1. Introduction
Human activities contribute to the air pollution, especially in industrial and urban areas. Airborne particulate matter (PM), consisting of a mixture of solid and liquid particles suspended in the air, varies in terms of physical properties (shape, dimension, density) and chemical composition [
1,
2,
3]. As a result of atmospheric deposition, metals migrate to water and soil, where they can be taken up by plants and animals and further transferred through the food chain directly into humans. Once released into the environment, metals are continuously undergoing physical and chemical transformations that affect the forms of their occurrence and determine their toxicity. Currently observed changes in the structure of particulate matter emission into the air [
4], involving a significant reduction in coarse particles (emitted mainly from industrial sources) and an increase in ultrafine particles (combustion by car engines) allow us to conclude that the actual environmental pollution by metals is primarily caused by the smallest particles. The finer particles the deeper penetration into the human tissues thus the stronger and more dangerous health impact [
5], since they penetrate deep into the lungs and deposit on the respiratory tract at an increasing rate [
6]. In nasal-breathing, coarse PM 10 fraction settles quickly and tends to lodge in the trachea (upper throat) or in the bronchi causing sneezing and coughing [
7]. Particles smaller than 10 μm are deposited in the tracheobronchial tree, the respiratory bronchioles and the alveoli [
8]. These particles can penetrate the lung and escape into the blood stream to cause significant health problems [
9]. Particles smaller than PM2.5 tend to possess a higher radical generating capacity than larger ones [
10]. Particles smaller than 1 μm behaving similar to gas molecules can penetrate down to the alveoli (deposition by diffusion forces) and translocate further into the cell tissue and/or circulation system [
11].
Previous studies indicate that environmental magnetism methods can be used to estimate the quantity, distribution, range and source of environmental pollution [
12,
13,
14,
15,
16,
17,
18] due to magnetic properties of Fe-rich PM derived from industrial and urban activities, fossil fuel combustion, vehicle emission and other anthropogenic as well as natural sources. It is well known that such magnetic particles are excellent absorbers and carriers of heavy metals and other pollutants due to their large specific surface area. The Cu, Pb and Zn were absorbed onto the surface of magnetite like minerals or incorporated into the structure of hematite [
19,
20,
21]. The Fe-rich particles being a component of PM and originating from atmospheric pollution were detected as a deposit on topsoils, plants (especially tree leaves) and other surfaces by means of reliable, simple and quick magnetic susceptibility measurements [
22,
23,
24,
25,
26,
27,
28,
29,
30,
31]. Several studies are concerned with the application of magnetic methods in order to characterize industrial or street dust samples [
32,
33,
34,
35], while some authors used bioindicators (as tree leaves, lichens, mosses or spider webs) for air pollution monitoring [
28,
29,
36,
37,
38] and filters from the air monitoring stations [
39,
40,
41,
42]. In this case, an empirical linear correlation was found between the magnetic susceptibility of filters from air-monitoring stations and the PM10 (particles with aerodynamic diameter not greater than 10 µm) and TSP (total suspended particles) concentrations as well [
39,
40]. Moreover, the magnetic properties of PM2.5 (particles with aerodynamic diameter not greater than 2.5 µm) and PM10 show significant seasonal patterns and weekly cycles [
42]. Górka-Kostrubiec et al. [
43] discovered a relationship between monthly magnetic susceptibility and temperature and absolute humidity which tend to correlate positively in the cold periods and negatively in warm months of the year. Actually, magnetic characterization was determined for particles with aerodynamic diameter not greater than 2.5 and 10 µm, using standard PM filters. Previous investigations on PM1 have concerned chemical composition, origin and meteorological dependence [
44,
45,
46,
47,
48].
This is one of the first studies in which PM1 filters were subjected to such diverse analyses, and its most innovative feature is the estimation of non-carcinogenic and carcinogenic risk assessment for selected elements in TSP and, especially, PM1. Therefore, the determination of the geochemical and mineralogical properties of particulate matter (TSP and especially PM1) suspended in the air of a selected urban area in southern Poland was the aim of this preliminary research. Measurements of magnetic susceptibility as a proxy for atmospheric pollution collected on filters were carried out in order to test the applicability of this method in air pollution monitoring. Moreover, finding similarities and differences in the particle size, morphology and chemical composition between emitted dusts, fly ashes and airborne PM will be the basis for the identification of the emission sources of metals and metalloids bound to TSP and PM1. The assessment of potential risk of urban ambient air to human health using hazard indices was also an objective of this investigation.
