SEM-EDX Characterization of Individual Coarse Particles in Agra , India

Aerosol samples were collected during winter and summer season in Agra, India. The mass concentrations of TSP ranged from 206.1–380.5 μg/m with the average concentration of 306.1 μg/m. The seasonal average concentrations of TSP were 273.4 ± 85.5 μg/m in summer and 338.6 ± 89.1 μg/m in winter. The high levels of mass concentration during winter may be attributed to different emission sources and meteorological conditions at this time of year. The morphology, size and elemental composition of individual aerosol particles were examined using a scanning electron microscope (SEM) coupled with an energy dispersive X-ray system (EDX). The particles analyzed in the study were mostly of large size, with equivalent diameters ranging from 2 to 70 μm. Based on the results of the elemental composition and morphology, 3,500 particles were classified into three groups: biogenic aerosol, geogenic and anthropogenic particles. Different groups of particles have varied morphologies. The soil related aerosols were dominant during the sampling period, showing that crustal materials are the primary contributor to airborne particles at this site. A distinct seasonal variation in the amount of carbonaceous particles was observed. The significant increase in mineral dust particles found during summer may be attributed to the contribution of dust storms, which is also supported by a trajectory cluster analysis.


INTRODUCTION
Atmospheric aerosols have been found to play an important role in the human health problems (Pope et al., 2002;Pope and Dockery, 2006;Campos Ramos et al., 2009), visibility degradation, cloud formation (Ramanathan et al., 2001) and scattering and absorption of solar radiation.These atmospheric particles are emitted from natural sources such as sea spray, crustal erosions, volcanic emissions, dust outbreaks as well as from anthropogenic processes such as fossil fuel combustion and industrial emissions (D'Almeida, 1991).Because of diverse sources, these particles greatly vary in their size, morphologies and chemical compositions (Salma et al., 2001).Poschl (2005) indicated that particle size, chemical composition and mixing states of atmospheric aerosols pose significant impact on climate and human health.Therefore, it is essential to understand size distribution and chemical composition of aerosol particles particularly in urban atmosphere.
In recent years, physical (size and morphology) and chemical (composition) characterization of individual atmospheric particles has received significant importance due to their effect on radiative and chemical properties.A detailed characterization of individual atmospheric particles also provide useful information about their sources, atmospheric history, formation, reactivity, transport and removal of atmospheric chemical species (Lu et al., 2006;Adachi and Buseck, 2010;Li et al., 2010a).Thus, in lieu of the importance, several studies have been conducted on characterization of individual atmospheric particles (Post and Buseck, 1984;Esbert et al., 2001;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).Scanning electron microscopy with energy-dispersed X-ray analysis (SEM/EDX) is commonly used for single particle study (Shi et al., 2003;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 (Esbert et al., 1996;Petrovic et al., 2000;Conner et al., 2001;Conner and Williams, 2004;Bernabe et al., 2005;Cong et al., 2010).
In Indian context only few studies using SEM-EDX have been done but they are largely confined to bulk particle analysis (Sachdeva and Attri, 2008;Srivastva et al., 2009;Agarwal et al., 2011;Pipal et al., 2011).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 (Bérubé et al., 1999;Li et al., 2010a).Keeping the view in mind, the present work has been carried out to characterize TSP aerosols on the basis of SEM-EDX method.An attempt has also been made to classify the airborne particles in groups and correlate them with potential sources (natural and anthropogenic).

