Seasonal Variation of Selected Metals in Particulate Matter at an Industrial City Kota , India

This study investigates seasonal variation in concentration of some heavy metals in suspended and respirable particulate matter (SPM and RPM) collected from five different zones situated in Kota city during both summer (March, April, May and October) and winter (January, February, November and December) seasons of 2011–2012. Mean concentrations of anthropogenic origin metals (Pb, Zn, Cu and Cd) were higher in winter and lower in summer with their relative abundance in order: Zn > Pb > Cu > Cd while reverse trend was observed for crustal origin metals (Ca, Mg and Fe) at all zones. Meteorological conditions such as temperature, relative humidity, wind velocity and wind direction during winter and summer were found affecting the metals concentration trends in different seasons. Wind roses indicate that the zones lying in predominant North-east wind direction from point source (KSTPS) in winter (25.74%) and summer (15.31%) faced higher metal burden following zone 1, which is suffering most owing to its closest location to the source. Statistical analysis by Pearson’s correlations, enrichment factor and principal component analysis indicates that coal based Thermal Power Plant is the major source of heavy metals besides other industrial activities in the study area. It is to be noted that because of higher residence time, significant concentration of Pb is found at all the zones in the city which, probably, has its origin in earlier vehicular exhaust as well.


INTRODUCTION
Particulate matter (PM) in the atmosphere is a complex mixture of elemental and organic carbon, mineral dust, trace elements and water (Lim et al., 2010).It originates from both the natural sources such as wind erosion, crushing, abrasion of surfaces, suspension and re-suspension of soil minerals, volcanic eruptions, dust storms and anthropogenic sources namely industrial processes, traffic sources, coal combustion etc. (Florence et al., 2012).Particulate matter has serious environmental impacts on climate, biogeochemical cycling in ecosystems, outside visibility and the health of living things (Colette et al., 2008;Chen et al., 2010).
Size, concentration, composition and toxicity of PM are important factors which can greatly affect the possible human health problems associated with their exposure (Hu et al., 2014;Kim et al., 2015).It is, particularly, the fine and ultra-fine PM that harbours the capacity to outsmart the natural defence functions within our respiratory system (Samara et al., 2003;Quiterio et al., 2004;Shah et al., 2006b;Pérez et al., 2008;Begum et al., 2010).The particle size appears to have a carrier capacity for associated chemicals.The fine and ultra-fine particles with their potential of entering the blood stream, carry along with them their constituents enabling them to cause all sorts of systemic effects.Among those constituents, metals such as iron, zinc, copper, cadmium and lead etc. have toxicological and carcinogenic effects (Gilmour et al., 1996;Moja and Mnisi, 2013).
Over the last few decades, intensive monitoring programs of heavy metals in atmospheric precipitations have been carried out worldwide, which are mainly focused on the chemical characteristics and long-term temporal trends (Halstead et al., 2000;Wong et al., 2003;Gabrielli et al., 2008;Garcia et al., 2009;Kyllonen et al., 2009).According to these studies, the loadings and sources of heavy metals in precipitation have great spatial variability over different locations, which is mainly caused by different meteorological conditions (prevailing wind directions and type, frequency, temperature, relative humidity and amount of precipitation) and the emission patterns of pollutants (emission sources, distance from emission sources and the sampling sites).
City Kota is having coal based Kota Super Thermal Kota has three distinct seasons; summer, rainy and winter.We have selected only two seasons (winter and summer) for particulate matter analysis because during rainy season, relatively high amount of rainfall and humidity is experienced causing very less amount of particulate matter so we did not consider the rainy season.Therefore, the present study has been conducted during both summer (March, April, May and October) and winter (January, February, November and December) seasons of 2011-2012 with the main objectives: i) to determine the composition of suspended and respirable particulate matter in terms of crustal (Fe, Ca and Mg) and anthropogenic (Cu, Cd, Zn and Pb) metals at various sampling sites located in five different zones covering entire Kota city area; ii) to identify possible sources of heavy metals associated with SPM and RPM using statistics namely enrichment factor, Pearson's correlation coefficient and principal component analysis; iii) to study the effect of climate on the concentration levels of heavy metals as a function of sampling sites, distance from KSTPS, seasons and meteorological parameters such as temperature, relative humidity, wind speed and wind direction.

