Morphological and Chemical Properties of Particulate Matter in the Dammam Metropolitan Region : Dhahran , Khobar , and Dammam , Saudi Arabia

1Geosciences Department, King Fahd University of Petroleum and Minerals (KFUPM), Dhahran 31261, Saudi Arabia 2Department of Industrial Hygiene, College of Engineering, West Virginia University, Morgantown, WV 26505, USA 3Physics Department, King Fahd University of Petroleum and Minerals (KFUPM), Dhahran 31261, Saudi Arabia 4Department of Physics, Faculty of Science, Alexandria University, Alexandria, Egypt


Introduction
Particulate matter (PM) is a very vital pollutant that alters the ecosystem, human health, and environment as well as air quality.Equivalent Aerodynamic Diameters (EAD) method is the basic technique used to classify PM [1], where particles could be classified into PM 10 , PM 2.5 , and PM 1.0 representing their 10, 2.5, and 1 m sizes, respectively [2,3].PM 10 particles are referred to as "inhalables" and usually trapped in mucus linings and may cause significant health effects to the respiratory system.Human health, ecosystem, and climate are affected by PM [4,5], with high correlation found between daily mortality and PM concentration in urban areas [5].PM has been universally regarded to be of extreme importance with worldwide organizations established to implement and monitor air quality standards and levels.
Several efforts have been conducted to determine the characteristics of particulate matter samples collected in both indoor and outdoor environments around the globe.Wu et al. [6] studied the attributes of PM 10 and PM 2.5 at Mount Wutai Buddhism Scenic Spot, Shanxi China.Ny and Lee [7] studied the size distribution of PM and some elements it contains within an industrial city in Korea.In another research, Ahmady-Birgani et al. [8] investigated the mineralogy and geochemistry of PM in Western Iran.
Field-Emission Scanning Electron Microscopy (FESEM) coupled with energy dispersive microanalysis has been used globally to study the chemical and morphological characteristics of airborne particles [9][10][11].FESEM-EDX has been exploited in many reviews [12,13] to ascertain the difference in morphology and structure, as well as estimating the elemental component of the particulate aerosols.Also, it was used to identify PM's likely emission sources and their link to the transport of pollutants from differently polluted areas [14].
Kingdom of Saudi Arabia is well known for its major dust storms which have great impacts on the distribution of the particulate matter in the Kingdom [15].Numerous studies investigated PM categorization over the Arabian Peninsula and the Mideastern region using airborne and Aerosol Robotic Network (AERONET) ground observations [2,3,16]; however ground observations are very limited due to the existence of only two AERONET stations in Saudi Arabia, namely, the Solar Village (24 ∘ N, 46 ∘ E) and KACST (22 ∘ N, 39 ∘ E).Hussein et al. [17] carried out a study, in Jeddah's urban atmosphere, on the concentration of particles heavier than one-quarter of a micron and particulate matter, during the year 2012.An investigation by Alharbi et al. [16] showed that the chemical characteristics of PM concentration in Saudi Arabia capital, Riyadh, during weekdays were 17% higher compared to weekends during the period between September 2011 and September 2012.
Eastern Province (EP) of Saudi Arabia is prominent for PM due to mega construction activities [18]; however, despite the fame, there is little information on the mineralogical composition and characteristics of the PM samples.Hence, this research was embarked on to study the pattern of particulate matter less than 10 m (PM 10 ) produced within 3 major cities (Dhahran, Dammam, and Khobar) of EP, Saudi Arabia, to determine their elemental composition, which may enter human's respiratory system through inhalation and compare the elemental concentration with available worldwide standards.The research also investigates the size, shape, and elemental composition of the pollutants as well as the mineral composition of the particles.These observations would provide public and private organizations, government regulatory bodies, and scientific communities with vital information that would be useful in protection of public health and welfare.

