Particulate Matter Ionic and Elemental Composition during the Winter Season: A Comparative Study among Rural, Urban and Remote Sites in Southern Italy

Abstract

We present an overview of the concentrations and distributions of water-soluble ion species and elemental components in ambient particulate matter for five measurement sites in southern Italy with the aim of investigating the influence of the different site characteristics on PM levels. The sites encompass different characteristics, ranging from urban to coastal and high-altitude remote areas. PM${}_{10}$ and PM${}_{2.5}$ fractions were collected simultaneously using dual channel samplers during the winter period from November 2015 to January 2016 and analyzed for water-soluble ion species, using ion chromatography, and elemental composition, using inductively coupled plasma mass spectrometry (ICP-MS). In all sites, PM${}_{2.5}$ represented the higher contribution to particulate mass, usually more than two times that of the coarse fraction (PM${}_{2.5-10}$). At the coastal site in Capo Granitola (Western Sicily), sea salts constituted about 30% of total PM${}_{10}$ mass. On average, ion species accounted for 30% to 60% of total PM${}_{10}$ mass and 15% to 50% of PM${}_{2.5}$ mass. We found that secondary ion species, i.e., SO${}_4^{2-}$, NO${}_3^{-}$ and NH${}_4^{+}$ dominated the identifiable components within both PM${}_{2.5}$ and PM${}_{10}$ fractions. The chlorine–sodium ratio was usually lower than that expected from the natural level in sea salt, evidencing aged air masses. At the monitoring site in Naples, a highly urbanized area affected by high levels of anthropogenic source emissions, an increased contribution of ammonium was found, which was imputed to the increased ammonia emissions from industrial combustion sources and road traffic. The concentrations of the investigated elements showed noteworthy differences from one site to another. The PM${}_{10}$ fraction was highly enriched by sources of anthropogenic origin in the samples from the most urbanized areas. In general, the enrichment factors of the elements were similar between the PM${}_{10}$ and PM${}_{2.5}$ fractions, confirming common sources for all elements.

1. Introduction

Aerosol particles are key components of the atmospheric system and affect not only clouds, rainfall and circulation, but also air quality, the environment and human health. Their characterization, in particular their chemical composition, is fundamental to understanding the potential impacts of both radiative properties and toxicity and, more generally, the potential dangers to human health.

The Intergovernmental Panel on Climate Change (IPCC) has identified the Mediterranean region as a climate change and air quality hotspot [1].

Being a semi-enclosed area, the central Mediterranean region experiences high particulate matter (PM) levels that could be attributed to sources originating from both local and remote locations. It is not uncommon for PM${}_{10}$ and PM${}_{2.5}$ annual averages to exceed the stringent World Health Organization (WHO) guidelines (20 and 15 $\mathsf{\mu }$g/m${}^3$, respectively) and even the laxer recommendations from the European Environment Agency (EEA) (40 and 25 $\mathsf{\mu }$g/m${}^3$, respectively). According to the latest EEA report [2], 97% of the urban population is exposed to levels of fine particulate matter that are above the latest guideline levels set by the World Health Organization. A total of 19% of the EU-28 urban population was exposed to PM${}_{10}$ levels above the daily limit value in 2015 and 7% was exposed to levels above the EU limit value for PM${}_{2.5}$. Southern Italy continues to be one of the most problematic areas with numerous exceedances, especially in urban environments [2,3,4]. Besides pollution and wildfires, mineral dust transported from northern Africa is one of the main factors responsible for PM${}_{10}$ exceedances in southern Italy. Water-soluble ionic species account for a significant fraction of atmospheric particle mass and are ubiquitous compounds, found in both urban and rural areas. They are associated with adverse effects on human health, the acidification and eutrophication of ecosystems and visibility reduction [5,6,7]. Atmospheric ions originate from both primary, mainly marine (Na⁺, Cl⁻) or crustal (Ca²⁺), and secondary sources, e.g., the oxidation of inorganic and organic compounds leading to products characterized by low volatility. The spatial and temporal variations of ionic compounds in PM can be very significant since they are controlled by numerous factors, such as climatic and orographic features, the emission rates of gaseous precursors and the long-range transport of pollutants. Moreover, in Mediterranean countries, natural PM sources related to Saharan air mass intrusions contribute to the increased levels of primary, as well as secondary, ions [8]. Several studies have demonstrated the mutual influence of mineral dust particles and trace gases on the respective concentration levels. For instance, the formation of sulfate and nitrate is favored by the reactions of the corresponding oxides on mineral dust particles [9,10,11] and mineral dust influences the concentration of gaseous compounds, e.g., ozone [12,13].

Sulphate, nitrate and ammonium are the dominant secondary species. Although these ions may be directly introduced into the atmosphere by natural processes and human-related activities, e.g., sulfate and calcium from marine aerosols or ammonia from cars with catalytic converters [14], the most considerable contributions are produced by chemical reactions. Starting from gaseous precursors, a complex network of reactions and physical transformations, involving gas-to-particle and reverse processes, produce (NH₄)₂SO₄, NH₄HSO₄, NH₄Cl and NH₄NO₃ [15], depending on the relative humidity and temperature.

Thanks to the I-AMICA project (Infrastructure of High Technology for Integrated Climate and Environmental Monitoring), the territory of southern Italy was equipped with several environmental and climatic observatories with the aim of improving scientific knowledge on the processes involved in the climatic and environmental changes in the Mediterranean region. The present study focused on the analysis of the secondary inorganic, elemental and carbon content of PM${}_{10}$ and PM${}_{2.5}$ composition, based on a one-month field campaign performed simultaneously at five measurement sites with different characteristics. Each observatory is representative of a specific environment, from remote to urban, covering different environmental scenarios. In more detail, the environmental characteristics of the observatory locations are: Lecce, a suburban background site; Capo Granitola, a pristine coastal site; Lamezia Terme, a coastal site close to several industrial facilities and an airport mainly serving domestic flights; and Monte Curcio, a remote high-altitude site. Moreover, Napoli San Marcellino, an urban site that was not initially included in the I-AMICA project, joined the field campaign to add specific information on aerosol particles in strongly urbanized areas. All of these sites are located in southern Italy at an average inter-site distance of about 300 km, except for the Lamezia Terme and Monte Curcio sites, which are relatively close at a distance of about 50 km.

Due to their geographical locations, they are affected by local and long-range transported marine, desert and anthropogenic aerosols [16]. Daily samples were collected for about 40 days during the winter period from 25 November 2015 to 1 January 2016. This is the most problematic period in southern Italy, when numerous exceedances for atmospheric particulate matter occur, especially in urban areas [2]. To our knowledge, this was the first time that a comparative and in-depth analysis of PM content was performed for these sites, exploring the contribution of different transport and ageing processes.

