Measurements of major ion concentration in settled coarse particles and aerosols at a semiarid rural site in India

Environment International 28 (2002) 1 – 7 www.elsevier.com/locate/envint

Measurements of major ion concentration in settled coarse particles and aerosols at a semiarid rural site in India Gur Sumiran Satsangi1, A. Lakhani, P. Khare, S.P. Singh, K.M. Kumari, S.S. Srivastava* Department of Chemistry, Faculty of Science, Dayalbagh Educational Institute, Dayalbagh, Agra 282 005, India Received 1 July 2001; accepted 16 November 2001

Abstract Deposition rates and deposition velocities of water-soluble ions (F, Cl, NO3, SO4, NH4, Ca, Mg, Na and K) were measured at a rural site (Gopalpura, Agra). Dry deposition samples were collected throughout the year from December 1995 to August 1997, while the aerosol samples were collected only during the winter season of 1996. Surrogate technique was used to collect the dry deposition samples, while aerosol samples were collected on PTFE membrane filter. Deposition velocities (Vd) of SO4 and NO3 are
0.01 m s  1, while Ca, Mg, Na, K, NH4, F and Cl exhibit greater than 0.01 m s  1. Equivalent concentration ratios of K/Na, Ca/Na and Mg/Na conform with the corresponding ratios of local soil, indicating the dominant contribution of local sources. Deposition rates are maximum in winter, followed by summer and monsoon. No significant differences are found in dry deposition rates of all ions or in atmospheric concentrations of soil-derived elements with respect to wind direction. However, in aerosols, concentrations of F, Cl, NO3 and SO4 are higher with winds from southwesterly and westerly directions corresponding to pollution sources located in these directions. Deposition data have been used to calculate the critical load of S and N for soil with respect to Triticum vulgaris. The critical load of actual acidity was found to be 622.4 eq ha  1 year  1 within the range of 500 – 1000 eq ha  1 year  1 as assessed by the RAINS-Asia model for this region. The present load of S and N (77.4 and 86.4 eq ha  1 year  1) was much lower than the critical load of S and N (622.4 and 2000 eq ha  1 year  1), indicating that at present there is no harmful effect on ecosystem structure and function. D 2002 Elsevier Science Ltd. All rights reserved.

1. Introduction Dry deposition is the process by which gases and aerosols are deposited directly from the atmosphere to vegetation, soil or materials in the absence of precipitation. It is governed by the concentration in air and turbulent transport processes in the boundary layer, by the chemical and physical nature of the depositing species and by the capability of the surface to capture or absorb gases and particles (Wesely and Hicks, 1977). Particle dry deposition is responsible for the atmospheric load to ecosystem of compounds such as sulfate, nitrate and ammonium and base cations (such as Ca, Mg, Na and K; Ruijgrok et al., 1995). Deposition of particle containing sulfate, nitrate and ammonium contributes to the potential acidification and eutrophication of ecosystems. Base cation * Corresponding author. Tel.: +91-562-281545; fax: +91-562-281226. E-mail addresses: [email protected] (G.S. Satsangi), [email protected] (S.S. Srivastava). 1 G.S. Satsangi is currently working as a Research Associate in the Department of Chemistry, St. John’s College, Agra 282 002, India.

deposition may be important for nutrient cycling in soil and ecosystems and may also neutralize acidity (Khemani et al., 1985; Varma, 1989). Particle dry deposition may cause detrimental effects, which include widespread acidification of soil, surface water and ground water, corrosion of building materials and decreased atmospheric visibility and formation of secondary pollutants that are toxic to living organisms (Haneef et al., 1992). Particle dry deposition has received far less attention compared with the experimental and modelling work on the dry deposition of the related gaseous compounds. Reasons for less attention are the complexity of deposition processes of particles (Hicks et al., 1986). Current knowledge of deposition is insufficient to give an adequate assessment of dry deposition of sulfur and nitrogen. However, there is a need for such quantification to evaluate critical load exceedance and abatement strategies of atmospheric pollutant. Critical load is defined as the maximum deposition of acidifying compounds that will not cause chemical changes leading to harmful effects on ecosystem structure and function (Nilsson and Grennfelt, 1988). Evaluating critical loads is thus an attempt to link the

0160-4120/02/$ – see front matter D 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 1 6 0 - 4 1 2 0 ( 0 1 ) 0 0 1 2 2 - 2

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emission abatement strategy with the capacity of ecosystem to withstand and buffer the effect of acidic deposition and to set goal or future deposition rates of acidifying compounds such that the environment is protected (Chadwick and Kuylenstierna, 1990). In the present communication, we report the results of deposition rates of particles along with their atmospheric concentration at Gopalpura (a rural site). Data obtained from the study have been used to evaluate critical load of sulfur and nitrogen for soil with respect to wheat (Triticum vulgaris).

