Armosphmc Envrronmvnt Vol 31, No. 15. pp. 2361-2366, 1997 \c; 1997 Elsev~er Scum Ltd

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DRY DEPOSITION OF SULPHATE AND NITRATE TO POLYPROPYLENE SURFACES IN A SEMI-ARID AREA

OF INDIA

ANITA SAXENA, U. C. KULSHRESTHA, N. KUMAR, K. M. KUMARI, SATYA PRAKASH and S. S. SRIVASTAVA

Department of Chemistry, Faculty of Science, Dayalbagh Educational Institute, Dayalbagh, Agra 282 005, India

(First received 25 March 1996 and in final jbrm 25 Nocember 1996. Published May 1997)

Abstract-Dry deposition rates of the major ionic species Cl-, NO;, SO:-, Na+, K+, Ca” and Mg’+ were estimated by the surrogate collection technique using polypropylene surfaces. The dry deposition rates of the soil derived cations Ca’+, Na+, K+ and Mg’+ were of the order of 0.4-6.1 mgm-‘d-l. The dry deposition rates of the acidic ions NO; and SO:- were of similar magnitude, suggesting that they are also soil derived or associated with the soil elements. On a seasonal basis, deposition rates were maximum during the winter followed by summer and minimum during the monsoon. 10 1997 Elsevier Science Ltd.

Key word index: Sulphate, nitrate, dry deposition.

INTRODUCTION

Besides wet deposition, dry deposition is another ma- jor atmospheric removal process of both gases and particulates to the earth’s surface including soil, water and vegetation. The dry deposition of small acidifying substances containing SO:-, NO; and NH: con- tribute to the total acid input to ecosystems. For large particles containing base cations the understanding of deposition is important for interpretation of through- fall measurements, nutrient cycling and assessment of critical loads. Continuous rise in emission of air pollutants has increased the necessity for developing reliable methods for quantifying atmospheric depo- sition. So far, dry deposition of particles has recieved far less attention than the deposition of related gas- eous compounds. Particle deposition measurements are surely hampered by the dependence of V, on particle size. Attempts have been made to estimate dry deposition by leaf washing techniques, micro- meteorological methods, throughfall method, water- shed mass balance method, inferential and surface accumulation methods. Reviews by Sehmel and Hodgson (1980), Voldner et nl. (1986), Nicholoson (1988) and Davidson and Wu (1990) show large uncer- tainty in deposition velocities derived from field ex- periments using different techniques. Most of these techniques have restricted use due to stringent site requirements, expense and sampling problems (Vandenberg and Knoerr, 1985).

Surrogate surfaces have been widely used although they may over collect or under collect the depositing particles and they have uncertainty in the extent to which they are representative of a natural collecting surface. Surrogates are potentially useful as they pro- vide a common surface available for application in a wide variety of environments (Vandenberg and Knoerr, 1985; Hicks et al., 1986). It is also the only method by which a chemical sample can be obtained with convenience, approaching that of wet deposition (Davidson et al., 1985). Surrogate surfaces which have been used include wet/dry open buckets (Krey and Toonkel, 1974; Volchok and Graveson, 1975; Servant, 1976), petri dishes (Lindberg et al., 1982), filter paper (Pierson and Cawse, 1979), Teflon plates, coated and uncoated glass (Huntzicker ef al., 1975; Davidson and Friedlander, 1978).

Monitoring of precipitation and assessing the chemical input to ecosystems from precipitation pro- cesses has been underway in India since 1960 and a sizeable databank on wet deposition has been gener- ated (Mukherjee, 1964; Handa et u/., 1982; Mahadevan et al., 1986, 1989; Naik et al., 1988; Pillai et al., 1988; Khemani et al., 1985; Khemani, 1989; Sharma et al., 1990; Saxena et al., 1991). However, while it is probable that dry deposition is by far the dominant deposition process in this geographical re- gion, relatively little experimental work has been conducted on dry deposition (Zutshi et al., 1980; NEERI, 1980; Tripathi et al., 1991). Published data on

2362 A. SAXENA et al.

chemical composition of dry deposition rates is scarce (Prakasa Rao et al., 1992). Therefore dry deposition montoring studies were undertaken at Agra (27”lO’ N and 78”02’ E, 169 mASL), a semi-arid area. Results from a previous short term study on dry deposition of SO:- and NO; at this site (Saxena et al., 1992) have shown that polypropylene petri dishes show max- imum deposition. Studies on aerosol composition in- dicate that it is dominated by coarse particles which are largely soil derived (Kulshrestha et al., 1995). Hence in order to chemically characterise and quan- tify the dry deposition, polypropylene surfaces were used. In the present work we report on the major constituents of dry deposition and infer their fluxes and deposition velocities to the polypropylene surfaces.