2. Experimentals

2.1. Site Description

The map and acronyms used for the observation sites are shown in Figure 1 and

Table 1. The characteristics of sites and ambient PM${}_{2.5}$ and PM${}_{10}$ concentrations.

Acronym	Type	Altitude (m)	# Samples	PM10 ($\mathsf{\mu }$g/m${}^3$)	# Samples	PM2.5/PM10 (%)
NAP	Urban	70	38	$56.2\pm 22.2$	38	70.1
LEC	Urban Background	36	35	$33.2\pm 12.9$	37	76.5
LAM	Industrial	7	41	$10.2\pm 3.7$	38	72.5
CGR	Marine	5	18	$24.0\pm 9.0$	15	45.0
MTC	Remote	1763	26	$3.6\pm 1.3$	31	83.3

. The San Marcellino Observatory (NAP, 40.5° N, 14.0° E; 53 m a.s.l.) is an urban site located in downtown Naples. It is within a restricted traffic area and near to the marina, but not far from heavily trafficked roads. It is influenced by multiple sources, e.g., vehicular traffic, port emissions and biomass combustion from the numerous pizza restaurants nearby, combined with widespread pollution conditions [17,18,19,20,21,22].

Lecce (LEC, 40.3° N, 18.1° E) is an urban background site located on the roof of CNR-ISAC (Istituto di Scienze dell’Atmosfera e del Clima, Italian National Research Council) at 12 m above ground level and about 4 km south-west of Lecce town center. It is referred to as an urban background station because it is not directly influenced by traffic or industrial emissions [23]. However, this area is sometimes downwind of the most extensive industrial settlements in the Apulia region: the area of Taranto (about 80 km away in the north-west direction) and Brindisi (about 30 km away in the north direction).

Capo Granitola (CGR, 37.6° N, 12.6° E; 10 m a.s.l.) is located within the CNR-IAS (Istituto per lo studio degli impatti Antropici e Sostenibilità in ambiente marino) research base, at the southwestern-most tip of Sicily, directly facing the Strait of Sicily and away from any direct pollution sources. It is well representative of the background conditions of western Sicily/the central Mediterranean basin. The site is affected by the sea–land breeze regime, with prevailing (49% of hourly occurrences throughout the measurement period) gentle wind breezes (up to 4 m/s) from inland (NW–NE) during the night and prevailing (80% of hourly occurrences throughout the measurement period) winds from the sea (W–SE) during the daytime.

The CNR-ISAC observatory of Lamezia Terme (LAM, 38.8° N 16.2° E; 6 m a.s.l) is located on an isolated plain, 600 m from the Tyrrhenian coast and about 4 km from the center of its urban area, which has about 125,000 inhabitants. A quarry for the extraction of aggregates and two plants for the production of lime are located approximately 7 km to the north-east, while a sludge purification platform, a plant for the treatment of municipal solid waste and a plant for the treatment of special waste of various kinds are located at distances ranging between 800 and 1500 meters to the south. Moreover, natural sources are also present (occasional volcanic ash from Stromboli 80 km to the west and Etna 200 km to the south-east; Saharan dust outbreaks).

The Monte Curcio observatory (MTC, 1780 m a.s.l.) of the CNR-IIA (Istituto sull’Inquinamento Atmosferico) is located (39.3° N, 16.4° E) in a strategic and isolated position within the natural reserve of the “Parco Nazionale della Sila”. The observatory is located on the top of the mountain after which it is called and has a completely unobstructed horizon, thus guaranteeing atmospheric monitoring measurements with large spatial representativeness [24,25]. Occasional wildfires occur, especially during dry summer periods.

The Capo Granitola, Lecce, Lamezia Terme and Mt. Curcio observation sites are regional stations of the Global Atmospheric Watch (GAW) network, while Mt. Curcio also belongs to the Global Mercury Observation System (GMOS) network, which is dedicated to the monitoring of atmospheric mercury levels.

2.2. Chemical Analysis

Sample Treatment and Analysis

The ion mass concentrations were quantified by ion chromatography (IC), using PM${}_{10}$ and PM${}_{2.5}$ samples collected on quartz fiber filters (suited for the analysis of carbon content) from 25 November 2015 to 1 January 2016. Whatman^® QM-A quartz microfiber filters (47 mm), without bindings, were first pre-treated at 700 °C for 2 h in a muffle furnace and then the samples were collected continuously every 24 h. During the whole study, the mass concentrations of PM${}_{10}$ and PM${}_{2.5}$ were simultaneously measured and sampled at each site using an automated dual channel beta attenuation monitor (SWAM5a Dual Channel Monitor, FAI Instruments, Fonte Nuova, Rome, Italy), equipped with two head samplers for selecting PM${}_{10}$ and PM${}_{2.5}$ and working with a flow rate of 2.3 m${}^3$/h.

For the characterization of the soluble ion fractions, a quarter of each filter was first treated with 15 mL of ultra-pure water using a closed vessel microwave digestion system (Milestone StartE), following the multistep temperature ramp proposed by [26], and then filtered and analyzed using a Dionex ICS1100 system. For anion detection, we used an AS22 column and a buffer solution of 3.5 mm of sodium carbonate–bicarbonate as eluent; for cations, we used a CS12A column and 20 mm of methanesulfonic acid solution as eluent. Calibration curves were defined using certified multistandard solutions. The compliance and accuracy of the are discussed in [27].

Another quarter of each quartz filter that was sampled was chemically analyzed using ICP-MS (inductively coupled plasma mass spectrometry) for the determination of the concentrations of certain metals: V, Cr, Mn, Fe, Ni, Cu, As, Cd and Pb. The results of the analytical determinations were obtained by removing the average levels present in the blank samples for each chemical species.

The sample preparation for the multi-element analysis of the PM${}_{10}$ and PM${}_{2.5}$ fractions involved acidic microwave-assisted digestion, in accordance with the standard method EN 14902:2005 [28,29]. Each filter was digested in a microwave oven with 8 mL of HNO${}_3$, 2 mL of H${}_2$O${}_2$ and 0.2 mL of HF. After digestion, the analysis was performed using ICP-MS (7500CE, Agilent, Santa Clara, California ) for the determination of elemental content. The analytical batch was composed of the calibration standards, the samples and a minimum of three blank filter samples. A calibration standard of the intermediate concentration in the linear dynamic range was measured after every 20 samples to assess for any instrumental drift during the runs. The quantitative analysis of the elements was carried out with six-point calibration curves covering the range of 0.1 to 1000 $\mathsf{\mu }$g/L and internal standardization with the online addition of a 200 $\mathsf{\mu }$g/L Rh solution via a Y-connector.

