Effect of dry deposition of NOx and SO2 gaseous pollutants on the degradation of calcareous building stones

Atmospheric Environment Vol. 26A, No. 16, pp. 2963-2974, 1992. Printed in Great Britain.

0004-6981/92 $5.00+0.00 © 1992 Pergamon Press Ltd

EFFECT OF DRY DEPOSITION OF NOx AND SO2 GASEOUS POLLUTANTS ON THE DEGRADATION OF CALCAREOUS BUILDING STONES S. J. HANEEF, J. B. JOHNSON, C. DICKINSON, G. E. THOMPSON a n d G. C. WOOD Corrosion and Protection Centre, University of Manchester Institute of Science and Technology, P.O. Box 88, Manchester M60 1QD, U.K. (First received 16 December 1990 and in final form 10 April 1992) Abstract--A laboratory-based atmospheric flow chamber, using realistic presentation rates of SO2, NO and

NO 2 pollutants directed to various dry and wetted stone surfaces, has been employed to quantify the effects of the individual pollutants and the role of ozone as an oxidant. For the individual pollutant gases reacting with stone surfaces coming to equilibrium with 84% relative humidity (r.h.), chemical reaction in the presence of a moisture film proceeds and the extent of this reaction is related to pollutant gas solubility in the moisture film, i.e. SO2 > NO2 > NO. After dissolution in the moisture film,the pollutant gases are oxidized in the presence of catalysts associated with the stones. The additional presence of ozone promotes oxidation of the pollutant gases and thus their reaction with the stones. For SO2 pollutant, oxidation in the gas phase is not significant compared with that in the moisture film, with enhanced oxidation in the presence of catalysts. Ozone increases oxidation of NO and NO 2 pollutant gases in the gas phase and moisture film; however, the oxidation of SO2 in the moisture film is more significantthan that of NO or NO2. Wetting of the stone surfaces, in the absence of ozone, reveals the consistently greatest chemical reaction with SO2 compared with NO and NO2, which is related to SO2 solubility, oxidation in the presence of catalysts and production of sulphuric acid. Generally similar behaviour is evident of NO and NO2, but NO shows a reduced extent of chemical reaction, implying that its oxidation in surface water, in the presence of catalytic species, is slow and hence the reactants are lost in the form of run-off.In the additional presence of ozone, the SO2 pollutant gas gives rise to enhanced chemical reaction, whereas both NO and NO2 show lower extents of chemical reaction than for the dry stones. This arises from the relatively slow conversion of N205 in the liquid phase to nitric acid, allowing loss of reactants in run-off. Key word index: Stone degradation, exposure chambers, dry deposition, pollutant gases, oxidants, surface water.

1. INTRODUCTION Stone monuments undergo various forms of degradation outdoors (Winkler, 1978; Gauri, 1980; Gauri and Holden, 1981; Amoroso and Fassina, 1983; Hoffman, 1985), although precise separation of natural weathering effects from those of man-made pollutants is not yet possible (Bawden and Ferguson, 1989; BERG, 1989). The apparent acceleration of decay in recent decades has been attributed to the increase in manmade pollutants above natural levels (Guidobaldi and Santariga, 1976; Babu Rao, 1983; Livingston and Baer, 1983; Youngdahl and Doe, 1986; Rosvail, 1988). Sulphur dioxide, from oil and coal combustion, is the most important man-made pollutant (Perkins, 1979; Brimblecombe and Rodhe, 1988; Wayne, 1990), and oxides of nitrogen from automotive exhausts (Rosvall, 1988). The pollutants are transported to the stone surfaces through dry and wet deposition processes (Hicks, 1982; Topoi and Vijayakumar, 1983). Sulphur dioxide causes deterioration of calcareous stones (Winkler, 1966; Spedding, 1969; Braun and Wilson, 1970); relatively short-term tests involving immersion of stones in sulphate solution have also indicated the potential importance of salt crystalliza-

tion within stones (BRE, 1983; Easters and Salisbury, 1986; Ross and Butlin, 1989). Such information has naturally led to models of stone degradation involving reaction of stone with the pollutant, transforming calcite to gypsum which, under appropriate conditions, crystallizes within pores of the building material. One possible consequence is rupture and spalling of the near surface layers of the stones (Easters and Salisbury, 1986). The degradation of building stones by nitrogen oxides has received comparatively little study. Oxidation of NOx species in the presence of water vapour can result in nitric acid generation (PORG, 1990). Such acid can react with calcareous stone although the lack of nitrates within the stone, or in run-off water, has led to the general conclusion that NOx species have little effect on stone (Livingston 1985; Crnkovic, 1985). Conversely, in the laboratory, Johansson et al. (1988) have shown that at 90% r.h., NO z causes a weight increase of calcium-carbonate-containing stones. No crystalline products were identified in the stones, as opposed to exposure to SO2, although leaching experiments revealed nitrite and nitrate. For a mixture of the gaseous pollutants, i.e. SO2 plus NOz, NO z enhances adsorption of SO z by the stone syn-

2963 AE(A) 26:16-fl

2964

S.J. HANEEFet al.