METHODS

Site description

Agra is located in the north-central part of India, about 200 km south east of Delhi. Two-thirds of its peripheral boundaries are bounded by the Thar desert of Rajasthan. The soil is calcareous, sandy and dusty. Vegetation is scarce, typically characterized by Xerophytic plants. Climatically, the city observes dry climate during summers and winters. Annual rainfall is about 650 mm with 90% being received during the monsoon season (JulyyAugust). Winters are asso- ciated with greater calm periods (75%) while summers are characterized by strong surface winds with calm periods reduced to about 40%. There is a wide variation in temper- ature over the summer and winter period. Maximum and minimum temperature in summers are normally 45 and 25”C, respectively, while in winters temperatures range be- tween 3°C to 10°C. Relative humidity varies from 81-30% maximum and 18-75% minimum.

Agra has developed industrially in the last two decades. The major industrial activities are ferrous metal casting, ferro alloy and non-ferrous industries, rubber processing, tanneries, lime oxidation and pulverisation, engineering, chemicals and brick industries.

Sampling and analysis

The site for the deposition collection was the roof of the Faculty building on the Institute campus at Dayalbagh located to the north of the city. The site is about 10 km away from the industrial sectors of the city. Dry deposition was collected by the surrogate collection technique using passive collectors made of polypropylene. The collectors were up- ward facing (12” x 12”) trays, mounted on iron stands about 1 m from the floor of the roof. Collectors were also provided

with a (1 m x 1 m) fixed rainshield made of fibreglass, moun- ted about 30 cm above the trays. The trays were exposed for 10 consecutive days from November 1990 through January, 1992. After the end of the exposure period, the deposits were collected in prewashed polyethylene bottle by a minimum quantity of deionized water, using a polypropylene police- man rod. The volume of the extract was finally made to 100 ml. The extract was stirred and left overnight to obtain the maximum water soluble ions. The unfiltered samples were first measured for their pH and conductance and then centrifuged and filtered. An aliquot of the sample was refrig- erated and used for determination of the anions. Remaining sample was acidified and used for cation analysis. The con- centration of the anions Cl-, NO,, SOi- were determined by ion chromatography (Dionex 2OOOi/SP) using 1.8/1.7 mM Na,CO,/NaHCO, eluant and 25 mN H$O, regenerant. Concentrations were determined by peak height measure- ments of standards and samples. The cations Na+, K+, CaZ + and Mg’+ were analysed by Atomic Absorption Spectro- photometry (Perkin Elmer, 2380). Concentrations were con- verted to deposition fluxes in mg m-’ d - ‘. Field blanks were determined in a similar manner as the deposition fluxes by exposing washed trays only for one minute. Uncertainty was determined by simultaneous collection of deposition on two trays at the site. Field blanks and the uncertainties asso- ciated with the ionic analyses of the dry deposition collectors are listed in Table 1. The table also includes the precision and accuracy of the ions analysed.

RESULTS AND DISCUSSION

Seasonally averaged dry deposition rates in mg mm2 d ’ for the various water soluble ions are listed in Table 2. Deposition rates are maximum for Cazf and decrease in the order SOi- >NO; > Cl- >Na+ >Mgzt >K+. In this semi-arid area, the dry deposited Ca ‘+, Mg’+, Na+ and K+ are essen- tially the soil derived aerosols carried into the atmo- sphere by wind blown dust (Saxena et al., 1991; Khemani, 1989). The observed dry deposition rates of Ca’+, Mg’+, K+ and Na+ are similar to those ob- served at Pune, where they have been attributed to be of soil origin (Prakasa Rao et al., 1992).

SOi- and NO; particles which are normally known to be released by anthropogenic activities showed deposition rates of similar order as the soil aerosols Cazf, Mg’+, Naf and K+. Anthropogenic SOi- and NO; are known to exist in the fine mode, on account of their small mass median diameters they possess lower deposition velocities and therefore exert lower deposition rates. The higher fluxes of SO:-

Table 1. Field blanks, uncertainties, precision and accuracy

Field blank” Uncertainty (%)

Accuracyb Precisionb

cl- NO; SOi- Na+ K+ Ca2* Mg’+

0.8 1.6 0.5 1.3 2.6 5.1 2.5 9 6 2 21 15 9 10

0.09 0.03 0.02 0.02 0.01 0.07 0.05 0.17 0.06 0.04 0.05 0.02 0.13 0.15

“Value x 10-7mgm-2s-1 bin figml-‘.