2.3. Meteorological Scenarios

The origin of air masses collected during the monitoring campaign was interpreted based on:

(a)

the daily determination of 5-day isentropic back trajectories (starting at 12 h and 500 m agl) using the HYSPLIT (Hybrid Single-Particle Lagrangian Integrated Trajectory) model [30];

(b)

the evaluation of daily NCEP/NCAR reanalysis data [31]. Their salient characteristics were a modest spatial resolution (2.5° × 2.5°), a temporal resolution of 6 h, global coverage and the inclusion of observations in the final analysis by means of state-of-the-art data assimilation techniques.

For each day of simulation and for each site, 24 trajectories were generated by varying the arrival time, i.e., making the trajectories arrive every hour, and then setting the arrival vertical level at 850 hPa, the approximate limit of the boundary layer during the midday hours. Different time levels were chosen because it was difficult to select a single level that would be representative of the origins of the air masses during conditions that were characterized by rapidly changing synoptic conditions; in that case, the assignment to a unique trajectory, arriving at a predefined time level, may not have been representative.

3. Results

3.1. PM${}_{10}$ and PM${}_{2.5}$ Mass Concentrations

The ion concentrations are reported in Figure 2 and Figure 3 as box and whisker plots. Furthermore, in the supplementary information, the main statistics are reported together with other parameters derived from the ion balance, for example, sea salt concentration, the quantity of the ions derived from sea salt and the charge balance. PM${}_{10}$ concentrations showed levels comparable to other Mediterranean environments and remote sites [32,33,34,35,36,37,38]. Not surprisingly, PM${}_{10}$ was high at the NAP urban site (average 52.6 ± 22.2 $\mathsf{\mu }$g/m${}^3$) and, to a lesser extent, at the LEC site (33.2 ± 12.9 $\mathsf{\mu }$g/m${}^3$). Significantly lower concentrations were measured for the other sites, especially at the mountain site of MTC. These results lay at the lowest end of the range of data distribution for other Mediterranean and European continental remote sites at a similar altitude [39,40,41,42,43].

The contribution from fine particulate represented more than 70% of the PM${}_{10}$ mass with the sole exception of the CGR site, the pristine coastal site, for which the contribution of PM${}_{2.5}$ was, on the average, less than 50%, with a high variability of the ratio. In other words, PM${}_{2.5}$ concentrations were almost always larger than those of the coarse particles (PM${}_{2.5-10}$), with an average PM${}_{2.5}$ mass of more than twice the average coarse particle mass.

Figure 4 shows the values of the simultaneously measured PM${}_{2.5}$ and PM${}_{10}$ mass concentrations for the five sites used in this study. The mass concentrations were correlated with a mean PM${}_{2.5}$ vs. PM${}_{10}$ slope of $0.796\pm 0.008$ (95% confidence level), evidencing that the intensities of fine and coarse aerosol sources co-vary. The most robust correlations were for the LEC, LAM and MTC sites. The urban NAP site was affected by a more scattered data distribution, spanning a larger interval, both for the PM${}_{10}$ and PM${}_{2.5}$ fractions, probably due to the more complex and unpredictable emission strengths that mix fresh pollution, resuspension from the soil and other natural sources. Clearly, CGR had a different pattern with a much larger fraction of coarse particles that was probably linked to its location, which is very close to the coastline. Figure 4 clearly shows that PM${}_{10}$ concentration varied independently from PM${}_{2.5}$ for the CGR site; instead, the points were distributed according to two distinct periods (Figure 5) with a different PM${}_{2.5}$–PM${}_{10}$ ratio: 0.7 and 0.3. Both periods were separately well correlated: $R^2=0.98$ and $R^2=0.90$. During the first period (approximately from 25 November to 1 December), there were strong prevailing winds from the north-west, while during the second period (approximately from 3–9 December), calm conditions were prevalent. Figure 6 shows the 5-day isentropic back trajectories (starting every day at 12 h and 500 m agl from the CGR site) from the HYSPLIT model superposed onto the average geopotential field at 925 mbar. The analysis of the meteorological scenarios confirms this hypothesis, with trajectories originating from the north-west Atlantic area during the first period, where they are presumably enriched with sea salt. In subsequent days, the trajectories highlighted an increased contribution from local areas, characterized by the lack of significant air mass advection and the prevalence of the recirculation of air masses. During the first period, the PM${}_{10}$ concentration achieved very high values that were nearly as high as 50 $\mathsf{\mu }$g/m${}^3$. The increase in PM mass was prevalently due to the coarse fraction, since the increase in PM${}_{10}$ was accompanied by a decrease in PM${}_{2.5}$ (see the left-hand panel in Figure 5); alongside the increase in the coarse fraction, the contribution from sea salt was revealed by an increase in the sodium concentration (Figure 5, right-hand panel), which achieved a concentration of nearly 10 $\mathsf{\mu }$g/m${}^3$. During the second period, when there was no prevalent advection pathway, the contribution from fine particles increased and the sodium concentration decreased in parallel.

3.2. Ion Composition

The percentage of total ion mass for PM${}_{10}$ varied within a range of 30% for urban and remote locations (NAP, LEC, MTC) and 60% for coastal sites (LAM and CGR). On average, the coarse fraction was more enriched with ion concentrations, which were less than the equivalent contribution to PM${}_{2.5}$ (about 20% for NAP, LEC and MTC and 55% for LAM and CGR). Sodium, chloride, calcium, nitrate and sulfate were the most abundant ions, though with different percentages depending on the site; in nearly all cases, K${}^{+}$ and Mg${}^{2+}$ represented minor components.

Sulphate and nitrate were usually the most represented anions in the PM${}_{10}$ and PM${}_{2.5}$ samples. For all sites except for MTC, the average sulfate concentration was around 2 $\mathsf{\mu }$g/m${}^3$ and was mainly concentrated in the fine fraction, while the nitrate concentration usually exceeded the corresponding sulfate concentration and was more evenly distributed among the coarse and fine fractions. The NAP and LEC sites showed the highest sulfate and nitrate concentrations in both fractions, highlighting the contribution of anthropogenic activities to secondary ion formation.