Methodology
2.1.Description of the Sampling Sites.Dammam metropolitan area is the largest metropolitan area in the EP of Saudi Arabia; it is formed by three main neighboring cities: Dhahran, Khobar, and Dammam.It has an estimated population of ∼3 million (https://www.stats.gov.sa/).These three cities are known for their major chemical, petrochemical, and oil industries.The area also includes a variety of natural landscapes, parks, and beaches with a recreational coastline to the Arabian Gulf.
Three (3) locations were selected in each city to collect air samples using PM 10 filter.The samples were collected over a period of 24 hours for 7 days at each location, as shown in Figure 1, from Oct. 10 to Dec. 12, 2015.Detailed latitudelongitude, elevation, and residential and traffic conditions for each site are displayed in Table 1

Sample Collection.
Air samples were collected using HiVol 3000 air sampling device (Ecotech, Australia) with PM 10 size selective inlet.The sampler was set to a steady sampling speed of 50 liters/mins, with a volumetric flow rate ranging between 45 and 96 m 3 /hr and having a flow precision and correctness exceeding ±1 m 3 /hr.The flow repeatability was 1.0% of the reading and the capability of the vacuum is 140 bars.The particles (PM 10 ) were collected on 8 × 10inch Pallflex EMFAB-TX40H120-WW filter membrane made with glass microfibers of borosilicate origin that have been protected with laced glass cloth.

Sample Preparation for Trace Metal Analysis.
Onequarter of the collected filter was cut into small pieces and weighted using Mettler Toledo JL602GE analytical digital balance after which the filter pieces were put into a 100 mL beaker and sprayed with distilled and deionized water.Aqua regia solution was prepared using distilled nitric acid (HNO 3 ) and concentrated hydrochloric acid (HCl) (ratio 1 to 3).More distilled water was added to the mixture until the mark and was then stirred inside the fume hood.The solution, including the pieces of filter sample, was then heated in an oven for 3-4 hours at a temperature of around 130 ∘ C until it reaches near dryness after which the solution was cooled down to room temperature.The mixture was then filtered with size 42 Whatman filter paper.The volume of the filtrate was then topped up to 50 mL using water.
The acid digest (filtrate) was finally analyzed as per the work of Sadiq and Mian [19].A total of 9 samples were analyzed for each sampling site along with 2 blank filter samples for QA/QC work.

Gravimetric Analysis.
Masses of the airborne PM collected in different locations were gravimetrically determined by calculating the difference in weights of the filter before and after sample collection.The filters were weighed using the Mettler JL602GE scale which has a maximum weighing capacity of 620 ± 0.01 g.In order to remove humidity effect and to obtain accurate PM measurements, filters were stored in silica gel desiccators for 24 h prior to weighing.Field blank filters were also collected to reduce gravimetric bias due to filter handling.Filters were handled with Teflon tape-coated tweezers to reduce the possibility of cross contamination.It is assumed that the particulate deposited on filter papers were uniformly distributed over the entire area of the filter.The average of three (3) weight readings were recorded for each filter (sample).The volume of air pumped by the sampling machine during the sampling period was also recorded and the mass of the PM was calculated in g/m 3 .

Morphology and Mineralogy Characterization.
The morphology (size and shape) of PM was identified using the TESCAN Lyra3 FESEM unit (Czech Republic).One-fourth of each of the filter samples was taken for the analysis, from which 1 mm 2 used for gold coating was at the center and set on tiny stubs for the coating.Ion sputtering was used to place a very thin gold membrane on the samples already placed on the stubs and images were taken for each sample subsequently.The energy dispersive X-ray (EDX), coupled with the FESEM, was also used for spot elemental analysis of the samples.X-Ray Fluorescence (XRF) spectrometer was used to obtain the elemental constituents of the sample.The XRF model used was JSX-3400RII from JEOL Co., Japan.JSX-3400RII with X-ray generator of 5 to 50 kV 1 mA 50 W is the general purpose XRF instrument.It utilizes Silicon (Lithium) detector which has a huge sensitivity to heavy metals and can measure the trace metals within a small time.

Trace Metals Analysis. A dual-view PerkinElmer
Optima 8000 Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES) with full-wavelength-range custom designed CCD array detector was used in this study to determine the concentration levels of the trace metals associated with the PM.After PM sampling, the filter paper was cut into 4 equal quarters.One-quarter was weighed, then further cut into small pieces, placed into a 100 mL glass beaker, and wet with water.A 3 : 1 (HNO 3 /HCl) aqua regia solution was used to digest the filter paper sample.The beaker that contains the filter paper and the aqua regia solution was placed on a heater for 3-4 hours at about 130 ∘ C until it reaches near dryness.The beaker was then cooled down to room temperature and their contents were filtered with Whatman filter paper (size 42), and the volume of the filtrate was made up to 50 mL with distilled water.The digested (filtrate) was used for heavy metal determination using ICP-OES.The digestion procedure is similar to that proposed by Sadiq and Mian [19].Similarly, for each of the 9 samples analyzed 2 blank samples were also prepared and analyzed in a similar way.

