Indian Journal of Radio & Space Physics Vol. 31, April 2002, pp. 88-92

Measurements of dry deposition of gaseous and particulate sulphur on marble at a suburban site

Ranjit Kumar, Abha Rani , S P Singh, K Maharaj Kumari & S S Srivastava

Department of Chemistry , Faculty of Science, Dayalbagh Educational Institute, Dayalbagh, Agr:1 282 005

Received 9 April 2001; revised 3 September 200 I; accepted 12 October 200 I

Dry deposition of S (sulphur) as sulphate on white marble (natural surfaces) has been measured at Dayalbagh, a subur­ban site of semi-arid region of Indi a, in winter season (December 1999-March 2000). Samples were collected by exposing the white marble for 24 h and analyzed by Dionex Dx-500 Ion Chromatograph system. Atmospheric concentration of S02

was 7.8 ± 2.0 !lg m-3 and particulate sulphate was 7.9 ± 1.8 !lg m-3. The dry d.eposition of S as sulphate has been found to

be 0.19 ± 0.12 mg m-2 h- 1. The percentage con tribution of the particulate su lphate (78.9) was hi gher than that of gas phase

(21. 1 ). The calculated dry deposition velocity ( Vd) of particulate sulphate has been found to be 0.53 ± 0.27 em s - I. The Vd calculated by present method is lower than that reported by throughfall method, while Vct range is comparable to other methods like foliar extraction, micrometeorological and surrogate methods.

1 Introduction The deposition of pollutants is of major concern

due to their detrimental effects including widespread acidification of soil, surface water, ground water, in­jury to vegetation, corrosion of building materials, decreased atmospheric visibility and formation of secondary pollutants, etc. Acidic deposition and their adverse consequences in the form of nature degrada­tion have attracted attention of scientific community to-day.

Two mechanisms for transferring pollutants to material surfaces are defined as wet and dry deposi­tion, i.e. transport of pollutants in the form of either gases or small particles to ground level (dry deposi­tion). The dry deposition process involves a close in­teraction between the atmosphere and the surface in which the characteristics of individual underlying sur­faces often determine the mass-transfer rates and it has been reported that dry deposition plays a major role in the deposition of acid substances on buildings 1•

Dry deposition is usually characterized by a depo­sition velocity (Vct). which is defined as the flux (F) of the species (S) to the surface divided by the concen­tration [S] at some reference height, i.e.

vd = [~J . .. (1)

The amount of the species deposited per un it area per second in a geographical location, i.e. the flux, can be calculated if the deposition velocity and the

pollutant concentration are known. The deposition velocity is also frequently related to resistance (r) as

1 vd =- ... (2) r

By analogy to electrical systems, the resistance r can be thought of as being comprised of several com­ponents in dry deposition2-4, such as the aerodynamic resistance Ra. the quas i-laminar resistance Rb and can­opy or surface resistance Rc. The inverse of the sum of the resistance gives the deposition velocity,

1 vd = ... (3) (Ra + Rb + Rc)

Of all the constituents produced by the combustion of fossil fuels, so2 is the most potent for stone decay processes. Dry deposition is the prevailing mechanism during which the pollutants accumulate as aerosols on stone surfaces and are activated by subsequent humid conditions. Dissolved in dew drops (highly concen­trated state) they descend as acid solutions on the stone surface and penetrate into the intergranular spaces. Acidic gases readily react with marble calcite (CaC03) and form CaS04.2H20 , Ca(N03h, CaCh.6H20 and so on. Because the solubility of these products is higher than that of CaC03, which is formed at the surface by the reaction of Ca(OHh with C02 (carbonation), they are dissolved in rain water and penetrate into the inner pores of concrete. After the evapora1:ion of water drops, these salts redeposit and give rise to stress in the buildings, which causes

RANJIT KUMAR et a/.: DRY DEPOSITION OF GASEOUS & PARTICULATE SULPHUR ON MARBLE - 89

their deterioration5. Depending on the characteristics

of the surface, the ability to capture pollutants from the air may vary by more than an order of magnitude, causing highly varied dry deposition rates even over small scales.

For S species, studies are concentrated on S02, S04

and S containing vapours. There is a wide variation in the deposition velocit/ of so2 ranging from about 0 to 3.4 em s- 1

• This variation may be due to differences in rates of aerodynamic transport. In addition, stoma­tal opening and closing corresponding to stomatal resistance also account for difference in vegetative canopies, reflecting higher Vct during daytime due to open stomata and lower in night owing to closed sto­mata. Dry deposition rates of S02 are also found to be increased by as much as ten-fold on vegetative cano­pies when leaves are wet6

. Deposition velocity of S04

has also been estimated by various methods including micrometeorological, surrogate 7-

9, foliar extraction 10

and throughfall studies". But all the studies per­formed earlier are by using indirect methods. Direct measurements are rare, and in India such studies do not exist.