Site Description
The sampling site is located in Agra (North Central India, 27°10'N, 78°05'E).It lies in a semi-arid zone, adjacent to the Thar Desert of Rajasthan.Agra's climate is tropical and strongly influenced by the Aeolian dust blown from the Asian subcontinent and Thar Desert of Rajasthan.Agra falls in Indo-Gangetic region which has the largest alluvial tract in the world and their deposits include a thick layer of sand, clay, loam and silt contributing largely to soil related aerosols at the sampling site.Meteorologically the year is divisible into three distinct seasons; summer (April-June), monsoon (July-October) and winter (November-March).Summer season is associated with strong hot dry westerly winds and high temperature ranging between 38-48°C.Relative humidity in the summer ranges between 18 and 48%.The monsoon season is hot and humid, temperature ranges from 24 to 36°C and the relative humidity ranges from 70 to 90%, while in winter season temperature drops below 2°C.The major industrial activities are ferrous and non-ferrous metal casting, rubber processing, chrome and nickel plating units, copper wire and diesel engine, electroplating industry, pulverization and engineering works.Agra is famous for Petha (famous Indian confectionary) and shoe industries which contribute to aerosol loading through their solid waste dumping and incineration.Apart from local sources, Mathura refinery, Firozabad glass industries and brick kiln factories are also situated within 40 km from Agra.
Study was carried out at Dayalbagh Educational Institute Campus in Dayalbagh, lying towards north of Agra city.It is a suburban site with a small residential community lying immediately outside the city where agricultural activities predominate.The meteorology of Agra is such that prevailing winds are mostly from the northwest so that Mathura lies upwind while Firozabad downwind.Population around the sampling site is about 25,000.The sampling site lies by the side of a road that carries mixed vehicular traffic, moderate (of the order of 1000 vehicles) during the day and minimal (of the order of 100 vehicles) at night.The campus lies about 2 km north of the National Highway-2 (NH-2) which has dense vehicular traffic (10 6 vehicles) throughout the day and night.Fig. 1 shows the geographical map sampling site.

Ambient Sampling
Sampling was carried out in Dayalbagh on the roof of Science Faculty building (12 m above the ground) in the Institute campus.All TSP samples were collected using High Volume Sampler, Envirotech APM (460 BL) Respirable Dust Sampler operated at a flow rate of 1.2 m 3 /min for 24 hours.The aerosol samples were collected on Quartz fiber filter paper at an interval of 15 days from January to June 2010.Total 12 samples were collected in both summer and winter season.Before exposure, the quartz fiber filters were pre-heated in a muffle furnace at 800°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 (Mettler, AJ 150) to determine the particulate matter mass.Each filter was weighed at least three times before and after sampling, and the net mass was obtained by subtracting the pre-sampling weights from the post-sampling weights.After weighing the samples wrapped in aluminum foil were sealed in polyethylene ziplock bags and stored in deep freezer at -4°C until the time of analysis to prevent the evaporation of volatile components.

SEM-EDX Analysis
TSP samples were analyzed by SEM-EDX at National Institute of Oceanography, Goa.The SEM-EDX analysis was carried out with the help of computer controlled field emission scanning electron microscope SEM (JSM-5800 LV) equipped with an energy dispersive X-ray system (Oxford 6841).The dry and loaded quartz fiber filter papers were punched in 1 mm 2 from the centre of each sample.All the samples were mounted on plastic stubs for gold coating.A very thin film of gold (Au) was deposited on the surface of each sample using vacuum coating unit called Gold Sputter Coater (SPI-MODULE) which can prepare 6 samples at a time.The fine coating of gold makes the samples electrically conductive.The samples were placed in the corner of SEM-EDX chamber.The working conditions were set at an accelerating voltage of 20 kV, a beam current of 40-50 μA and a Si (Li) detector 10mm away from the samples to be analyzed.X-ray detection limit is ~0.1%.The Oxford ISIS EDS system with 133 eV resolutions is capable of collecting spectrum from multiple points, lines across the interface and elemental mapping.
The EDX analysis was carried out at each analysis point and the elements present were both qualitatively and quantitatively measured.The weight percentage of each element present in the spectrum was identified.On normalization to 100% for C and O the weight percentage of different elements were also identified.Approximately 300 particles were analyzed on each filter paper.The EDX spectra of blank quartz fiber filter was also obtained and its composition was manually subtracted during the evaluation of the EDX spectra of individual aerosol particles.SEM-EDX spectra of blank fiber filter paper is shown in Fig. 3(a).

Trajectory Cluster Analysis
In order to identify the source and transport pathways of the airborne particles arriving at the sampling site, the air mass backward trajectory analysis was carried out.These air mass back-trajectories were obtained from the final run data archive of Global Data Assimilation System model using NOAA (National Oceanic and Atmospheric Administration) Air Resource Laboratory (ARL) Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) Model (http: //www.arl.noaa.gov/ready/hysplit4.html (accessed via NOAA ARL Realtime Environmental Applications and Display System (READY) website http://ready.arl.noaa.gov)).The three-day trajectory cluster analysis of summer and winter season was simulated at 12:00 hrs (local time) at 500 m above the ground level and has been represented in Fig. 6.The results indicate long range transport of aerosols originating from Thar Desert (western India) and other neighboring countries (Iran and Pakistan) of India during summer season whereas during winter season, short trajectories originating from local areas around Agra indicate the dominance of anthropogenic emission sources.