2. Experimental 2.1. Site description Gopalpura is located SE of Agra (27
100N, 78
050E) city at a distance of about 52 km. In Gopalpura, 60% of area is under agricultural use (wheat is common crop), 25% of the area is covered by forest with deciduous trees and rest 15% is barren land. Detailed description of site is cited elsewhere (Satsangi et al., 1998a). There is a wide variation in temperature over the summer and winter periods. In summer, temperature varies between 46 and 25
C, while in winter, temperature ranges from 3 to 10
C. Average relative humidity was 36% and 70% in summer and winter, respectively. 2.2. Sampling and analysis Dry deposition samples were collected from December 1995 to August 1997, and the aerosol samples were collected only in the winter season of 1996. Dry deposition was collected by the surrogate collection technique using polypropylene collectors, i.e. bottle and funnel, funnel of 14-cm diameter fitted on 1L polyethylene bottle. Although a funnel is not the ideal surface for dry deposition collection since it does not collect the gases (SO2, NO2 and HNO3; Dasch, 1984) and the shape of the funnel may restrict the entry of small particles (Hicks et al., 1980) if particle deposition is controlled by gravitational settling of large particles, a

funnel may be an adequate surrogate to determine particulate dry deposition. Moreover, surrogate surfaces, despite their limitation, provide the only possibility for routine monitoring and complete chemical analysis. Dry deposition samples were collected on a 24-h basis. Collector was previously rinsed with deionized water. During the monsoon season, the occurrence of rain is not an everyday affair. A break in monsoon season is an observed natural phenomenon. We collected dry deposition samples during this season as and when such conditions prevailed. The deposits collected on the funnel were extracted using 50-ml deionized water. pH was determined in collected samples at the site and transported to laboratory at Dayalbagh (Agra) every day. To minimize atmospheric influences during the transportation, the collected samples were tightly sealed with clean plastic lid. In the laboratory, samples were centrifuged, filtered, transferred into two clean polyethylene bottles, one of which was treated with CHCl3 (0.1 ml/10 ml of sample) to prevent microbial degradation, stored under refrigeration and used for analysis of anions. The other part was acidified with HNO3 and used for cation analysis. The number of samples collected in summer, monsoon and winter were 28, 23 and 27, respectively. As the dry deposition process leads to substantial uncertainty, it was measured by simultaneous collection of deposition on two collectors at the site. Field blanks were also collected by exposing washed collectors for 1 min. These were estimated from 10 triplicate samples. Table 1 comprises the precision, accuracy, field blanks and collection uncertainties of major cations and anions. The deposition rate of a pollutant at the surface is usually expressed in terms of deposition velocity. Deposition velocity for the analysed ions were calculated by dividing deposition rate ( F ) by their airborne concentration (C ), i.e. Vd = F/C. Aerosols were collected on PTFE membrane filters of 1.2-mm pore size using a single-stage, open face (47 mm diameter) filter holder at a flow rate of 12 ± 1 LPM for 24 h. Water-soluble particulate matter from the filter was extracted. The filter was wetted with methanol and then agitated with 50 ml of deionized water on a mechanical shaker for 2 h. The samples were then filtered and the filtrate was divided into two parts and treated in a similar manner as the dry deposition

Table 1 Accuracy, precision, uncertainties and field blanks

F Cl NO3 SO4 NH4 Ca Mg Na K

Accuracy (%)

Precision (%)

Collection uncertainties

Field blanks (mg m  2 day  1  10  7 )

3.3 4.3 4.2 6.3 5.0 7.0 5.0 2.0 1.0

1.2 0.8 1.2 1.2 10.0 13.0 15.0 5.0 2.0

8.0 3.0 11.0 0.6 8.4 18.0 10.0 12.0 8.0

0.4 0.3 0.5 0.3 0.4 1.1 0.5 0.5 0.4

G.S. Satsangi et al. / Environment International 28 (2002) 1–7

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Table 2 Comparison of average dry deposition (mg m  2 day  1) and seasonal variation of dry deposition at Gopalpura Average (mg m  2 day  1) b

