Magnetic, Chemical, and Microscopical Characterization of Urban

Research Communications Magnetic, Chemical, and Microscopical Characterization of Urban Soiling on Historical Monuments Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ United Kingdom

Here we report a study on the magnetic fraction in soiling crusts on urban building stones which, due to their high porosity and surface roughness, act as preferential depositional sites for airborne particles including Fe-rich ones (16, 17). A new analytical procedure involving magnetic measurements of bulk samples followed by magnetic separation of the particulate fraction in the crusts for scanning electron microscopy (SEM) combined with quantitative energy dispersive spectroscopy (EDS) has been tested to assess its efficiency in differentiating between sources of magnetic airborne particles deposited at the surface of historical buildings in the U.K.

LIPING ZHOU

Sampling Sites and Experimental Section

NICK SCHIAVON*

The Godwin Laboratory, University of Cambridge, Free School Lane, Cambridge, CB2 3RS United Kingdom, and The McDonald Institute for Archaeological Research, University of Cambridge, Downing Street, Cambridge, CB2 3ER United Kingdom

Introduction Sulfate-rich dark crusts are a common decay feature on the surface of urban monuments and buildings. They are composed mainly of a framework of gypsum crystals (CaSO4‚2H2O) originating from the reaction between atmospheric SO2 and Ca-rich building materials; within the gypsum crystalline framework, soil dust and anthropogenic airborne particles derived from the burning of fossil fuels for domestic and industrial purposes have accumulated through wet and dry deposition processes. In order to determine the relative contribution of past and present air pollution in soiling and to gain insight into the ability of building materials to act as sinks for atmospheric air pollutants, it is important to develop analytical techniques that allow us to distinguish between different emission types and atmospheric particulate sources responsible for the composition of the soiling crusts. Sulfur isotope studies have been used to determine the provenance of the SO2 (atmospheric, groundwater, de-icing salts, etc.) in gypsum crusts on building materials (1, 2). Chemical, morphological, and microscopic characterization of the particulate fraction of weathering crusts on monuments has also been used as a tool for distinguishing between oil and coal airborne particulate source contributions (3-5). It is well known that anthropogenic aerosols have a significant magnetic component (6-9) and that magnetic measurements have been found to be well suited to differentiating atmospheric dusts and aerosols from different types of emission sources (10-14). In recent review articles on environmental magnetism, Flanders (14) and Oldfield (15) have stressed the need to combine magnetic measurements with mineralogical, chemical, and morphological investigation by electron microscopy to characterize the magnetic fraction of airborne anthropogenic aerosols. * Corresponding author telephone: ++44-1223-333400; fax: ++441223-333450; e-mail address: [email protected].

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Samples of sulfated surface patinas from a few millimeters to several centimeters thick were collected from the following British limestone monuments: 1, St. Paul’s Cathedral, London; black crusts from roof parapet, southwestern side. 2, Westminster Cathedral, London; black crusts from the major nave. 3, Ely Cathedral, Ely; black crusts from the Lady Chapel, eastern side. 4, St. John College Chapel, Cambridge; southwestern side. 5, St. Luke’s Church, Chelsea, London; Black crusts from the west tower. 6, Salisbury Cathedral, Salisbury; black crusts from the major nave. All patinas come from areas sheltered from rain washout where active growth of soiling crusts is known to be favored (3). Climatic conditions do not vary appreciably between sites. Typical data for the Cambridge site are as follows: average temperature (January) ) 5 °C (average daily maxima and minima of 7.7 and 2.5 °C); average temperature (July) ) 18.2 °C (average daily maxima and minima of 23.9 and 12.62 °C); average annual rainfall ) 606 mm/year; and average relative humidity ) 67.52% (in London, annual means as high as 82% were recorded). Levels of atmospheric SO2 in the U.K. show a general decreasing trend in the last 30 years, and they are now well below the EEC limit of 113 µg/m3. No historical record of SO2 monitoring is available for Ely and Salisbury (in the latter site a shortterm monitoring exercise in August 1995 showed a mean SO2 concentration of 8.2 µg/m3). In Cambridge, SO2 levels have decreased from >150 µg/m3 in 1964 to 16 µg/m3 in 1994. In London (at the Lambeth site in the south of the city), there has been a 10-fold decrease in mean SO2 levels from 271 µg/m3 in 1963 to 29.7 µg/m3 in 1993; exceptional high SO2 pollution episodes such as the Great Smog of 1952 (where daily mean SO2 levels as high as 4460 µg/m3 were recorded; 18) do not occur anymore. Untreated lumps of samples were weighed and packed with cling film firmly into 10-mL plastic specimen pots for magnetic measurements. The magnetic measurements

