Transition metals concentration within the PM 10 fraction of atmospheric suspended particulate matter – case study, Craiova, 2021

. The current case study had the main purpose of determining and reporting the concentrations of the most toxic transition metals present in the composition of the particles suspended in the atmosphere, namely: lead, nickel, arsenic and cadmium. In the collection of experimental data, the fraction of suspended particulate matter represented by inhalable particles with an aerodynamic diameter smaller than 10 microns (PM 10 ) was studied, by taking, during two weeks within different seasons of the year 2021, two set of seven filters, each used for one day – from an area of heavy traffic, on one of the main arteries of the municipality of Craiova, near a continuous air quality monitoring station capable of monitoring PM 10 . Thus, it was possible to permanently compare the results obtained through this application with the official data – registered during the same time intervals – by the local division of the EPA, founding a remarkable data concordance.


Introduction
As is known, environmental policy has become, at present, a priority policy of the European Union, environmental protection requirements being considered as mandatory exigencies.
By adopting the strategy of sustainable development as the main element of its field of action (i.e., concerning for nature as a legacy and resource for future generations), the environmental policy is permanently connected to the global trends of environmental protection, as nowadays the European Union imposes a new approach to this kind of issues, looking at them from the point of view of their effects and pressure exerted on the environment, but also from that of their socio-economic consequences.
In this context, within modern society, it has been reached the point of finding correlations between environmental pollution and damage of biotopes, climate change, global warming and destruction of the protective ozone layer -worrying phenomena for the whole population of the planet.

Air quality -a foremost environmental issue 2.1 Air pollutants -a major health threat. PM10 and the metals within them
The main air pollutants are either gazeous or solid.The first category primarily includes nitrogen oxides, sulfur dioxide and carbon monoxide.The second category refers to the particles in suspension in the atmospheric air, originated firstly from industrial pollution pollution and secondly from road traffic [1].
Identifying and minimizing the particulate matter sources must be a great concern [2,3], as particle size is directly related to the potential to cause effects.Particles in the PM10 fraction, i.e., those with diameters smaller than 10 microns, are inhalable particles, which can pass through the nose and throat, potentially causing inflammation and intoxication [4,5].
People with cardiovascular diseases and those with respiratory diseases are particularly affected, especially asthmatics, as dust pollution worsens asthma symptoms, respectively causes coughing, chest pain and breathing difficulties (children inhale more air, consequently more pollutants, thus being more vulnerable).
Air pollution with suspended particles represents a priority environmental problem, especially because of its aggressiveness on human health, more within the urban environment than in the rural one [2], this problem being aggravated by the presence, in the composition of these solid particles in atmosphere, of some toxic metals, especially from the category of transition ones, especially the following: lead, nickel, arsenic and cadmium [6].

Air quality automatic monitoring
At European and international level, an integrated air quality management program was initiated, on the basis of ambient air quality data [7,8].
Air pollutants' concentrations are very strictly regulated [9].In Romania, from the National Air Quality Monitoring Network, consisting in 38 local centers, with 117 continuous air quality monitoring stations, fitted with automatic equipment for measuring the concentrations of the main atmospheric pollutants, which collect and, after primary validation, transmit the data provided by the monitoring stations firstly for certification and afterwards to the public information panels.Between them, there are 5 in Craiova, that is Dolj county residence, within which the study was realised [10].

Air quality regulations in Europe -transposed in Romania
Air quality protection represents a field of great importance in ensuring human health, as well as the general quality of life, in the spirit of the concept of sustainable development; that is why, in the context of the current requirements formulated at the global level, air quality status is permanently presented to the public [11].
The main air quality regulations are the European directives and, the main purpose of which is to evaluate and manage air quality in a comparable way, based on unitary criteria, at the level of the entire European Union [7].Of course, in Romania, there are specific laws that transpose the European directives at national level [7,8].
Air quality assessment in Romania is, therefore, the essence of the way to implement the requirements of the European directives, which tend to be tougher every year [12].
Establishing a balance between the need to increase the standard of living through economic progress, the quality of environmental factors and the health status of the population is decisive in order to integrate Romania into the environmental requirements of the European Union [10].

