Current World Environment

Introduction

Particulate matter is complicated mixture of fine particulates and liquid droplets. Its contains acids, organic chemicals, metals and dust particles. Due to rapid urbanisation and industrialisation, the aerosol load in the atmosphere has reached a threshold limit in most of the cities in India¹. Uttarakhand state in the northern part of India has high tourist activity due to its diverse landscape, religious places and high biodiversity². Dehradun city in the state of Uttarakhand is one of the major tourist cities in India; more than one million tourists visit the city every year. Although it has only small-scale industries, it is facing serious air quality problems³. In the recent years, there is a steady rise in the number of vehicles in the city. Particulate matter (PM) can be composed of suspended soil, street dust, pollen, sulfate, nitrate, ammonium, calcium carbonate, sea salt, organic and elemental carbon⁴. Determining the chemical characteristics of atmospheric particulate matter is important for elucidating the toxicity of the particles. Substantial variation in the concentration of particulate matter with seasons was reported earlier in several cities in India^{4, 5, 6}.

One of the most commonly used indicator for PM monitoring is PM₁₀ (particles that are 10 µm or less in mass median aerodynamic diameter are referred to as the PM₁₀). Heavy metals associated with respirable particles could reach the lungs and may be able to cause severe health problems⁷. Particularly in males, lung cancer mortality and non-malignant respiratory mortality was significantly associated with ambient PM₁₀ ⁸. Previously, several researchers have used SEM-EDS for determining the morphology and elemental composition of the particulates^9,10.

The aims of the present study has been attempted to (a) assess the chemical characteristics of particulate matter, (b) to determine the morphology and elemental composition of the particulates.

Materials and Methods

Study Area

Dehradun (30.00 °N to 30.50 °N and 78.30 °E to 78.60 °E) is a valley situated in the Shiwalik range of the Himalayas with elevation of 600-800 average mean sea level (amsl) (fig.1). The meteorological data of last two years showed that the average annual temperature of Dehradun was 29.2ºC. In addition, Dehradun received an average annual rainfall of 2456.1 mm (average value for the year 2012 and 2013). According to 2011 census, Dehradun district had a population of 1,696,694 and population density of 550. The PM₁₀ concentration and heavy metals associated with the particulate matter were determined in thirteen sites in Dehradun, the sites which have similar surroundings were clubbed accordingly and designated as institutional area (C and S10), commercial area (S1, S2, S4 and S5), industrial area (S11 and S12) and residential area (S3, S6, S7, S8 and S9).
 

Figure 1: Atmospheric PM10 Sampling Sites at Dehradun, Uttarakhand. Click here to view Figure

Location of Sampling Sites

Sample Collection

The respirable particulate matter concentration (PM₁₀) was determined using RDS (Envirotech, New Delhi) and EPM2000 grade filter paper (Glass fiber, Whatmann) was used for the study. The duration of sampling was 24 hours, from 6 AM on first day to 6 AM local time (LT) of the next day. The sampling was carried out during post-monsoon (post-m; December, 2012 and January, 2013), monsoon (mon; July and August, 2013) and pre-monsoon (pre-m; May and June, 2013). In each site nine samples were collected, in total 117 were collected (13 sites). The sampler was placed in an open area where there is free flow of wind. The filter papers were desiccated for 48 hours before sampling and weighted with electronic balance¹¹. The filter papers were kept in a polyethylene cover while carrying to the study site.

Chemical Analysis-AAS

The filter papers were desiccated for 48 hours after sampling and weighted in an electronic balance. One third of the glass fiber filter paper was cut and it was put in 100 ml teflon beaker and 5.55% Nitric acid (HNO₃) and 16.75% of Hydrochloric acid (HCl) were added^5,12. Reagents used were of AR grade. Subsequently, it was placed on a hot plate for two hours and filtered with whatmann-42 filter paper and further analysed with AAS (AAS4141A, ECIL India). For blank filter paper similar procedure was followed.

Morphological and Elemental Analysis-SEM-EDS

For determining the morphology of the particle, a 1cm² of the filter paper was cut and it was coated with Gold (Au) particles of thickness (15-30nm) for using a vacuum coating unit (Quorum Q150 RES, Germany) for making it electrically conductive and it was analysed with SEM, the positioning of samples and imaging were controlled by computer system¹⁰ . A portion of the subjected filter paper is focused and images were taken at three different magnification (fig. 3A, B and C) (2k´ with 10µm, 5k´ with 10µm and 10k´ with 1µm). For determining the elemental composition of the particulates, EDS was used at accelerating voltage of 20kV. A 1cm² of filter paper was cut and coated with carbon (C) particles of thickness (15-20nm) for 2-3 minutes through a vacuum coating unit. The filter paper was subjected to EDS through computer controlled field emission SEM (Carl zeiss SMT EVO series) equipped with quantax200 esprit 1.8.1 software.