Dry deposition of sulphate and nitrate 2363

and NO; implies their association with soil aerosols. Significant correlations exist between fluxes of SO:- and NO; with the fluxes of cations Ca’ ‘, Mg’ +, Na + and K+ (Table 3) indicating a close relationship be- tween them. Partial correlations between the cations and sulphate and nitrate indicate that sulphate is large- ly deposited with K, Ca and Mg (rk+ soI;noJ = 1.00; rCa: + ,SO:TNOi = 0.85; rMg2+,SO~~NO; = d.87) while ni- trate is normally associated with Ca2+ and K’ (r k+ NO;.sO:-= 0.52; rC.~+,N03.SO~- = 0.4). Mass size distribution studies of aerosols conducted at this site indicate that both SOi- and NO; show a bimodal distribution with an appreciable fraction occurring above the submicron mode, showing a peak in the l.l- 2.1 pm range (Kulshrestha et al., 1995). On a per- cent basis 58 and 67% of SO:- and NO; are present in fine mode and 42 and 33%, respectively, in the coarse mode.

Assuming particle deposition to be controlled by gravitational settling of large particles and the collec-

Table 2. Seasonal dry deposition rates (mgm-* d-l)

Species Summer Winter Monsoon

cl- 1.2 2.1 0.5 NO, 2.6 3.2 0.4 so:- 1.8 4.6 0.6 Ca’ + 4.1 6.1 3.9 Na’ 0.4 1.6 0.3 K’ 0.4 1.1 0.1 Mg”+ 0.4 0.5 0.2

tors serving as adequate surrogates to determine particulate dry deposition, deposition velocities of particulate species were calculated. The ambient coarse fraction concentrations of the various species from mass size distribution study have been used to calculate dry deposition velocities (Kulshrestha et al., 1995). The results are shown in Table 4. The deposi- tion velocity is less than 1 cm s-l for Cl-, SO:-, Na+ and K+ and greater than 1 ems-’ for Ca’+, Mg’+ and NO;.

Being a semi-arid area, it is possible that the local soil has high concentration of SOi- and NO;. Quantitative determination of SOi- and NO; in several local soil samples revealed a mean SO: con- centration of 330 and 65 mg kg- ’ NO; , showing that wind blown soil has the potential to affect the com- position of dry deposition. Moreover, it is also known that gypsum and lime are added to the adjoining barren and unfertile lands to increase fertility. Both CaSO, and Na,SO, are sufficiently water soluble to be completely dissociated in the aqueous extracts (Wall et al., 1988). Hence it is speculated that a frac- tion of the dry deposited SOi- and NO, are derived from soil, a natural input. This is further corroborated by the fact that this region of the country has high TSP concentrations ranging between 200 and 800 pg m- 3 basically composed of soil derived aero- sols (NEERI, 1980). The pH of the dry deposition sample extracts ranged between 6.5 and 7.3, which is the reported pH of soil in this region (ICAR. 1982).

The oxidation of SO, to SO:- on soil particles forming coarse sulphate may also contribute to the observed deposition rates (Winchester et al., 1986;

Table 3. Correlations between fluxes of ionic species

Cl- NO, SOi- Na’ K+ Ca’+ Mg”

CT 1.00 NO, 0.59” 1.00 so:- 0.78” 0.49” 1.00 Na+ 0.04 0.58” 0.48” 1.00

c”,+ 0.41 0.08 0.53” 0.51” 0.79b 0.87b 0.91b 0.38 0.39 1.00 1.00 Mg’+ 0.18 0.15 0.81b 0.46 0.41 0.73” 1 .oo

Note. N = 40. “p = 0.01. hp = 0.001.