F Cl NO3 SO4 NH4 Ca Mg Na K a b c

Seasonal variationa (mg m  2 day  1) c

Pune

Agra

Gopalpura

– 1.2 1.5 0.6 0.07 2.6 0.3 0.6 0.2

– 1.5 2.1 2.3 – 4.7 0.4 0.8 0.6

0.7 2.7 1.0 1.5 3.1 2.6 1.7 1.5 1.0

a

Summer

Winter

Monsoon

0.4 ± 0.2 1.7 ± 0.9 1.7 ± 0.6 1.3 ± 0.8 2.7 ± 1.1 2.8 ± 1.1 1.6 ± 1.0 1.5 ± 0.6 1.0 ± 0.3

1.3 ± 1.0 4.4 ± 1.0 1.4 ± 0.6 2.1 ± 0.9 3.5 ± 1.0 3.0 ± 1.1 1.8 ± 1.0 1.8 ± 0.7 1.3 ± 1.0

0.5 ± 0.2 2.1 ± 1.0 0.6 ± 0.3 1.0 ± 0.6 3.3 ± 1.0 2.1 ± 1.1 1.4 ± 1.0 1.4 ± 0.4 0.5 ± 0.2

Present study. Rao et al., 1992. Saxena et al., 1996.

samples for storage and analysis. To test for complete extraction, filters that had been extracted in the above manner were again subjected to the same procedure. Similarly, field blanks were also determined by putting a clean filter in the holder and then removing it after passing air for 1 min. The filter was then subjected for the same analytical procedure. The levels of anions and cations were below detection limit in these test samples and field blanks. Total number of aerosol samples was 11. The major water-soluble ions were also determined in soil samples. Twenty-five samples were collected from separate locations spread randomly in the vicinity of sampling site and 0.5-g dry sieved (250 mm) soil was shaken with 100 ml of double-distilled water for 2 h. The solution so obtained was filtered. The concentration of different ions were determined in soil extract with the same techniques used for deposition and aerosol samples. The anions F, Cl, NO3 and SO4 were analyzed by Dionex DX 500 Ion chromatograph using AS4A-SC column with self-regenerating suppressor (SRS). Mixture of Na2CO3 (1.8 mM) and NaHCO3 (1.7 mM) was used as the eluent for inorganic ions. The major cations Ca, Mg, Na and K were analysed by atomic absorption and emission techniques using PerkinElmer atomic absorption spectrophotometer (AAS-2380

model). NH4 was determined colorimetrically by the indophenol blue method (Weatherburn, 1967).

3. Results and discussion Geometric mean and standard deviation of all watersoluble components (F, Cl, NO3, SO4, NH4, Ca, Mg, Na and K) in dry deposition samples are presented in Table 2. pH of extract of dry deposition samples varies from 6.8 to 7.2. Deposition rate is maximum for NH4 and decreases successively for Cl >Ca >Mg>SO4  Na >NO3 >K>F. The observed dry deposition rates are compared with the rates reported at Agra (suburban; Saxena et al., 1997) and Pune (urban; Rao et al., 1992). Table 2 also comprises the average dry deposition rate at Gopalpura, Agra and Pune. Deposition rates of Mg, Na and K at Gopalpura are higher than those reported at Pune and Agra (Rao et al., 1992; Saxena et al., 1997). Higher rate at this site suggests that deposition rate is influenced by local sources preferably soil. Dry deposition rate of NO3 at Gopalpura is 1.5 and 2.1 times lower than the rate of NO3 reported at Pune and Agra, respectively, while SO4 is 2.5 times higher than Pune and 1.5 times lower from Agra. Higher SO4 deposition rate at Gopalpura as compared

Table 3 Comparison of atmospheric concentration (mg m  3) and calculated dry deposition velocity Atmospheric concentration in winter (mg m  3)

F Cl NO3 SO4 NH4 Ca Mg Na K a b c

Puneb

Agrac

Gopalpuraa

Dry deposition rate (winter; mg m  2 day  1)a

Deposition velocitya (10  2  m s  1)

– 0.8 1.3 1.4 0.5 0.8 0.2 0.4 0.4

5.7 6.0 10.5 – 1.5 7.6 15.5 1.0

0.5 ± 0.3 3.0 ± 1.1 1.6 ± 0.9 4.6 ± 1.1 2.1 ± 0.9 1.0 ± 0.4 0.8 ± 0.6 1.2 ± 1.0 1.0 ± 0.8

0.4 ± 0.2 1.7 ± 0.9 1.7 ± 0.6 1.3 ± 0.8 2.7 ± 1.1 2.8 ± 1.1 1.6 ± 1.0 1.5 ± 0.6 1.0 ± 0.3

2.9 1.7 1.0 0.5 2.0 3.5 2.5 1.7 1.5

Present study. Rao et al., 1992. Saxena et al., 1996.