Results and Discussion

Aerosol Mass Concentration

Table.1 shows the variation of PM₁₀ according to different seasons. The average mass concentration of particulate matter during pre-m was 216.5 ± 73.9 µgm^-3, post-m 245.4 ± 72.1 µgm^-3 and monsoon was 142.0 ± 59.5 µgm^-3. During winter season particulate matter concentration was found to be high at S4 (336.0 ± 42.5 µgm^-3). S4 showed maximum PM₁₀ concentration during pre-monsoon season (337.3 ± 50.5µgm^-3) and monsoon season (235.6 ± 57.2µgm^-3). The particulate matter concentration at C (control site) was found to be minimum during all the seasons, the minimum concentration of particulate matter may be due to tall trees and less vehicular movement. Along with C, S10 and S7 showed less concentration of PM₁₀ during all seasons. The average value of PM₁₀ concentration was found to be maximum at S4 followed by S1 and S12. ¹²have also reported this kind of trend, were the value of particulate matter was found to be maximum during the post-m season in comparison to pre-m season. ¹⁴ have mentioned that the high value of particulate matter in post-m season may be due to more energy consumption for industrial purpose or may be due to biomass burning for space heating¹⁵. Four times increase in particulate matter concentration during winter season from other seasons was documented by¹⁶. The average of concentration in particulate matter in all the sites during pre-m and post-m exceeded 24-hour permissible limit of 100 µg/m³ and during monsoon season 72% of the sampling days had violated the permissible limit¹⁷. The correlation coefficient between concentration of PM and vehicular count showed positive correlation with all the four categories of vehicles (r=0.7 with 2-w; r=0.5 with 3-w; r=0.7 with 4-w; r=0.4 with HV).

Table 1: Showing the Variation in Concentration of Particulate Matter and Heavy Metals with Different Sites.

Metallic Elements in Particulate Matter

Nine heavy metals viz. Cu, Zn, Fe, Mn, Co, Cr, Ni, Pb and Cd were analysed in PM₁₀ samples. S1 showed maximum concentration in Cu (10.05 µgm^-3), Zn (9.55 µgm^-3), Fe (16.0 µgm^-3), Mn (0.42 µgm^-3), Co (0.20 µgm^-3), Pb (1.29 µgm^-3) and Cd (0.066 µgm^-3). Concentration of Cr (0.56 µgm^-3) and Ni (0.58 µgm^-3) were maximum at S4 site (Table.1). The analysed heavy metals were observed in the order of Fe> Zn> Cu> Pb> Cr> Ni> Mn> Co> Cd. Previously, several researchers have reported maximum concentration of Fe and Zn among the analysed elements in PM₁₀^{12, 13, 18}. In addition, ¹⁹ showed that considerable amount of Fe can be released from fly ash from municipal waste. A study by²⁰ found Fe to be maximum in Raipur city. The toxic metals like Pb, Cd, Cu and Zn were observed in the traffic pollutants²¹. The particles with high concentration of Fe usually composed of Magnetite and Hematite¹³. On all the days of sampling the 24 hour concentration of Pb was found to be higher than the permissible limit in site S1 and S4¹⁷.

Enrichment Factor

Enrichment factor (EF) was widely employed to identify whether a metallic element has an anthropogenic source other than its natural source.⁶



Where El _sample is the element in sample, X means the reference element, X _sample means the concentration of reference element in sample, El _crust means the concentration of the element in the crust, X _crust means the concentration of the reference element in the crust.

The earth crust mean abundance of metals given in CRC handbook was used for calculating the enrichment factor²². Fe was chosen as a reference element²³. On the basis annual enrichment factor analysis, the chemical species were grouped into five categories.

Not enriched = EFs < 1; meagrely enriched = EFs is 1-10; little enriched = EFs is 10-50; moderately enriched = EFs is 50-500; highly enriched = EFs is 500-1000; very highly enriched = EFs > 1000. The measured enrichment factor varied in the range of 0.39 to 1128.8, which shows substantial contribution of anthropogenic activity on the heavy metal load in the ambient air (fig.2). In the category of non-enriched, only manganese (Mn) was found, as manganese is found in soil largely as MnO₂²⁴. The elements with moderate EFs were Co, Cr and Ni. The high EFs of Cu, Zn and Cd indicate high anthropogenic contribution.
 