BACKGROUND OF THE STUDY AREA
Kota is one of the major industrial city of Rajasthan state in India.It is situated at 25°11 N and 75°51 E on the eastern bank of river Chambal in the southern part of Rajasthan with an elevation of 271 meters (889 feet) above sea level on the South-east of Aravali ranges.The total geographical area of the district is 5,21,133 hectares as per land.Kota has a semi arid climate with temperature varying from minimum of 6°C in winter (January) to maximum 47°C in summer (June) and an average annual rainfall of about 885.6 mm.The monsoon season follows with comparatively lower temperature with higher humidity and frequent torrential downpours.It is concluded from the wind rose analysis that the prevailing wind directions in the city during this study are North-east in winter (25.74%) and summer (15.31%).Many large and small scale industries are present due to availability of river water and power supply.A popular cost effective building stone i.e., Kota stone is being excavated, cut to various sizes and polished in more than 200 units generating huge amount of slurry waste containing mainly CaO, MgO and SiO 2. In addition, Kota district is a major power production centre of the country having coal based KSTPS identified as point source for heavy metal pollution in the present study.

Sampling Sites
The sampling sites for atmospheric precipitation were chosen with the help of cartographic charts, field job and GPS (Global Positioning System).The choice of the sampling sites followed some criteria laid by ASTM D 5111 Standards (ASTM, 1996a).These criteria were: i) distance from point source (approximately a radius of 12 km from KSTPS); ii) predominant wind direction; iii) the distance from obstacles that could interfere in sampling (twice the height of obstacles); and iv) logistics (security, access, electric power supply).The entire city area was divided into five zones (Fig. 1).The location of the zones with respect to the point source KSTPS and other possible source of heavy metal in the study area are presented in Table 1.

Sampling Methods
A high volume sampler with respirable dust sampler attachment was used for the measurement of suspended particulate matter (PM 10-100 ) and respirable particulate matter (< PM 10 ).A total of 626 samples of both matter i.e., SPM (313 samples) and RPM (313 samples) were collected using above mentioned high volume air sampler model IPM 165 (Fig. 2) placed at about 6 meter height from the ground level at various sampling sites on 8 hourly basis and afterwards taking their average for 24 hour data (twice a week) during both summer (March, April, May and October) and winter (January, February, November and December) seasons of 2011-2012 on the clear day without wet precipitation activities.
The IPM 165 sampler first separates the coarser particles (larger than 10 microns) from the air stream before filtering it on the 0.5 micron pore size filter allowing the measurement of both suspended particulate matter and respirable fraction of this total suspended particulate matter (TSP).Ambient air, laden with suspended particulates, enters the system through the inlet pipe where coarse and non-respirable particulate matter is separated from the air stream by centrifugal forces acting on the solid particles.These separated particulates fall through a cyclone ' s conical Hooper and collected in the sampling pan placed at its bottom.The fine dust forming the respirable fraction of the TSP, is carried by the air stream to the pre-weighed filter paper of size 20 cm × 25.4 cm (with rough surface upwards) clamped between the top cover and filter adaptor assembly, to be retained by this filter paper.Afterwards, the carrier air is exhausted from the system through the blower.The folded filter paper is transferred to a clean petri dish and kept in the hot air oven for about 30 minutes followed by transfer to a dessicator for cooling.SPM samples (from pan) and the filter papers (re-weighed with RPM samples up to 4 decimals) were analyzed for their total metal content after digestion.

