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ELSEVIER The Science of the Total Environment 187 (1996) 79-91

Role of particulate matter from vehicle exhaust on porous building stones (limestone) sulfation

Carlos Rodriguez-Navarro*, Eduardo Sebastian htituto Andaluz de Geologia Mediterranea, Departamento Mineralogia y Petrologia. CSIC- hive&&d de Gran&,

Fuente Nueva s/n 18003, Granada, Spain

Received 14 December 1995; accepted 6 March 1996

Abstract

This work, for the first time, experimentally demonstrates the relationship between motor vehicle emissions and the decay of ornamental calcareous stone, by means of sulfation processes (the well-known phenomenon of Black-crust formation). The critical catalytic effects of carbon (soot) and metal-rich particles from vehicle exhaust result in the acceleration of the rate of fixation of atmospheric SO, to form gypsum on the limestones (calcarenites) used to build Granada Cathedral (Spain). The analysis of particulate matter deposited on the building (carbonaceous and metal-rich particles), as well as of emissions from both leaded-gasoline and diesel motor vehicles confirms that the origin of the particulate matter found in the surface of decayed building stones from Granada Cathedral is consistent with having been contributed by motor vehicle exhaust. Experimental data indicate the role played by this particulate matter in the fixation of atmospheric SO1 as sulfates (gypsum) on calcareous materials in the presence of humidity. We have also experimentally demonstrated that there is a close relationship between the composition of the particulate matter and the fixation rates of the SO, in the form of sulfate: (a) diesel engine exhaust, which is primarily composed of soot and metallic particles bearing Fe and Fe-S as major elements and of Cr, Ni, Cu, and Mn as trace elements, plays the largest part in the catalytic oxidation rates of SO,; (b) the emissions from gasoline engines, composed of minor quantities of soot and high concentrations of Pb- and Br-bearing particles, cause a lower rate of SO, fixation as gypsum on limestones. From these experimental findings, a new hypothesis is proposed concerning the sulfation of the limestones.

Keywords: Limestone; Vehicle emission; Building stone, decay; Black-crust formation; Granada, Spain

1. Introduction

The effects of atmospheric pollution and acid deposition on lakes, animals, forests, humans and buildings arouse considerable interest and are the

* Corresponding author. Present address: The Getty Conser- vation Institute, 4503 Glencoe Avenue, Marina de1 Rey, CA 90292, USA. Fax: +l 310 821-9409, e-mail: cnavarroagetty. edu.

subject of extensive discussion. Air pollution has since the mid-19th century been suspected of accelerating the degradation of many types of construction materials [l]. Moreover, air pollution has been suspected to be a major factor in the degradation and, in some cases, the loss of large parts of our cultural heritage [2-51. Pollutants such as SOz and particulate matter, primarily from the combustion of oil-derived fuels, have

0048-9697/96/$15.00 0 1996 Elsevier Science B.V. All rights resewed PIZ SOO48-9697(96)05124-8

80 C. Rodriguez-Navarro. E. Sebastian / The Science of the Total Environment 187 (1996) 79-91

been related by several authors [6-91 to the alteration by sulfating of ornamental stones (particularly carbonate ones: marbles, limestones, and dolostones), involving acid attack of these materials, and formation of sulfate compounds (i.e. gypsum). Nevertheless, many questions on the dynamics of this decay process remain unsolved.

Novakov et al. [lo] established a relationship between sulfating processes from atmospheric SOI and the presence of carbonaceous particulate matter. This process, according to experimental data by Urone et al. [l I], accelerates in the presence of some transition and other metals (Fe, V, Cr, Ni, Pb, etc.), which catalyze the oxidation and hydrolysis of the SO, to form sulfuric acid, which is responsible for both the ‘acid rain’ and the stone sulfation. Del Monte et al. [7] made it clear that carbonaceous particles are invariably present in the weathered gypsum crusts (the so-called ‘Black Crusts’) that form on the surface of calcareous materials used in historical buildings. Cheng et al. [S] demonstrated under laboratory conditions that metallic-sphere pollutants exert a catalytic action in the oxidation of SOz and determined the role played by carbonaceous particles in aiding the nucleating reactions of gypsum on marble surfaces. Pye and Schiavon [12] demonstrated by means of S isotope ratios that the S found in gypsum crusts on building stones always comes from an atmospheric source. Hildemann et al. [13], by means of C isotopes, identified the origin of carbonaceous particulate matter in the urban environment and concluded that a major source of these particles is the emissions from motor vehicles, with diesel engine vehicles being the major vehicular source.