Mass Concentration of TSP
Mass concentration is the key criteria for the assessment of air quality.At this site, the mass concentrations of TSP ranged from 206.1-380.5 μg/m 3 with the average concentration of 306.1 μg/m 3 .The average mass concentrations during the sampling period are shown in Table 1.The average daily mass concentration during the measurement period exceeded the 24 hour National Ambient Air Quality Standard (NAAQS, CPCB 1994) of India (200 μg/m 3 ).Higher level of particulate matter at this sampling site may be attributed to the combined impact of climatic conditions and anthropogenic emissions by various local sources such as vehicular exhaust, waste incineration and biomass combustion.
The seasonal average concentrations of TSP was 273.4 ± 85.5 μg/m 3 in summer and 338.6 ± 89.1 μg/m 3 in winter.The high levels of mass concentration during winter season may be attributed to different emission sources (increased biomass, coal and fossil fuel combustion) and meteorological conditions.The stable and cold conditions during winter months favor the prolonged life of ambient particles in the atmosphere leading to elevated levels of particulate matter.The results indicated that the average mass concentration during summer season was lower than winter but some samples collected on 21 April and 25 June 2010 show very high concentrations due to the contribution of dust storms which frequently originate from Thar Desert of Rajasthan.Thus, in addition to local sources, long range transport also plays a significant role during summer season.

Size and Morphology of Individual Aerosol Particles
The particles analyzed in the study were mostly of large size with equivalent diameter ranging from 2 to 70 μm.Total 3500 particles were analyzed for SEM/EDX out of which 1600 and 1900 particles were analyzed during summer and winter seasons, respectively.The seasonal number size distribution of all the analyzed aerosol particles is shown in Fig. 2. The fractions of particles within size range of 40-70 μm were found to be dominant during summer season.This significant increase may be attributed to the contribution of dust storms which has larger proportion of coarse particles.Earlier study from the same site on seasonal variation of number abundances of fine and coarse particles reported that number of coarse particles was 2.4 times higher during summer season which was attributed to local as well as long range transport of particles during dust storms (Pachauri et al., 2012).The dust particles which were transported from Thar Desert of Rajasthan carried Aeolian dust which particularly comprised of large size mineral dust particles.These mineral dust particles mixed with the local soil dust and contributed significantly to coarse mode.Several studies have also reported that coarse particles were more pronounced during the dust events (Wang et al., 2006;Pandithurai et al., 2008;Wu et al., 2008;Stone et al., 2011;Zhao et al., 2011).Further, Chun et al. (2001) and Lin (2001) had also shown that mineral dust was the major component of coarse mode particles.Most of these large sized mineral particles including Al, Si, Ca, Na, Mg and K were quite irregular.During winter season, the particle number within size 0-20 μm were higher than summer season.However, the particles within size range: 20-30 μm were quite similar during both the seasons.Different groups of particles display variable morphology.The shapes of the particles vary from tubular structure of silica to irregular shaped biological particles.It is interesting to note that the aluminosilicate group was dominated by diverse shapes of particles, from spherical to triangular and from rectangular to irregular.Soot particles were dominated by chain like aggregates of carbon bearing spheres while some individual carbonaceous spherules with traces of chloride were also found.

Major Particle Groups
Based on the result of the elemental composition and morphology, total 3500 particles on 12 filters obtained using SEM-EDX equipment were classified into 3 groups: biogenic aerosol (fungal hyphae with root like outgrowth and organic plant fragment), geogenic particles (derived from soil sediments and weathered rock surfaces) and anthropogenic particles (particles derived from industrial and combustion activities) (Table 2).