Chemical Analysis
For analysis of heavy metal content in SPM, 1 gm of sample was allowed to remain over night at ambient temperature in a covered beaker (to avoid the loss of Cd and Pb) containing a mixture of high purity HNO 3 (5 mL).After slow evaporation to dryness, 2 mL of HNO 3 was added and the solution was extracted with 0.1 N HCl.It is then filtered through pre-washed Whatman filter paper no.42 and diluted with 1% HNO 3 in a 50 mL polyethylene bottle (Rashed, 2008).
Each filter disc (containing RPM) was extracted using digestion reagents; hydrogen peroxide 35%, hydrochloric acid 37% and nitric acid 65% (Merck, Germany) following similar open acid digestion procedures as reported in literature (Loyola et al., 2009;Gharaibeh et al., 2010).Each filter paper was divided into four equal pieces using a clean paper cutter.One portion of the divided paper was further torn into small pieces and transferred to beaker to which 30 mL of HCl (1+1) and 5 ml of H 2 O 2 were added.The beaker was covered with a watch glass and heated on a hot plate at 120°C for 1 hour.After cooling, the solution was separated using a filter paper (Whatman 42).Another 20 mL of HCl (1+1) was added to the residue in the beaker and heated for 15 minutes for complete metal extraction.The beaker content was filtered again and the filtrate was added to the previously collected filtrate.This solution was concentrated on a hot plate till small amount of liquid left in the beaker.Then, HNO 3 (2+98) mixture was added to the beaker.The prepared sample was transferred to a 25 mL volumetric flask and filled up to the mark with HNO 3 mixture.After transfer to polyethylene bottle the digested samples were analyzed for their metal content.After digestion, the concentrations of 6 metals (Fe, Zn, Cu, Cd, Mg and Pb) were measured by Direct Air -Acetylene Flame method (Atomic Absorption Spectrophotometer -Shimadzu-6300).The Ca metal concentration was determined using Flame Photometer (Systronics -128) method.Certified standard solutions (CertiPUR * -MERCK) were used for calibrating the instruments.Blanks, quality control standards and standard reference materials were inserted during the analytical measurement to detect contamination and drift.The elemental concentrations of the blanks were lower than 1% of the mean analyte concentration for all metals and the precision (RSD) of the control standards and replicates were generally lower than 5%.

Statistical Analysis
The meteorological data during the measurement period [summer (March, April, May and October) and winter (January, February, November and December) seasons of 2011-2012] were collected from the Automated Weather Station (model number: DCPAWS02) mounted at the Kota Aerodrome, India.With the help of above meteorological data, monthly wind roses of study period were drawn using enviroware wind rose software.The results were summarized into a multielemental database using MS -Excel 2007.Analysis of principal components was performed with SPSS version 16 software.