A number of recent articles [ 14-181 have underscored the role played by the deposition of particulate matter (as a product of high urban pollution) in the formation of black crusts, mainly comprised of gypsum, (for an extensive review of this subject, see Lipfert [19] and Ross et al. [20]); however this paper presents the first experimental work supporting this theory. Our study provides new data on the action of particulate matter coming from motor vehicle exhaust in the sulfation of carbonate stones (limestones), that were so exten- sively used in the construction of historic buildings

in all of the Mediterranean Basin and especially in the South of Spain. Following building construction (and especially accelerated in the last few decades), limestones become covered by abundant black crusts in which gypsum and different types of particulate matter are normally found.

The black crust and other products deposited in the initial stages of weathering in the limestones used in the construction of the Cathedral of Granada (southern Spain) have been examined This historic building is located in an area where the degree of environmental pollution is low (there is no highly contaminating industry nearby). The only source of pollution therefore is from traffic and, to a lesser extent, from the seasonal use of furnaces. We have also analyzed the dust deposited in the molding, under cornices, and in other areas of the building where black crusts had not yet developed (the so-called gray areas [21]). This layer of dust reaches a thickness of several centimetres in particularly protected areas and is considered to be the precursor of black crust formation, Data from Skiotis et al. [22] and Yocom [23] indicated that this type of dust is normally found in the early stages of decay of building stones. They also pointed out the possible role of metallic elements, such as Fe, Mn, Cu and Zn that are always present in the dust, as catalysts in the oxidation of SO*, which in the presence of humidity results in the formation of sulfuric acid.

The main goal of this work is to demonstrate that the dust which enclosed particulate matter coming from vehicular traffic sources, plays a major role in fixing atmospheric SO* as CaSO,. 2H20, and as a consequence, is the first stage of black-crust development. A secondary goal is to demonstrate the differences in catalytic power of particulates from gasoline and diesel vehicle exhausts on the oxidation of SO2 and its fixation as sulfates.

2. Materials and methods

2.1. Materials

Fifteen samples were taken of the weathering products (black crusts) and dust deposits from different heights and orientations of the Cathedral of

C. Rodriguez-Navarro, E. Sebastian/The Science of the Total Environment I87 (1996) 79-91 81

Granada. This XVI Century monument was primarily constructed of a Tortonian limestone (calcarenite) of high porosity (average value: 32%, [24]). Other characteristics of this material include a microsparitic matrix which, in some cases, may include a second generation of sparitic carbonate cement containing a great variety of bioclasts (molluscs, echinoderms, and foraminifera). Given their porosity, distribution of pore size, degree of cementation, and primarily carbonate composition, these materials are very susceptible to weathering processes caused by the generation of gypsum [24-271.

The black crusts and accumulated dust from the most protected parts of the building were analyzed by chemical, mineralogical, and petrographical methods, using X-ray diffraction (XRD), polarizing microscopy (PM), scanning electron microscopy with energy dispersive spectrometry microanalysis @EM-EDS), X-ray fluorescence

(XRF) and ion coupled plasma emission spectrometry (ICP). Once the weathering products were identified, a study was made of the role of each of the components of the accumulated dust and gypsum crust, in limestone sulfation, focusing on the role of the enclosed particulate matter.

2.2. Experimental SO, attack

Unweathered blocks of limestone (from the ancient quarries) used to build this monument were analyzed with XRF, ICP, XRD, PM, and SEM with EDS, before being submitted to SOZ attack in a static chamber with controlled temperature and relative humidity. Fig. 1 shows a schematic view of the chamber used, the sample dimensions, and the different runs carried out, as well as the experimental conditions.

