Laboratory measurements of sulfur dioxide deposition velocity on marble and dolomite stone surfaces

Atmospheric Environment Vol. 27B, No. 2, pp. 193 201. 1993.

095%1272/93 $6.00+0.00 ~': 1993 Pergamon Press Ltd

Printed in Great Britain.

LABORATORY M E A S U R E M E N T S OF S U L F U R D I O X I D E D E P O S I T I O N VELOCITY O N MARBLE A N D D O L O M I T E STONE SURFACES W. GEOFFREYCOBOURN Department of Mechanical Engineering, University of Louisville, Louisville,Kentucky, U.S.A. and K. LAL GAURI, SANJEEV TAMBE, SUHAN Ll a n d EMINE SALTIK Department of Geology, University of Louisville,Louisville,Kentucky, U.S.A. (First received 30 June 1991 and in final form 5 September 1992)

Abstract--The deposition velocity of SO2 on marble and dolomite stone surfaces in a humid atmosphere was measured as a function of time in the laboratory using continuous monitoring techniques. The deposition velocity of SO2 on marble varied between 0.02 and 0.23 cm s- 1, and was generally observed to decrease with time. The deposition velocity of SO2 on dolomite varied between 0.02 and 0.10 cm s-1, and gradually increased over the first 2000 ppm-h of exposure. For both types of stones, the deposition velocity increased significantlywhen condensed moisture was observedon the stone surface. Chemicalanalysis of the stone samples indicated that the SO2 deposited reacted with the stone materials to form gypsum (CaSO4.2H20) on the marble surfaces and gypsum and epsomite (MgSO4-7H20) on the dolomite surfaces. Key word index: Stone deterioration, sulfur dioxide, deposition velocity.

l. INTRODUCTION The deterioration of artistic and building stone is greatly accelerated by the action of air pollutants such as sulfur dioxide, nitrogen dioxide, sulfuric acid, nitric acid and particulates (Amoroso and Fassina, 1983). The notion that the chemical attack by air pollution is much more severe than natural weathering is evidenced by the fact that stone damage is generally much more severe in polluted areas than in non-polluted areas (Feddema and Meierding, 1987), and also by the fact that much of the damage to ancient monuments has occurred in modern times (Gauri, 1990). The species most clearly associated with stone damage is sulfur dioxide, SO2, which is deposited directly onto stone surfaces from the gas phase in a process known as "dry deposition." In humid atmospheres, the sulfur dioxide reacts with the calcium carbonate in the stone. This is indicated by the reaction products found on stone surfaces, which consist largely of gypsum, CaSOa'2H20, as reported by several workers (e.g. Gauri and Holdren, 1981; Lindqvist et al., 1988). In the outdoors, stone deterioration is a complex phenomenon, and the rate of deterioration depends on many factors, including stone type, surface condition, pollutant concentrations, presence of catalysts, temperature, humidity and the frequency and duration of surface rain washings. There have been many field studies which have documented stone damage, but

without adequately explaining the relationship between the rate of deterioration and these various factors. Recently, however, Lipfert (1989a) has reviewed published data on rates of mass loss from calcareous stones exposed to rain and SO2 in the outdoors, and reported on statistical analyses of these data which were used to form various "damage functions" based mainly on rainfall amount and SO2 concentration. In order to understand the importance of each factor, acting independently or in combination, the problem must be studied in the laboratory so that conditions can be varied in a precise and systematic manner. To date there have been a limited number of laboratory studies concerning stone deterioration or the rate of SO 2 uptake by stone surfaces. Spedding {1969) reported on the uptake of sulfur-35 sulfur dioxide by oolitic limestone. Autoradiographs revealed a preferential uptake of sulfur dioxide by the matrix limestone material as compared to the fossil limestone material. Braun and Wilson (1970) exposed slabs of limestone to outdoor air and analysed stone scrapings for sulfur. Based on measurements of average SO 2 concentrations, they estimated an average "deposition velocity" in the range 0.2-0.3 cm s- 1. The deposition velocity is an overall mass transfer coefficient which is equal to the average rate of migration of a dilute gas towards an absorbing surface. They also reported that in laboratory studies of stone exposed to 193

