Recognition of the Source and Nature of Atmospheric Aerosols in Tehran, Iran

The size, morphology and chemical characterization of individual atmospheric particles have received significant attention due to their effects on radiative and chemical properties. Tehran is one of the most polluted cities not only in Iran but in the world, which is continuously affected by air pollution. In this study, 25 sites were sampled in urban and nonurban areas. The sampling was performed with a high-volume sampler for a period of 14–24 hours. Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDS) were used to characterize the aerosols in the urban and non-urban areas during each season. The SEM results indicate that during summer, flaky, irregularly shaped aggregate particles dominated the urban areas, whereas non-urban areas were relatively clean. During winter, the non-urban sites were observed to be dominated by spherical and irregularly shaped particles, suggesting combustion sources, whereas particles were observed in a more concentrated form at the urban sites. EDS analysis shows varying percentages of C, O, Zn, Si, Ca and Fe at both the urban and non-urban sites for all seasons. Furthermore, this study reveals the variability of the morphological and elemental composition of PM in different seasons at urban and non-urban locations and highlights the various probable sources associated with them.


INTRODUCTION
Industrial development, rapid population growth, urbanization, transportation development, and increasing fuel consumption are the main factors that contributed to making pollution in the world.Air pollution is a major problem of recent decades, which has a serious toxicological impact on human health and the environment (Waked et al., 2014;Balasubramanian et al., 2017).The particulate matters (PM) are the most important air pollutants in the world's major cities, according to the United Nations Environment Program.The particulate matters referred to as particles that are dispersed (liquid or solid) in a gas medium.The suspended particles are of both human origin and natural origin.Their anthropogenic sources are fossilfuel vehicles and dust from human activities (Superczynski and Christopher, 2011;Molnár et al., 2016;Jing et al., 2017).According to the World Health Organization, the reduction of suspended particles from 70 to 20 micrograms per cubic meter, air pollution deaths can be reduced by 15 percent (WHO, 2016).Several studies on the phenomenon of dust have been conducted on the factors affecting occurrence to determine the distribution areas.In some studies, the relationship between climate, vegetation and the occurrence of dust storms is expressed using satellite imagery, which indicates characteristics of the earth's surface can be effective in storm surges (Zou and Zhai, 2004;Superczynski and Christopher, 2011;Jalali et al., 2012;Verma et al., 2017).
Also, in recent years, morphology, size and chemical composition characterization of individual atmospheric particles have received significant importance due to their effect on chemical and radiative properties.A detailed characterization of individual atmospheric particles also provides useful information about their sources, formation, atmospheric history, transport, reactivity and removal of atmospheric chemical species (Lu et al., 2006;Adachi and Buseck, 2010;Li et al., 2010a).In the past, several investigations have been carried out on the measurements of size distribution of aerosol and associated elemental concentration in urban areas (Krejci et al., 2005;Lu et al., 2006;Zhang et al., 2006;Iordanisdis et al., 2008;Cong et al., 2009;Li and Shao, 2009;Li et al., 2010b;Moffet et al., 2010;Posfai and Buseck, 2010;Hu et al., 2012).Commonly, for study particles matter is used scanning electron microscopy with energy-dispersed X-ray analysis (Li et al., 2010b).It provides useful information on the morphology, elemental composition and particle density of aerosols and also gives us a better insight about the origin of particles that whether emitted from anthropogenic or the natural processes (Conner and Williams, 2004;Bernabe et al., 2005;Cong et al., 2010).Identify the origin of particles always was important with the use of the SEM study and has been considered.Ramirez-Leal et al. (2014), in Hermosillo, Sonora, showed some of the elements presented are directly related with human activities, and are of much interest from the public health and environmental perspectives.Research of Vineyard et al. (2015), indicate significant concentrations of sulfur in small particles that can travel great distances, and that this sulfur may be in the form of oxides that can contribute to acid rain.Since the average composition obtained by the traditional bulk analysis only gives us the overall information about the elements or ions present.Therefore, the elemental composition of individual particles is more useful than bulk analysis in determining their sources, formation, and influence on climate and human health.But there are not many studies on the field in Iran.Tehran is one of the most polluted cities in the world, and air pollution one of the greatest challenges for its inhabitant.The increase in the frequency of dust storms, especially in recent years, has led to an increase in the high level of suspended particles.Due to the presence of many human pollutants, Tehran's main air pollutants include CO, NO x , SO 2 , O 3 , HC and PM that 85% of car fuel and the remainder are created by factories and homes heating equipment (Givehchi et al., 2013).The high density of cars, high buildings, quiet air and no wind has increased the concentration of air pollutants at a dangerous level and has made air pollution one of the most important environmental problems in Tehran.Air pollution is considered as one of the environmental problems of Tehran's citizens and solving the air pollution problem requires more effective programs.So, in this work, we have tried to study the origin, chemical composition, and particle morphology by studying PM in the atmosphere of Tehran.We can make new control decisions by understanding the nature of these particles to reduce their concentrations.