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were performed following the procedures described by Thompson and Oldfield (19). The following magnetic parameters were used: Mass Specific Magnetic Susceptibility (χ): ratio of the magnetization produced in a substance to the intensity of the magnetic field to which it is subjected. This measure of “magnetizability” is often approximately proportional to the concentration of ferrimagnetic oxides in a sample. Frequency-Dependent Susceptibility (χfd): difference between the low frequency (0.47 kHz) and high frequency (4.7 kHz) susceptibility measurements. The frequencydependent component is mainly controlled by magnetic grain-size and is characteristic of ferrimagnetic grains with a diameter just below the lower limit for stable single domain grains (=0.03 µm). Isothermal Remanent Magnetization (IRM): IRM is the magnetization that remains after a sample has been subjected to a magnetic field at room temperature. IRM increases nonlinearly with increasing field strength and saturates in high field. The maximum remanent magnetization is known as saturation isothermal remanent magnetization (SIRM). Coercivity of saturation isothermal remanent magnetization (B0)CR: magnetic field strength required to reduce the IRM to zero after saturation. (B0)CR may be regarded as a measure of the stability of magnetization and is affected by magnetic mineral type and size rather than concentrations. The magnetic fraction was extracted using the following procedure: Gentle crushing of the bulk crust with mortar and pestle into a fine powder. Cold dissolution of the sulfate component in 5% HCl solution overnight. Centrifuge cleaning of the insoluble residue in distilled water (3 times). After ultrasonic dispersion of the insoluble residue in distilled water (adding as a deflocculant a few drops of NH3 solution), a strong hand magnet (wrapped with cling film) was applied to the outside surface of the glass beaker. During settling, magnetic particles clustered close to the magnet and were washed with distilled water into a clean beaker. The procedure was repeated (at least 6 times) until no separation of the magnetic fraction was visually detected. Filtering of the desegregated magnetic particles into Nuclepore polycarbonate membrane filters for SEM imaging and analysis. This technique proved successful in obtaining a scattered distribution of the magnetic particles on the filters avoiding clumping and chaining of particles (14) and enabling singleparticle analysis by EDS. A software program (LINK ZAF4/PB) specially formulated for quantitative electron analysis of particles was used; this program permits quantitation of data from rough specimens and particles by the calculation of peak to local background ratios rather than the more conventionally used net peak intensities (20). Peak to background ratios are less geometry dependent than net peak intensities. SEM+EDS operating conditions were Si(Li) detector; Be window; lightest element analyzed, Na; 1.25 nA beam current; 15 kV accelerating voltage; 23 mm working distance; 10 eV/channel; 100 s count time. Fifty particles per sample were analyzed. Ten random fields of view (using backscattered images) were selected, and five particles in each field were analyzed. The analysis points were manually selected in the centre of each particle. At 15 kV accelerating voltage, the volume of analyzed X-ray

FIGURE 1. Low frequency (χlf) and frequency dependent (χfd) magnetic susceptibility (×10-8 m3/Kg) measurements on weathering crusts on British historical monuments: 1, St. Paul’s Cathedral; 2, Westminster Cathedral; 3, Ely Cathedral; 4, St. John’s College; 5, St. Luke’s Church; 6, Salisbury Cathedral.

FIGURE 2. Isothermal remanent magnetization (10-3A m2 Kg-1) acquisition curve of weathering crusts on British historical monuments: 1, St. Paul’s Cathedral; 2, Westminster Cathedral; 3, Ely Cathedral; 4, St. John’s College; 5, St. Luke’s Church; 6, Salisbury Cathedral.

emissions in each particle is in the order of a few cubic micrometers (µm3); given that most particles fell within the 3-5 µm grain-size range (measured as the geometric projected diameter on the SEM screen), these analytical conditions were considered to be suitable for producing analyses that were representative of the bulk composition of each particle with minimum contamination from the substrate.