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flowing SO2-air mixtures, the rate of SO2 uptake was directly proportional to SO2 concentration for a fixed flowrate, while at fixed concentration the uptake was reported to increase with flowrate. Judeikas and Stewart (1976) reported on laboratory-measured SO: deposition velocities on selected building materials and soils. The reported deposition velocities varied, depending upon the surface material, from 0.04 cm s- 1 for asphalt to 2.5 cm s- 1 for a particular type of cement. Payrissat and Beilke (1975) have reported laboratory-measured deposition velocities over seven European soils ranging from 0.19 to 0,60 cm s- 1 Gauri et al. (1982-83) have studied the effect of grain size and relative humidity on the reaction between marble and SO 2. They reported that the reaction rate increased with relative humidity and decreased with grain size. For powdered marble the reaction rate was faster by two to three times as compared with the solid marble. Borgwardt and Harvey (1972) have studied the reactivity of SO 2 with calcines of various carbonate rocks at 980°C, and found that the physical properties of the original stone, particularly pore volume and surface area, strongly influenced the reaction rate. Johansson e t a l . (1988) have compared samples of red Kinnekulle limestone, white Carrara marble and yellow Rome travertine exposed to atmospheres containing SO2, NO2 and S O 2 - N O 2 mixtures at 90% relative humidity, using sample weight gain to monitor the reaction. After roughly 1000 h of SO2 exposure at 1.6 ppm, the limestone samples exhibited the largest weight gains, roughly 0.25 mg cm-2, as compared to roughly 0.1 mg cm-2 for marble and travertine. These experiments clearly indicated that the reactivity of limestone with SO 2 is significantly greater than that of marble. The difference is probably due to the much greater porosity of limestone, which creates a larger effective surface area for reaction. Van Houte et al. (1981) have reported that the rate of reaction of SO 2 with limestone can be increased considerably by treatment with certain additives such as CaCI 2. In addition, Johansson et al. (1988) have shown that the SO2 reactivity of various types of carbonate stone is enhanced in the presence of NO2. These results suggest that in the outdoor environment, where a variety of possible catalytic materials such as NO 2, carbon particles and vanadium particles are present, the reaction between SO 2 and carbonate stone may be accelerated. In fact, evidence from field studies have strongly suggested that carbon particles catalyse the reaction (e.g. del Monte and Vittori, 1985; Ross and McGee, 1989). Skoulikidis and Papakonstantinou-Ziotis (1981) and Skoulikidis and Charalambous (1981) have reported that gypsum films found on sulfated marble samples were formed on top of the original stone surface, and also that cavities were found below the surface of the samples. According to the authors, once a critical thickness of gypsum had formed, the forma-

tion of additional gypsum was governed by solid-state diffusion of Ca 2 ÷ through the layer, and was proportional to the square root of time. The observed slowing down of the reaction rate suggests that the gypsum product layer had added an additional diffusional resistance to the system, which increased as the layer thickness increased. Kulshreshtha e t a l . (1989) have studied the kinetics of the SO2-marble reaction at 100% relative humidity using SO2 concentrations of ~ 1 0 p p m for total time-concentration exposures of up to 10'~ppm-h. They reported that the reaction slowed down considerably after about 200ppm-h of exposure, but continued to even out to 104 ppm-h of exposure. They have calculated the characteristic thicknesses of the calcite removed by the reaction, based on weight loss of the samples obtained by leaching in water. These thicknesses were reported to be in the order of 50/~m after 104 ppm-h of exposure, which, based on a quantitative SO2-CaCO 3 reaction, corresponds to an average deposition velocity of about 0.09 cm s- 1. This research was undertaken to study the comparative reaction rates of dolomite and marble stones with SO2 in humid atmospheres. It is important to establish the basic rate of SO 2 deposition and attack on various stone types in the laboratory before more complex systems such as those involving catalysts are systematically explored. In addition, we wished to study in detail how the reactions proceed with time in order to determine if changes occur with the build-up of reaction products. In order to study the time variation in detail, it was necessary to continuously monitor the SO2 uptake by the stone. Otherwise, analysis of individual samples exposed over varying time periods gives only limited information in this regard. The deposition velocity, being the ratio of SO2 flux to SO 2 bulk concentration, is a convenient parameter to use to express the rate of SO2 uptake. The deposition velocity is a widely used parameter for describing uptake of gases by surfaces because of this convenience. However, it has the drawback that it is a bulk coefficient which accounts for both the surface effect and the aerodynamic mass transfer effect, which is highly variable, depending on the flow conditions. Therefore, care must be taken in applying chamber studies to either indoor or outdoor conditions, as pointed out by Lipfert (1989b). In the case of surfaces such as marble and dolomite which have relatively high surface resistances to transfer, as compared to the aerodynamic resistance, the chamber data may be adjusted to a given outdoor condition within a reasonably low margin of error. In order to verify that the SO2 uptake resulted in an irreversible reaction with the stone as assumed, it was necessary to demonstrate a sulfur mass balance between the sulfur dioxide removed and sulfate product leached from the stone samples. For marble, the overall reaction with SO2 is given by: CaCO3 + SO2