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to Pune might be due to local soil influences. The deposition rate of NH4 at Gopalpura is much higher than Pune. Higher NH4 concentration are probably because this site is exclusively a rural area and also samples collected were of 24-h duration, whereas Pune samples were collected on a weekly basis, which could have resulted in losses of NH3 due to its biodegradable and volatile nature. However, postdeposition of gaseous NH3 onto particles deposited on dry deposition collector cannot be discounted. The ionic concentration of water-soluble aerosols collected at Gopalpura varies from 0.5 to 4.6 mg m  3 (Table 3). Average concentration of anions inPaerosols P is 9.7 mg m  3, 3 while cations is 6.0 mg m . The cations/ anions is 1.3. Ratio of SO4/NO3 is 2.9 and approximately twice higher than the ratio of their deposition rate (1.5). Table 3 also shows the comparison of present study (Gopalpura) with Pune (Rao et al., 1992) and Agra (Kulshrestha et al., 1995a). It is evident from the table that the concentration of NO3, SO4 and Cl at Gopalpura are 3.8, 2.3 and 1.9 times lower than their concentrations observed at Agra (Kulshrestha et al., 1995a) but higher than observed at Pune (Rao et al., 1992). The concentration of NH4 in Gopalpura samples are observed to be 4.2 times higher than Pune. NH4 were also observed to be higher in rainwater samples at this site (Satsangi et al., 1998a). The concentration of alkaline species mainly Ca is 1.5 times lower than Agra and 1.3 times higher than Pune. Observation on aerosols in different environments [urban (Pune), suburban (Agra) and rural (Gopalpura)] have shown that the aerosol is basic in nature and is responsible for alkaline pH of both rainwater and dry deposition (Saxena et al., 1992, 1996, 1997; Rao et al., 1992; Khemani et al., 1982; Satsangi et al., 1998a). Deposition velocity (Vd) was only calculated for winter season because the data were only available for that period. The deposition velocities of different ions are given in Table 3. Deposition velocity is less than and equal to 0.01 m s  1 for SO4 and NO3, respectively, while greater than 0.01 m s  1 for Cl, Na, Ca, Mg and NH4. An earlier study on size distribution of aerosols at Agra (Kulshrestha et al., 1995b, 1998) revealed that mass median diameter (MMD) of soil origin particles namely Cl, Na, Ca and Mg are around 4.1 mm containing 64%, 69%, 53% and 60% in coarse mode, respectively, while MMD of NO3 and SO4 varies from 1.1 to 2.1 mm comprising 67% and 58%, respectively, in the fine mode. The deposition velocity of ions are directly proportional to their MMD. Therefore, the deposition velocity of soil origin particles Cl, Na, Ca and Mg is consistent with their MMD and lower deposition velocity of SO4 and NO3 are also expected due to their smaller MMD. In particular, it is observed that deposition of Ca is seven times greater than SO4. This implies that these large particles are efficiently removed by the dry deposition phenomenon. The variation between the dry deposition rate and ambient particulate concentration for Ca and Mg, NO3 and SO4 are shown in Fig. 1a and b, respectively. Variation of dry deposition rates of NO3 and SO4 were similar to their

particulate concentration. The correlation coefficient between the ambient concentration and deposition rate for soil-derived elements Ca, Mg, Na and K are  0.57,  0.76,  0.64 and 0.65, while for Cl, NO3 and SO4 are 0.59, 0.42 and 0.30, respectively. All the correlations are significant at the 95% level. Inverse relationship between the ambient concentration and dry deposition rates for soilderived elements, except K, reflects that maximum concentration of these species in dry deposition occurs at times of their minimum concentration in air. This shows that Vd increases much when the atmospheric concentration decreases. This is consistent with their higher MMD resulting in greater deposition velocities. Davidson et al. (1989) have observed higher concentration of NO3 in snow when their concentration in air were lower and this has been attributed to increased scavenging. Ambient concentration of K is well correlated with its dry deposition rate. It is emitted by both natural and anthropogenic sources. At this rural site, anthropogenic emissions of K include wood burning and combustion of vegetative matter. Hence, it is speculated that K exists in substantial amounts in both coarse and fine modes. Low correlation was found between the ambient particulate concentration and dry deposition rate for NO3 and SO4 (r =.42 and .30). A possible reason could be that the large particle NO3 and SO4 may control the dry

Fig. 1. (a) Variation between the dry deposition and particulate concentration of Ca and Mg. (b) Variation between the dry deposition and particulate concentration of NO3 and SO4.

