Seasonal Behaviours and Weekdays / Weekends Differences in Elemental Composition of Atmospheric Aerosols in Cairo , Egypt

The aim of the present study was to investigate the seasonal distributions and weekdays/weekends differences in ambient aerosols and their elemental contents in the atmosphere of the 15 May City, Egypt. Aerosol samples were collected from March 2009 to February 2010. The concentrations of 14 elements including Na, Mg, Ca, K, Al, Fe, Mn, Co, Cu, Zn, Ni, Cd, Pb and Cr were measured by Atomic Absorption Spectrophotometry (AAS). The mean aerosols concentrations were 290 μg/m (spring), 250 μg/m (summer), 330 μg/m (autumn) and 400 μg/m (winter). The measured elements showed a strong seasonal variation; with the highest concentrations in winter and the lowest in summer. Na, Mg, Ca, K, Al and Fe were the dominant elements, followed by Mn, Cu, Zn, Ni and Pb. The minimum concentrations were noted for Co, Cd and Cr. The dominant elements in aerosol represented 94%, 94.51%, 94.61% and 94.28% from the total measured elements and 5.08%, 5.74%, 6.11% and 5.44% from aerosol mass during winter, spring, summer and autumn, respectively. The aerosols concentrations on weekends were reduced by 31.25% in the winter and 15.2% in the summer. The concentrations of the measured elements were higher on weekdays than weekends. The weekday/weekend concentration ratios were 1.69 and 1.38 for total measured elements during winter and summer, respectively. The mean Ni and Cd levels were higher than the proposed WHO, USEPA and the European Community standards. Significant positive correlations were found between the concentrations of aerosols, Mg, Ca, K, Al, Fe, Mn, Co, Zn and Cr. The enrichment factors (EFs) and the non-crustal fractions of all elements indicated that Na, Mg, Ca, K, Fe and Mn originate mainly from soil sources, whereas Co, Cu, Zn, Ni, Cd, Pb and Cr are mostly emitted in the atmosphere of 15 May City from anthropogenic sources.


INTRODUCTION
Urbanization, industrialization and associated increase of energy demands have resulted in deterioration of urban air quality.In urban regions, anthropogenic activities linked with socioeconomics have significant bearing on aerosol scenarios and their trans-boundary implications (Mitra and Sharma, 2002).Ambient aerosols (suspended particulate matter, SPM) represent the most important air pollutant group, since it plays a serious adverse role in the atmosphere.It causes loss in visibility and cloud formation (Sloane et al., 1991;Lee and Sequerira, 2002), and acidification of precipitation and also affect climate (Dockery and Pope, 1994;IPCC, 2007).Aerosols may carry of acidic or toxic species and may have detrimental effects on human health and ecosystems (Cheng et al., 1996).Human health endpoints associated with exposure to aerosols include increased mortality and pulmonary and cardiovascular diseases (Lee et al., 2006;Perrone et al., 2010;Gualtieri et al., 2012).
Aerosols vary in size, shape, surface area, chemical composition, solubility and origin (Pope and Dockery, 2006;Zhang et al., 2009;Tai et al., 2010).They are complex mixture of anthropogenic and natural origin and may be emitted directly or formed in the atmosphere (Samura et al., 2003;Gualtieri et al., 2010).Industrial activities, energy production, construction, wastetreatment and vehicle exhausts are the main anthropogenic sources of aerosols in the atmosphere (Sabbak, 1995;Bilos et al., 2001).In urban environments, fine particles arise from anthropogenic sources, such as combustion processes and gas-to-particle conversions, whereas coarse ones originate mainly from natural processes, such as wind action on land and sea spray (Reichhardt, 1995;Jonathan et al., 1997).
Atmospheric aerosols are an analytically matrix that contains water-soluble inorganic compounds, organic carbons, elemental carbon and metals.Among these components, trace elements are of great concern in urban air pollution problems; as they can have severe toxic and carcinogenic effects on humans.Trace elements are released into the atmosphere both from natural and anthropogenic sources.
Resuspended surface dust makes a large contribution to the natural emissions, whereas the combustion of fossil fuels, automobile and industrial emissions are the principal anthropogenic sources of most toxic elements (Hao et al., 2007;Yatkin and Bayram, 2008).Short-term differences of atmospheric metal concentration has been observed in a day-to-day or even hour-to-hour (Gharaibeh et al., 2010).
The distribution of trace elements in airborne particulates shows significant variations with the meteorological conditions in different parts of the world (Jonsson et al., 2004;Shaheen et al., 2005).Metals associated with ambient aerosols catalyze oxidative stress (Muránszky et al., 2011), and may be responsible for production and release of inflammatory mediators by the respiratory tract epithelium, which contribute to the toxic effects of particulate air pollutants reported in epidemiological studies (Carter et al., 1997).Lead (Pb) can induce severe neurological and hematological effects, especially to children (Jedrychowski et al., 2005;Heinrich and Slama, 2007).Compounds of nickel (Ni), cadmium (Cd) and chromium (Cr) are susceptible for inducing carcinogenic effects in human (IARC, 1993).Exposure to high levels of manganese (Mn) can lead to neurotoxical impairments (Heinrich and Slama, 2007).
Studies on elemental concentrations in ambient particulates can give useful information on pollution sources, transportation of aerosols and their effects on human health (Qin and Oduyemi, 2003;Garcia et al., 2004).Thus, the use of elemental signatures to apportion aerosols into major regional sources requires an understanding of the behaviour of aerosols bearing element during different seasons.Industrial outputs are not generally seasonally dependent, whereas airborne concentrations of crustal material are seasonally affected.The aim of the present study was to: (1) investigate the seasonal variations and distribution of some atmospheric elements (Ca, Na, Fe, K, Zn, Mg, Cu, Pb, Mn, Co, Ni, Cr, Cd and Al), (2) evaluate the contribution of anthropogenic sources on the levels of these elements, and (3) study the differences in aerosols concentration levels and their elemental contents between weekdays and weekends during the winter and summer seasons.