H2O, 02 , C a S O 4 ' 2 H 2 0 + CO2.

(1)

SO 2 deposition velocity on stone surfaces At low humidities ( R H < 8 0 % ) it has been demonstrated that calcium sulfite is a n intermediate reaction p r o d u c t ( G a u r i a n d Gwinn, 1982-83), a n d this may be true at relative humidities near 100% also. W h e n exposed samples have been equilibrated to 100% relative humidity, however, gypsum is the reaction product. The overall reaction between SO 2 a n d dolomite is given by: H20, O2

CaMg(CO3)2 + 2SO2

' CaSO4 • 2 H 2 0

+MgSO4"7H20+2CO

2.

(2)

As reported by G a u r i et al. (1991), gypsum alone was found on exposed dolomite stone surfaces as observed by X-ray diffraction (XRD). However, analysis of leachate from the dolomite samples revealed calcium, m a g n e s i u m a n d sulfate ions in relative concentrations which were consistent with the above reaction. F u r t h e r m o r e , when droplets which were observed to form on dolomite samples after prolonged exposure to SO 2 were collected, dried u p o n a glass plate a n d studied by XRD, the deliquescent mineral epsomite, M g S O 4 . 7 H 2 0 , was found.

2. EXPERIMENTAL The experimental apparatus (Fig. 1) was designed to produce a humidified atmosphere at known SO 2 concentrations (,~ 10 ppm) for prolonged exposure of stone samples. Ex-

195

periments were carried out by placing several stone samples of dolomite or marble within a 10 f glass jar, suspended from a glass frame using nylon string. The jar contained four samples for each marble run, and for the dolomite runs the sample number varied from 9 to 27. Some additional runs on individual marble samples were also carried out in a smaller (~200ml) jar. The samples were each approximately 2.5 cm x 1.5 cm x 0.5 cm, and were placed so as to maintain approximately equal spacing between samples. All experiments were carried out in a temperature-controlled room, maintained at 20+2°C. Humidified air, maintained at a constant flowrate (800 ml min- 1) using an electronic mass flow controller (Datametrics, Wilmington, MA), was passed over a sulfur dioxide permeation tube (Vici-Metronics, Santa Clara, CA) maintained at a constant 20.0+0.01°C temperature by immersing the glass permeation tube holder in a temperature-controlled circulating bath (Neslab, Inc., Newington, NH). The SO2-air mixture was introduced into the glass jar by bubbling through water in the bottom of the jar to produce 100% RH air. A perforated glazed ceramic plate was used as a splash guard to prevent splashing the water on the samples. The system was allowed to run for a sufficient amount of time before introducing samples so that the H20 in the bottom of the jar could become saturated with SO2, and thus not remove SO 2 during the experiment. Certain experiments with marble were also carried out in jars without water. The SO 2 concentrations of air entering and leaving the jar were monitored continuously using a continuous flame photometric detection (FPD) analyser (CSI-Meloy Model 285). The SO2 concentration of the air entering the jar was monitored at the beginning and end of each experiment as a check on the stability of the system. The outlet concentrations were monitored during the experiment by diluting the gas at a constant ratio of 10:1 with "zero air" to bring the SO2 concentration within instrument range.

circulatingbath( 20°cI

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sampl

to analyzer dilutionair

IOL

2

Fig. 1. Diagram of the experimental apparatus. Flow control was achieved using electronic mass flow controllers. The humidity in the chamber was maintained at or near 100% and the temperature in the chamber was maintained at 20 + 2°C. For all experimental runs the SO 2 concentration entering the jar was approximately 10 ppm.