2. Materials and Methods
The study area was located in the very urbanized and industrialized city of Zabrze (southern Poland, Silesia Conurbation) (
Figure 1), adjacent to Bytom, Gliwice and Ruda Slaska, all known from their coking plants, steel mills and mines. The geological structure of this area is dominated by various sedimentary rocks (sandstones, conglomerates, limestones, marls, etc.) with numerous mineral resources: hard coal, zinc, lead and iron ores. In the close neighborhood of the sampling site (an air-quality monitoring station) a trunk road, blocks of flats and residential houses (heated with poor-quality stoves, qualitatively bad coal, and even garbage), and city center with commercial buildings are located. The monitoring station is located in a transformed area, on technogenic soil covered with grass. The TSP and PM1 were collected on quartz fiber filters (Whatman, UK, Ø 47 mm) during heating season, between 24 October 2018 and 17 January 2019. The impactor PNS3D15/LVS3D worked at the flow rate of 2.3 m
3/h.
The collected 70 filters of daily retained PM1 (35 filters) and TSP (35 filters) were divided by means of a guillotine specially designed for this purpose and adapted to the diameter of the filters and then weighted. One part of them was subjected to the non-destructive volumetric magnetic susceptibility (ĸ) measurements using an MFK1 Kappabridge device (Agico Advanced Geoscience Instruments Co., Brno, Czech Republic) that operated at a low frequency (976 Hz) and low magnetic field intensity (200 A·m−1). In order to ensure the exact positioning of the specimen, each filter was placed in the 10 mL cylindrical bottle (originally assigned for the Bartington MS2B device, Bartington Instruments Ltd., Witney, UK) which afterwards was placed in the central bottom of the plastic vessel of MFK1 for specimen fragments for use in static measurement mode (KLY VES40). Each sample was measured five times at least, and the mean value was calculated. Subsequently, the mass-specific magnetic susceptibility (χ, m3/kg) was computed by taking into account the weight of the sample as well as the mass and the χ (−0.02 × 10−8 m3/kg) of the clean filters.
Afterwards, SEM/EDS analysis was carried out at the Institute of Earth Science, Faculty of Natural Sciences of University of Silesia in Katowice. Industrial dusts and fly ash samples from neighboring industrial emission sources collected directly at the source (power and metallurgical plants) were analyzed using a scanning electron microscope Philips XL30 ESEM/TMP equipped with an EDAX EDS Sapphire system, detectors: SE, BSE and EDS. This microscope can work in the environmental mode, i.e., it enables analyses to be carried out without the need to spray the sample with a conductive layer, e.g., carbon (metallurgical dusts). Electrifying fly ashes were dusted with a thin layer of carbon and analyzed in the high vacuum mode. Magnifications from several to several thousand times were used, and the voltage accelerating the beam was 15 keV.
TSP/PM1 filters were analyzed using a Quanta 250 scanning microscope with Thermo-Fisher EDS Ultra Dry analyzer (Thermo Fisher Scientific Inc., Waltham, MA, USA). BSE images were recorded, in which, apart from the habit and grain size, the chemical composition variation was visible both between and within the sample grains. EDS microanalysis was performed at selected sample points to determine the chemical composition of the grains present in the samples.
The second parts of the filters were digested in a mixture of nitric acid and hydrogen peroxide solution in microwave oven in order to determine total content of the following elements, in majority considered as potentially toxic (PTE): V, Mn, Ni, Cu, Zn, As, Cd, Tl, Pb, Cr, Ti and Al using High-Resolution Inductive Coupled Plasma-Mass Spectrometry (HR-ICP-MS, 6100 DRC-e Perkin Elmer, Waltham, MA, USA). Iron content was determined by Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES; Avio 200 Perkin Elmer, Waltham, MA, USA).
The homogeneity of the loaded halves of filters was previously examined by the magnetic susceptibility measurements of experimental three filters (six halves). Obtained results varied from 51.25 to 61.32 × 10
−8 m
3/kg (mean 57.54 × 10
−8 m
3/kg). The favorable low relative error (7.9%) indicates the homogeneity of the collected material and the possibility of using filter halves for further analyses. Sysalová et al. [
49] tested homogeneity of divided filters in determination of trace elements in airborne particulate matter and they yielded sufficiently homogenous results (errors for particular elements between 1.7% and 12.5%).