Metal Analysis
Concentrations of crustal and anthropogenic metals in suspended (SPM) and respirable particulate matter (RPM) at five zones in Kota city were shown in Fig. 3.For both SPM and RPM, the concentration levels of crustal metals (Ca, Mg and Fe) were observed highest in Z 5 and lowest in Z 1 in the observed seasons.The activities related with the erosion and disturbance of upper layer of earth crust in zone 5 might be responsible for causing their higher concentrations while these activities were lower in Z 1 resulting in their reduced levels.
These metals (Ca, Mg and Fe), originating mainly from local soil, undergo a process of resuspension or mobilization and finally become incorporated in the local area.It is to be noted here that higher Ca metal concentration originates from carbonates generated from alkaline sources such as soil dust and earth crust whereas elevated levels of Mg metal can be the result of lime stone and mining activities carried out in the study area.Fe content is present in atmospheric particulate matter in the form of various oxides such as goethite, hematite, magnetite etc. indicating its origin from  earth crust.These findings of our study are in accordance with the results reported in earlier studies (Yusuf and Rashid, 1995;Banerjee, 2008;Teixeira et al., 2008).
The influence of fly ash emission (from KSTPS) and Kota stone factories could be seen from the higher values of crustal elements in comparison to anthropogenic elements at all zones.Furthermore, they could be original components from earth and dust (particulate matter) resulting in their elevated levels in the ambient atmosphere.
On viewing the results of anthropogenic origin metals associated with both SPM and RPM, among all the studied zones, zone 1 was found to have highest average arithmetic mean concentrations of heavy metals i.e., Cu, Cd, Zn and Pb in both the seasons (winter and summer) due to its closer vicinity to coal based Thermal Power Plant while zone 5, which is a residential area mainly with lesser number of industrial sources, was found to have lowest concentrations of all the measured metal species in both the seasons.As evident from wind roses shown in Fig. 4, North-east direction of wind blow from KSTPS encourages their worrying level in other zones (after zone 1) of study region.
The relative abundance of these heavy metals in SPM and RPM samples followed the order Zn > Pb > Cu > Cd in all zones.As volatile metals with low melting point are emitted more from the Thermal Power Station (KSTPS in the present work), Zn and Pb were quite readily transported in air owing to their lower melting point (Krolak, 2000).Besides, particulate Zn may also get suspended in air due to vehicular activities, source of which may be tyre wear or lubricants and fuel additives.In our study, Zn was found in highest concentration followed by Pb, Cu and Cd respectively which is similar to the reports of earlier research work (Banerjee, 2003;Fang et al., 2005;Wang et al., 2005).Despite the use of lead free petrol, high concentration of Pb could be due to Pb particles in street dust accumulated from earlier vehicular exhaust for a long time due to their higher residence time in environment (Kulshrestha et al., 2009).
Seasonal variation of the average arithmetic mean concentrations of all the analysed metals were, generally, consistent from site to site but were different depending on the metal species and then meteorological conditions.In all the zones, concentrations of heavy metal species viz.Cu, Cd, Zn and Pb were higher in winter and lower in summer while a reverse trend was observed for crustal metal species viz.Ca, Mg and Fe.The difference of concentration levels between the two seasons i.e., winter and summer could be explained by difference in meteorological conditions.During the sampling period in winter, Kota city had witnessed low average temperature, higher relative humidity and low wind speed leading to higher levels of anthropogenic metal species in ambient air while summers had high average temperature, low relative humidity and high average wind speed causing their decreased concentrations (Table 2).On looking at the results of crustal metals, high wind strength during summer is responsible for their deflation or entrainment and transport resulting in their higher levels whereas wind strength available to transport these metals drops significantly in the winter causing their reduced levels.
In summer, the frequent precipitation processes are effective for the removal of particulate matter from ambient air.In addition, influenced by the high pressure system and strong solar radiation, the higher wind speeds and temperatures favour the diffusion of SPM and RPM in summer, thereby reducing anthropogenic metal concentration.In winter, the atmospheric conditions are stable and prone to thermal inversion, breezes and calm winds inhibit the diffusion and dilution, resulting in an exponential increase of pollutants near the ground (Yang et al., 2005).

Enrichment Factor
Calculation of enrichment factor (EF) helps to determine whether a certain element has an additional or anthropogenic source other than its major or natural source.Calcium (Ca) has been used as a reference element for an EF evaluation, assuming that the contribution from its anthropogenic  2.0 ± 0.8 5.5 ± 3.1 Rain fall (mm) 10.0 10.0 sourcesto the atmosphere is negligible (Yaroshevsky et al., 2006).This study used the EF calculation formula (1) as follows: where x is the concentration of the metal of interest and c is the concentration of the reference metal.If the EF value approaches unity, then crustal sources are predominant while EF > 5 indicates that a large fraction of the element can be attributed to non-crustal or anthropogenic sources (Wu et al., 2007).Figs. 5 and 6 showed the seasonal mean enrichment factors based on average seasonal metal concentration of heavy metals measured in SPM and RPM collected from the five zones in winter and summer seasons.
For both Fe and Mg, the mean EF value was approximate 1.0 which indicates its origin from soil dust.These metals were mainly introduced into the atmospheric aqueous phase by influence of particulate resuspension of soil particles besides being emitted from lime stone mining activities carried out at study area.
The higher EF values suggest a substantial point source input for Cu, Cd, Pb and Zn as all these metals were components of fly ash generated at KSTPS.Pb is still persistent in road dust from earlier vehicular emission before ban of leaded gasoline because of its long residence time in the environment besides emission from coal burning power plant.
Seasonal EF values of Cu, Cd, Zn and Pb metal species showed similar trends in both the seasons.The higher EF values of metals (Cu, Cd, Zn and Pb) during winter season than summer may be attributed to transport of fly ash from coal combustion activity from point source KSTPS under the influence of stable and calm meteorological conditions in winter.
On comparing the EF values of SPM and RPM, every metal shows different characteristics.The EF values of Cu, Cd, Zn and Pb, suggesting their origin from mainly anthropogenic source such as coal based Thermal Power Plant, were higher in RPM than in SPM.This indicates that Kota city is more exposed to these toxic metals associated with fine particles than those associated with coarse particles causing more adverse health effects.This finding is supported by earlier reports (Zhu et al., 2002a;Vogt et al., 2003;Maynard, 2004).