ergistically (Rosenberg and Grotta, 1980; Johansson et al., 1988). Mangio and Johansson (1989) have also shown the important role of ozone in enhancing the absorption of SO2 on marble, especially at hig h humidities. Concerning the reaction of SO2 gaseous pollutant with stone, oxidation of SO2 in the gas phase, or in the moisture film retained on the stone and within pores, to form sulphuric acid is important (BERG, 1989). The oxidation is assisted by atmospheric oxidants, ozone and hydrogen peroxide, and by catalysts such as soot and smoke (Amorosso and Fassina, 1983; Johnson et al., 1983), as well as transition metal species (Cheng et al., 1971). Such metal species may be associated with the transition metal oxide impurities in the stone. F o r NOx, oxidation to nitric acid is also assisted by atmospheric oxidants and appropriate catalysts. Interestingly, reaction of limestone with sulphuric acid and nitric acid can thus lead to the development of calcium salts of relatively low and high solubilities respectively, with contrasting effects on the development of crystallization pressures within the stones. F o r quantification, the role of water must be determined as well as the possible influence of transition metal oxides associated with the stone on the catalytic oxidation of SO2 or NOx. A laboratory-based study has thus been undertaken, involving the dry and wet deposition of pollutant gases on stone surfaces. The separate effects of dry and wet deposition of individual pollutants can then be determined, prior to considering the detailed interaction of pollutants. In this paper, one of a series, the effects of dry deposition of the single pollutant species, SO2, N O and NO2, in the presence and absence of ozone as an oxidant, are determined. The reactions and interactions of mixed pollutants will be discussed in a future publication. Realistic presentation rates of pollutants and ozone to dry and wetted stone surfaces have been employed in the experimental m~ttrix, simulating outdoor exposure. A separate publication (Haneef et al., 1992) details the consequences of wet deposition of the pollutant species to the same stone surfaces.

After cutting, no further surface finishing was given to the stones. Duplicate specimens were prepared for exposure by drying at 328_+ 5 K, with the dry state being reached when the difference between two consecutive weighings, separated by a 24 h drying period, was within +0.05%. The dry state or, in reality, constant weight, was achieved over about 20 days. Drying at higher temperatures was considered but discarded because of temperature-induced transformations of possible reaction products, e.g. loss of water of hydration from gypsum at temperatures above 373 K. 2.2. Atmospheric flow chamber The dry deposition studies were performed in a carefully engineered and constructed atmospheric flow chamber. The chamber details, with major considerations in its design and construction, have been given elsewhere (Johnson et al.,= 1990). Briefly, the chamber allowed entry of humid air, 84 __+2% r.h. at 294 K, containing 10 ppmv of the pollutant gases, SO 2, NO or NO2. Although this gas concentration appears high at first sight, in fact this procedure allowed realistic presentation rates of the pollutants, i.e. comparable with those outdoors, to the stone surfaces (Table 1). Where required, ozone was employed as an oxidant; the ozone concentration was 10 ppmv, which again provided a realistic presentation rate. In addition to using the previously dried stones, the chamber allowed controlled application of CO2-equilibrated deionized water onto selected stone surfaces. Important considerations were the extent and rate of wetting of the various stone surfaces. As for the pollutant and oxidant gas concentrations, realistic levels of wetting were required. Thus, as a guide, the rate of entry of water onto the stone surface was controlled to give a run-off rate of approximately 2.5 x 10-6 ml cm- 2s- ], equivalent to the average Manchester, U.K., rainfall rate (Lee et al., 1986). The water was fed into the rig over 8 h, with the stones then drying in the ambient rig environment for 16 h. The wetting/drying cycle was repeated daily over a 30-day test period. Using a drip feed, the droplets of CO2-equilibrated deionized water ran over about one-fifth of the exposed stone surface, providing run-off in appropriately located receptacles. Clearly there are many possible variations of the wetting arrangement and schedule, e.g. slow or rapid wetting, locally or over the total exposed surface of the stone. The procedure employed was selected to give insight into the rate of uptake of the pollutant gases into the water running over the stone surface, the extent of oxidation and reaction with the stone and, importantly, the retention or loss of reaction products from the stone surfaces to the runoff solution compared with adjacent areas. 2.3. Analyses of stones After the 30-day test period, the individual stones were dried at 328 __+5 K until constant weight was achieved and weight changes determined.

2. E X P E R I M E N T A L

2.1. Stone preparation Portland and Massangis limestones, White Mansfield dolomitic sandstone (hereafter termed Mansfield sandstone) and Pentelic marble were used. The stones were initially characterized, by petrography, for identification of the individual mineral phases, chemical analysis and determination of porosity and density (Dickinson et al., 1987). The Mansfield sandstone is composed of approximately 50% quartz and 500 carbonate material; the latter can be considered as a magnesian dolomite, with a Ca/Mg ratio of 1 : 1. The stone blocks were cut to specimens of 50 x 30 x 5 mm, giving an exposed surface area of 1500 ram2; cutting employed a rock-cutting diamond saw with water as lubricant. Fresh lubricant was employed for each stone to avoid transfer of contaminants and debris.

