Ambient aerosol filter samples were collected at 16 EMEP and 4 other regional background sites located in 14 European countries, thus covering a wide range of the EMEP domain (Fig. 1 and Table 1). Most of these sites participated in both the summer (8 June to 12 July 2012) and the winter (11 January to 8 February 2013) IMP.

Northern Europe was represented by Aspvreten (SE12) in Sweden. The Atlantic and north-western European region included Mace Head (IE31) in Ireland, Auchencorth Moss (GB48) in Scotland and Harwell (GB36) in England, all with different marine/urban air mass influences. Puy de Dome (FR30), Revin (FR09), SIRTA (FR20), and OPE (FR22) in France represented central western Europe; Ispra in Italy (IT04), Payerne in Switzerland (CH02), Melpitz in Germany (DE44) represented the central European area. Eastern European stations included Amberd in Armenia (AM01), Leova (MD13) in Moldova, Starina (SK06) in the Slovak Republic, and K-Puszta in Hungary (HU02). In the south, the Spanish sites of Montsec (ES22), Montseny (ES1778), and Can Llompart, in Palma de Mallorca, (ESCLl) were taken as representative of south-western Europe, the Montelibretti (IT01) site in Italy represented central southern Europe, and Finokalia (GR02) in Crete (Greece) represented south-eastern Europe.

Daily PM10 sampling (24 h) was performed from 8 June to 12 July 2012 during the summer IMP, and from 11 January to 8 February 2013 for the winter IMP. PM10 samples were collected on Teflon filters (PALLFLEX, Pall Corporation Teflon Membrane Disc Filters cod. R2PJ047, 2 µm, 47 mm, 50 pkg-1) using low volume samplers (1–2.3 m3 h-1). One field blank was collected at each of the sampling sites for each IMP in addition to the analysis of laboratory blanks.

At Montseny (ES1778), PM10 filters were sampled concurrently by a DIGITEL high volume sampler (30 m3 h-1), loaded with Quartz microfiber filters (PALLFLEX, Pall Corporation QAO2500, 150 mm) to compare different methods for measuring mineral dust.

Two additional sites participated in the EMEP IMP, but PM10 filters were sampled and analysed by different methods. At Montsec (ES22) PM10 was collected on Quartz microfiber filters (PALLFLEX, Pall Corporation QAO2500, 150 mm) using a MCV/CAV-A MSb high volume sampler (30 m3 h-1), whereas at K-Puszta (HU02) PM10 was collected on Quartz microfiber filters (PALLFLEX, Pall Corporation, 47 mm) by using low-volume samplers (1 m3 h-1).

Information regarding sampling and filter analysis is summarised in Table 1.

Most samples collected during these campaigns were analysed by PIXE (Particle-Induced X-ray Emission). PIXE is a useful technique for the analysis of refractory elements in atmospheric aerosol samples. The convenience of PIXE for atmospheric aerosol research has been evaluated by Lucarelli et al. (2011) and Maenhaut (2015). This technique allows the simultaneous determination of concentrations for elements with atomic number higher than 10 with a good sensitivity. PIXE is a non-destructive technique that does not require a specific sample preparation. The capability of analysing samples with very low concentrations without sample pre-treatment lowers the risk of contamination and analyte losses.

In the present study, samples and both laboratory and field blank filters from all sites (with the exception of K-Puszta and Montsec) were analysed by PIXE with 3 MeV protons at the INFN LABEC (Laboratorio di Tecniche Nucleari per l'Ambiente e i Beni Culturali – Florence, Italy) with an external beam set-up (extensively described by Lucarelli et al., 2014). At LABEC, simultaneous high sensitivity detection of most mineral elements (Na, Mg, Al, Si, K, Ca, Ti, Mn, Fe, Sr, Zr) in a filter sample can be performed within a rather short time frame (30 s to 3 min), depending on the aerosol load. The minimum detection limit for each element measured by PIXE is listed in Table S1 in the Supplement.