Figure 2: Showing the Variation of Enrichment Factor with Different Sites. Click here to view Figure

To delineate the possible sources that contributed to the PM₁₀ in Dehradun city, principle component analysis with varimax rotation was applied. The components having eigen value greater than 1.0 were chosen for the identification of the possible sources^4,25. Table 2 shows the rotated component matrix for the collected PM₁₀ across the city of Dehradun for all the seasons. Three principle components were identified, explaining a cumulative variance of 81.75%. Factor loading greater than 0.5 for the individual metal in a component was considered as the emitted from the same source. First factor explaining the variance of 69.02 % was enriched with Mn, Co, Cr, Ni and Cd, which is possibly released from uncontrolled refuse and waste burning²⁶. Factor two relates the PM₁₀ with vehicular emission. Loading of Pb with Mn and Cd are correlated with the vehicular emission and vehicular induced re-suspension. Wear and abrasion of the vehicular parts enhances the loading of Mn and Cd in the atmosphere. Although unleaded gasoline is in use from long ago but its resistance to the environment after release marks its presence in the environment ²⁷. High enrichment of Fe with other trace elements are form dust re-suspension due to various human and natural activities like wind supported erosion of upper crustal dust, explains the third factor.

Table 2: Factor Loading of the Different Pollutants in Dehradun (Factor Loading >0.5 is Highlight in Bold)

Scanning Electron Microscope

For high resolution imaging, SEM was used. It employs high resolution electron beam and the images are dependent on the different quality of electrons emitted from the surface topographies. Other parameters are magnification: 7 to 1,00,000 ´; resolution 3.0 nm SE and HV; accelerating voltage of 0.2 to 30 kV; detector available: SE in HV. The morphology of the sampled particles were analysed using scanning electron microscope (fig.3 A, B and C), from the figure, it can be observed that the particles were rough, spherical and agglomeration types. The irregular shaped particles were attributed to natural and crustal source and can also be formed due to the result of reaction between the particles with gaseous or liquid phase.

SEM-EDS

Sampled filter paper during pre-m season from an institutional area (S10 site) was chosen for SEM-EDX analysis. It was observed that most of the particles were of natural origin. It was observed that O dominated over other elements which follows the trend as O>Si>Na>Ca>Mg>Al>S>K>Cl. The morphological studies show that the particles were irregular, spherical and agglomerate types. Among the elements, oxygen was found to have maximum percentage (53.98 % by wt) and chlorine (0.23 % by wt) was found to be minimum. Both qualitative and quantitative analysis was determined.

The elemental composition was determined using SEM-EDS (fig. 3 D). Oxides were present as silicon dioxide (Si0₂), aluminium oxide (Al₂0₃), magnesium oxide (MgO), sodium oxide (Na₂O), sulphur trioxide (SO₃), calcium oxide (CaO), potassium oxide (K₂O) and oxygen (O) and chlorine (Cl) was observed in the spectra (Table 3).

The Al and Si were associated with oxygen and the percentage of Al₂0₃ and Si0₂were 64.70 %. Cong et al. (2010) reported Alumosilicates as dominant in the examined filter paper and mentioned that it can include Montmorillonite, Illinite, Kaolinite and Feldspars. The morphology of Aluminosilicates were reported to be spherical and non-spherical²⁸. Ca and Mg were found in combination with oxygen. The contribution of CaO was 7.02 % and MgO was 3.81 %, these can be attributed to soil origin²⁹.

Table 3: Percentage Contribution of Various Elements Observed through the SEM-EDS System.

Figure 3: Micrographs obtained from scanning electron microscope through Au coating. Click here to view Figure

Conclusion

This study shows the diversity in the source of particulate matter including both of anthropogenic and natural origin. In most of the days of sampling, the 24-hour average mass concentration of particulate matter was higher than the permissible limit set by central pollution control board, New Delhi. EDS study revealed that the particulate loaded filter paper is composed mostly of oxides. Enrichment factor analysis shows substantial anthropogenic activity in the area. Elemental composition analysed through AAS revealed high concentration of heavy metals, particularly in two sites (viz) S4 and S1 (commercial sites). Source apportionment marked with the identification of the three important sources across the Dehradun city. Enrichment of toxic fraction of trace element form various anthropogenic sources is a matter of concern. So, this study would enhance the information on particular matters of chemical and morphological properties in a data deficit region i.e, India. In a long run such studies would also help in understanding the health effects of chemical components in the ambient air.

Acknowledgement

We are grateful to Dr. A K Tripathi (Scientist-F) for providing necessary support and infrastructure facility for our work. We extend our gratitude to Mr. Vinod Singhal, Director, Uttarakhand SPCB and Mr. N.K Juyal (Technical officer, WADIA) for providing technical support for SEM-EDS. Special thanks to Dr. Vijay Shridhar, Assistant professor, Doon University, Dehradun (Uttarakhand).

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