg cm−2 depending on the element. In the case of PM1, these analyses were performed on samples collected on Teflon filters since the use of quartz fiber filters as substrates only enables the reliable determination of elements from Ca due to the presence of silicon in the filter. 2.3. Meteorological data and back-trajectory analysis Temperature, solar radiation, relative humidity, rainfall, wind speed and wind direction were continuously monitored by a meteorological station located at the sampling site. Back trajectories of air masses arriving at Mt. Aitana were calculated using the HYSPLIT model developed by the National Oceanic and Atmospheric Administration (NOAA) (Draxler and Rolph, 2013). Ninety-six hour backward trajectories (for 12 a.m. modelling vertical velocity and for 3 different heights, 500, 1500 and 2500 m above ground level,
Please cite this article as: Galindo, N., et al., Regional and long-range transport of aerosols at Mt. Aitana, Southeastern Spain, Sci Total Environ (2017), http://dx.doi.org/10.1016/j.scitotenv.2017.01.108
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a.g.l.) were computed on each day of measurements. Air mass origins were divided into seven geographical sectors (see Fig. 1) as described in Ripoll et al. (2014). 3. Results and discussion 3.1. Concentrations and chemical composition of PM1 and PM10 The concentrations and chemical species of PM1 and PM10 averaged for the whole study period are shown in Table 1. In order to facilitate comparisons with previous studies, a table with PM values registered at other high altitude sites has also been included (Table 2). Since our previous works concern the PM1 fraction (Nicolás et al., 2015; Galindo et al., 2016), comparisons in the present study are mainly focused on PM10. PM1 and PM10 mean concentrations were similar to the values measured at the Montsec station, which is located at a similar altitude approximately 400 km north of Mt. Aitana. However, PM10 levels were considerably higher than those reported for puy de Dôme, in central France, although it is also situated at a comparable altitude. This can be explained by the high insolation degree that account for the greater concentrations of secondary inorganic ions at Mt. Aitana. The contribution of crustal elements to the levels of PM10 was also higher at Mt. Aitana since it is located in a semi-arid area closer than puy de Dôme to the African continent. Compared to the values measured at high mountain sites in Italy, the PM10 mean concentration at Mt. Aitana was slightly lower than the value reported for Mt. Martano and higher than that observed at Mt. Cimone, which is located in the Po valley, one of the most populated and industrialized areas in Europe. When interpreting this result, it is important to keep in mind that the average levels calculated in this work are more representative of the summer season, the period with maximum PM concentrations at high altitude sites (Galindo et al., 2016; Moroni et al., 2015). In fact, the average PM10 concentration at Mt. Aitana was very similar to the summer value obtained at Mt. Cimone (13.7 μg m−3; Tositti et al., 2013). These values were significantly higher than those reported for sites located at higher altitudes, such as the Jungfraujoch high alpine station (Bukowiecki et al., 2016) and the Nepal Climate Observatory-Pyramid (NCO-P), in the Southern Himalaya (Decesari et al., 2010). Organic carbon and sulfate were the main aerosol components at Mt. Aitana, being the joint contribution to the average mass of PM1 and PM10 around 60% and 25%, respectively. Crustal elements, such as calcium, and nitrate also accounted for a significant fraction of the PM10 mass. Due to the close proximity of the measurement location to the Mediterranean Sea, marine ions in PM10 showed concentrations notably higher than those measured at sites located farther from the coast, like Mt. Martano (Moroni et al., 2015) or puy de Dôme (Bourcier et al.,
Table 1 Average chemical composition (±standard deviation) of PM1 and PM10 at Mt. Aitana between March 2014 and September 2015. Concentrations are given in ng m−3, except for PM1 and PM10 (μg m−3).
Total OC EC SO2− 4 NO− 3 − Cl C2O2− 4 NH+ 4 + Na K+ Ca2+ Mg2+ S Si
PM10
PM1
13.3 ± 12.1 1927 ± 695 70 ± 41 1504 ± 895 817 ± 553 127 ± 165 156 ± 95 388 ± 268 250 ± 197 65 ± 57 467 ± 665 52 ± 39 270 ± 162
5.0 ± 2.8 2138 ± 727 92 ± 44 1013 ± 709 50 ± 77 67 ± 108 37 ± 36 373 ± 277 62 ± 98 45 ± 37 62 ± 86 6±8 400 ± 247 122 ± 221
Pb Al Zn Ti V Mn Fe Ni Cu K Ca Sr Ba Br
PM10
PM1
1±2
1±1 49 ± 84 2±2 4±6 2±1 1±2 24 ± 42 1±1
6 ± 11 20 ± 40 4±6 6 ± 10 173 ± 329 3±3 1±1 106 ± 187 413 ± 668 3±4 8 ± 21 3±2
55 ± 80 1±1 1±1 2±1
3
Table 2 Average concentrations of PM1 and PM10 measured at high mountain sites (μg m−3).