Ammonium was the most abundant secondary cation; its concentration was around 3–6% with respect to the total PM${}_{10}$ mass for all sites and was mainly concentrated in the fine fraction. ${\mathrm{NH}}_4^{+}$ combines with ${\mathrm{NO}}_3^{-}$ and ${\mathrm{SO}}_4^{2-}$ in the atmosphere, leading to the formation of ammonium nitrate and ammonium sulfate, respectively. To assess the availability of ammonia for the neutralization of acidic components, the ammonium availability index I and the sulfate neutralization index J were estimated. The ammonium availability index is defined as the molar ratio of the observed ammonium cation concentration to the amount needed to neutralize the nss${\mathrm{SO}}_4^{2-}$ and nitrate anion concentrations, and was expressed as:

$$\begin{array}{c}\hfill I=\frac{\left[{\mathrm{NH}}^{4+}\right]}{2\times \left[{\mathrm{nssSO}}_4^{2-}\right]+[{\mathrm{NO}}^{3-}]}\end{array}$$

(1)

Depending on the value of I, an ammonium deficit ($I<1$) or surplus ($I>1$) could be estimated [44,45]. The sulfate neutralization index is defined as the molar ratio of the observed ammonium cation concentration to the amount needed to neutralize the nssSO${}_4^{2-}$ anion concentration only, and was expressed as:

$$J=\frac{[{\mathrm{NH}}^{4+}]}{2\times \left[{\mathrm{nssSO}}_4^{2-}\right]}$$

(2)

The sulfate neutralization index averages $1.1\pm 0.4$ for all sites but NAP, indicating an almost complete neutralization between sulfate and ammonium. For the NAP site this ratio is equal to 2.4, indicating an excess of ammonium concentration for the neutralization of sulfate; nitrate is requested to counterbalance the excess of ammonium in PM${}_{10}$ for this site.

The ammonium availability index and the sulfate neutralization index are also shown in Figure 7 and Figure 8 for all measurements. It should be noted that ammonium did not completely counterbalance the sulfate and nitrate for neither PM${}_{10}$ nor PM${}_{2.5}$ (most of the time, the points are below the 1:1 line in Figure 7) and this effect was more pronounced for the coarse fraction. Figure 8 shows that the sulfate was only balanced by ammonium in all measurement sites except NAP, for which the sulfate was not sufficient to balance the excess ammonium. The high amount of ammonium for the NAP urban site was not unexpected. Although the agriculture sector (including the burning of biomass) contributes more than 90%, the energy sector, including industries and fuel combustion, contributes the rest of NH${}_3$ emissions on the global scale [46]. The contribution of vehicles to non-agricultural NH${}_3$ emissions has been considered to be negligible, up to [47,48]. Recent studies, however, have shown that ammonia concentration in urban environments has increased significantly since the introduction of gasoline-powered vehicles that are equipped with three-way catalytic converters (TWCs) and diesel-powered vehicles that adopt the selective catalytic reduction (SCR) system [49]. Although vehicles form a minor source of ammonia emissions, they can have significant impacts on a local scale. For example, after conducting measurements in a roadway tunnel [50], concluded that the contribution of motor vehicle emissions had risen from 2 to 15% of total NH${}_3$ emissions in the Los Angeles area since the introduction of catalysts. Roadside measurements in the UK, the USA and Europe have shown strong links between NH${}_3$ emissions and traffic [14,51,52,53]. According to the latest technical report from the European Environment Agency [2] on emission inventories, road transport is estimated to contribute 2% of total NH${}_3$ emissions, whereas industrial processes and waste decomposition contribute 1% each.

3.3. Sea Salt Contribution

In addition to the ionic chemical analysis, the contribution of sea salt to PM mass was also estimated. Specifically, the sea salt aerosol contribution was estimated as:

$$\mathrm{sea}\mathrm{salt}(\mathsf{\mu }\mathrm{g}/{\mathrm{m}}^3)={\mathrm{Cl}}^{-}+1.47\times {\mathrm{Na}}^{+}(\mathsf{\mu }\mathrm{g}/{\mathrm{m}}^3)$$

(3)

where 1.47 is the seawater ratio of (Na⁺ + K⁺ + Mg²⁺ + Ca²⁺ + SO₄²⁻ + HCO₃⁻)/Na⁺ [54,55]. Equation (3) was based on the assumption that the water-soluble Na⁺ concentration originates solely from seawater; Ref. [56] showed that soil dust has a minimal contribution to measured soluble sodium concentrations. Non-sea salt (nss) ionic contributions were calculated from the Na⁺ concentrations and the ratio of the corresponding ion to sodium in seawater.

Under the assumption that sodium originates solely from seawater, Tables S1 and S2 in the Supplementary Information show that the resulting sea salt percentage varied from 32% (CGR, corresponding to a concentration of 7.6 $\mathsf{\mu }$g/m${}^3$) to 3–4% (NAP and LEC, corresponding to concentrations of 2.0 and 0.9 $\mathsf{\mu }$g/m${}^3$, respectively) for PM${}_{10}$. On average, sea salt was mainly concentrated in the coarse fraction, since its percentage in PM${}_{2.5}$ was always less than that in PM${}_{10}$. The mean Cl^–/Na⁺ mass ratio depended on the site, with values close to those expected for fresh sea salt particles (1.8) at CGR and less than 1.0 at NAP, LEC and LAM. This finding is an indication of an aged sea salt with chlorine depletion due to chemical reactions involving NaCl and HNO₃ or H₂SO₄, which is a well-known event [57]. These reactions could be responsible for the disappearance of Cl${}^{-}$, thereby enriching particles with sodium.

Whereas it has been well demonstrated that HNO${}_3$ displacement dominates chloride depletion for coarse mode sea salt particles, the predominant mechanism for fine mode chloride depletion has been mostly attributed to sulfate substitution [58,59,60]. Coarse Ca–nitrate has also been observed in continental air masses as a result of the reaction of HNO${}_3$ with calcite CaCO${}_3$ in soil particles, forming Ca(NO${}_3$)${}_2$ [61,62,63].

The sulphate could be decomposed into its sea salt (ss) and non-sea salt (nss) contributions. ssSO${}_4^{2-}$ was a minor component in all cases except for CGR; at the CGR site, ssSO${}_4^{2-}$ accounted for about one third of the total sulfate. On the contrary, nssSO${}_4^{2-}$ was the major sulfate component with percentages close to, or even exceeding, 90% (Tables S1 and S2 in the Supplementary Information).

Table 2. The mass ratios of the major cations for PM${}_{10}$ (top three rows) and PM${}_{2.5}$ (bottom three rows).