Table 1. Comparison of the presentation rates of pollutant gases and ozone in the flow chamber and the natural environment

Gas

Presentation rate in the chamber (mg cm- 2 s- 1)

Presentation rate in the environment (mg cm- 2 s- 1)

NO NO 2 SO2 O3

7.41 × 10 -6 8.02 x 10 -6 16.2 × 10 -6 11.83 x 10 -3

10.21 × 10 -6* 6.4 × 10 - 6 * 12 × 10 -6* 80x 10-4"I.

*Calculated from the average annual concentration of gases in the Manchester area. I"U.K. average value.

Dry deposition of gaseous pollutants The retained anion contents within the exposed stones were obtained by crushing the individual stones in a pestle and mortar. A weighed mass of the crushed stone was refluxed in deionized water for 2 h; the solution was then filtered and made up to 250 ml prior to analysis of the extracted sulphate and nitrate anions by high-performance liquid chromatography (HPLC). For stones where run-off was collected, the run-off solution was analysed for calcium, magnesium, iron and aluminium cations by flame emission spectrophotometry and/or atomic absorption spectrometry. The possibility of discrete stone fragments being extracted through erosion by water running gently over the stone surface was also examined; no particulate material was found in the run-off.

3. RESULTS The four stone types were exposed to realistic presentation rates of SO2, N O or NO2, with or without ozone; the stones were either dry (really coming to equilibrium with the following humid air), or were wetted locally with deionized water and then allowed to approach equilibrium with the flowing air stream. In considering various stone-types, the stones are termed 'dry' or 'wetted', the former indicating the absence of wetting water and the latter the presence of water running locally over the stone. However, regardless of the state of the stone surface, the pollutant gases were transported to the stone surfaces by dry deposition processes.

(a)

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3.1. Weioht changes Weight changes are given for the exposed surface area of 1500 mm2; this approach has been adopted to allow subsequent comparison of stones of various mineral contents and porosities which were exposed 'dry', or locally 'wetted', to the pollutant gases. 3.1.1. Dry stones. The weight changes of the dry stones after exposure for 30 days to SO2, N O or NO2 reveal weight gains, with occasional small weight losses (Fig. la). The weight changes are too small to be significant, but lead to the conclusion that relatively little reaction proceeds over the period of the run. In the additional presence of ozone, all stones show a weight increase (Fig. lb), with the weight increases being generally greater than for the individual pollutant gases alone. Thus, for example, in the additional presence of ozone, Portland limestone shows weight increases of about 6 times those for N O and NO2 pollutant gases alone and about 1.5 times that for SO 2 alone. Other than for the weight increases experienced by all the stones, particular trends are difficult to identify without further analyses. This aspect is considered later, when the extents of reactions of the pollutant gases with the stones are quantified further. 3.1.2. Wetted stones. F o r the wetted stones, in individual pollutant gases, revealing behaviour is evident as all stones show weight increases after exposure to SO 2 and weight losses after exposure to N O and N O 2

(b)

NO

NO2 502 NO NO2 ,.,4

SO 2

¢9 Z

SO2 SO2 zG'I m

O

3 5 ~

502

Pentelic marble

Mansfield sandstone

Massangis limestone

Portland

limestone

70

6G

(c)

50 2

Pentelic mar ble

Mansfield sandstone

(d)

[ so

s°2

~" 4c

mll so,

M

~: 20

s°' II

Massangis

limestone

0

Portland limestone

7O

so2 ~'

60

_m

so~

--.t

5o2

'

40

c~

502 20 10

0 -10

Pentelic marble

Mansfield sandstone

Massangis limestone

Portland t imest one

Pentelic marble

Mansfield sandstone

Massangis I i mestone

Portland li mestone

Fig. 1. Weight changes of the various stones exposed to dry deposition of NO, NO 2 or SO 2 pollutant gases for 30 days: (a) dry stones, (b) dry stones with ozone as oxidant, (c) wetted stones, (d) wetted stones with ozone as oxidant.

2966

S.J. HANEEFet al.

(Fig. lc). The weight losses are generally small, whereas relatively large weight increases are evident. In the presence of run-off, with the possibility of reaction products being removed partly from the stone surface, the weight change measurement comprises weight increase and weight loss components, i.e. it is a net measurement. Consequently, weight change per se cannot be used as a measure of the extent of reaction of the pollutant gases with the stones. The trends of weight increase in SO2 and weight losses in NO or NO2 are generally maintained with ozone as oxidant (Fig. ld). Comparatively large weight increases are observed for all stones exposed to SO2 and ozone. The weight losses in N O or NO2 and ozone are generally slightly greater than those for the individual pollutants without ozone. After exposure, the wetted stones displayed channelling, where removal of material occurred during exposure (Fig. 2a). The channelling was developed relatively locally, where the drip water had run over the specimen surface prior to its eventual collection as run-off. Channelling was evident for all wetted stones and its extent was quantified as far as possible (Table 2). Generally, stone type influences the extent of channelling, with increased depths of channelling in the presence of SO2 pollutant and in the additional presence of ozone. Furthermore, at the boundary between the wetted channel region and the dry stone region, gypsum crystallization was evident after exposure to SO2 (Fig. 2b).