METHODS
A high-volume sampler was used to collect daily random samples of PM, where an air sample is drawn for 14-24 hours at a constant flow rate of 1.5 m 3 min -1 through a size-selective inlet and then collected on a fiberglass membrane (with the size of 8 × 10 inch).The filters were conditioned and weighted prior and after sampling to determine net weight gain due to the collection of the sample and eventually estimate the concentration.The aerosol samples were collected at an interval of 1 year from 2017 to 2018.Total 25 samples including 15 points in the urban areas and 10 points in the non-urban areas were collected in four seasons.The distribution map of the sampling points is shown in Fig. 1.Before exposure, the quartz fiber filters were pre-heated in a muffle furnace at 500°C for 3 h to remove organic impurities.Before and after sampling, the filters were equilibrated in the desiccator for 24 h and then weighed on an electronic microbalance to determine the particulate matter mass.
Preliminary SEM/EDS measurements were performed on some of the samples to get elemental information on individual aerosol particles; also particles were imaged with a Zeiss EV050XP SEM.The X-ray energy spectra were measured using a Bruker Quantax 200 EDS system with a Peltier-cooled X Flash silicon detector.The aerosol samples were coated with a thin layer of conductive material before the measurements were performed.The images of the samples were taken at a magnification of 500×, 1500×, 2500×, 5000×, 7500×, 10,000× and 50,000×.These selected magnifications allow analyzing the chemical and morphological parameters of particles in the entire particle size range considered.Spectra of individual particles were obtained after scanning an electron beam with an accelerating voltage of 25 kV for determination of the individual elemental chemical composition of the particles.

STUDY AREA
Tehran has a population of about 10 million people and an area of about 800 km 2 on the southern slopes of the Alborz mountain range.Also, from the southern part of Tehran, leads to open plains and agricultural fields as well as Qazvin plain.The Tehran Plain, located in the south of the Alborz mountains at an altitude of 900 to 1500 meters, is one of the main gatherings and human activities in the province of Tehran.In Fig. 2, the area is shown.Based on the Air Quality Index (AQI) calculated by the methods described in the Iranian National Standards in 2016, the period under study consisted of 17 clean days (AQI < 50), 260 days with healthy conditions (50 < AQI < 100), 80 days of unhealthy conditions for sensitive groups (100 < AQI < 150), and 9 days of unhealthy conditions (150 < AQI < 200) (AQCC, 2017).In total, the number of polluted days decreased by 22 days compared with the last year.Fig. 3 provides a summary of the air quality categories during this time interval (AQCC, 2017).The individual examination of source apportionment in the emission of each pollutant shows that mobile sources contributed to producing 70%, 6%, 98%, 86% and 47% of the city's PM, SO x , CO, VOCs, and NO x respectively, and the remainder was generated by stationary sources.Table 1 showed the annual emission of air pollution in Tehran (AQCC, 2017).According to reports, particulate organic matter (OM) was the dominant component during most of the year, with a contribution of 13-54% and an average of 35%.Organic matter (OM) and elemental carbon (EC) together comprised 44% of fine PM on average, reflecting the significance of anthropogenic urban sources, i.e., vehicles.Most of the organic carbon (OC) was formed from water-insoluble compounds (82.5 ± 4.3% on average), suggesting a large contribution from the incomplete combustion of fossil fuels (Oroji, 2018).
On the other hand, dust oxides dominated during the hot and dry months of June to September and comprised 32-56% of PM 2.5 .Fig. 4 showed the contribution of major mass constituents to PM 2.5 in the study area.The levels of major heavy metals, such as Cu, Cd, Ag, Ni, V, Sb, Tl, Pb, As, Rh, Sn, and Cl, were highly influenced by anthropogenic sources, such as industry, residual oil combustion, and vehicles.The levels of these heavy metals increased during cold seasons (Oroji, 2018).The determined major mass components of PM 2.5 together accounted for 89% of the particulate mass on average.The determined dominant   The effects of meteorological parameters on Tehran's air quality are significant, and this amount is from +5% to -5% (Oroji, 2018).Fig. 5 shows the impact of a meteorological station in the southwest of the region.The western winds are the prevailing winds for all seasons and the highest wind speed is observed in spring, which is associated with the best air quality in comparison with the rest of the studied year (Oroji, 2018).According to the result of the studies in Tehran city concentrations of criteria air pollutants are well beyond the national standards, and the WHO standards (Amini et al., 2014(Amini et al., , 2016)).The most air pollution in 2014 originated from mobile sources, including 97% of carbon monoxide, 86% of volatile organic compounds (VOCs), 70% of particulate matter and 46% of nitrogen oxides (Oroji, 2018).Also, the rest of 14% of VOCs in 2013 emitted from stationary sources including energy conversion 1.6%, industries 0.3%, gas stations 10.5%, terminals 0.15% and households and commercials 1.4% (Hosseini and Shahbazi, 2016;Shahbazi et al., 2016b).