	NAP	LEC	LAM	CGR	MTC
Mg${}^{2+}$/Na${}^{+}$	$0.17\pm 0.04$	$0.44\pm 0.07$	—   —	$0.18\pm 0.03$	—   —
Ca${}^{2+}$/Na${}^{+}$	$2.26\pm 0.19$	$2.80\pm 0.28$	—   —	$0.50\pm 0.06$	—   —
K${}^{+}$/Na${}^{+}$	$2.03\pm 0.21$	$2.39\pm 0.25$	—   —	$0.27\pm 0.04$	—   —
Mg${}^{2+}$/Na${}^{+}$	$0.20\pm 0.12$	—   —	—   —	$0.15\pm 0.04$	—   —
Ca${}^{2+}$/Na${}^{+}$	$2.61\pm 0.61$	—   —	—   —	$0.52\pm 0.07$	—   —
K${}^{+}$/Na${}^{+}$	$4.38\pm 0.93$	$2.44\pm 0.35$	—   —	$0.30\pm 0.05$	—   —

shows the mass ratios of several cations, specifically ${\mathrm{Mg}}^{2+}/{\mathrm{Na}}^{+}$, ${\mathrm{Ca}}^{2+}/{\mathrm{Na}}^{+}$ and ${\mathrm{K}}^{+}/{\mathrm{Na}}^{+}$. They all share a marine origin but may derive from other sources too, e.g., biomass burning for potassium and crustal erosion for calcium. The expected mass ratio in bulk seawater is often used to determine whether their origin is solely from a marine environment or not; the bulk seawater mass ratios for Mg²⁺/Na⁺, Ca²⁺/Na⁺ and K⁺/Na⁺ are 0.12, 0.04 and 0.036, respectively. As shown in Figure 2 and Figure 3, the concentrations of Mg${}^{2+}$, Ca${}^{2+}$ and K${}^{+}$ at the LAM and MTC sites were often close to the detection limits, so the uncertainty about their ratios prevented a robust estimation. At the NAP and CGR sites, the Mg²⁺/Na⁺ ratios were comparable to the expected seawater values, while at the LEC site, the value was much greater than 0.12 for PM₁₀; this could be explained by the large contribution of the magnesium carbonates of crustal origins at LEC that were observed in a previous work in this area [64]. It should also be noted that the Ca²⁺/Na⁺ and K⁺/Na⁺ ratios were greater than the expected seawater values in all cases, pointing to the importance of the non-marine sources of these salts. In particular, the enrichment in Ca²⁺ could be attributed to dust from soil resuspension in urban and suburban environments (no transport of desert dust was observed during this field campaign), while K⁺ enrichment could testify to the significant use of biomass burning for heating systems; this observation was particularly true at the NAP site, due to the accumulation of K⁺ in the smaller particles (PM_2.5 fraction). The higher contribution from anthropogenic sources was also supported by the measured organic/elemental carbon (OC/EC) content at the same site [19,65]. In that work, the authors analyzed the carbon content in PM from the same monitoring campaign and showed that the highest levels were achieved in Naples (12.8 ± 5.1 and 11.8 ± 4.6 μg/m³ for organic carbon in PM₁₀ and PM_2.5; 2.3 ± 1.1 and 1.8 ± 0.5 μg/m³ for elemental carbon in PM₁₀ and PM_2.5). The EC/OC ratio was also the highest out of all of the sites, evidencing the contribution of elemental carbon, e.g., combustion sources that are primarily from anthropogenic sources [66,67].

3.4. Charge Balance

The charge balance (Figure 9) was calculated from the concentrations of anions and cations. The satisfactory correlation between cation and anion equivalents for the PM${}_{10}$ and PM${}_{2.5}$ samples ($R^2=0.95$ and $R^2=0.90$, respectively) indicates that cations and anions were the prevalent ions extracted from the filters. As found in other studies [68,69], the linear best fit for PM${}_{10}$ (intercept = 24 nmol/m${}^3$, slope = 1.04) was always above the 1:1 line, indicating an excess of cations in all conditions. The explanation for the deficit of negative charges is probably the presence of undetected carbonates and other organic anions (although the contribution of water-soluble organic acids was negligible in this study). To test this possibility, we first calculated the excess Ca${}^{2+}$ relative to the charge surplus:

$${\left[{\mathrm{Ca}}^{2+}\right]}_{\mathrm{ex}}=\left(2\times \left[{\mathrm{Ca}}^{2+}\right]-(\left[\mathrm{Pos}\right]-\left[\mathrm{Neg}\right])\right)\times 0.5$$

(4)

where [Pos] and [Neg] are the sum of positive and negative charges. Then, the excess sodium that was not balanced by chlorine was calculated:

$${\left[{\mathrm{Na}}^{+}\right]}_{\mathrm{ex}}=\left[{\mathrm{Na}}^{+}\right]-\left[{\mathrm{Cl}}^{-}\right]$$

(5)

Finally, nitrate is presented vs. the excess sodium plus calcium in the coarse fraction, i.e., their content in PM${}_{2.5-10}$, in Figure 10. The significant correlation that was obtained confirms that the amount of coarse NO${}_3^{-}$ that was not neutralized by sodium was associated with excess calcium.

The average atmospheric concentration of CO${}_3^{2-}$, calculated from the anion deficit, was 24 nmol/m${}^3$ in PM${}_{10}$, which accounted for about 1.5 $\mathsf{\mu }$g/m${}^3$ (averaged over all sites), and was negligible in PM${}_{2.5}$. Similar results have also been obtained worldwide [32,70,71]. Therefore, the most likely explanation for the anion deficit is that a certain amount of CO${}_3^{2-}$ and HCO${}_3^{-}$ existed in the coarse fraction. The differences in the content of carbonate species between the sites did not seem significant. Apart from the NAP site, charges in PM${}_{2.5}$ were well balanced regardless of unaccounted species, suggesting that carbonates were mainly contained in the coarse fraction. A slight excess of positive charges seemed to contaminate the fine fraction as well.

To highlight the associations of the ionic components in the water extractions, we calculated the ionic balance from the concentrations. For this purpose, the following considerations were taken into account [72]: (1) Na${}^{+}$ has a mostly marine origin and is balanced with chlorine; (2) ammonium is preferentially associated with sulfate as (NH${}_4$)${}_2$SO${}_4$; (3) in the case of an excess of ammonium with respect to sulfate, this excess is balanced with NO${}_3^{-}$; (4) excess sulphate and nitrate after balancing with ammonium are first associated with excess Na after (1) and with the rest of the water-soluble cations (Ca${}^{2+}$, K${}^{+}$ and Mg${}^{2+}$); (5) in the case of an excess of sodium from (1), the excess Na${}^{+}$ is balanced with NO${}_3^{-}$; (6) if after balancing (5) and (3), there is Na${}^{+}$ and SO${}_4^{2-}$ available, the Na${}^{+}$ preponderance is associated with an excess of SO${}_4^{2-}$. If, by contrast, there is an excess of NO${}_3^{-}$ after (5), this is balanced with Ca${}^{2+}$; and (7) finally, if after (6), there is an excess of SO${}_4^{2-}$, this is associated with Ca${}^{2+}$. The results are reported in

Table 3. Ionic balance for NaCl, (NH${}_4$)${}_2$SO${}_4$ and NH${}_4$NO${}_3$ and an estimation of other sulfate and nitrate phases. The values are reported as $\mathsf{\mu }$g/m${}^3$ and the percentages concerning the total PM masses are in parenthesis.