Air quality regulation regarding PM10 and the metals within them
As a harmful pollutant, PM10 itself is not allowed to exceed 50 µg/m 3 per day and no more than 35 times a year, keeping an yearly medium below 40 µg/m 3 , according to the Law 104 /June 15 th , 2011 [9], Moreover, according to the same law [9], in order to preserve health protection, there are also clear regulation imposed on the concentration values of the main toxic transition metal within the particulate matter with diameters smaller than 10 microns, as it can be seen from Table 1.*referring to the maximum allowed annual mediated value; **referring to the maximum allowed daily mediated value.
3 Case study description

Case study purpose
As pollution of the environment -in particular, of the atmospheric air -becomes a problem of utmost importance, this case study, carried out during the year 2021, in Craiova city (the seat of Dolj county), regards heavy metals contained in the inhalable particles found within the air.
As mentioned before, between the 117 continuous air quality monitoring stations present of the Romanian teritory, there are 5 in Dolj county, being representative for Craiova urban agglomeration [12,13].
The present described case study aims of determining and reporting the concentrations of the most toxic transition metals whitin the composition of the suspended particles with diameters smaller than 10 microns (PM10), which are the following: lead, nickel, arsenic and cadmium.
The ultimate goal of this case study was to permanently compare the results obtained through our own application -that will be described in what follows -with the official data registered and reported for the same time intervals by the local division of the EPA (Dolj Environmental Protection Agency) [10], in order to observe whether or not these data are in concordance with our own findings.

General presentation of the case study
Experimental data were collected [15] by taking, during two weeks in different seasons of the year 2021, two set of seven filters -F1, F2, …, F7, each one being used for an entire day.For more clarity, they have been denoted also taking into account the part of the study in which they were used: P1F1, P1F2, …, P1F7 for the seven filters used between August 16 th and 22 nd , 2021 and P2F1, P2F2, …, P2F7 for the seven filters used between 13 rd and 19 th December, 2021 (in other words, the codes which the filters were denoted with were written as PiFj, i taking either the value 1 or 2 for the parts of the study and j varying from 1 to 7 (e.g., P2F3: filter 3 used in part 2).
The samples prelevation was made from a place very near to one of the 5 continuous air quality monitoring stations existing in Dolj county that were above mentioned, representative for Craiova urban agglomeration, which is capable of monitoring PM10 pollutant (not all of them are [14]).
The analyzes include several stages: preliminary conditioning of the Quartz microfiber filters inside an oven on Petri plates, initial mass determination for each filter, collecting particulate samples on these filters, re-putting the filters inside the oven on Petri plates, final mass determination for all filters, mineralization of the filters and finally the actual analyzes of heavy metals from the solutions obtained upon mineralization.This is to say that particulate matter, collected on Quartz microfiber filters, are to be gravimetrically evaluated, as to be further subjected to quantitative analyzes for determining the concentration of their components, such as, in our case, the toxic transition metals, namely: lead, nickel, arsenic and cadmium [15].

Materials
For the particulate matter sampling, we used Whatman Grade QM-A Quartz microfiber filters 8x10 in, provided by Cytiva [16], which are suitable for PM10 and also for further heavy metals concentration determination.
For conditioning the Quartz microfiber filters, we used a G-030 temperature-adjustable 80 L laboratory oven with forced ventilation, produced by Alfa Testing Equipment [17].
An XPR10 Microbalance Type purchased from Mettler Toledo [18] was used in order to weight the filters before and after using them.
Afterwards, for the mineralization of the filters, a CEM MARS 6 mineralizer (MARS meaning "microwave accelerated reaction system") [20] was used.
Finally, for the analysis of transition metals from the solutions obtained by mineralization, we made use of a Varian AA 240 spectrometer, based on the AAS technique, which is driven using Varian's easy to use worksheet-based SpectrAA software system [21].