To speed up the process, SO, was administered at the beginning of the experiments at a dose of

CHAMBER (500 I) CHAMBER (500 I)

Limestone (4 slabs) a) Blank b) Dust

Simultaneous c) Gasoline exhaust

5cm

Experimental conditions

SO, static chamber (100 ppm SO,) Timing: run a) 24 h

run b) 48 h Temperature: 30” C Relative Humidity: 100 %

Fig. 1. SO2 chamber scheme, sample dimensions, and experimental protocol (for detail see text).

82 C. Rodriguez-Navarro. E. Sebastian/The Science of the Total Environment 187 (19%) 79-91

100 ppm (50 ml of SO2 were introduced into a 500-l chamber, at 1 atm and 25%). This concentration is 2500 times higher than the average SO2 concentration near the cathedral (0.04 ppm), but, since one of the main goals of the experiment was to demonstrate that pollution particulate matter enhances SO2 fixation as sulfates on the stone surface, it was consider appropriate to obtain a high reaction rate. To evaluate the role of particulate matter coming from vehicular sources on stone sulfation, two types of particulates were collected directly from the exhaust systems of vehicles using leaded gasoline and diesel as fuels. These particles were analyzed by chemical, mineralogical and morphological means using SEM-EDS, ICP, XRF, and XRD, after which they were deposited on the limestone slabs (evenly spread over the upper surface with a brush). Then, each of the following limestone slabs were simultaneously exposed in the above described chamber:

(a) a fresh slab of limestone as a control; (b) a slab of limestone covered with a thin layer

of dust collected from the stone surface of the building (concentration: 100 mg cm-*);

(c) a slab of limestone covered with a thin layer of solid residue emissions from a vehicle using leaded gasoline (concentration: 10 mg cm-*);

(d) a slab of limestone covered with a thin layer of particulate matter emitted by a diesel engine (concentration: 10 mg cme2).

To accelerate the process of SO2 attack on the limestone surfaces and to reproduce the natural condition of the building, all slabs were wetted. It was pointed out [28-301, this would accelerate the sulfate attack by means of SO2 dry deposition process.

After 24 h from the beginning of the exposure, one set of samples (a-d) was removed from the chamber and immediately, their surfaces exposed to the attack, were submitted to XRD and SEM- EDS analyses. After 48 h from the start of the experiment, the very same operation was perform- ed on a second set of samples.

3. Results and discussion

3.1. Material from the building

In rock samples with incipient weathering it was

Fig. 2. Polarizing microscopy photomicrographs of black crusts: (a) Incipient crust with particulate matter (indicated by arrows), plane light; (b) developed crust, mainly composed of acicular gypsum crystals (crossed Nicols). Bar scale, 200 pm.

possible, as a rule, to observe by means of polarizing microscopy, a thin layer of tiny acicular gypsum crystals surrounded by clay minerals and calcium oxalate (weddellite). These phases were immersed in a dark matrix (Fig. 2a) of very porous particulate matter (soot) in conjunction with smooth, metal-rich spheres (as deduced from SEM-EDS data). The samples of black crusts (Fig. 2b) contained acicular gypsum crystals (up to 200 pm). Also, in central areas that were in contact with the limestone, several types of pollutants enclosed in a matrix of calcite crystals and microcrystalline gypsum (Fig. 2a,b) were also seen. However, there were not properly identified by this technique, due to their small size (average diameter < 2 pm).

XRD analyses of rock samples with incipient weathering (limestones with very thin, - 1 mm thick, black crusts), clearly indicated that calcite is

C. Rodriguez-Navarro. E. Sebastian / The Science oj the Total Environment 187 (19%) 79-91 83

Table 1 Comparison of the concentration of principal metallic elements of undecayed quarry limestone (fresh) and different samples from the building