Aitana Montsec Puy de Dôme Martano Cimone Jungfraujoch NCO-P
Location
Altitude (m)
PM10
PM1
References
Spain Spain France Italy Italy Swiss Alps Nepal Himalaya
1558 1570 1465 1100 2165 3580 5079
13.3 12 5.6 14.6 8.8 b7 6
5.0 5 3.9
This work Ripoll et al., 2014 Bourcier et al., 2012 Moroni et al., 2015 Tositti et al., 2013 Bukowiecki et al., 2016 Decesari et al., 2010
2012). Actually, the concentrations of PM10 major components at Mt. Aitana were comparable to the values found at Mt. Martano, except for sodium and chloride. As expected, anthropogenic elements like Pb or Cu exhibited concentrations much lower than the values found at urban areas (Nicolás et al., 2011; Padoan et al., 2016) and were frequently under the detection limit. OC and EC average concentrations in PM1 were slightly higher than in PM10 because not all the samples of both fractions were concurrently collected. From the standard deviations shown in Table 1, it can be inferred that PM10 concentrations were more variable than those of PM1. This can be explained by the large variations in daily concentrations of crustal components (represented by Ca, Fe, Ti, Mn and Sr), which are much more abundant in PM10. One of the reasons for such variations is the important and episodic contribution of long-range transported mineral dust coming from the Sahara desert, mostly observed in spring and summer. Table 3 presents the correlation coefficients calculated between specific components of the PM10 fraction. The correlation obtained between calcium concentrations measured by X-ray fluorescence and ion chromatography (slope = 0.84) indicates that, in the study area, calcium is in the form of water-soluble calcium salts, such as CaCO3 and CaSO4. In contrast, soluble potassium did not correlate with total potassium, suggesting different source emissions for soluble and insoluble potassium. Water-soluble potassium is primarily released by biomass burning (Decesari et al., 2010; Lee et al., 2011) or sea spray (Almeida et al., 2006), while insoluble potassium has a mineral origin. The significant positive correlations between total potassium and crustal elements confirm the geological origin of insoluble potassium at the measurement site. Although the concentration of mineral matter was considerably lower in PM1 than in PM10, good correlation coefficients between the most abundant crustal elements were also obtained in PM1, particularly between Fe, Si and Al (r N 0.99). Sulfate showed an excellent positive correlation with sulfur, both in PM10 and PM1 (r = 0.97 and 0.94, respectively), suggesting that sulfur is mainly in the form of sulfates. This was confirmed by the slope of the linear regression between both species (2.6), close to the expected to S mass ratio of 3.0. SO2− 4 3.2. Influence of air mass origin on aerosol levels and composition Average concentrations of PM1, PM10 and selected chemical species were calculated from daily values classified according to the origin of air masses as indicated by HYSPLIT. The results are shown in Fig. 2. Regional Table 3 Pearson correlation coefficients between individual PM10 components. Only significant values are shown.
Ca K Fe Ti Sr SO2− 4
Ca2+
K
Fe
Ti
Sr
Mn
0.99
0.89
0.81 0.97
0.72 0.94 0.99
0.77 0.84 0.89 0.86
0.86 0.95 0.99 0.98 0.84
S
0.97
Please cite this article as: Galindo, N., et al., Regional and long-range transport of aerosols at Mt. Aitana, Southeastern Spain, Sci Total Environ (2017), http://dx.doi.org/10.1016/j.scitotenv.2017.01.108
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episodes were divided into summer and winter episodes (REG-S and REG-W, respectively). During the study period, air masses mainly came from the Atlantic North West (ANW, 29%) and North Africa (NAF, 26%). Summer regional episodes (REG-S) occurred on 18% of the sampling days, while winter regional (REG-W) prevailed for 4% of the days. Air masses coming from the Mediterranean (MED), European (EU), Atlantic North (AN) and Atlantic South West (ASW) sectors occurred for b 10% of the sampling days. As expected, the highest PM10 concentrations (24.9 μg m− 3) occurred under Saharan dust outbreaks, together with those of crustal elements. The percentage increase in the levels of these components with respect to the average value for the whole study period was around 190% for Ti and Fe, 170% for K, and 130% for Ca. NO− 3 concentrations also showed a remarkable increase, most likely due to the formation of coarse mode nitrate on mineral dust particles (Galindo et al., 2008; Karydis et al., 2016). Interestingly, a concurrent increase in K+ concentrations was not observed, indicating again a different origin of soluble and