Site Description
Cairo is characterized by the presence of Mokattam hills to the east and southeast, besides the Shoubra-El Kheima and Helwan industrial areas in the north and south, respectively (Fig. 1).Industrial activities, heavy traffic density, as well as, the surrounding hills and desert are the main sources of air pollution in Cairo.About 52% of the industries and about 40% of electrical power stations in Egypt are found in Greater Cairo, besides more than 2 million vehicles are running in the streets of the Greater city, about 60% of which are over 10 years old and therefore lack modern emission cutting features like catalytic converters (Shakour et al., 2011;Hassan and Khoder, 2012).The high rate of emission coupled with low wind speeds and the frequent inversions in the area resulted in high local pollution load.Vehicle fuels used in Greater Cairo are mainly unleaded gasoline and diesel, and some vehicles are using compressed natural gas (Hassan and Khoder, 2012).The general climate of Cairo city is cold, moist and rainy in winter, and high temperature, high solar radiation, clear sky and rainless in summer.During the period of the present study, the daily temperature ranged from 17.4°C to 27.5°C (with an average of 22.3°C) in spring, 26.1°C to 35.4°C (with an average of 31.3°C) in summer, 20.5°C to 30.6°C (with an average of 25.5°C) in autumn and 10.7°C to 17.9°C (with an average of 15.5°C) in winter.The daily relative humidity ranged from 15.9% to 62.2% (with a mean value of 48.6%) during spring, 31.9% to 73.6% (with a mean value of 61.9%) during summer, 29.9% to 76.2% (with a mean value of 58.6%) during autumn and 50.9% to 76.2% (with a mean value of 67.6%) during winter.The dominant wind directions were north, north east and northwest during the four seasons.The average wind speeds were 13.

Sample Collection
Aerosols samples were collected using a low volume air sampler.The samples were collected using a calibrated vacuum pump (Gast, USA) to draw 15 L/min, for 24 hours..The air was drawn through the filter holder containing a pre-weighed cellulose nitrate membrane filters (pore size 0.45 µm, diameter, 25 mm).The flow rate of a calibrated vacume pump was meaured before and after sampling time.The differences were found to be very small, so, it is considered that no overload of filter media occurred for the sampling condition.Sampling was conducted from the roof of a ~12 meter height building.Samples were collected during the period from March 2009 to February 2010.They were collected twice per week (every weekend (Friday), i.e., 12 samples and in any of the weekdays chosen between Saturday through Thursday, i.e., 12 samples) during summer and winter, and once per week during autumn and spring seasons.