Correlation Analysis
The Pearson's correlation coefficient (r) was calculated from the elemental concentration in order to predict the possibility of a common source of SPM (Table 3) and RPM (Table 4).
In SPM, significant positive correlations were found between Mg and Ca (r = 0.846 and 0.887); Mg and Fe (r = 0.709 and 0.777); Ca and Fe (r = 0.812 and 0.819) in both winter and summer seasons, respectively.These significant positive correlations indicate that these metals have a common source, possibly natural soil.Similarly, significant positive correlation were found between Cu and Cd (r = 0.890 and 0.881); Cu and Zn (r = 0.837 and 0.825); Cu and Pb (r = 0.854 and 0.853); Cd and Zn (r = 0.743 and 0.838); Cd and Pb (r = 0.840 and 0.818); Zn and Pb (r = 0.826 and 0.845) in both winter and summer seasons, respectively.
In RPM, significant positive correlations were found between Mg and Ca (r = 0.855 and 0.828); Mg and Fe (r = 0.779 and 0.791); Ca and Fe (r = 0.802 and 0.813) in both winter and summer seasons respectively.These significant positive correlations indicate these metals have a common source, possibly earth crust.Similarly, significant positive correlation values were found between Cu and Cd (r = 0.799 and 0.771); Cu and Zn (r = 0.822 and 0.732); Cu and Pb (r = 0.754 and 0.767); Cd and Zn (r = 0.783 and 0.774); Cd and Pb (r = 0.820 and 0.798); Zn and Pb (r = 0.756 and 0.814) in both winter and summer seasons respectively.These significant positive correlations indicate these metals have a common anthropogenic origin i.e., fly-ash emission from point source KSTPS along with earlier vehicular emission and other industrial activities.

Principal Component Analysis
Principal component analysis (PCA) is a numerical approach to account for statistical variance by deriving the least number of major factors.It is useful in reducing the dimensionality of the large data sets and in clarifying the relationship between the variables and thus can effectively be used in interpretation of SPM and RPM composition data (Zhang et al., 1992;Shukla and Sharma, 2010).
To further identify pollution sources of SPM and RPM in the present study, PCA was carried out with varimax rotation.The results of principal component analysis (PCA) showed that only two eigenvalues were >1 which explains over 66.49% and 68.03% of variance in SPM and 64.67% and 65.05% of variance in RPM in both winter and summer season respectively.The results in rotated component matrix (Table 5) showed that all the analyzed seven metal species are explained by two factors (varimax factors 1 and 2).
The first factor (VF 1), which explained over 37.12% and 36.89% in SPM and 35.90% and 33.43% in RPM in winter and summer seasons of variance, showed high loading of the heavy metals such as Pb, Zn, Cu and Cd

Fig. 3 .
Fig. 3. Seasonal average concentrations of Ca, Mg, Fe, Pb, Zn, Cu and Cd measured at the five zones in SPM and RPM.

Fig. 4 .
Fig. 4. Wind roses of Kota city during (a) winter and (b) summer sampling periods.

Fig. 5 .Fig. 6 .
Fig. 5. Enrichment factor of analyzed metals in SPM at five zones in Kota city in (a) winter and (b) summer seasons.

Table 1 .
Location and characteristics of different zones of Kota city for present study.

Table 2 .
Meteorological conditions of Kota city during sampling periods.

Table 3 .
Correlation coefficients between the concentration values of analyzed metals in SPM at Kota city during 2011-2012 ( * significant at 5% level).

Table 4 .
Correlation coefficients between the concentration values of analyzed metals in RPM at Kota city during 2011-2012 ( * significant at 5% level).the influence of anthropogenic activities mainly coal combustion at KSTPS.VF 2 that accounted for 29.37% and 31.14% in SPM and 28.77% and 31.62% in RPM in winter and summer seasons of the layout variance showed high loading of Ca, Fe and Mg indicating the influence of crustal aerosols. indicating