Present knowledge is insufficient to give an ade­quate assessment of the dry deposition of particulate and gaseous sulphur on natural surfaces. The meas­urement of dry deposition of air pollutants continues to be a challenging task. It appears to be an important process for Indian climatic conditions, as dry condi­tions prevail for most part of the year, while rains are confined to a short monsoon period. Studies on dry deposition is scarce, only a few are available 12

-18

•

Studies reported from India are all on surrogate sur­faces. The use of surrogate surfaces leads to uncon­trolled deposition of larger particles. It provides accu­rate measurements for rapidly falling particles 19 and also gains credibility as particle size increases7

, but performance of these devices to measure deposition of small particles , like sulphate, which lies in the sub-

. 20 - f . d7 s mtcron range ts o ten quest10ne . urrogate sur-faces are not true representative of natural surfaces. To date, no study on natural surfaces has been re­ported from Indi a. The present study proposes to fill this void . In thi s paper, we have reported dry deposi ­tion rates of sulphur as sulphate on white marble and tried to estimate 'percentage contribution of gas/parti­culate phase in dry deposition of sulphur species.·

2 Methods and Materials 2.1 Site description

The sampling site (Fig. I ) is located in Agra (north-

central India), which is about 200 km south-east of Delhi. It is situated in sub-tropical Indi a in a semi-arid zone. Two thirds of its peripheral boundaries are bounded by the Thar Desert of Rajasthan . The soil is calcareous, sandy and dusty in nature. The major in­dustrial activities are ferrous metal casting, ferro alloy and non-ferrous industries, rubber processing, tan­neries , lime oxidation and pulverization, engineering, chemicals and brick industries .

Samples were collected at Dayalbagh, a suburban site that is located in the north of the city. The site is I 0 km away from the industrial sector of the city. Due to agricultural practices, vegetation predominates.

2.2 Sample collection system

Dry deposition samples were collected on white marble. The white marble was exposed in the atmos­phere for 24 h in winter season (December 1999-March 2000) on clear days (other than dewy or foggy days). A 0.05 m2 white marble was kept at an angle of about 80° to an iron stand system (1.5 m height) on the roof of the faculty building in the Institute campus

" ,, ,, " " " ,, ,, ,, ,, ,, ,, ,, ,,

" ,,

,, " " " " .. ,, ,,

,, " ,,

Fig !-Location of sampling si te at Dayalbagh

t

90 INDIAN J RADIO & SPACE PHYS, APRIL 2002

at Dayalbagh. Samples were collected by surface washing method. Before the exposure the marbles were washed carefully with deionised water by a sprayer to minimize the blank values. After drying for lO minutes under IR lamp, marble slabs were exposed for dry deposition. After 24 h exposure, dry deposits were collected by washing with deionized water. The collected samples were centrifuged and supernatant was used for analysis of major ions. The supernatant of sample was divided into two parts-one part was stored in refrigeration with chloroform and was used for anion analysis, while the other part was acidified with dilute HN03 and used for cation analysis. All the plasticwares used for storage were cleaned with de­ionized water.

Field blanks were collected prior to sampling by exposing the slabs for one minute. The field blanks for white marble, analytical precision, accuracy and detection limits for so4 are 0.5xl0-2 mg m-2 h- 1

,

1.1 %, 6.3% and 0.12 J.tg r 1, respectively .

In the present sampling programme measurements of dry deposition flux along with gaseous and par­ticulate sulphur were carried out on the roof of faculty building. Atmospheric concentration of S02 was monitored using a low volume sampler comprising a diaphragm reciprocating type of air pump (Model Dynamax 2, Charles Austin Pvt. Ltd, England). The S02 is absorbed by aspirating a measured volume of air through a solution of potassium tetrachloromercu­rate (TCM) at the flow rate of 2 I m- 1 for 24 h and analyzed colorimetrically by West and Gaeke method using UV-Visible spectrophotometer (Hitachi-2000i) at 548 nm. Aerosol samples were collected with Ki­moto size segregated impactor using high volume sampler (HVS) equipped with automatic flow rate controller. Predessicated Whatman 41 filters were used as collecting surface. After 24 h collection pe­riod, filters were withdrawn and after extractions in deionized water, concentration of sulphate was deter­mined by Ion Chromatograph System.

2.3 Chemical analysis The concentration of S04 was measured by Ion

Chromatography using a Dionex Dx-500 Chromato­graph system with guard column (AS 11A), separator column (AS 11 ASC) and self-regenerating suppres­sor (SRS) using 5.5 rnM NaOH as eluent.

3 Results and discussion The measurements of the values of dry deposition

of sulphur on white marble were integrated into an

estimate of the atmospheric dry deposition. The ob­served dry deposition value for sulphate is 0.19 ± 0.12 mg m-2 h- 1

• The mean concentration of S02 during the course of sampling was 7.8 ± 2.0 J..lg m-3

, while par­ticulate SO,. was 7.9 ± 1.8 J..lg m-3

.