	PM10					PM2.5
	NAP	LEC	LAM	CGR	MTC	NAP	LEC	LAM	CGR	MTC
NaCl	0.9 (1.6%)	0.5 (1.4%)	0.7 (7.2%)	5.3 (22.2%)	0.1 (2.1%)	0.4 (0.9%)	0.1 (0.4%)	0.1 (1.5%)	1.7 (15.4%)	–
(NH${}_4$)${}_2$SO${}_4$	2.4 (4.6%)	2.7 (8.0%)	1.9 (18.5%)	0.8 (3.4%)	0.6 (15.7%)	2.1 (5.4%)	1.8 (7.1%)	0.6 (8.0%)	0.5 (4.6%)	0.2 (6.2%)
NH${}_4$NO${}_3$	5.7 (10.8%)	1.4 (4.2%)	0.2 (2.1%)	–	0.1 (1.6%)	5.2 (13.3%)	0.6 (2.5%)	–	–	–
NaNO${}_3$	1.7 (3.2%)	1.0 (2.9%)	0.9 (8.4%)	1.4 (5.7%)	–	0.3 (0.9%)	0.4 (1.6%)	0.4 (4.9%)	0.6 (5.6%)	–
Na${}_2$SO${}_4$	–	–	–	–	–	–	0.1 (0.5%)	–	0.6 (5.8%)	–
CaSO${}_4$	0.1 (0.3%)	0.1 (0.5%)	0.2 (2.2%)	1.4 (6.1%)	–	0.1 (0.3%)	–	0.1 (1.4%)	0.5 (5.3%)	–
Ca(NO${}_3$)${}_2$	2.1 (4.0%)	2.1 (6.4%)	0.3 (2.6%)	0.1 (0.4%)	0.1 (3.1%)	0.8 (2.1%)	–	–	–	–
KNO${}_3$	0.1 (0.3%)	–	0.2 (2.2%)	0.3 (1.5%)	–	0.1 (0.4%)	0.7 (2.8%)	0.5 (7.0%)	–	–
K${}_2$SO${}_4$	0.3 (0.6%)	–	–	–	–	–	0.1 (0.4%)	0.1 (1.8%)	–	–

. When comparing the major contributions to the ion phases, sea salt for urban and remote sites ranged between 1 and 2%, both in PM${}_{10}$ and PM${}_{2.5}$. The contribution at coastal sites (LAM and CGR) was significantly higher, mainly for the coarse fraction; it achieved a percentage as high as 7.2% in PM${}_{10}$ and 1.5% in PM${}_{2.5}$ at the LAM site, and even higher percentages at the CGR site. As outlined in Section 3.1, during the first period, Capo Granitola was subject to intense wind conditions, which enhanced the lifting of sea salts in the marine boundary layer; because of the remoteness of this site and its distance from other emission sources, the sea salt contribution reached percentages as high as 22% in the PM${}_{10}$ fraction and 15% in the PM${}_{2.5}$ fraction.

Sulfate was mainly concentrated in the fine fraction at all sites. At the urban NAP site, more than 80% of (NH${}_4$)${}_2$SO${}_4$ was present in the PM${}_{2.5}$ fraction. In contrast to the other sites, nitrate was needed to counterbalance the excess ammonium in the fine fraction collected at the NAP site. The remote and coastal sites were affected by a smaller amount of ammonium nitrate.

The residual nitrate was mainly present as coarse NaNO${}_3$ and Ca(NO${}_3$)${}_2$ at all sites and as a minor amount of KNO${}_3$. As already noted from Tables S1 and S2 in the Supplementary Information, the mean Cl⁻/Na⁺ mass ratio was often lower than expected, indicating aged sea salts. The formation of coarse NaNO${}_3$ has been widely reported in the literature by the reaction of HNO${}_3$ with sea salt particles, resulting in NaNO${}_3$ and chloride depletion, which is an effect that is common to other Mediterranean sites [38,73].

3.5. Concentrations of Elements and Enrichment Factors

The concentrations of the nine elements investigated (V, Cr, Mn, Fe, Ni, Cu, As, Cd and Pb) showed noteworthy differences between the sites. In

Table 4. The mean, minimum, maximum and standard deviations (SD) of the element concentrations in PM${}_{10}$: the concentrations are in ng/m${}^3$; LOD, limit of detection; NAP, Naples; LEC, Lecce; LAM, Lamezia Terme; CGR, Capo Granitola; MTC, Monte Curcio. ${}^a$ [75]; ${}^b$ [76]; ${}^c$ [74].

	NAP (Urban)				LEC (Urban Background)				LAM (Indutrial)
	Mean	SD	Min	Max	Mean	SD	Min	Max	Mean	SD	Min	Max
V	2.7	2.0	0.2	11.6	3.6	2.2	0.2	9.3	1.9	1.2	<LOD	5.3
Cr	10.2	4.6	0.8	19.9	40.6	35.5	0.3	90.7	2.2	2.5	0.1	7.1
Mn	10.0	5.3	0.8	24.1	7.7	3.9	<LOD	20.2	4.4	3.9	<LOD	17.2
Fe	612.6	332.1	48.6	1273.8	386.5	230.8	38.0	1082.4	120.0	83.0	7.3	341.8
Ni	3.0	1.2	0.4	6.1	4.2	3.8	0.1	13.4	5.4	3.9	0.2	16.2
Cu	22.4	11.6	1.6	45.9	10.6	7.2	1.1	33.1	8.7	23.7	0.1	153.6
As	0.5	0.3	<LOD	1.3	0.7	0.7	0.1	3.9	0.3	0.4	<LOD	2.0
Cd	0.8	1.4	<LOD	9.1	0.3	0.1	<LOD	0.6	<LOD	<LOD	<LOD	2.2
Pb	25.0	15.7	1.4	61.0	11.2	8.3	0.8	33.5	7.2	11.8	0.1	50.7
	CGR (Marine)				MTC (Remote)
	Mean	SD	Min	Max	Mean	SD	Min	Max
V	2.4	2.3	<LOD	7.9	<LOD	<LOD	<LOD	<LOD
Cr	4,3	1.0	1.9	6.1	3.9	10.3	<LOD	34.9
Mn	2.9	0.9	1,4	4.6	1.1	1.2	<LOD	2.7
Fe	117.4	31.9	60.1	168.1	28.0	36.4	0.2	139.2
Ni	4.0	4.1	0.8	18.6	3.8	7.0	0.2	16.2
Cu	3.5	3.0	0.6	12.0	0.2	0.2	<LOD	0.4
As	0.3	0.3	0.1	0.8	1.2	1.2	0.1	4.2
Cd	0.2	0.1	0.1	0.5	<LOD	<LOD	<LOD	<LOD
Pb	4.7	3.2	0.6	13.2	0.3	0.4	<LOD	0.6
	Lahore, Pakistan a (Urban)		Düzce, Turkey b (Urban)		Düzce b, Turkey (Semi-Urban)		Tehran, Iran c (Remote)
	Mean	SD	Mean	SD	Mean	SD	Mean	SD
V	<LOD	<LOD	2.6	2.1	1.1	0.6	2.99	1.04
Cr	30	10	14.7	6.4	15.8	6.4	12.55	5.66
Mn	300	100	29	28.1	11.1	8.4	28.04	13.41
Fe	8200	1900	2500	2200	1000	700	1749.4	626.3
Ni	20	7	11.8	4.4	11.7	6.4	6.98	2.62
Cu	73	33	12.4	10.3	4.3	3.6	147	63.24
As	<LOD	<LOD	1.8	1	0.7	0.4	2.61	2.53
Cd	77	77	0.6	0.3	0.3	0.1	3.88	1.08
Pb	4400	3400	21.1	16.2	8.8	4	26.68	11.52