Levels of PM and Their Distribution.
Results show that the PM 10 concentration in the three cities is within the range of ∼127-380 g/m 3 .The average PM 10 measured at various sampling sites within Dhahran, Khobar, and Dammam is shown in Figure 2. In Dhahran, an overall mean daily PM 10 concentration was approximately 177 g/m 3 and this is above 50 and 150 g/m 3 , World Health Organization [20] and United States Environmental Protection Agency [21]   standards, respectively, but less than PME standards (350 g/ m 3 ).Dhahran locations 1 and 2 recorded an overall minimum and maximum PM 10 concentration of about 67 and 303 g/m 3 , respectively [22].As shown in Figure 3(a), PM in the 3 samples' collection sites in Dhahran showed a trend which suggests that variations occur in the PM 10 measurement obtained for all the period of samples (7 days), and the largest level was recorded in location 1 on October 5, 2015.The high PM 10 concentration could be attributed to high wind conditions and widespread dust observed on Oct. 5 as shown in Table 2.
In Khobar, Figure 3(b) the average concentration of PM 380 g/m 3 was recorded and this is more than the standard level set by USEPA, WHO, and PME which is 150 g/m 3 , 50 g/m 3 , and 350 g/m 3 , respectively.The minimum PM 10 concentration of 94 g/m 3 was recorded in Khobar (location 2) on Oct. 23, 2015, while a maximum PM 10 concentration of 1575 g/m 3 was recorded in location 1 on Oct. 22, 2015.Table 2 shows that Haze-widespread dust-mist-cloudy weather conditions were recorded on Oct. 22, 2015.This high widespread weather conditions could be responsible for the high PM 10 concentration observed on Oct. 22.The dissimilar trend observed in Khobar location 3 (Figure 3(b)) also indicates that temporal variation affects the concentration level of PM 10 .Hence, particulate matter concentration could be affected by temporal and spatial variation.
As observed from Figure 3(c), Dammam PM 10 sample has an average concentration 127 g/m 3 , and this is lower than the standard set by both USEPA (150 g/m 3 ) and PME (350 g/m 3 ) but above the standard set by WHO (50 g/m 3 ).In Dammam, the maximum concentration of PM 10 was recorded on Nov. 18, 2015, at location 1 (233 g/m 3 ), whereas on November 23 at location 2, the lowest PM 10 concentration of 57 g/m 3 was recorded.It is important to note that the comparatively smaller temperature and serene weather condition witnessed in Dammam during the sampling period (Table 2) may be the reason why the PM concentration measured in the city is lower when compared to other cities investigated for this study.
As it is mainly a residential area, Dhahran records lower PM 10 concentration than Khobar, which is a high traffic and business district.It was expected that the city of Dammam would have higher PM 10 concentration than Dhahran as it is a highly populated district hosting the main port and airport of the Eastern Province; however, results showed that the PM 10 concentration in Dammam was lower than in Dhahran and Khobar.This could be attributed to clear weather conditions reported in Dammam during sample collection.In order to identify local weather impacts on PM 10 concentration, simultaneous sample collection should be performed over the three cities considered in the study, but as this study is limited to use only one air sample collector, we could not perform simultaneous measurements.Based on the collected air samples, it is concluded that the mean daily PM 10 concentration around the city of Khobar was the highest among the three cities.(0005-1000 m).The shape of the PM may vary according to the source, type, or mode of formation from spherical (pollen) to irregular (dust) to platy (clay particles).The FESEM micrographs present in Figures 4(a)-4(d) show the morphology of the particulate matter collected from Dhahran, Khobar, and Dammam cities, respectively, along with an image of a blank filter prior samples collection.PM 10 samples from Dhahran have size range between 2 and 6 m and formed agglomerates, PM 10 samples collected in Khobar were between 1 and 4 m and dense, while those from Dammam shows that the samples are dispersed and their size is between 1 and 6 m.The images show that the size of particles from all the sampling cities is below 10 m.

Morphology
Based on locations, there are different particle shapes.The shape of particles collected from Dhahran is mostly rodlike and platy in shape and from Khobar is mostly irregular in shape while particles collected from Dammam are nearly spherical.Images of the particles from the 3 cities suggest that the particles are predominantly from geogenic sources such as sea spray or windblown dust.The difference in particles' shape and size indicates that their physical properties are spatially dependent.