Gravimetric and Chemical Analysis
Cellulose nitrate membrane filter was carefully equilibrated in desiccator before and after sampling to eliminate the effect of humidity, which can affect weight.The mass of particulate collected on each filter was determined by the difference in weight before and after sampling, then, particulate concentration was calculated from the volume of air filtered.Field blanks were collected through putting another set of cellulose nitrate membrane filters in the sampling site for the same duration with the same steps without operating the air sampler.After weighing, each filter was extracted by nitric acid (Merk Suprapure 65%) and hydrochloric acid (Merk Suprapure 37%) mixture on a hot plate to estimate the selected elements (Method IO-3.1, 1999).The extracted solution was filtered with washing by double distilled water (Gonzalez et al., 1997).The filtered solution was diluted to 50 mL with Double-distilled water in a volumetric flask and refrigerated in pre-cleaned strong polyethylene bottle until analysis was conducted.Selected elements were determined by an Atomic Absorption Spectrophotometer (Varian 220 Spectr A A).All glassware and plastic vessels were treated by dilute (1:1) nitric acid and rinsed with double distilled water before use.Doubledistilled water was used for preparing the stock solutions of target elements and all dilutions.Calibration curves for each target elements were obtained by using suitable standard solutions prepared from the stock solutions.In order to determine the precision of the analytical process, sample from one season was analyzed four times.The relative standard deviation (RSD) for this sample was found to be 2.5%.Laboratory blanks were prepared by adding the same volume of a 1:3 mixture of concentrated hydrochloric and nitric acids to a conical flask containing none of the samples being investigated.This consisted of all components added to the matrix during digestion.Blank samples were analyzed for total element concentrations.Field and laboratory blanks were used to evaluate external elemental contamination introduced by sample handling and analytical procedures.

Meteorological Parameters
The temperature and relative humidity during every sampling were measured using a Sigma-II thermohygrograph (no.7210), SK Sato Keiryoki MFG-Co., Ltd., Japan.Wind speed and direction, and solar radiation data were obtained from Egyptian Meteorological Organization.

Statistical Analysis
Data were analyzed using SPSS version 17 (SPSS, Chicago, Illinois, USA).The mean and standard deviation were used as summary statistics.Pearson correlation coefficient was used to test the correlation between two variables (Norman and Steriner, 2000).