and

Table 5. The mean, minimum, maximum and standard deviations (SD) of the element concentrations in PM${}_{2.5}$: the concentrations are in ng/m${}^3$; LOD, limit of detection; NAP, Naples; LEC, Lecce; LAM, Lamezia Terme; CGR, Capo Granitola; MTC, Monte Curcio. ${}^a$ [77]; ${}^b$ [78].

	NAP (Urban)				LEC (Urban Background)				LAM (Indutrial)
	Mean	SD	Min	Max	Mean	SD	Min	Max	Mean	SD	Min	Max
V	1.5	1.1	0.2	5.9	1.5	1.2	0.1	4.5	1.5	0.9	0.2	3.8
Cr	2.7	2.6	0.1	12.9	18.8	15.3	0.1	34.5	1.9	2.1	0.1	5.6
Mn	3.2	2.4	0.1	14.5	1.8	2.1	0.2	8.3	1.1	0.8	0.2	4.7
Fe	109.9	51.7	15.0	257.4	54.9	43.7	1.2	167.9	34.5	29.3	1.1	145.7
Ni	2.1	0.9	0.3	4.3	1.3	1.3	0.2	4.2	1.2	1.0	0.1	3.1
Cu	5.2	3.8	1.1	21.8	2.3	1.7	0.1	8.0	2.4	4.5	0.1	23.5
As	0.4	0.1	0.2	0.5	0.5	0.4	<LOD	2.5	0.4	0.3	0.1	1.0
Cd	0.4	0.2	0.1	0.7	<LOD	<LOD	<LOD	0.2	<LOD	<LOD	<LOD	0.9
Pb	16.6	11.9	1.5	55.6	8.0	6.2	0.5	24.0	4.7	7.0	0.1	34.0
	CGR (Marine)				MTC (Remote)
	Mean	SD	Min	Max	Mean	SD	Min	Max
V	1.5	1.3	0.2	4.8	<LOD	<LOD	<LOD	<LOD
Cr	<LOD	<LOD	<LOD	<LOD	<LOD	<LOD	<LOD	<LOD
Mn	1.5	0.2	1.2	1.8	<LOD	<LOD	<LOD	<LOD
Fe	32.3	11.5	15.8	62.7	<LOD	<LOD	<LOD	<LOD
Ni	1.1	1.6	<LOD	3.5	<LOD	<LOD	<LOD	<LOD
Cu	1.3	0.8	0.3	3.3	<LOD	<LOD	<LOD	<LOD
As	0.2	0.1	0.1	0.4	<LOD	<LOD	<LOD	<LOD
Cd	<LOD	<LOD	<LOD	<LOD	<LOD	<LOD	<LOD	<LOD
Pb	2.4	1.1	0.7	4.1	<LOD	<LOD	<LOD	<LOD
	Saint-Omer, France a (Urban)		Taipei, Taiwan b (Urban)		Hualien, Taiwan ${}^b$ (Rural)		Chiayi, Taiwan b (Suburban)
	Mean	SD	Mean	SD	Mean	SD	Mean	SD
V	3.98	3.44	4.32	2.86	2.55	1.74	5.28	3.27
Cr	0.74	0.53	13.20	92.7	5.82	39.20	2.95	5.04
Mn	4.18	4.94	6.43	4.89	2.36	2.69	8.55	5.75
Fe	52.1	41.8	142.0	78.4	57.2	95.9	138.0	211
Ni	2.91	2.18	10.90	10.20	1.69	3.70	3.33	5.00
Cu	3.11	4.79	6.12	4.63	2.02	2.86	5.94	5.33
As	1.35	1.28	0.91	0.78	0.79	0.81	1.24	1.04
Cd	0.51	0.53	0.36	1.03	0.33	1.21	0.64	1.30
Pb	18.2	19.7	6.58	5.42	4.46	4.34	20.3	20.5

), we report the metal concentrations in PM${}_{10}$ and PM${}_{2.5}$ that were observed in our study and, for comparison, the metal concentrations reported by other studies in urban, suburban and remote sites worldwide.

In our study, the total concentration of the covered elements represented, on average, 1% and 0.4% of the PM${}_{10}$ and PM${}_{2.5}$ mass concentration at all sites, respectively. The concentrations of metals such as Pb, Ni, As and Cd, classified as carcinogenic by the IARC (International Agency for Research on Cancer), did not exceed the EU’s limits (500, 20, 6 and 5 ng/m${}^3$, respectively). Fe was the element with the highest concentration in both PM fractions, thus accounting for 1.2% of the PM10 mass concentrations at Naples, Lecce and Lamezia, and 0.5% and 0.7% at Capo Granitola and Monte Curcio, respectively. Meanwhile, Fe accounted 0.5% of the mass concentration of PM${}_{2.5}$ at Lamezia Terme, 0.3% at Naples and Capo Granitola and 0.2% at Lecce (<LOD for Monte Curcio). In order of abundance, we successively found Cu < Pb in Naples, Lamezia Terme, Capo Granitola (in this case Cr < Pb) and Lecce (in this case Pb < Cr). Different abundances, with Ni < Cr, were observed at Monte Curcio.

A comparison of our data to those of other international studies revealed that, in all I-AMICA sites, the concentration of Fe in PM${}_{10}$ was significantly lower than in the other studies, while in PM${}_{2.5}$, the Fe concentration at the NAP site was comparable to the values of the urban/suburban sites of Taipei and Chiayi. As for Cd, the concentrations in PM${}_{10}$ at I-AMICA stations were comparable to the levels detected at similar sites in France, Pakistan, Turkey and Taiwan. The only difference found was with values in Iran, where the concentrations are 10 times higher, on average [74]. Meanwhile, in PM${}_{2.5}$, the Cd concentration in our study < LOD for all sites, except for NAP. The concentrations of V, Cr, Cu, Mn, Ni and As were in good accordance with those reported by other studies, while the detected Pb concentrations were typically lower than those of international sites under comparison, with the exception of NAP, where lead levels were similar to those of the Tehran and Duzce sites for PM${}_{10}$ and the sites of Saint-Omer and Chiayi for PM${}_{2.5}$.