RESULT AND DISCUSSION
Several studies in this field have been conducted in India by Badarinath et al. (2009), Central Europe by Von Bismarck-Osteal et al. (2015), Hyderabad (India) by Sandrini et al. (2016) and Italy by Amato et al. (2016).In these studies, the physical and chemical differences in the aerosol particles in the urban and non-urban environment have been pointed out and have revealed the fundamental differences between these particles.Monthly variations of PM mass concentrations are given in Table 2.The highest mass mean concentrations were observed for all size fractions in October primarily due to the atmospheric inversion phenomenon.The pattern of TSP during September to April is consonant with the wind speed pattern, which indicates that there is a notable influence of wind speed on large particulate levels.As expected, the lowest PM levels were recorded during spring season as a result of the humid and occasionally rainy conditions.Based on the results of samples taken in the urban areas, apart from the dominating elements were determined elements such as calcium, iron, chlorine, and potassium-rich particles, silicon, aluminum, and magnesium.It is trend represents the presence of soot particles in the case study of the area.Fig. 6 shows an SEM image of the aerosol particles in the area.As well the results of studying the images of SEM in particles PM < 1 micron showed that these particles show irregular, spherical, fine-rod-like and crystalline shapes.From the SEM image of the particles, it is observed that the particles of the size 2.5 microns show spherical, irregular and cluster shapes.In these SEM images inferred Ca as the most dominated particle among others, which could be considered as the form of calcium carbonate related to the calcite phase, CaCO 3 .In the winter season, the concentration of O, C, and Si was comparable in the non-urban and urban areas.While in summer, the concentration of Zn and Ti was observed to be higher in the urban area than the non-urban area.In non-urban areas, Cu concentration was observed to be higher in summer season than winter whereas, in urban areas, was found to be slightly higher in winter than summer.Table 3 shows chemical composition of particles in the southwest region of the study area.Major elements found in the PM 2.5 aerosol   Morphology and shape of the particles are generally irregular and coarse.Elements such as Zn, Ti, Fe, and C were observed in varying concentrations in all over the area, especially in the northeastern regions of the study area.Also, these results show the indicating the presence of anthropogenic sources in the composition of the particles.Fig. 7 shows a scanning electron micrograph of the particles.Due to the fact that in the studied area winds and winds in the direction from south to north and southwest to northeast, the concentration of particles resulting from combustion of cars is always in the eastern regions more than western regions.However, due to atmospheric stability in most of the year in the region, the concentration of air in the central areas is more than other areas.The study of particle type, combustion of fuel by moving resources has the highest volume of suspended particles in the region.
Table 4 shown concentrations of elements of aerosol particles produced by stationary sources in the area.The elemental composition of these particles indicates that Zn, Ca and K-rich particles were dominant.The other dominated elements were O, Si, and Al.This could be inferred as the fly ash origin.These particles can be related to originating from construction activity, regional transport from the urban zones as well as agricultural vegetative burning and natural dust.These particles are composed primarily of feldspar (Ca, Si and Al) and clay (Al, Fe and Si), their origin is mainly crustal, but they can also come from erosion of building products and road dust.Other elements are present in minor concentration in the alumino-silicate particles and they are Mg, Na, Ti, and Zn.Due to the large volume of construction in different parts of Tehran and the lack of management in the activities of sand and gravel factories, its suspended particles are high in the atmosphere of the region.Since the location of these factories is located in the dominant wind direction in the region, hence the emission of particles from these sources is always one of the main particles forming the atmosphere of Tehran.Fig. 8 shows the SEM micrograph spectrum of aerosol particles in the study area, it is obvious that the particles show irregular, spherical and cluster shapes.The particles   Ca, Fe, K, O, Si and Al follow the similar trend.Table 5 showed concentrations of elements aerosol particles in central regions of the study area.All of these images inferred Ca as the most dominated particle among others, which could be considered as the form of calcium carbonate related to the calcite phase, CaCO 3 .Fig. 9 shows the SEM micrograph spectrum of aerosols in the size range PM 2.5 .The lack of suitable vegetation and the occurrence of droughts in the marginal areas of the region is the main source of particle emissions in the western parts of the region.With the inflow of wind from these areas to the region, the concentration of suspended particles in the atmosphere of the region increased and during the occurrence of intense currents in the region a large volume of particle concentration is included.Table 6 shown chemical compositions of aerosol particles produced in the western regions of the study area.Fig. 10 is the SEM micrographs of the PM 10 in    10 is an SEM image of aerosol particles and also concentrations of elements in the suburbs area shown in Table 7.In addition to mineral particles released by wind currents in the atmosphere of Tehran, there are other sources in the region that include the main suspended particles in the western and central parts of the region.These particles are formed from burning biomass in the marginal areas of Tehran and are flowing through the winds in the western and southwestern parts of the region and extending to the northern and northeastern areas.
The number 126 images were taken of PM 10 particles where its morphology and chemical composition of each particle individually was analyzed by SEM-EDS.The results show compositional data of elemental chemical constituents and atomic percentage (%) from PM 10 particles which showed the following elements: Al, Ca, Cl, Fe, K, Mg, Na, Pb, Si and Ti; and the predominant elements were Al (12.71%),Si (34.22%) and Fe (7.31%).As shown in Table 8 very strong to moderate inter-correlations are found between PM of different size fractions.The weak correlations are between PM 2.5/10 and temperature.The moderate correlation between TSP and wind speed is noticeable by the influence of wind on the diurnal variations of TSP.Also, weak Due to atmospheric conditions in the region, the suspended particles in the atmosphere are divided into two major groups: urban particles that are the result of combustion of fossil fuels in the region.In the second group, resources are generated outside of the urban boundaries and in the marginal areas of the study area, which mainly consist of particles from dust storms, biomass, as well as moving and fixed resources.Land use change in the western parts of the region and the conversion of vegetation cover and rangelands to residential and industrial areas is the main reason for the production of pollutants in these areas.The shape of these particles is irregular due to the origin of the mineral and due to the presence of biomass particles; they are regular and sometimes spherical forms.Carbon and silica are mainly found in the composition of these particles.This illustrated the presence of feldspar and clay minerals.Al and Si might be the form of alumino-silicates which in tum might include kaolinite, illite, and feldspar.