RESULTS AND DISCUSSION
The average aerosols concentrations (± SD) on seasonal basis are shown in Table 1.The maximum levels were found in winter and the low levels in summer and spring seasons.This is consistent with the results reported by Cong et al. (2011) and Kulshrestha et al. (2009).The lowest average aerosols concentration in the present study was recorded in summer, at 250 μg/m 3 , and the highest 400 μg/m 3 in winter (Table 1).The winter/summer concentration ratio of aerosols was 1.60.The highest concentration of aerosols in winter may be attributed to lower dispersion rate of pollutants and low level ground based inversions during winter (Shridhar et al., 2010).Increasing mixing layer heights and consequent better dispersion conditions are the probable cause for lower concentrations during summer season (Almeida et al., 2006;Srimuruganandam and Nagendra, 2011) .
In the present study, aerosols mean levels exceeded the WHO primary (80 μg/m 3 ) and secondary (60 μg/m 3 ) standards during the four seasons (WHO, 2000).They also exceeded the annual average of the Egyptian Ambient Air Quality Standard (90 μg/m 3 , EEAA, 1995).The daily concentrations of aerosols in winter exceeded the Egyptian Ambient Air Quality Standard (230 μg/m 3 ) for 24-h during 100% of the investigated days.Therefore, from these results, it is assumed that 15 May City is heavily polluted by particulate matter.
Table 1 also shows the summary of mean seasonal variations in elemental levels during the study period.Based on the average concentration, Na, Mg, Ca, K, Al and Fe were the dominant elements; the sum of which was 20330 ng/m 3 in winter, 16655 ng/m 3 in spring, 15275 ng/m 3 in summer and 17940 ng/m 3 in autumn, followed by Mn, Cu, Zn, Ni and Pb; the sum of which was 1173 ng/m 3 in winter, 879 ng/m 3 in spring, 797 ng/m 3 in summer and 990 ng/m 3 in autumn.The minimum concentrations were  (Lindgren, 1996).Soil dust is a source for Na and K (Rahn, 1999).Mn, Ni and Cr in aerosol mass reflect the bulk matrix of road dust.The presence of Zn, Cu , Mn, Fe, Co, Ni, Cd, and Pb in aerosols mass can be linked emissions from brake and tire wear (Legret and Pagotto, 1999;Adachi and Tainosho, 2004).Ca and Mg come from combustion of motor oil additives and Fe and Cr are linked to emissions from the brake lining materials (Garg et al., 2000), automobile rusts (Hopke et al., 1980) and motor car exhausts (Weber et al., 2000).Another emission source of Mn and Ni are automobile exhaust fitted with catalytic convertor (Young et al., 2002).Pb also comes from fuel and motor oil combustion and re-suspension road dust (Young et al., 2002;Tüzen, 2003).
The highest average levels of Ca, Mg, Na, K, Al, Fe, Mn, Co, Cu, Zn, Ni, Cd, Pb and Cr were recorded during winter, whereas the lowest levels were found in summer (Table 1).The sum of element concentrations were 21627, 17623, 16144 and 19028 ng/m 3 in winter, spring, summer and autumn, respectively, representing 5.41%, 6.07%, 6.46% and 5.77% from aerosols mass during the corresponding seasons, respectively.The winter/summer concentration ratios were 1. 44, 1.18, 1.24, 1.31, 1.34, 1.47, 1.46, 1.67, 1.65, 1.45, 1.66, 1.65, 1.44 and 1.84 for Na, Mg, Ca, K, Al, Fe, Mn, Co, Cu, Zn, Ni, Cd, Pb and Cr, respectively.These results agree with Srimuruganandam and Nagendra (2012) who found that the average elemental concentrations in particulate matter were found to be higher in winter season followed by monsoon and summer seasons.
The correlation coefficients matrix between aerosols mass and their elements content during the period of study are presented in Table 2. Statistically significant correlation coefficients (p < 0.001) are highlighted in bold.Significant correlations between different pairs were found, some of which were common in the four seasons.Significant positive correlations were found between the concentrations of aerosols, Mg, Ca, K, Al, Fe, Mn, Co, Zn and Cr.This may be attributed to the natural input of trace element and aerosols in the local atmosphere, as most of these metals are well known for their crustal abundance (Hien et al., 2001;Loyola et al., 2006).
The average concentrations of atmospheric aerosols and their related airborne elements in the present study in comparison with those reported from other regions around the world are shown in Table 3. From this table, it can be concluded that, the mean values of atmospheric aerosols and their related element in the atmosphere of the study area were lower/higher or similar to those detected in other cities of the world.Generally, this variations among the different locations of the world was presumably due to the difference in the traffic density, industrial activities, and intensity of human activities, land use patterns and the frequency of rainfall prior to sample collection.The average element concentrations in the atmospheric aerosols of the 15 May City can be compared with the safe limits proposed by international agencies.WHO and USEPA standards for atmospheric Pb, Mn, Cr, Ni and Cd are 500, 150, 1100, 0.38 and 5 ng/m 3 , and 1500, 500, 100, 0.24 and 6 ng/m 3 , respectively (WHO, 2000;ATSDR, 2002).In the present study, the average concentrations of Cr were lower than the WHO and USEPA standards.However, Ni and Cd levels were many times higher than the proposed WHO and USEPA standards.Likewise, average concentrations of Cd and Ni were also higher than those recommended by the European Community (EC, 2000).Similarly, the mean concentration of Cd in the atmosphere of 15 May City was much higher than the safe limit range of the metal (0.24-0.55 ng/m 3 ) for cancer (EC, 2000).The mean concentrations of Pb and Mn were lower than the USEPA standard and relatively higher than the WHO standard in winter and autumn seasons.
Enrichment factors (EFs) can give an insight into differentiating an anthropogenic source from a natural origin, and hence, can also assist in the determination of the degree of contamination (Yongming et al., 2006).To obtain preliminary information about the sources of elements in the aerosols of the study area, the enrichment factors (EFs) for each element were calculated.The enrichment factor for a generic element X with respect to a reference crustal element Y is defined as EFX = (X/Y)air/(X/Y)crust, where the ratio (X/Y) is the concentration ratio of X and Y in either aerosol sample or earth crust.In the present study, Al was used as the reference element Y, and the earth crust chemical composition was taken from Taylor (1964) and Taylor and McLennan (1985).Our calculation of EFs was   done assuming that contributions of man-made sources to Al are insignificant in the study area.The use of average crust values provides meaningful comparison to many other studies that commonly use this technique.The average local soil profiles are not available and will be investigated in the follow up study.The mean EF values of the elements measured in aerosols are summarized in Fig. 3. Since trace element EFs include some degree of uncertainty related to the natural variations of the earth crust composition, only elements with EFs greater than 5 can be regarded as enriched in atmospheric aerosols and associated with sources other than the local soil (Hien et al., 2001).Generally, EFs close to unity point to a crustal origin while those greater than 10 are considered to have a predominantly non-crustal source.In aerosols samples from the study area, EF values lower than 5 were found for Na, Mg, Ca, K, Fe and Mn during the four seasons.The similarities between EF values in different seasons suggest that the main sources of these elements are of a crustal type (e.g., soil and re-suspended dust), while anthropogenic sources have a lesser contribution (Hien et al., 2001;Hao et al., 2007).The moderately higher EFs for Na, Mn and Ca than Mg, K, and Fe denote that part of Na, Mn and Ca are derived from anthropogenic sources and human activities such as construction, windblown road dust, cement factories and building material manufacturing facilities.The EFs of Cr is relatively higher than the aforementioned elements which means that anthropogenic sources have a somewhat higher contribution to this element (Hao et al., 2007).The atmospheric aerosols in the study area were found to be extremely enriched by Co, Cu, Zn, Ni, Cd, Pb and Cr during the different seasons.
The high EF values for these elements indicate that they are present in atmospheric aerosols in concentrations too high to be explained in terms of normal crustal weathering processes.Consequently, these elements are immensely originating from the anthropogenic sources (Loyola et al., 2006;Yongming et al., 2006).In urban environments, the anthropogenic sources of the majority of the enriched elements are the emissions of combustion from mobile sources (vehicle-exhaust emissions) and industrial activities (Lough et al., 2005;Birmili et al., 2006).The fraction of elements coming from non-crustal sources [(C x )*] can be calculated by the following formula (Mason, 1966) 4. According to the results, a substantial part of Na, Mg, Ca, K, Fe and Mn entered to the air from the Earth' crust, while Co, Cu, Zn, Ni,Cd, Pb and Cr are mostly emitted by anthropogenic sources in all seasons.
Weekday and weekend aerosols mass concentrations and their related elements concentrations are shown in Table 5.The average aerosols concentrations were higher during weekdays than weekends in the summer and winter seasons.The aerosols averaged 400 and 250 µg/m 3 on weekdays and 275 and 212 µg/m 3 on weekends, during the winter and summer seasons, respectively.The weekdays/weekends aerosols concentration ratios were 1.45 on winter and 1.18 on summer.The aerosols concentrations on the weekends compared with weekdays were reduced by 31.25% in winter and 15.2% in summer.This reduction could be attributed to the decrease of traffic density due to the official days-off of government institutions, schools and colleges.The effect of decreasing traffic density during weekends did not only cause low exhaust particulate emission, but also reduce emissions generated from tyre wears and re-suspension of street dust.In addition, the reduced emissions of nitrogen oxides (NO x ) from the traffic sources on weekends lead to a decrease in the formation of secondary aerosol.This is consistent with the observations of other investigators who have attributed the decrease in aerosols concentration on weekend days to the decrease in traffic density (Latha and   Highwood, 2006;Karar et al., 2006;Lonati et al., 2006;Lough et al., 2006).In the present study, although the aerosols mean concentrations were lower on weekend days than working days compared with other urban areas, the higher aerosols levels on weekends indicate that the emissions were not only derived from local source but a part of them are transported from other sources around the study area.
In the present study, the average concentrations of the elements in aerosols were higher during weekdays than weekends (Table 5).The weekday/weekend concentration ratios were 1.48 and 1.28 for Na, 1.58 and 1.36 for Mg, 1.68 and 1.35 for Ca, 1.55 and 1.25 for K, 2.10 and 1.62 for Al, 1.69 and 1.40 for Fe, 1.70 and 1.40 for Mn, 1.40 and 1.26 for Co, 1.35 and 1.23 for Cu, 1.80 and 1.60 for Zn, 1.40 and 1.24 for Ni, 1.41 and 1.33 for Cd, 1.40 and 1.25 for Pb, 1.39 and 1.22 for Cr, and 1.69 and 1.38 for total measured elements during the winter and summer, respectively.The higher concentrations of the measured elements on weekdays may be attributed to the higher weekday aerosols concentrations.The higher concentrations of crustal-mineral components on weekdays than weekends resulted from the influence of local and regional anthropogenic activities on the generation of crustalmineral dust, such as re-suspension of street dust by traffic and different anthropogenic activities, such as industrial processes and building construction.The effect is especially noted during winter season, because of colder and wetter weather conditions, natural emissions of dust by the action of the wind over bare ground are at a minimum.Anthropogenic activities, such as industrial processes and building construction are possible causes for increase the crustal species on weekdays, but the most probable source of anthropogenically emitted dust is re-suspension of road dust by road traffic.The latter is an efficient emission process for dust, because the intense road traffic rapidly dries road ground, even in winter and colder weather conditions (Almeida et al., 2006).This is also confirmed by higher levels of Zn at the same periods, which originates from motor oil and tire wear (Sternbeck et al., 2002).