L~, CO equilibrated deionized water feed

Pentelic

salt crystallization

marble

channel

(a)

run-off

3.2. Retained anions within the stones 3.2.1. Dry stones. After exposure, the stones were dried in the standard manner and water soluble anions, extracted from the crushed stones, were analysed by HPLC (Table 3). Clearly for exposure to SO2 or N O or NO2, all stones showed retained sulphates or nitrates. The retained anion content is increased dramatically in the additional presence of ozone. F o r dry stones, in the absence or presence of oxidant, the general trend is the reduced levels of retained anion associated with the more compact stones, Pentelic marble and Massangis limestone (Table 3). Figure 3 shows a scanning electron micrograph of a petrographic section of Portland limestone which had been exposed to SO2 plus ozone. A higher sulphurcontaining region at the outer surface of the stone, indicating the location of retained salts, is revealed (Fig. 3b). This contrasts with wet deposition results, where penetration into the stones is much deeper (Haneef et al., 1992). 3.2.2. W e t t e d stones. Generally, significant quantities of retained nitrates and sulphates were detected (Table 4). In the absence of ozone, wetting has resulted in increased levels of retained sulphates and nitrates from exposure to SO2 and NO2, respectively. However, nitrates resulting from exposure to NO are reduced from the levels for the dry stones. With ozone as oxidant, further increase in retained sulphate levels is detected. The nitrate levels from NO2

• "

n ) ~

;+

+

:

; ++:L+

....

,

Fig. 2. Channelling of wetted Pentelic marble exposed to dry deposition of SO2 pollutant gas in the presence of ozone for 30 days: (a) schematic diagram revealing the channel and the damp and dry stone regions, (b) scanning electron micrograph showing development of gypsum crystals on the surface of Pentelic marble at locations between the dry stone surface and the channel.

2967

Dry deposition of gaseous pollutants ']?able 2. Depth of channelling (#m) which results from exposure of the various wetted stones to dry deposition of the pollutant gases for 30 days Exposure regime Stone

Pentelic marble Mansfield sandstone Massangis limestone Portland limestone

NO

-. ---

NO 2

-. ---

SO 2

NO+O3

10 .

. 5

10 . 5 <5

NO2+O 3

SO2+O 3

20

80

10 <5

25 25

.

-Channelling is not quantifiable.

Table 3. Retained anion contents (mg) of the various dry stones exposed to the pollutant gases in the presence and absence of ozone for 30 days Exposure regime

Stone Pentelic marble Mansfield sandstone Massangis limestone Portland limestone

NO

NO 2

SO 2

NO+O3

Nitrate

Nitrate

Sulphate

Nitrate

5.3 13.9 5.2 14.7

6.3 12.0 17.8 6.8

4.9 13.7 9.8 17.5

24.7 16.2 100.8 172.8

NO2+O 3 502+O 3 Nitrate 50,1 85,6 46,4 88,8

Sulphate 49 75.3 63.9 67.9

Fig. 3. Scanning electron micrographs of a petrographic section of Portland limestone. The dry stone was exposed to the dry deposition of SO 2 pollutant gas in the presence of ozone for 30 days: (a) area showing detachment of the outer stone regions and representing the early stages of gypsum crust formation, (b) sulphur X-ray image of(a) showing high levels of retained sulphate in the detached stone region.

2968

S.J. HANEEFet al.

Table 4. Retained anion contents (mg) of the various wetted stones exposed to the pollutant gases in the presence and absence of ozone for 30 days Exposure regime

Stone Pentelic marble Mansfield sandstone Massangis limestone Portland limestone

NO

NO 2

SO 2

Nitrate

Nitrate

Sulphate

none detected 1.8 0.4 3.8

13.0 24.1 17.4 19.5

17.8 36.2 28.7 51.2

exposure remain broadly at the level measured for N O 2 alone, but nitrate levels for exposure to N O pollutant gas have increased markedly from the level from NO alone. Retained sulphites or nitrites from exposure to the appropriate gas were not detected. 3.2.3. Run-offanalysis. The wetted stones produced run-off solution, which was analysed for calcium content. Prior to analysis the run-off was filtered; the filtrates did not reveal the presence of any particulate material associated with detached or redeposited calcite. Table 5 shows that for the individual gases, a greater calcium concentration in the run-off is evident after exposure to SO2. Calcium levels, resulting from exposure to NO or NO2, are comparatively low. For SO2 plus ozone, the calcium content in the runoff increased four-fold, but this depends on stone type. Calcium levels after exposure to NO2 and ozone have increased and are significantly greater than for exposure to N O and ozone. About 20% more deionized water was run over the surfaces of Portland limestone and Mansfield sandstone than Pentelic marble and Massangis limestone to achieve the desired collection of run-off. This was because of the porous nature of the particular stones and hence additional water was necessary to saturate the pores after the daily drying periods. This particular requirement has not generally led to greater calcium levels in the run-off for Portland limestone and Mansfield sandstone compared with Pentelic marble and Massangis limestone. 3.3. General observations from pollutant #ases