Table 4. Calculated dry deposition velocities (cm s- ‘)

Species

Cl_ NO, so:- Ca2+ Mg2+ Na+ K’

Cont. of coarse particle Deposition flux Deposition velocity (Pgm-? (pgm ) -zs-1 (ems-‘)

4.33 0.03 0.6 2.76 0.03 1.1 6.16 0.05 0.7 1.60 0.06 3.8 0.73 0.02 2.2 2.04 0.01 0.5 0.77 0.01 0.6

2364 A. SAXENA et al

Ashu Rani et al., 1992). The alkaline nature of the soil particles would favour this reaction by greater adsorption of SO, (Fugas and Gentilizza, 1978). The atmospheric SOi- particles so formed are greater in size and efficiently scavenged by the dry deposition phenomenon (Gatz, 1977; Lindberg, 1982). Similarly coarse NO; particles formed by reaction of atmo- spheric HNO, with soil particles could explain the observed fluxes of NO,. In our climatic conditions, the formation of HNO, in the day is probably fa- voured by high daytime temperatures and intense sunlight leading to high concentration of OH radicals (Crutzen and Gidell, 1983). Under these conditions the instability of NH,NO, aerosols may further add to HNO, concentrations (Stelson et al., 1979; Stelson and Seinfeld, 1982; Tanner, 1982). In the dark, the following reactions probably result in coarse NO; particles (Richards, 1983).

NO, + NO,-M_t N,O, + M

N,O, + H,O ‘Oil s”rface, HNO,.

A number of investigators have observed coarse frac- tion SOi- and NO; to prevail in the atmosphere (Wall et al., 1988; Savoie and Prospero, 1982; Khemani et al., 1985; Khemani, 1989). Also depleted concentrations of gaseous SO, have been noted to correspond to high concentrations of super micron sulphate at dusty locations supporting the role of coarse particles as sites for oxidation of SO, (Winchester et al., 1986). Dry deposited SO,(g) and HNO,(g) could also be a factor determining the de- position fluxes of SOi- and NO,. Mean gas phase concentrations of SO, in Agra have been recorded to be about 6.5 and 10 pg mm 3 during summers and win- ters respectively (CPCB, 1987). However, the existence of HNO, vapour needs to be verified experimentally.

Seasonal variation in dry deposition rates

The seasonally averaged dry deposition rates (Table 2) show maximum deposition rates for all the constituents during the winter and minimum during the monsoon season. The low monsoon fluxes are evidently due to frequent rain showers, washout ef- fects and also low emission fluxes from soil which settles during this period.

In summers, occasional dust storms and strong surface winds can cause lifting of aerosols of local soil origin. This should have resulted in higher deposition rates. In contrast, the rates were observed to be higher in winter season. A similar trend has been observed at Pune (Prakasa Rao et al., 1992). Greater flux during the winters in this region can probably be explained to be due to differences in meteorological conditions during the two seasons. Agra being an inland station adjacent to a desert, observes large seasonal and diur- nal temperature variations. In summer, due to high temperature and strong winds, the atmosphere is un- stable and turbulent and leads to maximum disper- sion and dilution of pollutants. On the contrary, in winters temperatures are much lower and calm, stable conditions prevail promoting stagnation of pollutants which is further enhanced by frequent temperature inversions. Moreover, the persistence of humid and foggy conditions during the winter would tend to maximize deposition rates through deposition by fog and checking resuspension due to moistening of the collector surface. It is also probable that the deposited material may enhance the deposition characteristics of the surface. Additionally, the alkaline nature of the deposited material and moistening of the surface would favour adsorption of gases and particles. These factors may therefore qualitatively account for greater deposition during the winter. An examination of the monthly dry deposition rates reveals that maximum

Fig. 1. Monthly variation in dry deposition rates of NO; and SO:-.

Dry deposition of sulphate and nitrate 2365

flux were observed during December and January months, when the winter in Agra is at its peak (Fig. 1).

CONCLUSIONS

Input of elements to ecosystems by dry deposition involves supermicron as well as submicron particles. The dry deposition rates of the soil derived cations Ca2+, Na+, K+ and Mg2+ were of the order of 0.4-6.1 mgm-* d- I. The dry deposition rates of ni- trate and sulphate were also of similar magnitude, suggesting that they are also soil derived or are asso- ciated with the soil elements. The deposition velocities were less than 1 cm s - ’ for Cl-, SO:-, Na+ and K+ and greater than 1 ems-’ for Ca’+, Mg’+ and NO;. The study indicates that supermicron S and N con- taining particles may be an appreciable fraction of the total S and N dry deposition. However, the formation of extrinsic sulphate and nitrate on the surfaces should be further investigated. Temporal variability in dry deposition rates has been observed, with greater deposition rates during winters, intermediate in summers and minimum in monsoons. Temporal variability appears to be governed in part, by the meteorological factors.

Acknowledgements-The authors gratefully acknowledge the grants provided by the Council of Scientific and Industrial Research and the Ministry of Environment and Forests, New Delhi, India, and The India Meteorological Depart- ment, New Delhi, for the meteorological data.

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