CONCLUSIONS
Seasonal distributions, weekdays/weekends differences in ambient aerosols and their elemental contents in the atmosphere of the 15 May City, Egypt during the period from March 2009 to February 2010 have been discussed in this work.The lowest average aerosols level (250 μg/m 3 ) was found in summer, whereas the highest average (400 μg/m 3 ) was detected in winter.The average aerosols concentrations exceeded the WHO and annual average of the Egyptian Ambient Air Quality Standard, indicating that the 15 May City is heavily polluted by particulate matter.The winter/summer concentration ratios were 1. 44, 1.18, 1.24, 1.31, 1.34, 1.47, 1.46, 1.67, 1.65, 1.45, 1.66, 1.65, 1.44 and 1.84 for Na, Mg, Ca, K, Al, Fe, Mn, Co, Cu, Zn, Ni, Cd, Pb and Cr, respectively.Na, Mg, Ca, K, Al and Fe were the dominant elements in the atmosphere of 15 May City.The average concentrations of ambient aerosols and their elemental contents were higher during weekdays than on weekends.The weekday/weekend concentration ratios were 1.48 and 1.28 for Na, 1.58 and 1.36 for Mg, 1.68 and 1.35 for Ca, 1.55 1.25 for K, 2.10 and 1.62 for Al, 1.69 and 1.40 for Fe, 1.70 and 1.40 for Mn, 1.40 and 1.26 for Co, 1.35 and 1.23 for Cu, 1.80 and 1.60 for Zn, 1.40 and 1.24 for Ni, 1.41 and 1.33 for Cd, 1.40 and 1.25 for Pb, 1.39 and 1.22 for Cr, and 1.69 and 1.38 for total measured elements during the winter and summer, respectively.The mean Ni and Cd levels were higher than the proposed WHO, USEPA and the European Community standards.The correlation coefficients between the elemental concentrations of aerosols, enrichment factors (EFs) and the non-crustal fractions of all elements indicated that Na, Mg, Ca, K, Fe and Mn are mainly from soil sources, whereas Co, Cu, Zn, Ni, Cd, Pb and Cr are mostly emitted in the atmosphere of 15 May City from anthropogenic sources.