stone

exposure

NO+O

3

NO2+O

3

802-1-O 3

Nitrate

Nitrate

Sulphate

15.1 52.6 26.8 45.6

17.0 23.9 16.7 28.1

52.8 63.1 87.5 129.5

leached from the stones. Additionally, Mangio and Johannson (1989) recorded enhanced adsorption of SO2 in the presence of ozone and appropriate humidity; in this case, gypsum within the stone was identified by X-ray diffraction. The presence of additional water on the stone surface, as well as producing channelling, typical of damage outdoors, contributes to significant weight increases for SO 2, with weight losses generally evident for NO and NO2. Consistent with the weight gains, considerable retained sulphates are evident within the stones, as well as calcium ions in the run-off. 3.3.1. Pollutant interaction with stone. In order to allow more precise definition of the role of the pollutants in their interaction with stones, equivalent calcium yields have been determined. Such yields are derived on the basis that the anions present in the stones have resulted from reaction and neutralization of the appropriate acid with the stone constituents, i.e. calcite. In the presence of run-off, the calcium detected in the run-off is added to the equivalent determined from the retained anion contents to produce the overall equivalent calcium yield. In the case of Mansfield sandstone, due account is taken of the magnesium content of the stone but, for convenience, the conversion is still expressed as equivalent calcium yield. The equivalent calcium yields of Table 6 indicate the effective chemical reaction of the pollutant gases employed with the various dry and wetted stones and the effect of ozone as oxidant. Thus, they essentially give an overview of the overall processes in each case. Important features revealed are summarized below.

to

Generally, interfacing the different stone types to the pollutants results in weight changes and the presence of the respective oxidized species, i.e. sulphate or nitrates, on or within the stones. Ozone, as oxidant, performs this role effectively, with resultant greater retained anion contents within the stones and greater weight increases. No nitrites or sulphites were detected, indicating their oxidation if initially present. This contrasts with the constant humidity studies of Johannson et al. (1988), where calcium sulphite hemihydrate was identified for samples exposed to SO2 alone; for NO2 exposure, nitrites and nitrates were

1. For dry stones, in the absence of ozone as oxidant, the general order of pollutant gas effect is S O 2 > N O 2 ~ NO. 2. Dry stones, in the presence of pollutant and ozone, show significantly greater reaction than for the pollutant alone. NO with ozone is the least reactive combination, but the relative importance of SO2 and N O 2 with ozone depends on stone type. F o r the relatively compact stones (Pentelic marble and Massangis limestone) the order of reaction is SO2+O3>NO2+O3>NO+O

3.

Conversely for Portland limestone and Mansfield

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Dry deposition of gaseous pollutants Table 5. Calcium contents (mg)in the run-offsolutions from the various wetted stones exposed to the pollutant gases in the absence and presence of ozone for 30 days Exposure regime Stone

NO

NO 2

SO2

NO+O3

Pentelic marble Mansfield sandstone Massangis limestone Portland limestone

0.1 0.1 0.9 0.1

0.3 0.1 0.1 0.3

3.0 2.8 4.8 12.7

0.2 1.1 (0.01) 1.0

NO2+O3

SO2+O3

3.4 3.0 6.91 7.9

11.0 11.7 19.0 12.7

Table 6. Equivalent calcium yields (mg) derived from retained anion contents and calcium contents in the runoff, for the dry and wetted stones, exposed to the pollutant gases in the absence and presence of ozone for 30 days Exposure regime Stone

NO

NO 2

SO 2

NO+O 3

NO2+O 3

SO2+O 3

Dry

1.7

2.0

2.1

7.9

16.0

20.6

Wetted

0.1

4.5

6.3

4.9

9.1

33.2

Dry

4.5

3.8

5.8

27.4

56.4

31.8

Wetted

0.7

7.9

17.9

8.2

19.8

38.2

Dry

1.7

1.7

4.1

14.8

18.6

26.8

Wetted

1.1

5.7

16.8

8.6

12.2

55.7

Dry

4.7

2.2

7.7

28.2

55.3

28.5

Wetted

1.3

5.8

25.8

10.0

22.5

67.1

Pentelic marble

Mansfield sandstone

Massangis limestone

Portland limestone

sandstone, the order of reaction is NO2+O3 >SO2+O3 >NO+O3. 3. All wetted stones, with pollutant gases in the absence of oxidant, give the following clear order of reaction: S O 2 > NO 2 > NO.

Significantly greater reaction occurs upon wetting for SO2 and NO 2 but, interestingly, the extent of reaction of NO is below that of dry stones. 4. Wetted stones, with pollutant gases and oxidant, give the following order of reaction: SO2 > NO2 > NO. The wetting has enhanced further action of SO2, but the extents of reaction of NO2 and NO are again below the levels experienced by the dry stones. 5. Stone type has a significant role in the reaction with pollutant gases, with the more porous stones revealing greater calcium equivalent yields and hence reaction. From the summary of extent of chemical reaction, expressed as equivalent calcium yield, the role of water and oxidant in enhancing the effects of SO 2 pollutant gas on stone degradation is relatively clear. However,

for NO and NO v various subtleties are evidently introduced into the degradation mechanism. Thus, for NO and NO 2 interacting with dry or wetted stone surfaces, ozone as oxidant increases the equivalent calcium yield, indicating enhanced chemical reaction. However, wetting of the stone surface largely results in reduced equivalent calcium yields and extents of chemical reaction than evident for the dry stones in the presence of ozone. For further insight into the possible influences of stone type, oxidant and water on the pollutant reaction at the stone surface, the percentage conversions of the incoming pollutant gases have been calculated for the various exposure regimes. The percentage conversion is expressed as the ratio of the retained anions in the stone (and, where appropriate, their effective concentrations in the run-off) to the effective anion concentrations that would result from complete oxidation of the incoming pollutant gases to sulphate or nitrate (Table 7). For the individual pollutant gases interfacing with dry stones, relatively low percentage conversions, in the range of 1-6%, are realised. Ozone as an oxidant certainly fulfils this role on the dry stones (in the absence of local wetting) and is particularly effective for the NO and NO v The presence of surface water without ozone reduces the conversion of NO to nitrate, which is not the case for NO 2 conversion to nitrate and SO2 conversion to sulphate. In the pre-