Biogenic Aerosols
The particles of biological origin were quantified by the method used by Matthias-Maser and Jaenicke (1991,1994).The elemental composition and morphology of the particle was considered during EDX analysis.Biological particles (both dead and alive) contain minor amounts of Na, Mg, K, P, Si, Fe, Cl, Al and Ca (usually <10% of relative element X-ray intensity of the particle) which are essential tracers present in plants ( Matthias-Maser and Jaenicke, 1994;Artaxo and Hansson, 1995;Matthias-Maser et al., 2000a, b;Posfai and Buseck, 2010) and can be used to separate these particles from other types of particulate matter.To achieve this, the following clustering rule was used for each sample as determined by Coz et al. (2010): Bioaerosol: (C + O) > 75% and 1% < P; K; Cl < 10% (1) Using the above rule the particles belonging to the biogenic aerosol group were separated and particles that did not have specific morphologies associated to this group were removed.
In the present study, biological particles have been recognized with the size ranging from 20 to 50 μm.A club shaped outgrowth with root like aggregations is shown in Fig. 3(b).Such types of particles are abundant in C and O while other essential tracers (Na, Mg, K, Ca and Cl) are found in minor amount.Such biological particles include microorganisms and fragments of all varieties of living matter (i.e., viruses, bacteria, fungal spores, pollen, plant debris and animal matter).Similar particles in atmospheric aerosols have earlier been reported in many studies (Matthias-Maser, 1998;Yeo and Kim 2002;Puc, 2003;Després et al., 2007;Elbert et al., 2007;Deguillaume et al., 2008;Iordanidis et al., 2008;Cong et al., 2009;Coz et al., 2010;Chen et al., 2012).

Geogenic Particles
At the present site, geogenic particles (also called as minerogenic or natural particles) were about 70% of total particles analyzed.The particles with the equivalent diameter ranging from 2 to 70 μm have been observed.These particles with crustal origin include aluminosilicates, quartz, Fe/Ti oxides, calcium rich particles and chloride particles.In this analysis, elements are assumed to be present if their calculated concentrations are ≥ 1%.Single particle elemental analysis have revealed the presence of particles containing two or more of the following elements: Na, Mg, Al, Si, Cl, K, Ca and Fe mostly present in aluminosilicate group.This soil derived group contributes to highest percentage of the total particles analyzed indicating the dominance of local as well as transported aeolian dust from the Asian subcontinent and Thar Desert of Rajasthan.

Quartz/Silica
SiO 2 particles (commonly called silica) are characterized by high content Si and O (≈50% by weight).The diameter of silica particles ranged from 10-30 μm.These particles have tubular structure as revealed from Fig. 3(c).The pure silica particles have natural and anthropogenic origins (Li et al., 2010b).It is the most abundant chemical constituent of Earth's crust and a major component of sandstone and granite.Thus, the most abundant source for this particle type is soil related.Additionally, silica is widely used for making building materials like cement, glass, bricks, clays, ceramics etc. therefore, these particles also likely to be originated from building construction and demolition.

Aluminosilicates
The major type of chemical compound in the earth's crust is composed of about 72% of aluminosilicate group in terms of weight (Van Malderen et al., 1996;Cong et al., 2009).Our analysis show that soil derived aluminosilicate particles are mainly composed of Si and Al oxides with varying amount of Na, K, Mg, Ca, Fe and Ti.This aluminosilicate group contains about 55% of the total particles analyzed indicating the abundance of minerogenic particles i.e., mineral particulate aerosol derived from soil sediments and weathered rock surfaces.The size of aluminosilicates particles varied, ranging from 2 to 50 μm.Most of the particles in this group showed irregular shapes.In aerosols of this site, different kinds of aluminosilicates were identified which were made up of Al-Si-O along with other minerals like Ca aluminium silicate (Grossular) Fig. 3(d), Fe aluminium silicate (Almandine) with traces of other mineral dust particles, Mg aluminium silicate (Pyrope), Biotite (Si, Al, K, Mg, Fe), Na-feldspar (albite), K-feldspar (K aluminium silicate), Ca-feldspar (Anorthite), Mg-Fe aluminium silicate and Ca-Mg aluminium silicate.The most common source of these particles is crustal origin through windblown dust.

Calcium rich Particles
Particles composed of high content of Ca (< 50% relative contribution by weight) fall into this group.Ca silicate particles (Wollastonite) with irregular morphology indicate that these particles are often dominated by resuspended dust and crustal material from paved and unpaved roads or from windblown dust and construction activities (Fig. 4(a)).Carbonate minerals identified include calcite (CaCO 3 ) and dolomite (MgCO 3 .CaCO 3 ) along with traces of other dust related elements which are the common constituent of soil and often observed in the individual aerosol particle analysis (Lu et al., 2006;Shao et al., 2008).These particles are irregular fragments with distinct rough surfaces on all faces originated from process of building construction and demolition and commonly found in earth's crust.A small spheroidal shaped CaF 2 particle (40% relative contribution by weight of F) with traces of Al, Si Mg and Fe is shown if Fig. 4(b).The morphology of this small particle suggests its mineral origin.