Magnetic Properties All samples show high magnetic susceptibility (χ) values, suggesting that the main magnetic carrier in the magnetic fraction is ferrimagnetic grains (Figure 1). The presence of magnetite (Fe3O4) as the main magnetic contributor to the magnetic properties is demonstrated by the shape of the IRM acquisition curve up to 1 T; all the samples attain 9197% of the SIRM in a forward field of 300 mT (Figure 2). The back-field IRM displays typical “soft” magnetization behavior of magnetite as opposed to the “hard” one typical, for example, of canted antiferrimagnetic grains, i.e., hematite-goethite. The predominance of magnetite as op-

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TABLE 1

Composition (% Elemental Concentrations) of Magnetic Particle Types by Energy-Dispersive X-ray Analysis particle type no.

Na

Mg

Al

Si

S

Cl

K

Ca

Mn

V

Fe

Ti

1a 1b 2 3a 3b 4a

0.41 0.45 0.50 0.29 0.46 0.61

0.21 1.42 0.80 0.19 2.20 0.00

0.40 9.25 5.27 0.26 9.95 0.00

0.68 12.89 3.72 0.42 10.61 0.49

0.03 0.10 0.06 0.13 0.06 0.00

0.05 0.02 0.01 0.02 0.11 0.00

0.06 1.40 0.48 0.02 1.26 0.02

0.23 0.60 0.71 0.06 0.09 0.15

0.45 0.25 0.18 0.15 0.65 0.07

0.04 0.11 0.03 0.10 0.03 0.08

66.20 29.12 52.57 65.46 32.50 68.54

0.07 0.30 0.34 0.06 0.34 0.80

a

This analysis refers to magnetite octahedral particles.

common textures are dendritic, crystalline, (Figure 4b) and “brainy”. They constitute 16% of the total number of particles analyzed. In many cases, the surface pattern is intimately connected with the bulk texture of the particles (Figure 4b), whereas in others only its superficial nature is evident. The elemental composition is dominated by Fe with Al, Si, K, Mg, Ca, and, rarely, Ti as minor elements (Table 1). (3) Irregular Iron-Rich Fragments. Irregular iron-rich fragments with a solid and/or vesicular texture. A particular type is Fe-rich particles displaying a “grid” pattern (Figure 4c); a closer examination reveals that these fragments are made up of micron-size octahedra of magnetite. This class constitutes 19% of the total number of particles analyzed. Compositionally, two subclasses can be distinguished (Table 1): Fe as the major or only element (3a) and Fe in an Al-Si matrix with Mg, K, Mn, and Ca as minor elements (3b). FIGURE 3. Typical XRD chart of the magnetic fraction from sulfaterich weathering crust, St. Luke’s Church, London. Numbers are d-spacings in Å. M, magnetite; Q, quartz; F, feldspar; H, hematite.

posed to hematite is also confirmed by X-ray powder diffractometry (Figure 3). The low frequency-dependent magnetic susceptibility χfd values (Figure 1) suggest that the magnetic fraction is composed mainly of relatively coarse (single-domain or multi-domain) grains. The (B0)CR values are all around 43 mT, suggesting that the magnetic mineralogy does not vary between different sites. It should be mentioned that plots of -20 mT/SIRM versus -200 mT/ SIRM as used by Hunt et al. (10) to distinguish between vehicle-derived and fly ash particulate did not yield clustering of data points in the present study.