To characterize the chemical composition of airborne particulate matter, the enrichment factor $\left(EF\right)$ was calculated as follows:

$$EF={\displaystyle \frac{{\mathrm{X}/\mathrm{Fe}}_{\mathrm{air}}}{{\mathrm{X}/\mathrm{Fe}}_{\mathrm{crust}}}}$$

(6)

EF relates the concentration of an element (X) to that of a crustal element (Al, Ti or Fe) in the air, normalized to the ratio of these elements in the average continental crust [79]. In this case, the element chosen for reference was Fe. Even when the analysis of the $EF$ only provides qualitative information, it is a good indicator to assess the presence/intensity of anthropogenic contaminant deposition on the surface soil [80]. According to the degree of enrichment, the studied elements are shown in Figure 11. The findings show that, for all elements, the $EF$ of the PM${}_{10}$ fraction was lower than that of the PM${}_{2.5}$. Assuming that $EF<10$ is moderate enrichment, $10<EF<20$ is significant enrichment and $EF>20$ is very high enrichment [81], Cd, Pb, Cu (except at Monte Curcio), Ni (except at Naples and Lecce), As (except at Naples) and Cr (except at Naples and Lamezia Terme) exhibited higher enrichment factors in the PM${}_{10}$ fraction, indicating that the concentrations of these elements in PM were highly enriched by sources of anthropogenic origins. V and Mn (and Ni at Naples) instead showed $EF<10$, indicating that these elements were moderately enriched and mostly derived from crustal sources. In the PM${}_{2.5}$ fraction, the elements Cr, Ni, Cu, As, Cd and Pb showed $EF>20$ in practically all sites (except at Monte Curcio, which were not reported due to < LOD levels), underlining their predominantly anthropogenic origin. $EF<10$ was observed for Mn and V (only at Naples and Lecce), as already observed in the PM${}_{10}$ fraction. The Cd concentration is not reported for either fraction at Lamezia due to the few values available.

In general, the trend of element enrichment factors was similar between the PM${}_{2.5}$ and PM${}_{10}$ fractions (although with lower values) and this information was also confirmed by the good correlation ($R>0.6$) obtained between certain elements in both fractions, such as V, Ni, As, Cd and Pb at Naples and V, Cr, Mn, Cu, As and Pb at Lamezia Terme and Lecce, to which Cd was added, confirming the common sources of all elements, both natural (crustal origin) and anthropogenic (plant emissions and vehicular traffic).

4. Summary and Concluding Remarks

In this study, we presented the results of the mass and composition of atmospheric aerosol samples from five sites in southern Italy, which were collected on quartz filters during the winter season of 2015–2016.

All sites are located in southern Italy, at the center of the Mediterranean area, and are representative of very different environmental conditions: from urban (Naples) to remote sites (Monte Curcio), passing through urban background (Lecce), industrial (Lamezia Terme) and marine (Capo Granitola) sites. Despite the environmental heterogeneity in the dataset, the PM${}_{2.5}$ and PM${}_{10}$ samples showed well correlated mass concentration values with a correlation coefficient of $R^2>0.90$. The overall PM${}_{2.5}$–PM${}_{10}$ ratio (0.80) was fairly constant across sites, but the PM mass and composition reflected the contribution of different sources. The mean mass concentration of PM${}_{10}$ ranged from 3.6 $\mathsf{\mu }$g/m${}^3$ at Mt. Curcio, a remote high-altitude site, to $24\mathsf{\mu }$g/m${}^3$ at Capo Granitola, a low-altitude coastal site facing the Strait of Sicily, and to more than $50\mathsf{\mu }$g/m${}^3$ at Naples, a highly urbanized and densely populated area. A large fraction of the mass of the PM belonged to the fine fraction; the lowest percentage was 50% at the Capo Granitola site, but reached percentages of up to 75% in the urban area of Naples.

The monitoring site in Naples is located within a limited traffic area in the historic city center, but is still affected by traffic emissions outside of its borders and by neighboring port activities. The marina is one of the busiest ports in the Mediterranean, thanks to both cruise ships and numerous tourist ferries to/from nearby islands. Furthermore, the urban area of Naples is one of the most densely populated cities in the world (almost 10,000 inhabitants per square kilometer), a factor that enhances the contribution of anthropogenic sources.

Opposing results were obtained for the Capo Granitola site, with a much larger fraction of coarse particles reflecting its location, which is very close to the coast. The ionic composition also showed a strong dependence on the site, with a percentage ranging from 30% to 60% of the PM${}_{10}$ mass for urban/remote and from 20% to 55% for coastal sites.

The levels of the marine source fraction (i.e., Cl⁻, Na⁺ and ${\mathrm{SO}}_4^{2-}$ in PM₁₀ varied from 3 to 4% at the Naples and Lecce sites, 12% at the coastal site of Lamezia Terme and 30% at Capo Granitola. Chloride levels were generally lower than those expected from the Cl–Na ratio of seawater, with the sole exception of the Capo Granitola site, which indicated aged sea salt that was depleted in HCl as a consequence of its volatilization during the formation of Na₂SO₄ from gaseous HNO₂ and the formation of H₂SO₄.

The metal content in the PM of urban and extra-urban sites showed an enrichment factor for elements that were attributable to anthropogenic sources, such as Pb, Cd and Cr in both fine and coarse fractions, probably due to coal plants. The other sampling sites showed an increase in the enrichment factor of Cu, As and Ni from PM${}_{2.5}$ to PM${}_{10}$ (generally, for all sites), probably originating from anthropogenic sources due to the combustion of fuels and coal.

The data on the chemical composition and the relative analyses shown here represent a significant contribution to knowledge about the chemistry of PM${}_{2.5}$ and PM${}_{10}$ for the sites considered in this work; in addition, they provide a significant basis for various types of future studies. In particular, the considerations of the enrichment factors of metals and the correlation of ionic species provide an important insight into the chemical characterization of PM: a fundamental step for the development of possible prevention measures. Furthermore, the information on the elemental and ionic compositions, along with the previously published data on EC/OC, can be used as an input for source apportionment studies aimed at better understanding the different contributions to the generation of PM for the sites considered in this work; moreover, they constitute a complete dataset for comparison to PM collected in similar areas.

In addition to the work produced during this study, it is desirable for more extensive (at least year-round) and continuous monitoring campaigns to be carried out in order to study seasonal variability and other factors that control the composition of atmospheric aerosols in the central Mediterranean region.