The dry deposition of S (sulphur) consists of depo­sition of gaseous S02 and S04 as particulate. To find out the deposition of S02, the atmospheric concentra­tion (7.8 ± 2.0 J..lg m-3) of so2 was multiplied by deposition velocity vd (0.4 em s- 1

) for so2 as onto soil21

, since surface resistance for building materials was assumed to be similar to those for soils2

. The calcu­lated dry deposjtion rate for so2 was 0.14 mg m -2 h-I. In ambient air, rates of oxidation of S02 up to -30% h- 1 have been observed22

"23

. So, 30% of estimated dry deposition of S02, i.e. 0.04 ± 0.01 mg m-2 h- 1

, will be deposited as sulphate. After subtracting the deposition flux (0.04 mg m-2 h- 1

) due to gaseous S02 from total deposition fl ux of sulphate (0.19 mg m-2 h- 1

), the flux obtained is due to particulate phase (0.15 mg m-2 h- 1

).

The percentage contributions of particulate and gase­ous phase sulphur deposition towards total S deposi­tion are 78.9 and 21.1, respectively . The high contri­bution of particulate sulphate in total sulphur deposi­tion might be due to high concentration of particulate sulphate in atmosphere and basic nature of aerosols in this region24. It finds support from high mass median diameter (MMD) of S04 particles in this region.

In order to calculate deposition velocity of par­ticulate S04 onto marble, the average deposition flux (0.15 ± 0.1 5 mg m-2 h- 1

) of partie late deposition was divided by particulate sulphate concentration (7 .9 ± 1.8 J..lg m-3

) in atmosphere. The calculated deposition velocity was 0.53 ± 0.27 em s - I.

Table 1 presents deposition velocities derived from field experiments using different techniques25

·26 along

with presernt technique. The Vct calculated by present method is lower than that reported by throughfall method, while the Vct range is comparable to other methods Ii lke foliar extraction, micrometeorological and surrogate method. The Vct values derived from throughfall experiment give high values, because these dry deposition fluxes of so4 might not be parti­tioned into fluxes caused by gaseous and particle S deposition that is more difficult for throughfall ex­periments.

Surrogate surfaces are unable to simulate the proc­ess of gas exchange between the atmosphere and natu­ral surfaces. In micrometeorological methods, ex-

RANJIT KUMAR et al.: DRY DEPOSITION OF GASEOUS & PARTICULATE SULPHUR ON MARBLE 91

Table !-Summary of deposition velocities (em s- 1) derived from

field experiments using different techniques as reviewed by Nich­olson (1988) and Davidson and Wu ( 1985) along with present technique

Species Technique Mean

so4 M icro meteorological 0.55±0.65

Foliar extraction 0.23±0.24

Surrogate surfaces 0.26±0.25

Through fall i.0±0.4

Natura l surfaces (marble) 0.53±0.27

change between large area and the atmosphere is measured by using sensors and in tunnel conditions. Humidity profiles may have considerable biasing ef­fects on eddy correlation measurements27

• So, it can­not be denied that this technique is not direct method and sedimentation effects of large particles caused higher deposition velocities.

4 Summary During the present study the dry deposition rate of

S as sulphate onto natural surfaces (white marble) has been measured and an attempt was also made to par­tition the contribution of gaseous so2 and particulate S04 towards total sulphur deposition. The dry deposi­tion velocity CVct) of particulate sulphate on white marble (natural surfaces) was also calculated.

The mean dry deposition rate of S as sulphate onto white marble at Dayalbagh in the winter season was 0.19 ± 0.12 mg m -2 h- I. The atmospheric concentra­tion of so2 and particulate so4 were also measured along with dry deposition sampling programme. The average concentrations of so2 and so4 were 7.8 ± 2.0 J..l.g m-3 and 7 .9 ± 1.8 J..l.g m-3

, respectively. The dry deposition of sulphur consists of deposition of S02

and S04 . The calculated dry deposition rate of S02

and S04 were 0.04 ± 0.01 mg m-2 h- 1 and 0.15 ± 0.15 mg m-2 h- 1

, respectively. The percentage contribution of so2 and so4 towards total sulphur deposition was 21.1 and 78.9, respectively. This high contribution of particulate sulphate might be due to high concentra­tion in atmosphere and alkaline nature of aerosols24. In order to calculate dry deposition velocity of par­ticulate sulphate on white marble, the flux obtained for S04 was divided by atmospheric concentration. The observed dry deposition velocity of particulate sulphate was 0.53 ± 0.27 em s- 1

•

Acknowledgements The authors wish to thank Shri Dinesh Yadav for

his invaluable assistance during the course of sam­pling. They are grateful to Prof. Satya Prakash, Head, Department of Chemistry, for providing necessary facilities. Financial assistance from DST (Project No . ESS/661163/98) is gratefully acknowledged.

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