Fig. 1 .
Fig. 1.Map of the greater Cairo areas showing districts of residential (R), industrial (I), residential industrial (RI) activities and the sampling site (•).

Fig. 2 .
Fig. 2. Seasonal variations of the contribution of each element to total elements mass concentrations of the atmospheric aerosols.
: (C x )* = {(C x ) air -(C Al )air.(Cx /C Al ) crust }/(Cx) air (1) where: (C x ) air : the concentration of an element x in the air (C Al ) air : the concentration of Al in the air (Cx) crust : the concentration of an element x in the crust (C Al ) crust : the concentration of Al in the crust.The percentage of crustal and non-crustal fractions of the detected elements during the four seasons are shown in Table

Fig. 3 .
Fig. 3. Seasonal variations of the enrichment factors of atmospheric elements.

Table 1 .
Seasonal variations in atmospheric aerosols (µg/m 3 ) and their related elemental contents levels (ng/m 3 ) during the period of study.These results indicate that crustal elements were the most abundant constituents in aresols of the study area.This is because of re-suspension of road dust and wearproducts of Asphalt pavement 3 in autumn.The seasonal relative contribution of the individual element to total element concentrations in aerosol are presented graphically in Fig.2.The distribution and relative concentration of the individual measured elements were found to follow the following pattern: Ca (28.67%) > Fe (20.34%) > Al (18.96%) > Na (13.31%) > Mg (7.40%) > K (5.32%) > Pb (3.00%) > Mn (0.94%) > Zn (0.

Table 2 .
Correlation coefficients a between the element concentrations in the atmospheric aerosols of 15 May City during the period of study.

Table 3 .
Comparison of average aerosols (μg/m 3 ) and their elemental levels (ng/m 3 ) in the atmosphere of the study area with the counterpart data reported from other regions around the world.

Table 4 .
Crustal and non-crustal fractions (%) in atmospheric aerosols of the study area during the four seasons.

Table 5 .
Weekday and weekend mass concentrations (µg/m 3 ) of aerosols and their related elemental contents levels (ng/m 3 ) during the summer and winter seasons.