insoluble potassium. Regional episodes during summer were also characterized by relatively high PM10 values (13.3 μg m− 3) mostly caused by the production of secondary aerosols, both inorganic and organic, under high insolation conditions (Millán et al., 2002). The enhancement in SO24 −, NH+ 4 and OC concentrations in the submicron fraction is consistent with this statement. A similar effect was observed under the influence of Mediterranean air masses, although their occurrence was much lower (b 5%). It is worth mentioning that during summer regional episodes relatively high levels of crustal elements were also recorded, possibly because these events often alternate with Saharan dust intrusions during the summer season. The lowest PM levels were observed when air masses came from the Atlantic and during winter regional episodes. Winter events were characterized by elevated concentrations of EC, K+ and NO− 3 , particularly in the submicron fraction. EC and soluble potassium could be emitted by biomass burning in the surroundings of the sampling site. Regarding nitrate, previous
works performed at urban and suburban areas in the study region (Galindo et al., 2011; Yubero et al., 2015) have reported high levels of fine NH4NO3 in winter under stagnant conditions, which are associated with trajectories with short pathways. 3.3. Saharan dust outbreaks at Mt. Aitana It is evident from the results shown in the previous section that the transport of dust from the Sahara desert has an outstanding influence on aerosol levels and composition at Mt. Aitana. The relative impact of such events is considerably higher in PM10 than in PM1 (Fig. 3). As already reported (Galindo et al., 2016), PM1 concentrations exhibited the seasonal pattern characteristic of high altitude sites with maximum values in summer and minimum during winter time. Although PM10 levels were also highest in summer, the seasonal cycle is less marked than that of PM1 and peak concentrations can be recorded during winter on days affected by African dust events. The temporal variations of crustal elements in both fractions were clearly modulated by the occurrence of Saharan events (Fig. 3c, d, e). It is obvious that, differently from the PM1 fraction, the variability in total PM10 mass concentrations followed quite well those of crustal elements (the correlation coefficients of PM10 with Ca and Fe were around 0.85). Nevertheless, relative increases during these episodes were higher for elements such as Fe and Ti than for Ca, as pointed out in the previous section. This can be attributed to the high calcium background levels in the study area due to local dust resuspension, suggesting that in this region Ca is not the best tracer of Saharan dust events. In fact, the calcium to titanium and iron mass ratios could help to identify African dust intrusions. During these episodes the Ca/Ti and Ca/Fe ratios experienced a significant decrease with respect to non-intrusion days (Fig. 4), indicating that these ratios can be used as sensitive indicators of Saharan dust outbreaks. Similar results were reported in a previous work performed at an urban site in the study area (Nicolás et al., 2008).
30
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Fig. 2. Concentrations and chemical composition of aerosols at Mt. Aitana as a function of the air mass origin.
Please cite this article as: Galindo, N., et al., Regional and long-range transport of aerosols at Mt. Aitana, Southeastern Spain, Sci Total Environ (2017), http://dx.doi.org/10.1016/j.scitotenv.2017.01.108
N. Galindo et al. / Science of the Total Environment xxx (2017) xxx–xxx
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Fig. 3. Time series of PM mass concentrations and crustal elements at Mt. Aitana during the study period. The most important Saharan events are marked with yellow shadows on Panel a. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
In order to better understand the origin (natural or anthropogenic) of the elements present in atmospheric aerosols, enrichment factors (EFs) have been widely used in the literature (Clements et al., 2014; Nava et al., 2015; Fernández-Olmo et al., 2016). EFx ¼
Xsample =Rsample Xcrust =Rcrust
where X and R are the element (X) and reference element (R) mass concentrations in the sample and the upper continental crust, respectively. A recent work has used titanium as a reference element to distinguish between mineral dust originating from the southern U.S. and North Africa (Bozlaker et al., 2013). In this work, EFs in PM10 samples were also estimated referencing Ti in the upper continental crust (Rudnick and Gao, 2003). The results are presented in Fig. 5.