CONCLUSIONS
Studying the chemical composition and morphology of suspended particles in the Tehran atmosphere reveals both mobile and stationary sources.Due to wear and tear, the combustion of fuel in mobile sources, such as cars, motorcycles and city bus lines, emits suspended particles at levels that exceed global standards.Additionally, emissions produced by stationary and biomass-burning sources in the study area to the south are transported by wind currents from the west and southwest to the central part of the region.Although the wind may remove particles from the eastern and northeastern parts of the region, atmospheric stability in the winter and autumn and the occurrence of a temperature inversion phenomenon on most days generally increase concentrations at near-ground levels, resulting in critical levels of pollution for residents of the city.As mobile sources are the primary contributor of suspended particles in this region, controlling the movement, limiting the activity, and managing the emissions of these sources at the municipal level can prevent air pollution from reaching hazardous levels.

Fig. 1 .
Fig. 1.Map the sampling in the study area.

Fig. 4 .
Fig. 4. The contribution of major mass constituents to PM 2.5 in the study area.

Fig. 5 .
Fig. 5.The impact of the dust phenomenon on PM 2.5 and PM 10 concentrations.

Fig. 6 .
Fig. 6.Scanning electron micrograph of aerosols particles in the area.

Fig. 7 .
Fig. 7. SEM images of aerosol particles produced by stationary sources in the area.

Fig. 8 .
Fig. 8. SEM images of aerosol particles in central regions of the study area.

Fig. 9 .
Fig. 9. Scanning electron micrograph of aerosols in the size range PM 2.5 .

Fig. 10 .
Fig. 10.Scanning electron micrograph of the PM 10 in the suburbs area.

Table 1 .
Annual emission of air pollution in Tehran.

Table 2 .
Monthly concentrations PM 2.5 and PM 10 (µg m -3 ) during the study period.

Table 3 .
Chemical composition of particles in the southwest region of the study area.
are Si, O, C, Mg, Ca, Mn and K.In general, common major crustal elements found in atmospheric particles over all the sites are Si, Al, and Fe.Mineral particles usually derived from natural sources such as resuspension of dust from roads, soil dust crust.Other particles produced by anthropogenic activities are such as fuel combustion by vehicles, particles produced in various industries, roads and urban development and abrasion of metallic materials.

Table 4 .
Concentrations of elements of aerosol particles produced by stationary sources in the area.

Table 5 .
Concentrations of elements in central regions of the study area.thus indicate mainly the fine rod-like crystalline shapes, spherical as well as irregular shapes.Ca, Fe, O, Zn, Si, and K were more dominated followed by the Al, Na, and Mg.According to the EDS Al is associated

Table 6 .
Concentrations of elements the western regions.Mg, O, Na and Ca.Shown in Fig.

Table 7 .
Concentrations of elements in the suburbs area.

Table 8 .
Pearson correlation coefficient for aerosol and meteorological parameters in the study period.
correlations are between PM 2.5/10 and humidity.According to the results, the highest average concentrations of airborne particulate are associated with the wind coming from the west and southwest directions, where heavy highway traffic flow exists (resuspension of surface dust).