SEM+EDS Analyses No major difference in particle types has been found between samples from heavily polluted urban locations such as London and less polluted sites such as Cambridge, Ely, and Salisbury. Morphologically, magnetic particles are clearly distinguishable under SEM and can be grouped into four main particle classes: (1) Smooth Iron-Rich Spherical Particles (Figure 4a). Spherical particles with a smooth surface texture constitute 60% of the total number of particles analyzed. Particle sizes range between 1 and 20 µm in diameter, but most of them (83%) are in the 1-5 µm size range. Compositionally, they can be subdivided into two subclasses (Table 1); Fe as the major or only element (1a) and Fe in an AlSi matrix (1b). (2) Iron-Rich Spherical Particles with Surface Texture. Spherical particles (with diameter ranging between 3 and 10 µm) displaying several types of surface texture: the most

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(4) Iron-Rich Crystalline Particles. Individual octahedral iron oxide crystals interpreted as magnetite (Fe3O4) are common (Figure 4d; Table 1). Crystalline pyritohedra with cubic symmetry and a Fe-S elemental composition (interpreted as pyrite, FeS2) are also present (Figure 4e). Other Fe-rich crystalline oxides (such as ilmenite, FeTiO3) are also rarely found. Iron-rich particles with a well-defined crystalline habit constitute 5% (of which 20% are single magnetite octahedra) of the total number of particles analyzed.

Discussion Bulk magnetic measurements indicate magnetite (Fe3O4) as the predominant magnetic component in the particulate magnetic fraction deposited within sulfate-rich soiling crusts on selected British monuments. Powder X-ray diffractometry analysis on bulk samples (Figure 3) confirms the presence of magnetite with minor hematite and traces of quartz and feldspars as nonmagnetic contaminants. Under SEM examination, single crystals of magnetite with an octahedral habit are sometimes found (Figure 4d), but most of the Fe3O4 is microcrystalline (with individual crystallites occurring within the silicate matrix in each particle). The high saturation isothermal remanent magnetization and the low frequency-dependent magnetic susceptibility values suggest that the magnetic fraction in the weathering crusts is derived from anthropogenic pollution sources and not from soil erosion, because the magnetic particulate fraction eroded from modern and/or ancient soil profiles would contain significant amounts of ultra-fine ferrimagnetic grains near the superparamagnetic/ single-domain boundary (centered around 0.03 µm) with higher χfd values than the ones reported in this study (11, 12, 19, 21-23). Anthropogenic Fe-rich magnetic particles

FIGURE 4. BSEM images of (a, top left) smooth Fe-rich magnetic spherical particles; (b, top right) spherical Fe-rich magnetic particle with crystalline surface texture; (c, middle left) irregular Fe-rich magnetic particle made up of micron-size crystals (arrow) with an octahedral habit interpreted as magnetite (d, middle right) magnetite (M) crystals with octahedral habit; (e, bottom) pyrite crystals (FeS2) with typical pyritohedron habit.

in urban aerosols may be derived from several sources: fossil fuel power plants (mainly coal-fired), industry, and traffic (wear of brake shoes, exhaust and engines) (24, 25). Our combined magnetic and microscopical data are consistent with a main derivation of the magnetic fraction from coal combustion processes. Fly ash derived from oil burning has a low Fe content (typically =0.5% as opposed to =11.5% in coal fly ash; 25) and, accordingly, does not contain a large number of magnetic spherules (26-28). On the contrary, it is well known that pulverized fuel ash (PFA)from coal-fired power plants contains iron oxidebearing magnetic spherules (6, 7, 9, 29-33). The composition and morphology of magnetic particulate produced by coals of different composition are remarkably similar. Iron-

bearing mineral compounds in coal may provide the original material for these particles: sulfides (mainly pyrite), clays and silicates, carbonates, oxides and oxyhydroxides, and iron sulfates. A direct relationship between the occurrence of pyrite framboids in coal and limestones and ferrospheres in fly ash has been suggested (31, 34). The presence of pyrite crystals (Figure 4e) in the weathering patinas could then be interpreted as a result of incomplete combustion in the original coal and supports a coal origin for the magnetic particles examined in this study. Furthermore, particle surface textures found in this study are similar to the ones found in coal emissions (6, 7, 9, 32, 33). PFA’s morphology, crystallinity, and composition largely depend on combustion history: with increasing length of exposure