Please cite this article as: Galindo, N., et al., Regional and long-range transport of aerosols at Mt. Aitana, Southeastern Spain, Sci Total Environ (2017), http://dx.doi.org/10.1016/j.scitotenv.2017.01.108
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N. Galindo et al. / Science of the Total Environment xxx (2017) xxx–xxx
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Fig. 4. Relationship between (a) calcium and titanium concentrations and (b) calcium and iron concentrations during Saharan dust events and non-event days. Panel c shows the daily variability of Ca/Ti and Ca/Fe ratios during the study period. Black dots and red crosses denote those days with back-trajectories from North Africa. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Elements with a clear crustal origin (K, Mn, Fe and Sr) showed enrichment factors close to unity. Additionally, EF values for these elements were very similar during intrusion and non-intrusion periods, indicating that they are not emitted by other anthropogenic local sources. Enrichment factors for trace elements associated with traffic (Cu, Zn; Clements et al., 2014) or oil combustion (V, Ni; Becagli et al., 2012) were higher than 10, especially on non-intrusion days. Although these values are considerably lower than those calculated for urban areas (EFs N 100; Fernández-Olmo et al., 2016; Malandrino et al., 2016) the results suggest that these elements are predominantly emitted by anthropogenic sources, in particular when no transport from the Sahara desert occurs and the mineral dust load is low. The reduction in EFs of anthropogenic elements during African intrusions has been reported in previous works (Bozlaker et al., 2013). It is worth mentioning that the concentrations of vanadium and nickel exhibited a significant increase during Saharan dust events (average V and Ni levels were around 4 and 3 times higher, respectively, than on non-intrusion
days). This can be primarily attributed to the transport by Saharan dust plumes of pollutants emitted in the Mediterranean basin and northern Africa, since both elements were still enriched (EF ~ 7 and 11, respectively, for V and Ni) during Saharan outbreaks. The enrichment of Cl could be due to sea spray. Regarding calcium, EFs calculated for this element suggest Ca enrichment in this area given that the average value for non-intrusion days was double that obtained for intrusion periods (~6 and 3, respectively). The Ca/Sr ratio measured in the local soil (~ 200; Gallello et al., 2013) was high compared with the values (~80) for the upper continental crust (Rudnick and Gao, 2003) and Saharan dust (Bozlaker et al., 2013), supporting Ca enrichment in this region. 4. Conclusions Average concentrations of PM1 and PM10 measured at Mt. Aitana between March 2014 and September 2015 (5.0 and 13.3 μg m−3,
Please cite this article as: Galindo, N., et al., Regional and long-range transport of aerosols at Mt. Aitana, Southeastern Spain, Sci Total Environ (2017), http://dx.doi.org/10.1016/j.scitotenv.2017.01.108
N. Galindo et al. / Science of the Total Environment xxx (2017) xxx–xxx
60 Intrusion
50 Enrichment factors
Non-intrusion 40 30
20 10 0 Cl K Ca Ti V Cr Mn Fe Ni Cu Zn Sr
Fig. 5. Enrichment factors of elements in PM10 at Mt. Aitana on Saharan event- and nonevent days.
respectively) were of the same order as those found at other high mountain stations located in northeastern Spain and Italy. The main components of PM1 were organic carbon and ammonium sulfate, representing N 60% of the total mass. In the PM10 fraction, nitrate and mineral elements such as calcium were also major components. Significant day-to day variations in PM concentrations were observed, particularly for the PM10 fraction. This variability was mainly associated with changes in the concentrations of major and trace elements of crustal origin due to the important and episodic contribution of Saharan dust outbreaks. The analysis of the effect of air mass origins on the PM10 fraction revealed that concentrations considerably higher than the average were primarily recorded during African events, which occurred on 26% of the sampling days. During summer, Saharan intrusions often alternate with regional episodes that prevailed on 18% of the days in the study period. These events were also associated to relatively high PM10 levels due to the formation of secondary organic and inorganic species and the persistence of crustal elements. The relative influence of Saharan dust events on the PM1 fraction was much lower. As a result, PM1 concentrations showed a more defined seasonal cycle than those of PM10. For both fractions, the lowest levels were generally observed during Atlantic advections, which is the most frequent origin of air masses arriving at Mt. Aitana (N 30%). Enrichment factors calculated for intrusion and non-intrusion days pointed to Ca enrichment of local soil. In fact, the relative increase in the levels of titanium and iron during Saharan dust intrusions was appreciably higher than that of calcium, implying a significant reduction in Ca/Ti and Ca/Fe mass ratios. These ratios could be used as suitable indicators for the identification of Saharan events. Acknowledgements This work was supported by the Spanish Ministry MINECO CGL2012-39623-C02-2 (PRISMA-AITANA) project. This study was also co-financed by the European Union, through FEDER funds. We would also like to thank the Spanish Defense Ministry (EVA n. 5) for allowing access to its facilities. References Abdelkader, M., Metzger, S., Mamouri, R.E., Astitha, M., Barrie, L., Levin, Z., Lelieveld, J., 2015. Dust–air pollution dynamics over the eastern Mediterranean. Atmos. Chem. Phys. 15, 9173–9189.
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Please cite this article as: Galindo, N., et al., Regional and long-range transport of aerosols at Mt. Aitana, Southeastern Spain, Sci Total Environ (2017), http://dx.doi.org/10.1016/j.scitotenv.2017.01.108