Samples collected by high-volume samplers at ES1778, ES22 and HU02 were analysed by ICP-AES and ICP-MS at the IDAEA CSIC laboratories, following the method devised by Querol et al. (2001). At ES1778, the correlation between the concentrations obtained by PIXE and ICP-AES/MS was very high for most major and trace elements (Al, S, K, Fe, Ca, Ti and Mn), with R2= 0.95–0.99 and slopes close to 1 (1.01–1.05). Higher slopes were determined for Na (1.13) and Mg (1.26), although R2 > 0.9. Correlations were lower for V, Cr, Pb and Sr (R2= 0.60–0.69; slopes 0.3 to 0.9), and very low for Ni (R2= 0.19, slope 0.4; see Fig. S1 and Table S2 in the Supplement). For all those elements, concentrations were close to the MDL in PIXE.

The content of carbonate carbon was determined for 86 filter samples collected at 6 sites; i.e. ES1778, IT01, IT04, CH02, GR02 and MD13. The analysis followed the approach by Karanasiou et al. (2011), which by acidification (phosphoric acid) transforms the filter sample CO32- content into CO2. CO2 is subsequently determined by a flame ionisation detector of a commercial thermal-optical analyser, after reduction to CH4. The method detection limit was 0.2 µg C cm-2.

Some elements such as Na, Mg, Ca and K are associated with both mineral dust and sea-salt aerosol. The sea-salt contribution was estimated for each element prior to estimating the mineral load. The concentrations of these elements in sea water are well known and therefore it is possible to estimate the marine contribution to their content in PM once we know the concentration of one of these elements. In marine-influenced and background areas, Cl- has a major marine origin, therefore the marine fraction of each element could be estimated from Cl- concentrations obtained for each sample. However, this method may result in an underestimation of the marine aerosol due to the interaction (in the atmosphere or on the filter) of NaCl with acidic species resulting in the formation of HCl or NH4Cl, which may be volatilised in the atmosphere (Harrison and Pio, 1983).

It is well known that Na aerosols commonly have a marine origin, as halite associated with Cl-; however they can also derive from a range of other minerals such as clays, carbonates, feldspars and sulfates. Thus, assuming an exclusively or even dominantly marine origin for Na can cause significant errors, particularly in the southern European countries that are frequently affected by dust events. Average Na / Cl ratios obtained for each site during the summer EMEP IMP varied from 0.5 for Mace Head (IE31), between 1 and 2 for GB48, IT01 and GR02, and from 3 to 9 for the remaining sites. Therefore, given that the Na / Cl sea water ratio is 0.56 (Drever, 1997), there is a clear excess of Na (mineral or anthropogenic) and/or a depletion of Cl- (by volatilisation) for most sites, except for IE31.

The mineral fraction of Na was estimated from the content of Al by using the ratio determined by Moreno et al. (2006) for soils and dust in northern Africa. Thus, the mineral sodium (non-sea-salt sodium, nssNa) was obtained by multiplying Al concentration by 0.12 (nssNa = [Al] × 0.12). Hence, the sea-salt fraction of Na (ssNa+) can be estimated subtracting nssNa from the total Na.

The sea-salt fraction of calcium, magnesium, potassium and sulfate was estimated by using their sea water ratios with respect to ssNa+ (Drever, 1997; Nozaki, 1997). Finally, the total sea-salt load was determined by the sum of Cl-, ssNa+, ssCa2+, ssMg2+, ssK+ and ssSO42-. The non-sea-salt (nss) fractions of Ca, Mg, Mn, Na, K and SO42- were obtained by subtracting the previously calculated sea-salt fraction from their bulk concentration.

Correlation between nssK and Al concentrations for southern sites during the summer Saharan dust events (SDEs) permitted the identification of two major sources for nssK: a mineral (dustK) and a biomass burning (bbK) source. The mineral fraction is estimated from Al (dustK = 0.31 × Al; Moreno et al., 2006), and the biomass fraction by the difference: bbK = nssK - dust K.

Finally, the total mineral dust concentration was determined by addition of the concentrations of all mineral-related elements expressed as oxides, such as Al2O3, SiO2, Fe2O3, TiO2, P2O5, nssCaO, nssMgO, nssNa2O, and dustK2O.

Concentrations of elemental sulfur (S) were measured by PIXE. Assuming that most S is present as sulfate in PM, concentrations of S, as measured by PIXE, were converted to SO42- multiplying by 2.995.

The impact of African dust outbreaks on air quality over Europe during the periods of the study was traced by using publicly available information: i.e. dust maps supplied by the Navy Aerosol Analysis and Prediction System (NAAPS) model from the Navy research Laboratory (http://www.nrlmry.navy.mil/aerosol; data and/or images from the BSC-DREAM8b (Dust REgional Atmospheric Model) model, operated by the Barcelona Supercomputing Center (BSC, http://www.bsc.es/projects/earthscience/BSC-DREAM/); and NASA Terra – MODIS satellite imagery (http://modis.gsfc.nasa.gov/).

