The Discoloration of the Taj Mahal due to Particulate Carbon andDust DepositionM. H. Bergin,*,†,‡ S. N. Tripathi,*,§ J. Jai Devi,†,‡ T. Gupta,§ M. Mckenzie,‡ K. S. Rana,∥ M. M. Shafer,⊥

Ana M. Villalobos,⊥ and J. J. Schauer⊥

†School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, Georgia 30332, United States‡School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States§Department of Civil Engineering and Center for Environmental Science and Engineering, Indian Institute of Technology, Kanpur,India∥Archaelogical Survey of India (Science Branch), Delhi, India⊥Environmental Chemistry and Technology Program, University of Wisconsin at Madison, Madison, Wisconsin 53706, United States

*S Supporting Information

ABSTRACT: The white marble domes of the Taj Mahal are iconic images ofIndia that attract millions of visitors every year. Over the past several decadesthe outer marble surfaces of the Taj Mahal have begun to discolor with timeand must be painstakingly cleaned every several years. Although it has beengenerally believed that the discoloration is in some way linked with poor airquality in the Agra region, the specific components of air pollution responsiblehave yet to be identified. With this in mind, ambient particulate matter (PM)samples were collected over a one-year period and found to contain relativelyhigh concentrations of light absorbing particles that could potentially discolorthe Taj Mahal marble surfaces, that include black carbon (BC), light absorbingorganic carbon (brown carbon, BrC), and dust. Analyses of particles depositedto marble surrogate surfaces at the Taj Mahal indicate that a large fraction ofthe outer Taj Mahal surfaces are covered with particles that contain bothcarbonaceous components and dust. We have developed a novel approach thatestimates the impact of these deposited particles on the visible light surface reflectance, which is in turn used to estimate theperceived color by the human eye. Results indicate that deposited light absorbing dust and carbonaceous particles (both BC andBrC from the combustion of fossil fuels and biomass) are responsible for the surface discoloration of the Taj Mahal. Overall, theresults suggest that the deposition of light absorbing particulate matter in regions of high aerosol loading are not only influencingcultural heritage but also the aesthetics of both natural and urban surfaces.

■ INTRODUCTION

On the time scale of several years the outer marble surfaces ofthe Taj Mahal become discolored and must be cleaned in atime-consuming process.1 Figure 1 shows a cleaned section of amarble Mosque Dome at the Taj Mahal (where cleaninginvolves applying a layer of clay and removing the clay after itdries, followed by rinsing the surface with clean water) directlynext to an area being prepared for cleaning. An obvious contrastis seen between the clean, white marble surface and the darkerdiscolored surface of the Marble Dome. Many measures havebeen undertaken to avoid the impact of local air pollution,including restricting traffic within 1 km of the grounds andlimiting the emissions of industrial pollution in the city of Agra,where the Taj Mahal is located. While detailed scientific studieshave not been reported in the literature, past efforts focusing onthe discoloration have hypothesized that local air quality isresponsible and suggestions have included surface reactionswith gas-phase SO2, as well as aqueous phase chemistry linkedwith the deposition of fog droplets, and water condensation,2 as

well as dust deposition.1 Despite efforts to keep the outersurfaces of the Taj Mahal white, it continues to becomediscolored with time, and the reason for the discoloration is notcurrently understood.Recent work has reported poor air quality throughout the

Indo-Gangetic plain,3,4 including relatively high concentrationsof particulate matter in Agra.5−7 Particulate matter in the regionincludes the light absorbing components black carbon (BC),light absorbing organic carbon (a fraction of which can absorblight preferentially in the UV region and is often termed browncarbon, BrC), and dust.8 Both organic carbon and dust have thepotential to preferentially absorb solar light in the blue regionof the spectrum, which can give the atmosphere a brown hueand has thus been dubbed the Atmospheric Brown Cloud.9 The

Received: August 18, 2014Revised: November 24, 2014Accepted: December 3, 2014Published: December 3, 2014

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presence of these light absorbing aerosols, and in particularthose that can take on a dark hue against a light coloredbackground, suggest that the deposition of ambient particulatematter may be playing a role in the discoloration of the outerwhite marble surfaces of the Taj Mahal.

■ EXPERIMENTAL METHODSAmbient Particulate Sampling and Analyses. In order

to determine the influence of PM on the Taj Mahal, ambientaerosol sampling was conducted for a roughly one-year periodbeginning on Nov. 5, 2011 and continuing through June, 2012just prior to the monsoon season. Filters were collected everysixth day each month for both PM2.5 (fine particulate matterhaving diameters less than 2.5 μm) and total suspendedparticulate matter (TSP), and analyzed for major anions,organic (OC) and elemental carbon (EC), and trace elements.The PM2.5 cut-point was established using an upstream cyclone,while the TSP directly sampled ambient air. Portions of eachfilter were combined to make monthly composites that wereextracted and analyzed for source specific trace organiccompounds using GCMS. The trace organic concentrationswere used to estimate source contributions to particulateorganic carbon using chemical mass balance (CMB) model-ing.10 Additional information on the sampling and chemicalanalyses is presented in the Supporting Information (SI)section (sections S1 and S2).Marble Deposition Target Sampling and Analyses. In

addition to ambient samples, several precleaned marbledeposition targets (with dimensions 2 × 2 × 0.5 cm) wereplaced outdoors within roughly 300 m of the main Taj Mahaldome. Both the air sampling equipment and targets werelocated in a section of the Taj Mahal that was accessible only tostaff of the Archaeological Survey of India (ASI), and had verylittle foot traffic. Precleaned marble cuboids were fastened toTaj Mahal structures with double-sided tape at a variety oflocations, and exposed from April to June 2012. Some of themarble samples were placed horizontally and others vertically.Prior to, and after exposure, the marble samples were placed insealed, precleaned Petri dishes and stored in a freezer to avoiddegradation of deposited particles.Scanning electron microscopy (SEM) (LEO 1530, Carl Zeiss