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The 802 mass flowrate into the jar was determined by taking the permeation tube weights before and after each run to determine the average permeation tube emission rate. The SO z mass flowrate out of the jar at a given time was determined as the ratio of outlet to inlet measured concentration multiplied by the permeation tube emission rate. The mass concentrations could then be calculated by dividing the SO2 mass flowrates by the volumetric flowrate. The FPD analyser contains linearizing circuitry to achieve a linear output response. After performing an initial calibration to check this linearity, it was no longer necessary to calibrate the FPD independently for each run, since it was being used essentially only to determine the ratio of outlet to inlet concentration. The experiments were run for approximately 2000 ppm-h of exposure. The dolomite runs were conducted first. For these runs, a few samples were removed at selected time intervals during the run, for chemical and gravimetric analysis. For the marble runs, all samples remained in the chamber for the full exposure period, and were analysed upon completion of the run. For all experimental runs with multiple samples, at least one sample was selected for analysis by XRD. The remaining samples were each leached in deionized water for sufficient time to completely remove all soluble reaction products (e.g. gypsum in the case of the marble samples). Normally, one immersion for 48 h was sufficient to achieve this. The leachate was analysed for calcium and magnesium using atomic absorption spectrophotometry, and for sulfate using the barium chloride/barium sulfate turbidometric method. Some samples were analysed with a recently acquired Dionex model 100 ion chromatograph.

3. R E L A T I O N S H I P BETWEEN T H E D E P O S I T I O N VELOCITY AND THE CHARACTERISTIC CRUST THICKNESS

The removal of SO 2 from the atmosphere by surfaces such as soil, water, vegetation or stone is known as dry deposition. The process is governed by the mass transfer in the gas phase and by absorption at the surface. In the case of calcareous stone, the latter process results in an irreversible chemical reaction, leading to the formation of products such as gypsum. If the surface flux is assumed to be proportional to the SO2 concentration immediately adjacent to the surface, then a surface resistance can be defined as follows : r~ = C J F ,

r=rs+r~.

(4)

where Coo is the SOz concentration in the freestream above the concentration boundary layer. In the atmosphere, the gas-phase resistance is due to the sum of transfer resistances in the laminar sublayer near the surface and in the turbulent boundary layer above the surface. The gas-phase resistance is a complicated function of wind speed and surface roughness, but measured values reported in the literature are generally in the range 0.1-2.0 s cm-1. Several workers

(5)

F r o m conservation of mass it is clear that the flux can be conveniently expressed as F = C~/r.

(6)

The overall resistance has units of inverse velocity, It is standard practice, therefore, to define a "deposition velocity," Vd, equal to the inverse of the overall resistance. The surface flux is then the product of the deposition velocity and the freestream SO2 concentration. F = UdCzo.

(7)

The deposition velocity is thus a useful parameter for estimating the rate of uptake of SO 2 from air flowing over a reactive surface. In estimating the deposition velocity over a surface in the outdoor environment, three possibilities exist: (1) the surface resistance is relatively low (rs<0.1 s c m - ~ ) in which case the gas-phase resistance is rate controlling and v d ~ 1/rg; (2) the surface resistance is relatively high (rs > l0 s c m - 1 ) in which case errors in estimating rg would have a small effect on the estimated deposition velocity, and v d ~ 1/rs; and (3) the surface resistance is of the same order as the gas-phase resistance (0.1 s c m - 1 < rs < 2.0 s cm 1) in which case accurate estimates of both resistances are necessary. The surface flux can be related to the rate of reactant mass consumed per unit area through conservation of mass and stoichiometry 1 dm~ = M~ F, A dt ),

(8)

where A is the surface area, m~ is the mass of reactant (e.g. calcite), M~ is the reactant molecular weight, and 7 represents the stoichiometric coefficient of SO2 in the reaction (7 = 1 for marble and 7 = 2 for dolomite). It is useful to define a characteristic thickness of the reactant layer ~r -~- mr

(3)

where C~ is the surface SOz concentration in tool cm - 3, and F is the flux in mol c m - 2 s- 1. Similarly, a gas-phase resistance can be defined as r, = (C~ - Cs)/F,

(e.g. Garland, 1978; Fowler, 1984) have demonstrated that the resistances are additive, in analogy to electrical resistance. Thus, the overall resistance is given by

p~A'

(9)

where Pr is equal to the reactant density. The rate of reaction can thus be expressed in terms of this characteristic thickness as follows: d

1M r F

(10)

It is clear that expressions similar to Equations (8) and (9) can be developed for the mass of solid product formed, and hence the rate expression for the product layer characteristic thickness is: d 1 Mp d~(6p)=~ F.