During the summer 2012 IMP, two African dust outbreaks occurred: the first from the 17 to 23 June and the second one from 28 June to 7 July. The two episodes were initiated by the intense heating of the Sahara and the consequent development of the north African thermal low south of the Atlas Mountains, coupled with anticyclonic conditions at upper atmospheric levels. This is the most frequent scenario causing dust outbreaks over south-western Europe (Moulin et al., 1998; Rodríguez et al., 2001; Escudero et al., 2007; Pey et al., 2013), with the convective system pumping dust up to 5000 m a.s.l. Once the dust is injected into the mid-troposphere it may be transported toward western Europe by the eastern branch of the high (pressure) present over northern Africa (Rodríguez et al., 2001). In such cases, the air masses are heavily loaded with dust and are transported toward the north, covering most of the western Mediterranean basin, forming a wide plume of dust. As shown in the satellite image (Fig. S2b), by 25 June 2012 Saharan dust had spread to the west across the Canary Islands and Madeira. The dust started blowing a couple of days earlier in Algeria and Mali, and travelled hundreds of kilometres toward the north-west. Over the Atlantic Ocean, the dust made a giant turn toward the east, in the direction of the Mediterranean Sea. As shown by the NAAPS model, the plume first hits the Iberian Peninsula, moving north and eastwards successively affecting parts of central, south central, and south-eastern Europe (Fig. S2a).

Short but intense dust episodes occurred during the winter 2013 IMP, affecting mainly sites in the eastern Mediterranean. This is the typical scenario in winter, when the development of low pressure systems gives rise to the rapid transport of dust at surface levels towards the east (Moulin et al., 1998), resulting in severe dust episodes in the eastern Mediterranean region (Pey et al., 2013). As shown in Fig. S3, a dust plume blew off the coast of Libya and crossed the Mediterranean towards the east on 20 January, reaching Greece and eastern Europe by 21 January. Other episodes impacted the eastern Mediterranean on 3 and 7 February. The satellite image corresponding to 7 February reflects the occurrence of multiple dust plumes transported from the coast of Libya towards the northeast (Fig. S3). The transport at surface levels results in a low spatial dispersion of the plume.

Concentrations of PM10, measured by gravimetry or by automatic monitors (e.g. TEOM) were available for the samples analysed at most sites, with the exception of FR20, FR30 in winter and FR22 in summer. PM10 concentrations in summer were, in general, higher in the southern sites (Fig. 2). Mean levels ranged from 20 to 31 µg m-3 at DE44, ES22, GR02 MD13, ES1778, and IT01; from 12 to 20 µg m-3 at CH02, FR09, AM01, IT04, HU02, SK06 and ESCLl; and from 3 to 8 µg m-3 at FR30, GB48 and SE12. The high levels measured at IT01 are probably due to its proximity to the city of Rome, whereas similarly high levels registered in ES1778 are attributed to the occurrence of two African dust outbreaks (see below).

In winter, low PM10 levels were registered in northern and south-western Europe (3–9 µg m-3) and the highest PM10 levels were measured in southern, eastern and central Europe (20–40 µg m-3). High PM10 levels were recorded along a north–west to south–east transect, ranging from 22 to 40 µg m-3, with the highest concentrations observed for GR02, CH02 and IE31 (40, 37 and 36 µg m-3, respectively).

Average concentrations of major and trace elements determined for each site during the summer and winter IMPs are summarised in Tables S3 and S4. In the current section, the spatial variation of major elements is discussed. The elements were classified according to their main affinities into marine, mineral, or mixed origin groups.