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and higher temperature in the combustion chamber, amorphous, irregular, spherical-smooth, and sphericalcrystalline particles are produced (6). Cooling rates and temperature gradients are also controlling factors for the type of particle discharged into the atmosphere through chimney stacks. In the extreme case of a liquid droplet, for example, undergoing an almost instantaneous transition to the solid phase (i.e., very rapid cooling), nucleation will be suppressed and an amorphous rather than a crystalline particle will result. To explain the origin of such textures in PFA, theoretical work on powder fabrication has been performed (35, 36); these studies have shown that the particle microstructure may be dependent on nucleation and growth events. In the case of rapidly cooled, microscopic, spherical, alloy powders, once nucleation has occurred, the solidified microstructure is determined by the growth rate of the solid from the liquid. If the growth rate cannot be supported by diffusional repartitioning of the elements (a high growth rate or a low temperature), then a uniform crystal structure is anticipated. If the growth rate is low as compared to the diffusion rates in the liquid, a dendritic crystalline pattern is expected. Higher absolute temperatures with small temperature gradients (retarding growth rate) will also lead to dendritic crystalline surface patterns. Although coal fly ash may contain a high antiferrimagnetic component (i.e., hematite) and this has been used a criteria for distinguishing it from vehicle-derived particulate matter (10), the subordinate presence of hematite in our samples does not rule out a coal origin. Hematite may be transformed to magnetite at 1663 K, and its presence would be favored only when combustion temperatures are low and/or the duration of burning is short (8). Hematite, formed from the oxidation of pyrite, may also undergo partial or total dissolution by molten silicates and subsequently precipitate as crystalline magnetite as proposed by Hansen et al. to explain its absence from stack-collected particle-sized PFA (7). Air pollution from coal-burning activities has been widely considered to be responsible for the growth of sulfate-rich surface weathering crusts in the U.K. (5). High-sulfur coal has been used for centuries as the main fossil fuel for domestic and industrial purposes in the British Isles until the introduction of the Clean Air Act in 1956 following the Great Smog pollution episode (responsible for more than 4000 deaths in London in the winter of 1952; 18). For the London sites and in particular for St. Luke’s Church in Chelsea, the very close proximity of a major coal pollution source such as the Battersea Power Station (operational between 1933 and 1983) can be established (37). For other sites such as Cambridge and Ely, domestic coal fires are more likely to have acted as the main source of magnetic airborne particles. Source variability between sites, however, is not well reflected in the soiling patinas investigated; the very low χfd, the IRM acquisition pattern and the similar B0CR values, and the SEM particle characterization indicate that the magnetic species in sulfated crusts from the selected sites are magnetically similar. Even χ values (Figure 1), roughly proportional to the concentration of ferrimagnetic grains and, indirectly, of air pollution levels (11), are high not only in London but also in Cambridge and Ely where localized, high levels of air pollution surrounding the buildings under examination may have played an important role. As seen in previous studies (5, 16, 17, 38), the particulate fraction imbedded within gypsum crusts on

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building facades is compositionally quite complex and may be related to several distinct sources in each particular sample. Therefore, the use of representative portions in a non-homogeneous weathering crust sample is an important factor for ensuring the validity of the analytical data. The “history” of the building under consideration may also be very complex with unrecorded episodes of stone replacements, cleaning, etc. that may have affected the magnetic composition of the weathering crusts through the years. Notwithstanding the aforementioned considerations, we believe that the combination of magnetic analysis with SEM + quantitative EDS on extracted magnetic particles can provide an effective and quick analytical method for the characterization of magnetic airborne particulate deposition on buildings. In line with previous studies, our results confirm that the composition of these crusts is dominated by past pollution sources (such as coal-burning power plants and domestic fires) which, although no longer a serious threat to human health, can still contribute to the decay of historical buildings due to the well-recognized ability of iron-rich particulate pollutants to act as catalysts in oxidation mechanisms leading to stone decay. Further work is needed to extend the data set to other sites and to combine analyses on soiling patinas with air monitoring tests to assess the relative contribution of past and present particulate pollutant deposition on buildings.

Acknowledgments This work was financially supported by the Commission of the European Community under the Science & Technology for Environmental Protection Programme (Project STEP CT90-110). Cambridge Earth Sciences Contribution No. 4713.

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Received for review June 3, 1996. Accepted August 9, 1996. ES9604774

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