The potential health impacts associated with environmental exposures to inhaled contaminants were estimated on the basis of USEPA’s guidance [
50]. The non-carcinogenic risk assessment (mutagenicity, neurotoxicity, developmental and reproductive toxicity) posed by As, Cd, Cr, Ni, Pb and Mn and carcinogenic one posed by As, Cd, Cr and Ni (classified as class 1 carcinogenic agents) [
51] were determined by their hazard quotient (HQ) and carcinogenic risk (CR), respectively [
52,
53]. The hazard quotient (HQ) was computed as the ratio of the exposure concentration of the inhalable chemical (EC) to a reference concentration (RfC) meaning an estimate of a continuous inhalation exposure to the human population (including sensitive subgroups) that is likely to be without an appreciable risk of deleterious effects during a lifetime USEPA [
54]. RfC values for As, Cd, Cr, Ni, Pb and Mn were as follows: 0.000015, 0.00001, 0.0001, 0.00009, 0.008 and 0.003 mg/m
3, respectively:
The carcinogenic risks (CR) were computed by multiplying the exposure concentration of the inhalable chemical (EC) for the inhalation unit risk (IUR):
Values of IUR were specified by USEPA [
55]: 0.0043, 0.0018, 0.012, 0.00024 for As, Cd, Cr and Ni ((μg/m
3)
−1), respectively. EC was calculated according to the following equations [
50]:
All symbols concerning hazard parameters used in the above equations are explained in
Table 1. To assess the overall potential non-cancer and cancer health risk caused by multiple compounds the hazard index (HI) was introduced as the sum of hazard quotients of individual elements [
50]. In the present work HI values were calculated as follows:
Hazard index values exceeding unity provides evidence that potential health effects may occur. Otherwise (HI < 1), it is assumed that the risk is at an acceptable level. Carcinogenic risk is the chance of an individual causing any kind of cancer from lifetime exposure to carcinogenic hazardous substances. The acceptable or tolerable risk for regulatory purposes is 1 × 10
−6–1 × 10
−4 [
51].
The obtained results were analyzed using Microsoft Office Excel software and the package of Statistica for Windows, version 12 (StatSoft Polska Sp. z o.o., Kraków, Poland). The basic statistical functions of mean, median, standard deviation, as well as factor analysis were applied in order to interpret results and explain variations in the data. For data evaluation and presentation mean values of obtained results were used. As the data did not exhibit a normal distribution, a Spearman’s rank correlation coefficient was applied.
4. Conclusions
Obtained results revealed that the concentration of TSP and PM1 well correlated with their mass-specific magnetic susceptibility. The good relationship between the PM concentration and χ suggests that magnetic susceptibility measurements can be a good proxy of low-level atmospheric dust pollution.
The difference in
χ for TSP and PM1 was generally low, indicating a comparable content of magnetic minerals in both TSP and PM1. Thus, PM1 was the main carrier of technogenic magnetic particles in the total deposited dust. Generally, enhanced values of
χ indicated an increased number of Fe-rich particles in PM filters, but obtained results concerning Fe concentration and its correlation with
χ did not confirm this statement. Probably, a foggy weather condition occurring during almost the whole investigated period can be the reason for an unexpected negative correlation between magnetic susceptibility (concentration of ferrimagnetic iron oxides) and Fe concentration [
57]. On the other hand,
χ exhibited a positive and significant correlation with the majority of determined PTE in the case of TSP, but not for the PM1 fraction—Only with Cu, Mn, and a negative significant correlation with Ba. So, the other elements were linked to susceptibility when the particles were larger than 1 μm, as it happens for non-exhaust emissions. Comparing the content of chemical elements in TSP and PM1, it was observed that concentration of V, Mn, Ni, As, Tl, Cr, Ti, Ba and Al was slightly higher in PM1, but the daily variability of elemental concentrations in TSP and PM1 looked similar. PM1/TSP ratios for PTE concentrations and
χ were around or slightly above unity, which indicated that PM1 was the main carrier of PTE (with the exception of cadmium, copper and lead) and technogenic magnetic particles.
Correlation matrices and principal component analyses were applied in order to determine the sources of dusts aspirated on filters. In the case of both PM fractions two main groups of components were separated. A distinct group consisting of Al, Ba and Zn was distinguished for both size fractions of PM. High concentration of these elements was probably due to the influence of tyre abrasion, and Ba most likely originated from fly ashes of the coal used for heating in the study area. The second TSP cluster was characterized by high loadings of As, Cd and Pb which may be related to coal combustion, while the second PM1 cluster was associated with χ, As, Cd, Fe, Mn, Pb and Tl, elements which are produced as a by-product of non-ferrous metal smelting and refining. Obviously, the influence originated from steel metallurgy and vehicular emissions, but also that from soil and road dust re-suspension should not be excluded. SEM images and results of EDS analysis confirmed the presence of particles and minerals characteristic for the above-mentioned sources.
Surprisingly, despite not very high values of magnetic susceptibility and the PTE concentration, the human health risk indices (cancer and non-carcinogenic) determined on the basis of PTE concentration were very high, which confirms the relevance of this study and suggests that they should be continued.