Figure 7 shows the spatial variation of the average mineral dust load contribution determined for each site and IMP following the procedure explained in Sect. 2.4. In general, higher concentrations of mineral dust were recorded in summer compared to winter. The exceptions were GR02, strongly affected by Saharan dust events in winter, and IE31, with very low concentrations of mineral dust. The summer maxima of mineral dust were more evident in the southern and eastern countries, showing a spatial distribution similar to that described for PM10 (Fig. 2). The highest dust load (5 to 10 µg m-3) was determined at the MD13, IT01, ES22, AM01 and ES1778 sites. Intermediate levels (between 2.5 and 5 µg m-3) were obtained at GR02, ESCLl, HU02, SK06, FR09, FR22 and IT04, followed by CH02 and DE44 (1.5–2 µg m-3). At the remaining sites (FR30, SE12, GB48 and IE31) the dust load was < 0.5 µg m-3. Hence, the mineral load accounted for less than 10 % of PM10 in summer at the northern sites (2 % in IE31, 6 % in SE12 and 9 % in DE44), for 15 to 25 % of PM10 at most sites, and for more than 30 % at ES22 (34 %), MD13 (38 %) and AM01 (42 %).

The time series of the ambient air mineral dust concentration during the summer IMP is presented in Fig. 8. Mineral dust concentrations increased at southern and at certain central European sites during the two SDEs observed during the summertime IMP; i.e. from 17 to 23 June and from 28 June to 7 July. The mineral dust concentration first increased at the south-western sites (ES22 and ES1778), reaching 12 µg m-3 on 17 June (Fig. 8). As the plume moved eastward (see Fig. S2), levels increased at the two Italian sites, reaching 15 µg m-3 at IT01 on 22 June. On 28 June a second and more intense plume was observed over the European continent, spreading to distant areas such as Germany and the British Isles. The second SDE first impacted the Iberian Peninsula, which experienced daily mineral dust concentrations of 34 µg m-3 at ES22 and 20 µg m-3 at ES1778 on 28 June. Mineral dust levels decreased northwards, reaching maximum daily concentrations on 28 June at CH02 (14 µg m-3), ESCLl (11 µg m-3), FR09, FR22 and DE44 (7–8 µg m-3), FR30 and GB48 (3 µg m-3). On 29 June, the concentrations of mineral dust increased at ES22 (up to 39 µg m-3) and at ES1778 and started increasing at the Italian sites (IT01 and IT04). On this day, the concentration of mineral dust decreased at the mentioned central and north European sites (CH02, ESCLl, DE44, FR30 and GB48). On 30 June, the mineral load started decreasing at ES22 but still increased at ES1778, reaching 35 µg m-3. Subsequently, the mineral load decreased at these sites, but increased at IT04 and IT01, reaching 9 µg m-3 on 30 June, reflecting the transport of the dust plume towards the eastern Mediterranean region. The first of July, levels of mineral dust reached the maximum at ES1778, IT01 and HU02 (12–14 µg m-3). On 5 July the mineral load peaked at GR02, reaching 15 µg m-3. From 1 July and until the end of the month in 2012 EMEP IMP, levels remained relatively high in the central and eastern parts of the Mediterranean Basin. During these dust episodes, the mineral dust concentration was always higher at ES22 than at ES1778, reflecting the transport of dust at high altitudes in summer. Finally, during the SDEs, the mineral dust contribution to PM10 was on average 35 % at the affected sites, ranging from 25 % at DE44 to 55 % at ES22.

It should be mentioned that at the Spanish sites the contribution of the Saharan mineral dust during the sampling period accounts for two exceedances of the daily limit value of 50 µg PM10 m-3, established by the EC Directive (EC, 2008).

The SDEs only partly affected eastern Europe. Thus, the mineral dust loading observed in this region is presumably related more to the influence of local and/or regional sources. The presence of distinctive geochemical ratios (e.g. Si / Al and Mg / Al) characteristic for this region is confirmed in the present study which has allowed us to demonstrate the importance of crustal sources in the eastern European region. Further research is needed to identify these sources in more detail, e.g. to establish emission factors, and to quantify their impact on ambient PM levels over longer periods. Another complicating factor is that this area is likely impacted at times by emissions of dust from the Arabian deserts.

In winter, the concentrations of mineral dust were lower for all sites except for GR02 (Fig. 7), where an average concentration of 13.5 µg m-3 was calculated due to the influence of short but intense Saharan dust events (see Fig. S3). Levels between 1 and 2 µg m-3 were measured at IT04, IT01, MD01, and FR20. At the remaining sites, the dust load was < 1 µg m-3. In contrast to the summer IMP, the dust load did not reflect the temporal variation of PM10 during the winter IMP. For central and northern Europe, the mineral fraction accounted for less than 5 % of PM10 at most sites, whereas it ranged between 5 and 10 % for the southern and eastern sites. The 34 % contribution of mineral dust to PM10 at GR02 was an exception. The lower mineral dust loads in winter were attributed to a lower impact of the Saharan dust outbreaks (affecting only the eastern part of the Mediterranean) and reduced soil resuspension in this period.