2970

S. J. HANEEFet al. Table 7. Percentage conversions of the incoming pollutant gases to the respective oxidized forms, through reaction with the various dry and wetted stones, in the presence and absence of ozone as an oxidant for 30 days Exposure regime Stone

NO

NO 2

SO 2

NO + 03

NO2 + 03

Dry

2.0

2.0

Wetted

0.1

Dry

SO2 + 03

1.0

9.0

12.0

9.0

4.3

2.4

5.3

7.1

12.6

5.0

3.0

2.0

32.0

43.0

13.0

Wetted

0.7

7.8

6.8

18.8

9.1

14.5

Dry

2.0

1.0

2.0

17.0

25.0

11.0

Wetted

0.6

5.6

6.4

9.3

8.8

21.1

Dry

6.0

2.0

3.0

33.0

42.0

12.0

Wetted

1.4

6.4

12.9

16.3

12.9

25.4

Pentelic marble

Mansfield sandstone

Massangis limestone

Portland limestone

senee of surface water, ozone again acts as an oxidant; however, for N O and NO2 the levels of nitrate are below those for the dry stones.

4. DISCUSSION 4.1. General considerations The determination of equivalent calcium yields allows quantification of the extents of chemical reaction of the stones with the pollutant gases. From this base, the effects of the individual pollutant gases can be compared, as well as the role of the oxidant ozone and surface water. Concerning the latter, it is recognized that at 84% relative humidity the dry stone surface carries an extremely thin moisture film (2 nm), unless promoted by various factors, i.e. porosity (Barton, 1976). For the wetted stones, about 20% of the surface was influenced directly by dripping, resulting in a local moisture film thickness about 0.2 mm (Barton, 1976). The wetting of the surface was relatively gentle, ensuring that any solid products were not washed from the surface. 4.2. P o l l u t a n t interaction with stones without ozone

Dry stones show comparatively little reaction with the pollutant gases over the 30-day exposure period (Tables 6 and 7), with SO2 being more chemically reactive to the stone than N O 2 or NO. Evidence for reaction is available from the retained nitrate or sulphate contents of the stones; also, the percentage conversions of the pollutant gases range from about 1 to 6% which is similar to that experienced outdoors. In explaining this behaviour, it is known that gaseousphase oxidation of the pollutants is extremely slow (Eggleston and Cox, 1978), unless catalysts of various kinds are present (Cheng et al., 1971; Penkett et al., 1979; Novakov et al., 1974). Furthermore, oxidation in an aqueous phase is very slow in the absence of

suitable catalysts (Larson et al., 1978). From Table 7, it is evident that the percentage conversion of SO2 to sulphate far exceeds that expected from gas-phase reaction and hence, catalytic effects are evident. Thus, the SO2 pollutant gas will initially dissolve in the moisture film on the stone surface and resultant oxidation, assisted by Fe, Mn and Ti catalytic species on the stone surface (Dickinson et al., 1987), will generate a moisture film of pH reduced from the initial level of 5.6. Generation of sulphuric acid results in reaction with calcite to release calcium ions which will precipitate as gypsum. Analogous behaviour is expected for N O and NO2, but the extent of chemical reaction is limited by their low solubilities in the moisture film. F o r the locally wetted stones, the extent of chemical reaction is SO2 > NO2 > NO (Table 6) which follows the same trend as the pollutant gas solubilities in water (Weast, 1989). The consequence of this behaviour is evident in the percentage conversion factors (Table 7) which have increased to 2-13% for SO2, whereas the NOx gases lie in the range 0.1-8%. Thus, as a consequence of the greater solubility of SO2 than the NOx gases, eventually an increased H + ion concentration is available for reaction with the stone. An additional feature is evident for the N O pollutant gas, where the presence of surface water reduces the chemical dissolution. This impfies that oxidation of NO in the presence of catalysts on the wetted surface is relatively slow and some of the N O which dissolved in the surface water is carried away to run-off rather than reacting with the stone. The equivalent calcium yields (Table 6) reveal that the extent of chemical reaction of the wetted stones with SO2 and NO 2 is up to four times that of the dry stones. Clearly this acceleration by water depends on time of wetness of the stone surface and rates of oxidation of the dissolved gases in the presence of catalysts on the stone surface. Furthermore, a far greater enhancement is anticipated for complete wet-