G.S. Satsangi et al. / Environment International 28 (2002) 1–7 Table 4 Ratios of ionic concentration in aerosol samples, dry deposition rate, soil and sea salt ratio (meq/meq)

Aerosols Dry deposition rate Local soil Sea salta a

Cl/Na

K/Na

Ca/Na

Mg/Na

SO4/Na

NO3/Na

1.6 1.2 3.9 1.16

0.5 0.4 0.5 0.02

0.9 1.9 1.7 0.04

1.3 1.9 1.8 0.22

1.8 0.5 0.8 0.12

0.5 0.2 0.2 –

Tsitouridou and Samara, 1993.

deposition rate but make only a minor contribution to the suspended particle mass. 3.1. Seasonal variation We examined the variation in concentration of ionic species with respect to season. The mean dry deposition rate of the major ionic components during summer (March – June), monsoon (July – September) and winter (October – February) are presented in Table 2. It was observed that deposition rates of ions were maximum in winter followed by summer and monsoon seasons, except for NO3, which showed highest concentration in summer. Variation in deposition are, perhaps, influenced by the meteorological conditions. In summer, high temperature and strong winds make the atmosphere unstable and turbulent, leading to maximum dispersion and dilution of pollutants. Contrarily, in winter, because temperature drops very low, atmospheric conditions become calm and these factors promote the stagnation of the pollutants, which are further enhanced by the frequent temperature inversion. Besides, the alkaline deposits and surface moistening also favour the adsorption of gases and particles. In addition, certain local activities may also lead to higher input of these substances. These include burning of wood, coke and other combustible matter for domestic heating purposes and burning of sugarcane fields after its harvest. Most of the villagers prepare jaggery from sugarcane juice, which is heated in large pans using coke and cowdung cake as fuel. However, higher concentration of NO3 in summer probably results from greater formation of NO3 by photochemical reaction (Saylor et al., 1992; Rodhe et al., 1988). The ratio of mean NO3/SO4 varied from 1.3 in summer to 0.7 in winter. Summer/winter ratio for F, Cl, NO3, SO4, NH4, Ca, Mg, Na and K are 0.3, 0.4, 1.2, 0.6, 0.8, 0.9, 0.8, 0.9 and 0.7, respectively.

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3.2. Influence of local sources In an attempt to investigate the possible sources of ions in the aerosols and dry deposition, the ionic concentration ratio (meq/meq) of various ions (Cl, K, Ca, Mg, SO4 and NO3) with respect to Na were calculated and were compared with the ratio of sea salt and local soil (Table 4). It is evident from the table that the ratios are higher than the sea salt ratio, indicating the presence of nonmarine constituents. In dry deposition, K/Na, Ca/Na and Mg/Na concentration ratios were 80 –90% similar to local soil, suggesting an influence of soil. This is further corroborated by the correlation analysis in dry deposition samples, indicating that these elements are well correlated to each other (Ca and Mg = 0.75, Ca and Na = 0.55, Mg and Na = 0.65, Mg and K = 0.59 significant at .001 level). Similarly, in aerosols, concentration ratios were found to be close to the corresponding ratios of local soil. SO4/Na and NO3/Na ratios in aerosols are higher than soil ratio, while in dry deposition, respective ratios are lower than the aerosols. This may be due to the existence of particles in both coarse and fine modes in aerosols, which may lead to the higher ratios, while deposition of only coarse particles to the surrogate may result in lower ratios. SO4 and NO3 in this region are found to exist both in coarse and fine modes. On a percent basis, SO4 and NO3 are 58% and 67% in fine mode and 42% and 33% in coarse mode, respectively (Kulshrestha et al., 1998). 3.3. Wind direction influences In order to investigate the effect of distant sources on aerosols and dry deposition rate, the events were separated by wind direction into the following sectors: 0 –45
as N, 45 – 90
as NE, 90 –135
as E, 135 –180
as SE, 180– 225
as S, 225– 270
as SW, 270 – 315
as W and 315– 360
as NW. Since 11 samples of aerosols were collected and the prevailing wind directions were SW, W and E during the collection period (winter of 1996), the data could be divided into these three sectors only. Higher concentration of F, Cl, SO4 and NO3 are found when wind blows from westerly and southwesterly directions. These concentrations are significantly different from the other wind directions. On comparing the excess concentration of these ions with respect to soil, 6 –18% is affected from westerly winds and 26– 39% concentration by the southwesterly winds. Presumably, their concentrations are affected by emissions