Sample V Cr co Ni cu Zn Pb Fe Mn

Fresh 13 9 2 8 4.2 15.8 4 629 154 Initial crust (1) 18 17 2 11 7.3 30.8 18 1538 77 Initial crust (2) 10 10 2 3 8.1 16.1 2 1188 77 Initial crust (3) 14 15 3 6 12.2 54.8 37 1503 310 Crust (1) 10 44 2 5 56.5 34.8 83 3951 154 crust (2) 10 30 2 4 37.5 53.7 209 4720 232 Crust (3) 17 18 2 11 10.5 40.8 29 1608 77 Crust (4) 22 22 4 8 27.7 70.0 130 2308 77 CNst (5) 36 164 6 91 26.6 82.1 72 4557 481 CNSt (6) 26 31 3 7 48.8 53.7 11300 7203 309 Dust (1) 24 44 3 9 57.5 98.3 260 7448 310 Dust (2) 10 36 2 9 44.2 123.0 3740 4021 232 Dust (3) 13 29 2 8 39.4 142.0 193 4196 464 Dust (4) 44 38 6 15 100.1 113.0 439 11 644 154 Dust (5) 53 65 11 31 61.5 124.9 352 11480 225 Dust (6) 56 57 10 18 96.5 191.0 677 11 826 463

All element concentrations in ppm. The fresh sample was taken from an ancient quarry and reflects the average composition of these materials. Samples from the building (Initial crust, Crust and Dust) are identified by number (reflecting their different locations).

the major phase, followed by clay minerals, quartz, and small amounts of gypsum. In the most developed black crusts (thickness greater than 2 mm and up to l-2 cm), gypsum is the major phase, with calcite, quartz, clay minerals, and oxalates (weddellite) as minor components.

Analysis of the accumulated dust revealed the presence of a small amount of gypsum together

z 300

2 ;I 225

a + e 150 Y z 75

0 1 I

0 20 40 60 80 100

Gypsum (wt%)

Fig. 3. Gypsum concentration versus amount of Pb, Zn and Cu in black crust developed on limestone surfaces of Granada Cathedral.

with quartz and clay minerals, the latter being the most abundant. Minor quantities of weddellite, feldspars and iron oxides were also found. Detail- ed analysis of the <2-pm fraction that was ex- tracted from the acid-insoluble residue by centrifugation, and analyzed by means of XRD, using oriented aggregates, both untreated and ethylene glycol- and di-methyl sulfoxide-solvated, gave a composition of: illite, smectites, chlorite, kaolinite, and paragonite.

Chemical analysis of both the black crusts and the accumulated dust made it clear that there were significant variations in the metallic element content with respect to the fresh stone from the quarry (Table 1). The principal changes detected, when compared with unaltered limestone were:

(1) An increase in the proportions of chalcophilic elements (Pb, Zn and Cu) as well as Fe, Mn, V and Ni, in the black crusts.

(2) Fe, Cr, V, and Mn in high concentrations in the accumulated dust as well as the presence of the chalcophiles mentioned above.

Fig. 3 compares the gypsum content with the amount of chalcophilic elements (ECU, Zn, Pb) in decayed samples on the building, and the data show a clear, positive linear, correlation. As

84 C. Rodriguez-Navarro, E. Sebastian/The Science of the Total Environment 187 (19%) 79-91

Fig. 4. SEM micrographs oE (a) general overview of a black crust from the building, with considerable gypsum growth and enclosing different pollution particles: porous carbonaceous particles (s) and metal-rich particles (m); (b) star-shaped gypsum crystals experimentally developed on etched calcite surface 24 h after the start of the experiment. Particulate matter (from diesel emissions) is widespread over calcite grains (indicated by the arrow); (c) massive gypsum growth on limestone slab covered with diesel particulate matter (after 48 h); (d) gypsum (indicated by the arrow) growth inside broken Fe-rich sphere from diesel exhaust. Bar scale, 20 pm.

pointed out above, the decay level is proportional to the gypsum content, which is also proportional to the amount of metals within a gypsum crust. This fact has been observed in many buildings where different stone types were used [31].