(11)

SO2 deposition velocity on stone surfaces Physical evidence revealed by electron microscopy (Gauri et al., 1989) suggests that a product layer, or "crust," is present on top of the original surface, and there exists an inner layer which has empty cavities from which calcium ions have presumably migrated to form the surface crust. The characteristic layer thicknesses 6p and 6r are thus related to these two physical layers. An expression for the crust thickness in terms of deposition velocity and SO2 concentration is easily obtained by combining Equations (7) and (11). d

IMp

(12)

Integration of this equation would in principle yield the crust thickness as a function of time, but it is clear that the deposition velocity may depend upon the environmental conditions (relative humidity, temperature, wind speed, surface wetness, etc.) and on the crust thickness as well, which may be expected to add to the surface resistance. Nevertheless, integration of Equation (12) gives the formal relation between deposition velocity and crust thickness formed over a period of time. t

P

=MP ypp | vdC~ dt.

J

(13)

0

One purpose of this work was to measure and compare the deposition rate, or deposition velocity on two different stone types as a function of time. The instantaneous, area average molar flux of S02 onto the stone was measured by subtracting the outgoing SO 2 mass flowrate from the incoming flowrate, and dividing by total surface area and sulfur dioxide molecular weight. F

m.... in- m ...... t Mso2A

(14)

The instantaneous deposition velocity was then obtained from Equation (7), taking the average of the inlet and exit SO z concentration as C~ in the equation• It should be noted that although the exit concentration must be less than the inlet concentration, it should be reasonably close to the inlet concentration so that the concentration in the jar is reasonably uniform. In our experimental runs, the outlet concentration was typically about 70% of the inlet concentration, so that the average jar concentration was about 85% of the inlet concentration.

4. RESULTSAND DISCUSSION Deposition velocities for dolomite and marble samples exposed to humid air containing SO 2 have been determined from the procedures described above. Some dolomite samples were exposed for periods of upto 1000 h, but continuous SO 2 data was recorded out to 200 h only, and intermittently thereafter. The

197

marble runs were each approximately 200 h long. For all runs the average chamber concentration was in the vicinity of 10 ppm, so that a 200 h exposure represents an integrated total of 2000 ppm-h. Chemical analysis of the leachate from the marble and dolomite samples was performed, using the method described previously. An approximate mass balance for the element sulfur was confirmed by comparing the sulfate found in the sample leachate with that predicted from the sulfur dioxide measurements. For 16 dolomite samples, the average amount of sulfate found was 6.56 mg, compared to 5.68 mg predicted from the sulfur dioxide measurements. For seven marble samples taken from the 10• jar, the average amount of sulfate found was 40.46 mg, compared to 42.58 mg predicted from SO 2 deposition measurements. The average amount on the marble samples was so much higher because all marble samples remained in the jar for the full 200 h, whereas dolomite samples were removed periodically, as another way of monitoring the chemical reaction with time. For individual samples, the measured vs predicted sulfate was not always so close. For example, for the marble samples, the average absolute deviation between the measured and predicted values was 10.1 mg. This was partly due to random experimental error, partly due to errors involved in the extraction process, and partly due to a certain degree of non-uniformity in the SO2 concentration in the jar. The measured deposition velocities over dolomite varied between 0.02 and 0.10 cm s- 1 during the period examined (Fig. 2). Results of two of the runs agreed closely. The measured deposition velocity was typically about 0.03 cm s- 1 at the beginning of the exposure period, and gradually increased to about 0.05 cm s- 1 at the end of the 200 h period. For the third run, the deposition velocity started at about the same value, but increased to about 0.08cm s-~ at the end of the period. The fact that the deposition rate, and therefore the reaction rate, increased over the first 200 h may be explained by the fact that one of the reaction products, epsomite, is deliquescent. Hence, the formation of liquid water droplets or film on the surface of the stone could have served to increase the surface reaction rate, for example by increasing the mobility of the migrating calcium and magnesium ions. The data collected after 200 h was not continuous, but in two of the runs the deposition velocity peaked at about 0.12-0.14 cm s-1 and declined thereafter. The decline could possibly have been associated with the formation of a product layer which added another diffusional resistance to the system. Three marble runs were carried out using the same 10 ~ jar as for the dolomite runs. In these runs, the measured deposition velocities over marble varied mostly between 0.06 and 0.10cms -1 (Fig. 3). The results of the three runs were fairly consistent. The pattern of time dependence was markedly different from that of the dolomite runs. Except for a short

198

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60

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120

140

160

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180

200

Time of exposure (hr)

Fig. 2. The measured deposition velocity over the dolomite stone samples as a function of time. For each run the deposition velocity gradually increased during most of the exposure period.