Figure 9 shows the time evolution of the mineral dust concentration calculated for PM10 during the winter IMP. The dust load was low at most sites, but peaked frequently at GR02 due to the impact of short but severe dust episodes which severely deteriorate air quality in the eastern Mediterranean area (Dayan et al., 2008; Querol et al., 2009; Pey et al., 2013). Thus, daily concentrations of mineral dust > 50 µg m-3 were recorded at GR02 over 3 days (ranging 60–83 µg m-3). The lower spatial dispersion of most winter episodes is a consequence of the transport at surface levels. During 21 January a dust plume blew off the coast of Libya and crossed the Mediterranean towards the east (see Fig. S3). This plume extended to areas in northern Italy and dust concentrations increased simultaneously at distant sites such as GR02, HU02, SK06 and DE44, although it is difficult to attribute this increase exclusively to the impact of the dust plume.

In the current section, days not impacted by Saharan dust were studied to characterise the chemical profiles of local dust sources and to quantify their contribution to PM. Average dust concentrations were calculated for the two measurement periods without impact of Saharan dust (NO-SDE, Fig. S4). The results show a clear spatial pattern, as the dust concentration decreases towards the north and the west. Hence, dust attributed to regional/local sources accounted for more than 25 % of PM10 at the eastern sites MD13 (7 µg m-3) and AM01 (6 µg m-3; summer only). At the southern sites (western, central and eastern Mediterranean) local/regional dust ranged between 1.5 and 4 µg m-3, accounting for 8–15 % of PM10. The higher contribution at IT01 (4 µg m-3, 15 % of PM10) is likely related to anthropogenic sources in the urbanised areas nearby the site. At most central European sites, the mineral dust concentration ranged between 0.5 and 2 µg m-3, accounting for 5–10 % of PM10. Lower values were estimated for sites in northern and western Europe.

The chemical composition of mineral dust may vary considerably depending on the location, soil composition and influence of external sources (i.e. Saharan dust). As shown in Sect. 3.3, Saharan dust had a significant influence on PM10 levels at the southern sites during the summer IMP. As dust can be mixed with other sources and transformed during transport, the current set of data provides a unique opportunity to investigate the impact of Saharan dust on the chemical composition of mineral dust and on the variation of dust composition during transport.

Chemical composition of the mineral dust, expressed as oxides, was averaged at each site for those days with a significant impact of Saharan dust (Fig. 10). During the winter IMP only GR02 was considered, as it was the only site clearly impacted for a significant period (more than 1 day), although other sites such as HU02, SK06, and MD13 were also briefly impacted. Figure 10 shows a steep gradient in the concentration of mineral dust dominated by African dust, with the highest values seen for the areas located close to the source region; i.e. the eastern Mediterranean (in winter) and the southwest of Europe (summer). The relative composition (expressed as %, Fig. 10 bottom) is quite similar among all sites, with differences for GR02 in winter (GR02-W), IT01 and ESCLl, which have a larger contribution of CaO.

Ternary diagrams with average dust composition during Saharan episodes are presented in Fig. S5. With respect to SiO2, Al2O3 and Fe2O3 (Fig. S5), the average composition was almost identical for all sites, indicating a similar silico-aluminous composition. However, for similar SiO2/ Al2O3 ratios, the contribution of CaO varies (Fig. S5), being higher during the winter (GR02-W) events. This is probably related to the different source areas for the summer and winter periods. In summer (see Figs. S2 and S3), dust was emitted from southern Algeria and Mali, whereas in winter dust blew off the coast of Libya. This is in agreement with previous studies showing higher content of Ca-carbonates for dust coming from north-eastern Africa (Formenti et al., 2011; Scheuvens et al., 2013). Relatively higher contents of CaO were also measured at ESCLl and IT01. This could be related to local contribution from soil resuspension given that the geology in these areas is characterised by the presence of carbonate rocks (limestone). At IT01, an important fraction of calcium could be also related to the contribution of local anthropogenic sources, given its proximity to Rome.