Fe/Ti Oxides
In addition particles abundant in Fe, Ti and O along with the association of mineral dust elements like C, Al, K, Si, Ca and Mg have also been identified in aerosol of this region Fig. 4(c).This group of particles mainly corresponds to Fe oxides with irregular morphology which are assumed to be soil related.Earlier studies also reports that the crustal materials in mineral phase with characteristic edges, morphology, angles and size variability of 10-20 μm (hematite, magnetite, ilmenite and rutile) are common constituent of soil and most likely to occur during single particle analysis (Shao et al., 2008;Cong et al., 2009).

Chloride Particles
A small triangular shaped CaCl 2 particle with diameter less than 1 μm which is characterized by high relative Xray intensity of Cl (> 50%) together with traces of C, Na, Mg, K and Ca is shown in Fig. 4(d).The other chloride particles include KCl and carbon particle along with Cl indicating the presence of Ca from crustal dust and K from vegetation emissions (Kleinman et al., 1979;Parmar et al., 2001).Earlier studies on chemical characterization of aerosols at this site show that Cl, Na, Mg and K are soil derived (Kulshrestha et al., 1998;Parmar et al., 2001).In addition presence of Cl may also be due to spray of insecticides and pesticides in a large quantity which contain significant amount of Cl.Similar particles of chloride have earlier been reported in aerosols of Delhi by Shrivastava et al. (2009).PbCl 2 particles (Fig. 5(a)) show the presence of anthropogenic activities like vehicular emissions generated by leaded gasoline.

Anthropogenic Particles
This group consisted of carbonaceous and industrial particles.Local sources contribute significantly to these anthropogenically originated particles.These aerosol particles emitted from anthropogenic sources along with the huge amount of gases (SO 2 , NO 2 and organics) complicate atmospheric chemical reactions through homogenous and heterogeneous reactions.

Industrial Particles
Airborne particles at this site are rich in metals as shown in Figs.4(b)-4(c).The dominant metalliferous particles contain Cr (> 41%), Mn (> 50%) and Ni (> 10%) in combination with Fe, Si and O in traces, as these particles is similar in their rough texture.Therefore, the common source attributed to these metals at this site is resuspension of road dust due to vehicular activity, which is rich in Ni, Cr, Fe due to activities like wear and tear of tires, oil burning, abrasion of mechanical parts of road vehicles, oil lubricants.Earlier study by Adachi and Tainosho (2004) has also reported that anthropogenic metal elements are usually embedded into re-suspended dust particles.Mn is used as additive in  with minor relative intensities of Mn, Si and Fe is shown in Fig. 5(c).The oxides of Cr (Cr 2 O 3 and CrO 3 ) have anthropogenic origin from industries as these oxides are used in electroplating and ferrous and non-ferrous alloys.The Cr rich particles also contain elements such as Mn, Ni and Fe in appreciable quantity clearly indicate that these particles are likely to be originated by anthropogenic activities from small industrial processes around the region.Both Cr and Ni are used in electroplating and Mn is used in non ferrous alloys.The large scale construction activities also make use of building materials including cement, steel, and iron (used in welding) are also likely to attribute to the above mentioned particles.The lead particles observed was in strong association with different metals like C and Cl with irregular morphology shows the dominance of vehicular emissions.In India Pb has been phased out of gasoline from year 2000, but still it persists in road dust from earlier vehicular exhaust emission due to its longer residence time in environment (Kulshrestha et al., 2009).Some Ca silicate particles were also observed which may be mechanically generated in glass industries mainly situated at Firozabad (famous for glass works and is 40 km from the sampling site) during manufacturing of normal domestic glass for windows which is Ca alkali silicate.In addition to these particles, spherical aluminosilicate particles (fly ash) were also identified Fig. 5(d).As the site is away from all heavy industrial activities so we assume that these particles might have transported through windblown dust or strong air movement episodes.