Dry deposition of gaseous pollutants ti.ng of the stone surface as opposed to the 20% employed here. 4.3. Pollutants with ozone In the presence of ozone, gaseous-phase oxidation of NO proceeds most rapidly, with the oxidation of SOz being least effective (Jones and Seinfeld, 1984; Nahir and Dawson, 1987). However, aqueous-phase oxidation of dissolved SO 2 with ozone is high. For the dry stones, these additional features associated with ozone can be examined with reference to Tables 6 and 7. The equivalent calcium yields of Table 6 indicate that stone type has some influence on the extent of reaction, with the associated moisture film playing a role in the overall behaviour. Thus, the extent of chemical reaction of Pentelic marble and Massangis limestone is S O 2 + O 3 > N O 2 + O 3 > N O + O 3 , whereas for the more porous stones (Portland limestone and Mansfield sandstone) the order is NO 2 + O 3>SO 2 + 0 3 ~ NO + O 3. Furthermore, while SO2 + 03 provides a constantly high level of chemical reaction for all the stones, significantly higher levels of chemical reaction with NO + 03 and NO 2 + 0 3 are only evident for the more porous stones. The behaviour may relate to the masking of reaction sites on the stone surface, or within pores, by gypsum through the relatively rapid production of sulphuric acid in the presence of SO 2 + 0 3. For NO and NO 2 with ozone, oxidation and dissolution promotes nitric acid formation; reaction with calcite is not limited so readily by precipitation and reaction sites are available. For the more porous stones, with more extensive moisture films within pores, increased chemical reaction with NO and NO2 is evident. For the deliberately wetted stones, a single order of chemical reaction of S O 2 + O 3 > N O 2 + O 3 > N O + 0 3 (Table 6) is evident; with surface water, chemical reaction with SO2 + O3 has increased, whereas, reaction with NO + 0 3 and NO 2 + 0 3 has decreased. Such behaviour is also confirmed by examination of the percentage conversion figures (Table 7); for SO 2 the percentage conversion has increased to 13-25%, whereas for NO and NO2 it has fallen to 5-19% and 7-13%, respectively. In examining processes occurring on the wetted stones, at the outset the extents of gaseous-phase reactions are not influenced by surface water and a situation similar to the dry stones is envisaged. Consequently, for SO2 with ozone, the additional water provides a medium for the production of a low pH solution resulting in further chemical reaction of calcite than that for dry stones. For NO and NO2, an analogous process is expected, providing the rate of acid production in the surface water and/or rate of chemical reaction are sufficiently fast. It is evident that the former, oxidation of NO and NO 2 in the surface water, is rate-controlling. For NO and NO z with ozone, eventual oxidation to HNO 3 in the moisture film proceeds through formation of N 2 0 5 which is assisted by appropriate catalysts (Kamens et

2971

al., 1990). However, even with the various transition metal oxides associated with the stones, the conversion of N 2 0 5 to HNO 3 occurs sufficiently slowly for some of the pollutant species to be lost in the run-off prior to reaction with calcite. The depths of channelling (Table 2) support this; thus, where evident, channelling associated with SO 2 is greater tha~a that with NO and NO2. The various pollutant gas reactions with stones of different porosities are summarized in Figs 4 and 5. Figure 4 indicates eventual reaction through SO3taq), although it is recognised that the rate constant for dissolution of SO3 is extremely large (Castleman et al., 1974; Calvert et al., 1978). 4.4. Influence of stone type In the present study it is difficult to quantify precisely the catalytic role of particular transition metal oxides associated with the stones and their efficiency in promoting oxidation; the maximum Fe, Mn and Ti contents are 0.34, 0.15 and 0.46% wt, respectively (Dickinson et al., 1987). In broad terms, the percentage conversion of gases (Table 7) suggests a reduced influence of catalytic species associated with Pentelic marble than the remaining stones. However, comparison is further complicated by reaction of the pollutant gases with the stones, which relocates catalytic species. For example, the wetted stones revealed a brown coloration after exposure, suggesting movement of iron species. In considering SO2, oxidation on the dry stone surface (at 84% r.h.) proceeds much more than gaseous-phase oxidation; further significant oxidation occurs in the presence of water on the stone surface. In addition to inherent transition metal species, stones exposed outdoors will also be associated with catalytic sites through deposition of carbonaceous material (Del Monte et al., 1981). External oxidants, i.e. ozone, play a very significant role in the oxidation of NO, NO 2 and SO 2 on the stone surfaces; surface water also has a pronounced effect for SO 2 oxidation. Outdoors, the atmospheric concentration of oxidants will depend on particular environmental circumstances; thus during photochemical smog formation, oxidant concentrations can be very high (Rosvall, 1988). Stone morphology plays an important role in the observed behaviour; porosity influences both the development of moisture films and the sites of reaction. Even for the dry stones, the more porous materials show greater reaction with SO2, NO and NO2 than the less porous stones. With wetted stones revealing channelling, the more compact stones give comparatively deep channels. These result from reaction with the stone and the lack of sufficient porosity to retain reaction products which are washed away to run-off. 4.5. Influence of water Thin moisture films on 'dry' stones, or more macroscopic surface water on 'wetted' stones, play a major

2972

S.J. HANEEFet al.