During the summer episodes, southern sites (orange and red colours) tend to have a higher content of CaO and MgO and a relatively lower content of K2O and Fe2O3 compared to central and northern European sites. This could be attributed to the different composition of local dust or to the preferential settling during transport due to the particle size and morphology of specific minerals. Again, higher contents of CaO were obtained for IT01, GR02-W and ESCLl whereas ratios of MgO / CaO keep constant for GR02-W and ESCLl, indicating a soil-related source (carbonates). CaO / MgO was higher at IT01, probably due to an anthropogenic contribution (construction, demolition, road dust).

Ternary diagrams based on the average composition of samples not affected by Saharan dust are shown in Fig. S6. As for the African dust episodes, the SiO2/ Al2O3 ratio was fairly constant for all sites except IE31. The possibility of the low levels observed at IE31 affecting the results should not be excluded. For the other components, there is a wider variation in comparison with the African episodes (see Fig. S5). When plotting SiO2, Al2O3 and CaO, the contribution of CaO clearly decreases from the southern to the eastern and northern sites. The obvious presence of this trend in the high altitude sites (ES22, AM01 and FR30) emphasises that it is a likely marker for far-travelled particles derived from the local/regional geology in the source areas. An inverse pattern is observed when plotting SiO2 and Al2O3 with either Fe2O3 or K2O; there is a clear increasing trend of the relative contribution of K2O and Fe2O3 from the southern to the eastern and northern sites. However, in this case the average concentrations are very similar at the three high altitude sites, suggesting that this trend reflects the different influence of other sources such as biomass burning for K, or iron steel industry for Fe2O3.

Sulfur is typically present in ambient air as sulfate (SO42-). Therefore, although PIXE measures concentrations for S, we have chosen to use sulfate in the present text. Non-sea-salt sulfate is a major secondary component formed by the oxidation of SO2, mainly emitted by anthropogenic sources such as fossil-fuel combustion processes and metal smelters, as well as natural sources. Moreover, non-sea-salt sulfate (nssSO42-) may have a minor mineral association, mostly as coarse gypsum, and can also be released from marine biogenic and volcanic emissions (Bates et al., 1992 and references therein).

Concentrations of nssSO42- differed both spatially and temporally (Fig. 11). The highest mean SO42- concentration in summer was observed at the south-eastern site GR02 (5.8 µg m-3). Relatively high concentrations were also determined for the eastern, south central, and south-western sites, ranging between 2.6 and 3.4 µg m-3. Lower levels were measured at central and northern sites (0.5–1.1 µg m-3) with an intermediate concentration for DE44 (2.0 µg m-3). In winter, higher concentrations of SO42- were observed in eastern and central European sites (ranging 2–3.8 µg m-3), whereas the average concentration was < 2 µg m-3 for southern and northern Europe.

Given the significant impact of Saharan dust outbreaks in southern European countries, it can be speculated that a minor fraction of SO42- could be primary gypsum, despite that it is a minor (usually < 2 %) component of Saharan dust with particularly low concentrations in most north African source areas (Claquin et al., 1999; Scheuvens et al., 2013; Journet et al., 2014). To identify the contribution of primary gypsum to the observed concentrations of SO42-, we have investigated the correlation of SO42- with major mineral elements. Figure 12 presents ternary diagrams for average concentrations of SiO2, Al2O3 and SO42- for SDE and NO-SDE samples. For SDE the spatial pattern was different from that observed for Ca (Fig. S5). The highest relative SO42- contributions were obtained for GR02-S (summer) and central European sites, with lower contributions for GR02-W (winter) and some southern European sites, whereas higher relative contributions of Ca were determined for GR02-S and southern sites. A similar but opposite spatial trend of Ca was observed during NO-SDE (Fig. 12, right, and Fig. S6). The low concentrations of SO42- at the mountain sites, indicated a low contribution of primary gypsum. The increase in concentrations of SO42- at distant sites from northern Africa even during SDE suggests a major anthropogenic source, most probably related to the impact of regional SO2 emissions, and originating mainly from fuel oil and coal combustion. A major source of SO42- in the Mediterranean region can be related to shipping (heavy oil) emissions, whereas in central and eastern Europe high concentrations are more often related to stationary sources, such as individual coal heated ovens and coal power plants, which are discussed in more detail in the next section.

Figures 13 and 14 show the spatial variation of selected trace elements during the summer and winter IMPs. The concentrations varied substantially between sites, whereas there was typically a minor difference with respect to concentration between summer and winter at most sites.