Table 5 Computed value of wet and dry deposition (eq ha  1 year  1) of SO4, NO3 and NH4 Dry deposition Wet deposition Total

Sulfate

as S

Nitrate

as N

Ammonium

Reference

114.1 118.1 232.1

38.0 39.4 77.4

58.9 326.2 385.1

13.3 73.6 86.9

628.6 331.9 960.5

Present study Satsangi et al. (1998a,b)

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from industries and automobiles at Agra located approximately 300
(NW) of Gopalpura. No significant differences are observed for soil-derived elements. In dry deposition, no significant variations are observed in deposition rates of all measured ions, suggesting deposition rates are not dependent on wind direction. It further suggests that local sources contribute to deposition rate. 3.4. Calculation of critical load for soil with respect to T. vulgaris The link between atmospheric deposition and soil loads is important because critical loads refer to soil loads and atmospheric deposition estimates provide a link with emissions. Thus, if critical load exceedances are used to estimate emission reductions, the relation between atmospheric deposition and soil load should be known (Erisman et al., 1997). The steady-state mass balance (SMB) method was used to determined critical load. Critical load was calculated for soil with respect to T. vulgaris (wheat) using wet deposition data (Satsangi et al., 1998a) and dry deposition data (present study). Computed values of wet and dry deposition rates (eq ha  1 year  1) of NO3, SO4 and NH4 are given in Table 5. Critical load of actual acidity (CLAcact) was calculated as follows: CLAcact = BCw + Hcrit Q + Alcrit Q, where BCw = base cation weathering rate (eq ha  1 year  1), Hcrit = critical hydrogen concentration (eq m  3), Alcrit = critical aluminum concentration (eq m  3) and Q = runoff (m3 ha  1 year  1). After calculating critical load of actual acidity, critical load of sulfur (CL(S)) and critical load of nitrogen (CL(N)) were calculated. The detailed description of method and value of different parameters have been discussed elsewhere (Satsangi et al., 1998b). The calculated value of critical load of actual acidity was found to be 622.4 eq ha  1 year  1. On comparing the critical load of S and N with present load of S, it was found that the present load of S (77.4 eq ha  1 year  1) and N (86.4 eq ha  1 year  1) (Table 5) were much lower than the critical load of S and N (622.4 and 2000 eq ha  1 year  1) for soil with respect to T. vulgaris. RAINS-Asia model assessed acidity for Agra region with the value of 500 –1000 eq ha  1 year  1. The value of critical load of acidity is within the range of RAINS-Asia model.

4. Conclusions The calculated deposition velocities for SO4 and NO3 are
0.01 m s  1 and Vd for Ca, Mg, NH4, Na, K, F and Cl are >0.01 m s  1. Higher deposition velocities reflect that these components are soil-derived elements. Variation of dry deposition rates of SO4 and NO3 were similar to the particulate concentration, while deposition rates of soil-derived elements vary inversely with its airborne concentrations. The equivalent concentration ratios of K/Na, Ca/Na and Mg/Na are found to be close

to the corresponding ratios of local soil. Deposition rates are observed to be maximum in winter season. The deposition rates of various ions do not significantly depend on wind direction. For aerosols, concentration of soil-derived elements are independent of wind direction, while concentrations of anions (F, Cl, NO3 and SO4) are higher with westerly and southwesterly winds and are probably due to emissions from Agra. Deposition data have been used to evaluate the critical load of S and N for soil with respect to T. vulgaris. Critical load of actual acidity was found to be 622.4 eq ha  1 year  1. On comparing the present load of S and N with critical load of S and N, it was found that present load was much lower than the critical load, indicating that at present there is no risk to ecosystem.

Acknowledgments The authors are grateful to Prof. Satya Prakash, Head, Department of Chemistry at this Institute, for providing laboratory facilities. This work was completed under project SS/B11-126/94 funded by DST, New Delhi. One of the authors (G.S. Satsangi) acknowledges CSIR for providing the research grant. Sri Dharmendra Singh Yadav’s assistance during sampling is greatly appreciated.

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