SEM data reveal that both the black crusts and the accumulated dust on the building surface contain either irregular carbonaceous particles (OS-20 pm in diameter) that are very porous and largely pseudomorphed to gypsum; smooth- surfaced metal-rich particles that can be grouped according to chemical composition: (a) Fe-rich particles (similar to those described in [32]); (b) Al- and Si-rich particles [33-351; and (c) Fe-, Pb-, Cr-

Ni-, and Cu-bearing particles 135,361; and very irregular particles with an extremely variable metallic composition (Fe, Br, Ba, Cu, and Pb).

Aside from these particles, all the crusts, whatever their stage of development, invariably had irregular to acicular gypsum crystals, either planar or massive, always in contact with the above described particles (Fig. 4a). They also contained euhedral calcite rhombohedrons, as well as clay minerals and quartz. Organic materials (pal- len, roots, fungal hyphae, mycelia, bacteria, etc.) which may contribute to stone sulfation (371, were also detected.

These data revealed the clear relationship ex-

C. Rodriguez-Navarro, E. Sebastian/The Science of the Total Environment 187 (1996) 79-91 85

isting between the products of weathering in the building stone, (principally gypsum), and the presence of pollution-derived particulate matter of variable composition and morphology. Chemical and SEM-EDS analyses of the black crusts and of the accumulated dust from the surfaces of the calcareous stones made it quite evident there were high levels (compared with fresh limestone) of such elements as Fe, Pb, Cr, Ni and Co, and indicate this phenomenon is due to the abundance of smooth spheres and other metal-rich particles forming part of atmospheric pollution. The increased concentrations are especially spectacular in the cases of Fe and Pb.

The black crusts not only contain the above- mentioned types of particulate matter, but also carbonaceous particles with an entirely different morphology and composition. Both types of particles, however, appear associated with the gypsum, oxalates, and clays (the latter having the same composition as that of the soils of the surrounding area) in the accumulated dust. Thus, the transport of dust or of atmospheric particulate matter from distant industrialized zones may therefore be discarded.

The presence of these elements (especially the particles above mentioned) in the accumulated dust on the building, means that this dust could be a precursor to the initial stage of black crust formation. The only differences between the mineralogy of the dust and the black crusts were the morphology and the proportion of gypsum found in the crusts.

3.2. Particulate matter from diesel and gasoline car exhaust

Particles from the emissions of diesel and leaded-gasoline vehicle engines were analyzed by XRD. It was found that diesel-engine emissions are comprised of almost 70% (determined by semi- quantitative XRD analysis) carbon particulate matter (soot or carbonaceous particles), with graphite, iron oxide (hematite) and iron sulfide (pyrrhotite) diffraction lines being clearly detected. Many of these carbonaceous particles produce broad diffraction lines, indicating a high proportion of non-crystalline carbon compounds.

Diesel vehicles are responsible for 80% of the

estimated 140 000 tons of carbonaceous particulate matter emitted in Europe each year [38], and this represents a significant fraction (30-50% by mass) of the tine particulate matter found in urban atmospheres [13,39]. In contrast, gasoline engine emissions contain a lower proportion of carbon components (15%), but do contain Al and Fe (hematite) oxides as well as iron sulfide (‘pyrrhotite). Nevertheless, Hildemann et al. [40] quoted lower values for elemental carbon emitted by new diesel vehicles (40%), but pointed out that these values were at least five times greater than for non-catalyst leaded-gasoline vehicles.

Chemical analyses of both major oxides and sulfur (Table 2) and metallic elements (Table 3) revealed some compositional differences between the two types of exhaust particulate matter. In fact, some metallic element concentrations are markedly different. Pb and Br are major elements in gasoline exhaust, but appear as traces in diesel exhaust. This is easily understood if one considers that lead is added to the gasoline as Pb(CzH& and, separately, bromine is added as C2H4Br2 1401. Fe is a major element in both emissions, but V, Cr and Mn are preferentially concentrated in

Table 2 Composition of particulate matter from gasoline and diesel exhaust (weight percent oxide)

Sample

Gasoline

SiOz 1.55 A1203 0.82 CaO 1.78 MgO 0.33 Na20 0.01 K2O 0.06 Fe203 20.8 MnO 0.05 TiO, 0.051 PP5 0.12 Sa 2.86 Brb 13ooo LOP 74.9 Total loo.5

3 in weight percent. bBr concentration in ppm. cLOI, weight loss on ignition.