Elemental Composition of PM Samples.
In order to assess the variations in the levels of elements collected in the 3 cities as considered in this study, EDX and XRF analysis are used and the result is shown in Figures 5(a)-5(c).It is important to state that XRF analysis cannot detect elements lower than Na in the periodic table.It is also important to understand that both the EDX and XRF analysis represent the ratio of the elements in the filter sample tested and not the actual concentration.In order to make a comparison between EDX and XRF data, we determined the ratio between each element and the Si as a reference element as it was detected with a There is a high likelihood of the presence of calcite and silica in the samples due to large percentages of C, Ca, O, and Si found in the three cities samples.
The results of this finding show that spatial or temporal disparities do not affect the type of elements observed in sample while affecting their percent composition.This is consistent with outcome of earlier studies [13,23,24].3.As has a concentration of <0.01 g/L in the 3 cities, which is below the allowable limit of As concentration in ambient air [25].The concentration of Ba in all the cities considered for this study is high, and the highest level was recorded in Dammam (10.13 g/L) while 5.11 g/L and 8.91 g/L were recorded in Dhahran and Khobar, respectively.The most dangerous pollutant for organisms, Cadmium, has a concentration lower than 0.005 g/m 3 in all samples collected, which is lower than United States' typical industrial and urban area [26].In all sampling sites, Co was present at varying concentrations.Dhahran and Khobar samples have Cd concentration that is below 0.001 g/L, while Dammam samples have Cd concentration below 0.002 g/L.According to FDA 2013, the maximum allowable limit of Co is 0.005 mg/L; hence, this is not considered a significant threat.In Dhahran, Khobar, and Dammam, the concentration of Chromium (Cr) was found to be 0.06, 0.07, and 0.06 g/m 3 , respectively.Cr concentration is above the threshold recorded in metropolitan and industrialized areas in a typical ambient air in the United States [27].The concentration of copper was found to be 0.02, 0.03, and 0.01 g/L Dhahran, Khobar, and Dammam, respectively.Manganese (Mn) has a concentration of 0.01 g/L, 0.06 g/L, and 0.11 g/L in Dammam, Dhahran, and Khobar, respectively.The concentration of Mn in PM samples from Dhahran and Khobar is relatively higher than what had been reported in previous studies [28].In Dhahran and Khobar, the concentration of Molybdenum (Mo) is less than 0.03 g/L while it is below 0.005 g/L in Dammam.The concentration of Nickel in Dhahran and Khobar is 0.02 g/L whereas it is 0.01 g/L in Dammam.Based on EU and USEPA standards, the concentration of Ni in Dhahran and Khobar is the highest level allowed outdoors (ambient air).Vanadium concentration in Dhahran, Khobar, and Dammam was found to be 0.05 g/L, 0.07 g/L, and 0.06 g/L, respectively.Zinc concentration was very high in all three cities, with the maximum level measured in Khobar (12.64 g/L) while the concentration of 8.98 g/L and 8.87 g/L was recorded in Dhahran and Dammam, respectively.Dhahran, Khobar, and Dammam have titanium concentration of 0.66 g/L, 0.99 g/L, and 1.47 g/L, respectively.The lead was found at a very low concentration (<0.001 g/L) in Dammam but not detected in Dhahran and Khobar air samples.

Conclusion
Characteristics of particulate matter with an Equivalent Aerodynamic Diameters (EAD) below 10 m in tricities of Dhahran (residential district), Khobar (traffic hub), and Dammam (residential district, seaport/airport hub), Saudi Arabia, were determined.It was found that the average daily concentrations in Dhahran, Khobar, and Dammam are ∼177 g/m 3 , 379 g/m 3 , and 126 g/m 3 , respectively.Particles shapes were found to vary from irregular, platy, spherical, rod-like, and angular.Different elements like Al,

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Ba, C, Ca, F, Fe, K, Na, O, and Si were detected varying percentage by weight of the samples.Cr, Zn, Ni, Ti, and V measurements show concentration above the maximum allowable limit; however, other trace metals were found below the maximum allowable limits.It was found that the spatial distribution of the PM measurements affects the form, shape, and size of airborne PM but not the elements they contain.

Figure 1 :
Figure 1: Map of sampling site.

Table 3 :
Trace metal levels in PM samples collected from Dhahran, Khobar, and Dammam.