0.200 I

0.160 l >~ •~-

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Fig. 3. The measured deposition velocity over marble stone surfaces as a function of time. Except for an initial period of 5-10 h, the deposition velocityfor each run was relativelyconstant or slightlydiminishingthroughout the exposure period.

initial period of 5-10 h, the deposition rate was relatively steady with time, or perhaps slightly declining. The slight rate of decline could have been due to the build-up of reaction products on the surface of the marble, adding another diffusional resistance to the system. The increase in measured deposition velocity during the first several hours was perplexing. Previously we had observed that sometimes when the jar was opened and then released, several hours were required to re-equilibrate the SO2 in the jar, so these data were questioned. An additional six marble runs were subsequently carried out in a smaller jar ( ~ 200 ml) using only two samples in the jar per run. These runs were for shorter periods of time than the previous runs. The smaller jar made possible a re-examination of the first few hours of the reaction process, since the small jar required only several minutes to re-equilibrate after opening and resealing. Also, the average velocity over the

samples in the smaller jar (0.6 cm s-1) was roughly a factor of 20 greater than in the large jar. Using the small jar, we measured relatively high deposition velocities (0.25-0.35 cms -1) during the first several minutes of sample exposure (Fig. 4). These initial high rates declined fairly rapidly as the fresh, unreacted surface was depleted. After about 5 min, the measured deposition velocities were in the range 0.080.10 cm s-1. For three of the runs (M4, M5, M6) the deposition velocity decreased after this time, while for three of the runs (M7, M8, M9) the deposition velocity increased. The difference is explained by the fact that conditions were different for the two sets of runs. Though all runs were conducted at or near 100% relative humidity, for runs M4-M6 there was no condensed water on the sides of the jar or on the samples, while for runs M7-M9 there was. The latter condition corresponded to that of the marble runs in the large jar. This condition was produced, in both

SO 2 deposition velocity on stone surfaces

199

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6

12

18

24

30

36

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Time of exposure

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(hr)

Fig. 4. The measured deposition velocity over marble as a function of time and surface wetness. For runs M4-M6 the samples did appear to be wetted by condensed water, while for runs M7-M9 the samples did not appear to be wetted. Both sets of runs were conducted in a smaller jar, at or near ! 00% relative humidity, for periods of time ranging from 10 to 60 h.

cases, by the presence of standing water in the bottom of the jar. It is apparent that the existence of condensed water on the sample surface has the effect of lowering the surface resistance and hence increasing the rate of reaction to a significant degree. Since the reaction is believed to occur at the surface, this would suggest a change in the reaction mechanism from a solid-phase to a liquid-phase reaction with dissolved S02. For the "wetted" condition, the deposition velocity over marble measured in the small jar was generally much larger than that measured in the large jar after a period of about 5-10 h. This may be partly attributable to the fact that the gas-phase resistance in the small jar was less than in the large jar, but not entirely. From our own experimental results and theoretical calculation based on mass transfer to spherical pellets in laminar flow (e.g. see Tambe et al., 1991) we have estimated that the gas-phase resistance in the small jar was roughly 3.0 s c m - 1, while in the large jar it was roughly 5.0 s c m - 1. When these resistances were subtracted from the measured total resistances in order to estimate the surface resistance, the values measured in the small jar (after 10 h or exposure) came out smaller, indicating that for these particular samples at this particular time the surface conditions promoted a relatively faster reaction, perhaps because more condensed moisture was present. In comparing the measured deposition velocities for the marble and dolomite samples, it is evident that the time-averaged values were roughly comparable over the 200 h exposure period, for these experimental conditions. However, the initial deposition velocity of dolomite was significantly lower than that of marble. Also, the deposition velocity of the dolomite samples