Diesel

2.48 1.30 1.52 0.85 0.15 0.06

16.7 0.07 0.053 0.19 2.41

13 71.5

101.0

86 C. Rodriguez-Navarro, E. Sebastian / The Science of the Total Environment 187 (19%) 79-91

Table 3 Comparison of the concentration of principal metallic elements of particulate matter from gasoline and diesel exhaust (concentration in ppm)

Sample

V Cr co Ni cu Zn Pb Fe Mn

Gasoline Diesel exhaust exhaust

9 10 54 78 24 5 27 20 73.3 23.4

634.0 139.0 80000 34 12737 58399

387 542

diesel emissions, as are Co and Zn in gasoline exhaust.

It must be said that S is a major compound in both emissions, and was detected in proportions over 2% by weight. This explains how certain amounts of S have been found in soot particles (SEM-EDS analyses), where pyrrhotite was present. Nevertheless, no sulfates (i.e. gypsum) were detected in these particulate emissions.

SEM analysis with an EDS microanalyzer also revealed the following data:

(1) Gasoline exhaust: major presence of soot (carbon) under 2 pm in size, and Pb-rich particles, with a significant amount of Br. In a few cases, the Pb is associated with Fe, Si and Al.

(2) Diesel exhaust: a large quantity of well- developed Fe- and S-rich particles. The largest (up to 20 pm in diameter) and most abundant metal- rich particles are composed exclusively of Fe. Al silicates and quartz were also found, with the re- mainder being soot. The carbonaceous particles, being the most abundant, were difficult to study with this technique, due to their small dimensions (average diameter < 1 pm).

These data demonstrated the clear morphological and compositional correspondence between the emissions of diesel and gasoline vehicles and the particulate matter found on the building. It must be pointed out that vehicle emissions are almost the only sources of pollution in non-industrialized

a 2oo1

_ .

0 2 4 6 8 10 12 14 16 18 20 22 24

Time (h)

b 250

-- 200

E 3 3 150

B " 100 .- r

2 50

0 0 2 4 6 8 IO 12 14 16 18 20 22 24

Time (h)

Fig. 5. Daily (February 19th, 1993) SO, (a) and particulate matter (b) concentrations in the surroundings of Granada Cathedral (data source: [59]).

Granada, with the exception of sporadic emissions from urban fuel-oil furnaces. This is shown in Fig. 5 where the concentration of SO* and particulate matter in the surroundings of the Cathedral of Granada (center of the city) is plotted against time. Heavy vehicular traffic is normally present during rush hours (09:00-10:00 h and 2030 h) when max- ima for both pollutant concentrations occur.

3.3. Experimental weathering

Samples of limestone uncovered (fresh) and covered with diesel and gasoline exhaust particles


and with dust were exposed in the SOZ chamber star-shaped gypsum aggregates appeared growing and then analyzed by XRD. In the limestone on etched calcite grains, surrounded by soot and covered with diesel exhaust particulate matter, metallic particles (Fig. 4b). Intergrowths of gyp- large amounts of gypsum (L 15% by weight of the sum blades were also to be seen in other areas, powder obtained after mechanical removal of the mainly in pits or etched zones of calcite crystals. surface layer, of 1 mm thickness) were detected After 48 h, massive interwoven gypsum crystals after 48 h from the start of the experiment. Minor appeared, varying in size from 10 to 50 pm (Fig. quantities of gypsum, I 5% by weight, were found 4c). Associated with the gypsum were metallic and in the slabs covered with gasoline exhaust. How- carbonaceous particles (always in contact with the ever, larger quantities were found in the slabs limestone’s calcite grains and the newly formed covered with dust (15-20% by weight). It must be gypsum). Most of the metallic particles associated said that the dust originally contained gypsum in with the growth of the gypsum have a composition proportions ranging from 5 to 10% by weight. No rich in Fe (with trace amounts of Ni and Cr). Some gypsum formation was detected on clean broken Fe-rich spheres developed gypsum crystals limestone. inside (Fig. 4d).