Carbonaceous Particles
The carbonaceous materials are the significant component of total ambient particle mass.In the present study, this group of particles contributes about 25% of the total particles analyzed.The group of particles characterized by high Xray intensity contain varying fraction of carbonaceous particulates and aggregates with > 60% relative contribution by weight.The morphology of this kind of carbonaceous particles varied from soot chains to complex structures, which depend on fuels, burning conditions and atmospheric processes (Bérubé et al., 1999;Posfai et al., 2004;Cong et al., 2009;Posfai and Buseck, 2010;Tumolva et al., 2010).
In the present study, we were able to identify the branched clusters resulting from the interconnection of often hundred of carbonaceous spherules which stick together through a combination of adhesive surface forces and partial coalescence occurring at high temperature common during combustion (Murr and Bang, 2003).A mixture of carbonaceous particle and inorganic with varying amount of soil related components like Na, K, Mg, Ca and Al and crystalline particles forming complex aggregates and inorganic were also observed.Earlier study by Li et al. (2010b) has also reported such type of particles.
A single carbon particle with nearly spherical morphology observed in our study is dominated by C and O (> 90%) which is produced from biomass and biofuel burning Fig. 5(e).Earlier studies show that this spherical carbonaceous particle can scatter and absorb light efficiently and may play an important role in climate forcing (Hand et al., 2005;Alexander et al., 2008;Cong et al., 2009).The carbon particles also show a strong association with other adsorbed elements.The origin of these particles may be related to agricultural burning and waste incineration.

Seasonality of Number Abundances of Particle Groups
Table 3 shows the seasonality of number abundances for different particle groups while the relative abundances of each particle group in different size fractions are shown in Figs.7(a) and 7(b).The seasonal distribution of particle groups show that aluminosilicates were predominant in both summer and winter seasons, indicating the dominance of crustal sources.However, during summer season, increase of mineral dust particles may be attributed to the contribution of dust storms.The trajectory cluster analysis also indicate that during summer season most of the air masses come from the west with a rapid moving speed carrying aeolian dust from Thar Desert of Rajasthan (Fig. 6).Earlier studies have also reported the abundances of aluuminosilicate minerals (koalinite, illite, feldspars etc.) especially during dust storm periods in summer (Ganor et al., 1991;Falkovich et al., 2001;Sobanska et al., 2003).On the other hand, the number of carbonaceous particles was about two times higher in winter season.This significant increase of carbon rich particles may be attributed to  combined effect of different emission sources (increased biomass and biofuel combustion) as well as stagnant meteorological conditions (i.e., low wind speed and low mixing heights) which cause less dispersion of atmospheric particles.The presence of short trajectories during winter period also suggests the contribution of local sources.Our previous study by Satsangi et al. (2012) has also reported high concentrations of carbonaceous aerosols (organic and elemental carbon) during winter season at Agra.The number abundances of all other particle groups (i.e., industrial, biological, Fe/Ti oxide, quartz and Ca rich particles) except chloride particles do not show any significant seasonal variation.

CONCLUSIONS
The elemental composition and morphology of atmospheric particles were investigated using SEM-EDX system.The aerosol particles were characterized into 3 groups: biogenic particles, geogenic and anthropogenic particles.The results indicate that these diverse groups of airborne particles were both natural and anthropogenic in origin, but most of the particles were soil related indicating the dominance of crustal materials.The aluminosilicate group of particles was major contributor of minerogenic particles contributing highest percentage of total analyzed particles.The particles with equivalent diameter ranging from 2 to 70 μm were observed.The seasonal variation indicates that the fractions of particles within size range of 40-70 μm were found to be dominant during summer season due to occurrences of dust storms.However, the particles within size range: 20-30 μm were quite similar during both the seasons.
The trajectory cluster analysis indicate that there is significant increase of mineral dust particles especially during summer season indicating the dominance of long range transport of aerosols through westerly winds carrying aeolian dust from the Thar Desert of Rajasthan.While carbonaceous particles were more predominant during winter period due to enhanced use of biomass and biofuel combustion.The other particle groups were quite constant in both the seasons.

Fig. 2 .
Fig. 2. The seasonal number size distribution of aerosol particles.

Fig. 3 .
Fig. 3. Scanning electron images and energy -dispersive X-ray spectra: (a) Blank Quartz fibre filter (b) A club shaped outgrowth with root like aggregations with minor amount of elements Na, Mg, K and Ca of biological origin (c) A tubular structure of pure silica particle (d) Ca aluminium silicate (Grossular) with irregular morphology.

Fig. 4 .
Fig. 4. Scanning electron images and energy -dispersive X-ray spectra: (a) platy shaped Ca silicate particle (Wollastonite) (b) A small spheroidal shaped CaF 2 particle (40% relative contribution by weight of F) (c) Irregular shaped Fe oxide particle with traces of mineral dust elements (d) A small triangular shaped CaCl 2 particle with diameter less than 1 μm.

Table 1 .
TSP mass concentrations during summer and winter seasons at Agra.

Table 3 .
Table showing seasonality of number abundances for different particle groups at Agra.