AE(B) 27:2-F

increased by roughly a factor of two over the 2000 ppm-h exposure period, while that of the marble samples remained approximately constant or decreased slightly. The time-averaged values of deposition velocity (0.05 c m s - ' over dolomite, and 0.07 over marble) are lower than the average values of 0.2-0.3 cm s-~ reported by Braun and Wilson (1970) for samples of limestone exposed outdoors. This is not unexpected, since limestone has been shown to be more reactive with SO2 as compared with marble (Johannson et al., 1988), and since the reaction of carbonate stone with SO 2 is probably catalysed by a variety of agents in the outdoors (e.g. Ross and McGee, 1989). It is instructive to apply these experimental data to estimate the rate of stone weathering or of crust growth on stones in the outdoor environment attributable to chemical attack by sulfur dioxide alone. These estimates must be regarded as highly uncertain, and probably well on the low side, since in the outdoors there are many more variables which interact together to influence the weathering rate. For example, other chemical weathering agents, such as nitrogen oxides and fine particles, and in addition, outdoor weathering mechanisms such as freeze-thaw cycling and salt recrystallization all act to accelerate the weathering process. In making the estimates, it is important to recognize the possible influence in the time variation of SO 2 deposition. For surfaces which receive regular washings, the average SO2 deposition velocity based upon the first few ppm-h of exposure would presumably be more relevant than the longterm average. This is because, in this case, the soluble reaction products (gypsum or epsomite) tend to be removed by rain washings, so that the product layer,

200

W.G. COBOURNet al.

which could modify the deposition rate, would remain relatively thin. As an example, a 10-day exposure at a typical U.S. urban average outdoor concentration of 0.01 ppm would be equivalent to roughly 0.24 h in our apparatus, so taking initial rates of 0.10 c m s -1 for marble and 0.03 cm s - 1 for dolomite from Figs 2 and 4, and using Equations (7) and (10), rates of weathering based upon the characteristic reactant layer thicknesses were calculated, giving 0.96-/~m yr-1 for marble and 0.25/~m y r - 1 for dolomite. For surfaces unexposed to rain washings, it is perhaps relevant to calculate the average rate of crust growth, based upon the characteristic product layer thickness, using Equation (12). Gypsum crusts have been observed to form on statues and monuments protected from rain, and these may eventually exfoliate, causing severe damage (Skoulikidis and Papakonstantinou-Ziotus, 1981). The 2000ppm-h experimental exposure would be equivalent to 22.8 years at 0.01 ppm SO2 concentration, assuming that the reaction is first order with respect to SO 2. Taking average deposition velocities of 0.08 cm s- 1 for marble and 0.05 cm s- 1 for dolomite from Figs 2 and 3, and using Equation (12), the calculated average rates of crust growth are 1.10ttmyr 1 for marble and 0.79/~m y r - 1 for dolomite. In making these estimates, we have not corrected for the lower expected gas phase or aerodynamic resistance in the outdoors. For one thing, the aerodynamic resistance is variable, depending on time and location. Also, as previously mentioned, these estimates of the SO2 effect are expected to be low as compared to outdoor rates, due to the lack of catalytic and other weathering agents, and should be regarded as minimum estimates, attributable to the effect of SO2 acting alone in humid atmospheres. Further laboratory studies are required in order to assess the combined effect of the wide variety of environmental factors that influence stone weathering in the outdoors.

5. CONCLUSIONS A continuous monitoring technique has been applied to measure the deposition velocity of SO2 on marble and dolomite stone surfaces in a humid atmosphere. Over a 2000 ppm-h exposure period at approximately 10 ppm and 100% relative humidity, the measured average deposition velocities of SO2 over the two stones were comparable in magnitude. For dolomite, the measured deposition velocity varied between 0.02 and 0.10 cm s- 1, whereas for marble the deposition velocity varied between 0.02 and 0.23 eros -1. The measured deposition velocity for both types of stone changed as a function time. The deposition velocity over dolomite increased gradually with time. The increase was attributed to a gradual increase of liquid water on the surface, brought about by the formation of the deliquescent mineral epsomite. For marble, the measured deposition velocity varied

mostly between 0.03 and 0.23 cm s 1. The wide variation appeared to be associated with the absence or presence of condensed moisture on the marble sample surfaces. For most of the marble runs, the deposition velocity generally decreased slightly with time, after an initial period. The decrease could have been due to the build-up of reaction products on the stone surface. Acknowledgements--The authors wish to thank Mr Bruce Roy and Mr John James for technical assistance, and Mrs Pamela Withrow for manuscript preparation.

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