SEM with microanalytical EDS analyses gave the following results:

(1) Fresh limestone: No gypsum detected on the surface.

(2) Limestone coated with dust collected from the building (analysis carried out after 48 h from the start of the experiment): It is worth remarking on the presence of a massive growth of fungi due to the temperature and high relative humidity of the SO2 chamber. Underneath this organic layer, small gypsum crystals developed. In general, gypsum is found in higher amounts than in the initial dust from the building, and it is associated with metallic (iron mainly) and carbonaceous particles. The newly formed gypsum crystals were easily distinguished from the original crystals by their habits: star-shaped or needle-shaped crystals were formed in the slab attacked in the chamber, and globular or lenticular, poorly developed crystals associated as aggregates were found in the original dust.

These experimental results confirm the working hypothesis that there is a clear relationship between atmospheric pollutant particulate matter (both metallic and carbonaceous) and the formation of gypsum on the Granada Cathedral porous calcareous materials. The reasons leading to this conclusion are the following:

(i) Gypsum does not form on clean limestone that is exposed to an SO;, atmosphere at a high relative humidity. Gypsum formation by oxidation and wet deposition in an uncatalyzed system involving SO2 [41] can therefore be discarded. By the same reasoning, it is also possible to discard sulfate formation either by oxidation in the heterogeneous, liquid, or drop phases in the presence of O2 [42-451. These conclusions coin- cide with the fact that there is practically no acid rain in the area studied.

(3) Limestone coated with particles collected from gasoline engine emissions (analysis carried out after 48 h from the -tart of the experiment): An incipient growth of gypsum crystals appeared on the surface of the limestone in contact with the gasoline engine particulate matter. The crystals were amassed in globular aggregates and were very similar to those seen in the dust collected from the building (very small, 2-3 pm in size, poorly developed crystals).

(4) Limestone coated with particles coming from diesel car-engine exhaust: After 24 h of exposure,

(ii) Confirmation of the oxidation of SO, in a heterogeneous phase onto solid surfaces [46] in the presence of solid, metal-rich or carbonaceous particle catalyzers [11,47], is given by the occurrence of gypsum both on the limestone slabs on which dust was deposited as well as on the slabs on which particulate matter from gasoline and diesel exhausts were deposited. In this case, dry deposition of SO2 is the main process of SO2 removal from the chamber’s atmosphere. Furthermore, dry deposition is known to be a major process of SO, removal from the atmosphere [29,30,48,49] and, it is especially important in the decay of sheltered stony materials in monuments /15,17,19,28].

Gypsum formation by interaction with SO2 on

88 C. Rodriguez-Navarro, E. Sebastian / The Science of the Total Environment 187 (1996) 79-91

the surface of the limestone is thus strongly influenced by the presence of carbon particles [7,8,10] as well as by metal-rich [50] and metal-S- rich particles, which are the main constituents of diesel and gasoline exhaust. Our results are in agreement with those of Novakov et al. [lo] and Chang et al. [51], regarding the behavior of these particles in the catalytic oxidation of SOz. Fur- thermore, carbon particles play a decisive role as nucleation points for gypsum, and their potential for reactivity is clearly conditioned by their elevated porosity, which implies a high surface area and large absorption capacity for SO* [52]. The differences in the rate of gypsum formation, which was noticeable in the limestone with accumulated dust, was only incipient for gasoline exhaust, and spectacular in the case of diesel exhaust, should be basically due to variations in the composition of the particulate matter in each of the runs, since all the other variables were kept constant. In fact, diesel exhaust particulates have a greater amount of carbonaceous particles than gasoline exhaust. The gasoline exhaust, however, contains a large accumulation of Br and Pb particles [40,54]. This last type of particulate matter would seem to be less reactive, as confirmed by the slight formation of gypsum on the limestone slabs on which it was deposited. However further inves- tigation on the details of this process is warranted.

Not surprisingly, the experiment also demonstrated that the accumulated dust from the monument, which had a particulate composition halfway between that of the gasoline exhaust and the diesel exhaust, gave intermediate SO, fixation rates as calcium sulfate.