Methods
Figure 1 aims to show some of the main work stages within our case study application and also the main part of the equipment that we made use of, which was previously presented.
As formerly stated, the preliminary step was conditioning the Quartz microfiber filters [16] inside an oven with forced ventilation [17], on Petri plates, afterwards weighing each of them on a microbalance that allows reading with six decimals [18] (i.e., its readability is 1 µg).Placing the medium volume sampler MVS 6.1 [19] somewhere near to a continuous air quality monitoring station in Dolj county which is capable of monitoring PM10 pollutant, the air samples were directly drawn into it by a vacuum pump, which contains a plate with holes.The impact plate of the pump was cleaned and greased with silicone Vaseline (Figure 1a).On a digital screen, the volume of air sampled was displayed, in operated m 3 , with an accuracy of 0.01 m 3 (the flow rate was set at constant, at 2.3 m 3 /h, in accordance with the standard EN 12341:2014 [22], which was subsequently updated [23]).Changing filters at the measurement site was facilitated by using a quick-change filter holder (Figure 1b).The O-rings on the jet plate of the pump (Figure 1c) were also greased with silicone Vaseline, to increase lubrication.The used Quartz microfiber filters [16] were extracted every day (Figure 1d), both for analysis and replacement.At each replacement at the sampler, the drawn air volume was recorded.
The filters were re-put in Petri dishes (Figure 1e) to be conditioned once more in the oven [17] (Figure 1f); upon conditioning, each filter was weighed again at the microbalance [18], to determine the mass difference (i.e., the mass of PM10 dust fraction retained on that filter).
Then, conditioned filters were brought, one by one, into the mineralizer [20] (Figure 1g), as to extract the heavy metals from the samples with HP 500 type cartridges placed on a rotor that was formerly calibrated, by using the so-called "pressure ramp method".After finishing the operation cycle of the mineralizer, the cartridges were allowed to cool, then the obtained solution was put into volumetric flasks and brought to the mark by filling with distilled water.
After staying for a day, for all the particles to settle down, the solutions thus obtained were analyzed with the help of the atomic absorption spectrometer [21] (Figure 1h).As its own name shows, it has as operating principle the atomic spectrometry, which is based on the absorption/ emission of energy by the atoms of the studied chemical elements, brought into the gaseous state, consequently allowing the concentrations of different atoms to be determined according to the principles of Lambert-Beer law [24].Finally, the experimental data were processed by using the worksheet-based software system of the spectrometer, called SpectrAA (Figure 1i).

Reported concentration values of toxic transition metals whitin PM10
As previously stated, the local division of the EPA (Dolj Environmental Protection Agency) records, from all air quality monitoring stations fitted with automatic measuring equipment, the concentrations of every main atmospheric pollutant, permanently reporting the data thus obtained to the national EPA (National Environmental Protection Agency) [10].
As far as the concentration values of toxic transition metals whitin PM10 are concerned, Figure 2 offers a graphical comparative perspective performed by us on the basis of the values reported by the local division of EPA during both weeks of 2021 during which our study was carried out, namely: August, 16 th -22 nd and December 13 rd -19 th , respectively [15].

Results obtained through the case study
Regarding the determination of the concentration of the most significant toxic metals within PM10, it must be specified that, in all cases, the concentration, which was initially recorded in μg/L, was afterwards transformed into another unit, namely μg/m 3 , by dividing its value to twenty times the drawn air volume, expressed in m 3 ; in what follows (Table 2), both variants will be presented each time [15].
The concentration must be determined in every part of the study, every day, for each individual filter and for each of the four analyzed metals.
Table 2 [15] shows which are the concentrations (expressed in two units, namely μg/L and μg/m 3 ) that were registered in our application for all the fourteen PM10 filters used in August 2021 (denoted P1F1, P1F2, …, P1F7) and December 2021 (denoted P2F1, P2F2, …, P2F7), as well as how these concentrations are situated comparative to the regulated limit value within the air for lead and, respectively, to the lower and upper regulated limit values within the air for the other three toxic transition metals investigated [9].Furthermore, Figure 3 [15] presents a graphical comparison between the values registered in this application and the ones -previously presented -which were registered and reported for the same periods by the local division of the EPA collected from a near point, a notable concordance being observable between all sets of data.and the ones reported from near, for the same periods, by the local division of the EPA (front charts) [15].