0 2 (g) (very slow) SO2(g) ' --- SO3(g) Jdissoluti:n3 (g)(slow) (fast) I dissolution

i1(fast) 02 ,catal yst : [ 2 0 : : : l o w ) / 2 S 0 ' (fast)

1 03 (slow)

(aq)

/

S.O3 (aq) /

/

/

(g) Gas phase (aq) Aqueous phase

t J H2504(aq) stone [ CaCO3

removal in run-off; predominant in compact stones

diffusion into stone and crystallization in pores as CaSO4.2H20J predominant in porous stones

Fig. 4. Schematic diagram showing the conversion of SO2 pollutant gas to sulphuric acid and its reaction with calcium-carbonate-containing stones. The diagram considers the wetted region of the stone, from which run-offis evident.

NO2(g)

NO(g) dissolution

NO(g)

I di.olution

(slow)

(slow) 0 2 (slow) -- NO2(aq) NO(aq) 0 3 (fast) 03 (fast)

NO2(g)

102 0 (s~w) (fast) |

O2(siow)

NO2(g)

~N ~ (st°w)

: NO3(g) 0 3 (fast)

1' NO2

)3(acl)

~

Lo3°Z(s,ow)(fast)

dissolution~

N O5(aq)

N205(g) I H20(g)

~cs,ow)

HNO3(g)

no catalysts (slow)' cataysts (fast)

dis's°iuti°n ~ (fast)

HNO3(aq)~ stone CaCO3

(g) Gas phase (aq) Aqueous phase

Ca(NO3)2 (aq)

/',,,

removal in run-off

diffusion into stone and retention in porous stones

Fig. 5. Schematic diagram showing the conversion of NO~ pollutant gases to nitric acid and its reaction with calcium-carbonate-containing stones. The diagram considers the wetted region of the stones, from which run-off is evident.

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Dry deposition of gaseous pollutants role in stone degradation. In addition to providing a medium for calcite dissolution, they allow pollutant dissolution and subsequent transport to catalytic sites. With water running over the stone surface, the rates of oxidation of individual pollutant gases are important since they can be lost to run-off when oxidation is slow. For SO 2, surface water enhances dry deposition effects in the presence and absence of external oxidants. For NO, surface water reduces the dry deposition effects in the presence and absence of ozone; for NO2 plus ozone, a reduction in the dry deposition effect is also evident with additional surface water. This behaviour is again associated with pollutant species with relatively slow transformation kinetics to their oxidized forms, when surface water provides a means of partial removal of potentially damaging pollutants to run-off without reaction with the stone. Outdoors, where relatively large surface areas of stone are exposed, such effects may not be readily revealed. Additionally, as for channelling, surface water focuses chemical reaction in preferred regions and provides a means for removal of reaction products. Again, outdoors, such reaction products run onto adjacent stones and provide calcium ions for the crystallization of salts at potentially damaging sites in the near surface regions of the stones. Intermediate zones, or damp regions, between regions of relatively dry and water saturated stone, show aesthetically displeasing colorations due to crystallization of salts generated through stone interaction with pollutants. Whether such salts develop through transport of reaction products through the stone from nearby channel or are transported largely in surface water, or both, remains to be identified.

3.

4.

5.

6.

reacts extensively with the calcium carbonate minerals comprising the stone. Ozone does not contribute significantly to gaseousphase oxidation of SO2, but it does in the moisture layer, enhancing oxidation in the presence of catalysts. NO and NO 2 do not oxidize to any great extent in the gas phase in the absence of ozone. Furthermore, they only dissolve to a limited extent in moisture layers where they can be oxidized in the presence of catalytic species. The conversion of N2Os to nitric acid in the moisture layer is relatively slow and represents the rate-determining step. Ozone oxidizes NO and NO2 to NO 3 in the atmosphere, with the generation of N 2 0 s. N 2 0 s is soluble in the moisture layer, but its conversion to nitric acid remains the slow step in the generation of acid for reaction with the stone. With water running locally over the stone surface, providing run-off, the relatively slow liquid-phase conversion of NOx gases to nitric acid results generally in loss of reactants from the stone surface, limiting reaction with the stone. Conversely, under similar circumstances, SO2 shows enhanced reaction with the stone through its relatively extensive oxidation in the liquid phase.

Acknowledgements--The authors acknowledge support by

the Commission of the European Communities, contract EV4V.0053.UK(H), Effects of Air Pollution on Historic Buildings and Monuments and the ScientificBasis for Conservation.

REFERENCES

Amoroso G. G. and Fassina V (1983) Stone Decay and 5. CONCLUSIONS

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1. Using a laboratory-based atmospheric flow chamber, with realistic dry deposition presentation rates of pollutants and ozone, as oxidant, directed to various dry and wetted stone surfaces, important features in pollutant reaction with stone can be readily identified from measurement of weight change, retained anion contents in the stone and, where appropriate, cation losses to run-off. The total calcium yield is particularly indicative of the overall reaction of the pollutants with the calciumcarbonate-containing stones. 2. From the calcium equivalent yields and percentage conversions of the pollutant gases to their oxidized forms, it is evident that the SO2 pollutant gas is not oxidized to any significant extent in the gaseous phase in the presence or absence of ozone. The pollutant gas dissolves extensively in the moisture film or water layer on the stone surface, where it is oxidized by dissolved oxygen and ozone in the presence of catalysts. The resultant sulphuric acid

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