Thus, the process of oxidation of SO* occurs by means of the reaction:

Particulate matter (metal-rich and carbonaceous particles): oxidant and catalyst

2so2 + 02 - 2sos (1)

In the presence of water (in the experiment, the relative humidity was kept constant at values of lOO%, thus condensation took place on the slab surfaces), the following hydrolysis reaction applies:

SO3 + Hz0 - H2S04 (2)

The newly formed sulfuric acid attacks the limestone in the presence of humidity, resulting in the formation of gypsum, according to the following reaction:

H2S04 + CaCOs + Hz0 - CaS04-2HzO + CO2

(3)

The catalytic action of the metallic particles (in reaction 1) is probably increased by the synergetic action of different metals within them (Fe + Cr- Ni-Pb) [43]. Finally, it is worth remarking that, in contrast with other metallic catalysts (e.g. vana- dium, [8]), carbon particles induce SO2 oxidation at ambient temperature [53], a fact that is extremely important in this process.

3.4. Black crust growth hypothesis

The above data allow us to postulate the following hypothesis regarding the growth of black crusts on the studied limestone building:

Stage 1. Dust deposition: this is the first step in the formation of a crust, as it has been experimentally demonstrated that the dust has a high capacity for fixing atmospheric SO2 as sulfates in the presence of humidity (dry deposition process). It must be pointed out that, due to its mineralogical composition (rich in clay minerals) and high porosity, this dust can retain a high humidity while at the same time prevent quick evaporation of the water penetrating through to the surface of the porous underlying limestone. Several authors have argued that humidity has a key role in the formation of black crusts [55]. Many works [56-581, suggest that the roughness and porosity of a stone surface would affect both the deposition of particulate matter as well as the humidity contribu- tion. The high surface roughness and open porosity of the limestone would therefore aggravate deposition as well as affecting Stage 2.

Stage 2. Catalytic oxidation of SO2 in the presence of particulate matter and humidity: Once the oxidation of SO2 in the presence of particulate matter has taken place on the stone surface, the hydration processes necessary for the formation of sulfuric acid can then occur. Gypsum nucleation points are then produced as a consequence of the sulfuric acid attack on the calcite in the limestone.


Stage 3. Gypsum growth andfinal development of the black crust: The gypsum formed in Stage 2 is initially poorly crystalline (spherulitic aggregates and star-shaped gypsum), but in this stage it becomes involved in processes of dissolution and reprecipitation (forming bladed and tabular crystals) on both the surface and within the porous system of the stone, thus giving rise to a crust, which is black because of the incorporation of carbonaceous species. As the crust grows, it forms a discontinuous layer between itself and the unaltered calcareous substrate, thereby producing the loss of entire slabs of crust and limestone, as observed here [24-261 and elsewhere [ 151. The process can repeat itself indefinitely as new particulate matter and atmospheric dust are deposited on the newly exposed surface. Obviously, the presence of SOS and particulate matter as pollutants in the urban atmosphere surrounding the building are premises for this process to take place. Nevertheless, SO, from gasoline and diesel motor vehicles emissions is the main gaseous pollutant in this city and particulate matter (from vehicular traffic) is found in large quantities in the surroundings of the building.

In conclusion, it must be emphasised that this research strongly confirms the great interdepend- ence existing between the degradation of historic buildings (especially those constructed with ornamental carbonate rocks) and urban pollution. It is not necessary to have industrial complexes or power plants as the sources of this pollution, since it is plain that pollution by motor vehicles (especially diesel engines) is enough to destroy the historical and artistic heritage of a population center.

Acknowledgement

This work has been supported by the Resear- ching Group No 4065 of the Junta de Andalucia, the Project PB-93-1090 (DGICYT) and, a Grant of the MEC from the Spanish Government (Post Doctoral Fellowship, PF94-24232705). We also acknowledge the Research Technical Services of Granada University for the SEM-EDS study. We are indebted to the technical support of The Getty Conservation Institute, and Dr E. Doehne and Dr W. Ginell for their critical review and comments.

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