Discussions on the results obtained through the case study
As it was already observed, the variations in the concentrations of toxic transition metals in PM10 differ from one metal to another, being clear that, among the four toxic transition metals studied, the highest concentrations were presented by lead (although, as seen in Table 2, its limit value in air was never even close to the threshold); as far as the other three metals are concerned, it can be seen that nickel did not present any problems, unlike arsenic and cadmium, which, unfortunately, recorded frequent exceedances.
It is also important to note that, in general, the concentrations of these metals within the PM10 fraction of suspended particulate matter were higher in the second set of measurements than in the first one, which seem to suggest a quite strong influence of the residential heating during the wintertime, as well as of the augmented traffic caused by the approach of holidays.
Obviously, various findings can be made regarding the concentrations of toxic metals in PM10, in both parts of the study.
In August 2021, regarding lead, a low value of concentration is found for filters P1F1 and P1F5 and a high value of concentration for filters P1F2 and P1F4; it is observed that the recorded values are below the limit value of lead in the air.Regarding nickel, a lower concentration value is found for filters P1F1, P1F5 and P1F7 and a higher concentration value for filters P1F4 and P1F6; it is noted that the recorded values are below the lower assessment threshold for nickel in air, and obviously, implicitly, also below the upper assessment threshold.Regarding arsenic, a lower concentration value is found for filters P1F1 and P1F5 and a very high concentration value for filter P1F7; it is observed that the recorded values are twice below the lower assessment threshold for arsenic in the air, and obviously, implicitly, also below the upper assessment threshold, but two other times they are between the two thresholds, in the remaining three times being above the upper assessment threshold, and, implicitly, also above the lower assessment threshold.Regarding cadmium, a lower concentration value is found for filters P1F6 and P1F7 and a higher concentration value for filters P1F3 and P1F5; it is observed that the recorded values are only twice between the two thresholds, in the remaining five times they are above the upper evaluation threshold, and, implicitly, also above the lower evaluation threshold of cadmium in the air [15].
In December 2021, regarding lead, a low concentration value is found for P2F1 and P2F6 filters and a high concentration value for P2F2 and P2F4 filters; it is observed that the recorded values are below the limit value of lead in the air.Regarding nickel, a lower value of concentration is found for filters P2F1 and P2F6 and a higher value of concentration for filters P2F4 and P2F7; it is noted that the recorded values are below the lower assessment threshold for nickel in air, and obviously, implicitly, also below the upper assessment threshold.Regarding arsenic, a lower concentration value is found for the P2F1 filter and higher concentration values for the P2F4 and P2F7 filters; it is observed that, in all situations, the recorded values are above the upper threshold, and, implicitly, also above the lower threshold for assessing arsenic in the air.Regarding cadmium, a lower concentration value is found for the P2F6, P2F2 and P2F6 filters and a higher concentration value for the P2F3 and P2F5 filters; it is observed that the recorded values are three times between the two thresholds, in the remaining four times they are above the upper assessment threshold, and, implicitly, also above the lower assessment threshold for cadmium in the air [15].

Conclusions
At the end of the case study, it is worth comparing the two sets of results, obtained through the application developed by us in Craiova, during one week of August and another week of December 2021, regarding the concentrations of toxic transitional metals within PM10.
Generally, it was found that these concentrations were higher in wintertime.
Thus, we allow ourselves to state that this comparison suggested us that they seem to be strongly influenced by heating systems in general and especially by those that use solid fuel, as well as from increased traffic due to the approach of the winter holidays.
Indeed, the increase in the number of individual heating systems leaded to the appearance of multiple sources of diffuse toxic emissions, spread over the entire area of the Craiova municipality, which in the conditions of prolonged atmospheric calm do not disperse, but, on the contrary, accumulate and persist over the residential zone.
The Integrated Air Quality Management Program includes measures regarding the prohibition of individual gas power plants in the future blocks, as well as the enrichment of the city fleet of buses with environmentally-friendly ones (what the municipality has already implemented, during the last year and the current year), the intensification of controls at economic agents with atmospheric protection impact, and educational awareness activities for citizens regarding the great importance of an optimal air quality management, for a sustainable development of the Romanian society, in the context of increasing pollution.
However, compared to other locations analysed in the literature [2 -6], Craiova does not seem to exhibit alarming threats on human health, as far as PM10 pollutant is involved.

Fig. 1 .
Fig. 1.Some of the main work stages within our case study application and the equipment that we used: a) impact plate greased with silicone Vaseline; b) filter holder; c) jet tubes with O-rings; d) extracting the Quartz microfiber filters; e) placing the Quartz microfiber filters in Petri dishes; f) arranging the sets of Petri dishes containing the Quartz microfiber filters inside an oven; g) introducing one of the HP 500 type cartridges into the mineralizer; h) Varian spectrometer; i) processing the experimental data obtained by AAS technique with the worksheet-based software of the spectrometer, called SpectrAA.

Fig. 2 .
Fig. 2. Charts which we realized based on the sets of values registered, simultaneously with our study, i.e., during one week of August 2021 (16 th -22 nd ) and one week of December 2021 (13 rd -19 h ), by the local division of EPA [10, 15].

Fig. 3 .
Fig. 3. Graphical comparison between the values which were registered in our application (rear charts)

Table 1 .
Regulations imposed on the concentrations within PM10 of the four studied transition metals [9].

Table 2 .
Con15]trations registered in our application for all the fourteen PM10 filters which were used in August 2021 and December 2021 -compared to their regulated limit values within the air[9,15].