Microscopy) and energy dispersive X-ray (EDX) spectroscopy(Oxford Instruments Xmax detector) were carried out on two

horizontally facing marble targets. Images were taken at manydifferent magnifications to capture the particles having sizesranging from 100 nm to 100 μm. The particle sizes and shapeswere accessed through SEM images using image processing in aMatlab program. EDX analyses were carried out on the samemarble targets on ∼1000 particles. The information gainedfrom the SEM/EDX analyses allowed for the estimation of theparticle number and surface area concentration, and chemicalcomposition as a function of area of the marble target. Thisinformation was then used, as described in the next sections, toestimate the change in color of the marble surface. Moreinformation on the SEM/EDX analyses is given in the SISections S1 and S2.

Linking Deposited Particles to Marble Surface Color.In order to estimate the impact of particles deposited to themarble substrate on the perceived color change of the surface,we developed an approach that estimates the influence ofdeposited particles on wavelength-dependent surface reflec-tance. The method builds on previous work that estimated theinfluence of particles deposited to plant leafs on availablephotosynthetically active radiation.11 First, we use SEM/EDXanalyses of particles deposited to the marble targets to estimatethe optical depth of deposited particles as a function ofwavelength (τλ) as follows:

∑τ π= +λ λ λ=

D Q Q4

Ac [ ]i

n

i i i1

p,2

s a(1)

Where Ac is the areal particle number concentration (numberof particles deposited per area of the marble surface) for eachsize bin, i, Dp is the particle diameter for deposited particles,and Qsλ and Qaλ are the wavelength dependent Mie scatteringand absorption efficiencies that are determined based onparticle size and composition.After estimating the optical depth, the wavelength dependent

single scattering albedo, ωλ (ratio of light scattering toextinction) is estimated as

ω =∑

∑ +λλ

λ λ

=

=

D Q

c D Q Q

Ac

A [ ]in

i i i

in

i p i i

1 p,2

s ,

1 ,2

s a (2)

The single scattering albedo is a key parameter thatdetermines the relative amount of light absorption that occursover the white marble surface. For white, scattering onlyparticles the single scattering albedo is near 1.0 and the surfacereflectance of a white surface will not change.The change in the surface reflectance of the white marble

surface is calculated using SBDART, a radiative transfermodel12 with input values including τλ and ωλ estimatedfrom eqs 1, and 2 as well as the asymmetry parameter (relativeamount of light scattering in the forward direction) as afunction of wavelength using Mie theory.13

In order to determine the perceived color change of thewhite marble surface based on particle deposition, and therelated change in spectral surface reflectance we used the modeldescribed by D’Andrade and Romney14 to convert spectralreflectance to perceived color in the Munsell color system. TheMunsell color system is based on three components thatinclude the value (lightness/darkness), hue (color), andchroma (purity/saturation). The model used to estimate theperceived color of the marble surface with deposited particlesuses the spectral reflectance from the radiative transfer model ofthe marble surface loaded with particles to estimate the Munsell

Figure 1. A clean surface and an area being prepared for cleaning on amarble Mosque dome at the Taj Mahal.

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color. The Munsell color estimate also takes into considerationthe human eye response as a function of wavelength of incidentlight (the approach is described in more detail in the SI, sectionS3).

■ RESULTS AND DISCUSSIONAmbient Particulate Concentrations. Figure 2a shows

the average mass concentrations of particulate organic carbon

mass (OM), ions, dust and elemental carbon (EC) for bothTSP and PM2.5 over the sampling period. The mean dailyconcentrations (and standard deviations) of both TSP andPM2.5 are 135 (55) and 60 (39) μgm−3. The values aresignificantly higher than the annual World Health Organization(WHO) PM guidelines for PM10 and PM2.5 of 20 μgm

−3 and 10μgm−3 highlighting the poor air quality in the region. Thefraction of particulate matter greater than 2.5 μm is ∼60%, andis due in large part to the coarse mode dust that increases from15% of the PM2.5 mass fraction to 30% of the TSP mass. Inaddition to dust, other PM components that absorb light in thevisible spectrum, and hence have the possibility to influence thecolor of the outer white marble surfaces, are elemental carbon(EC) that is responsible for 2% of the TSP mass, as well as OMthat accounts for 39% of the TSP mass. Estimates of thesources of OM in the PM2.5 mass fraction shown in Figure 2bindicate that biomass burning, a known source of BrC, isresponsible for roughly half of the OM with significantcontributions from vehicular emissions. It should be pointed

out that biomass burning OM can be from a variety of activitiesincluding the combustion of wood and dung, crop residue, andthe burning of trash and refuse that is ubiquitous in the region.The prevalence of light absorbing aerosols in Agra (i.e.,elemental and organic carbon, and dust) suggests that PMdeposition to the white marble surfaces may be responsible forthe observed discoloration of the outer Taj Mahal structuresincluding the famous Taj Mahal dome.

Size and Composition of Particles Deposited toMarble Targets. Figure 3a shows a scanning electron

microscopy (SEM) image of a marble deposition target thatwas placed horizontally at the Taj Mahal, and was exposed for aroughly two month period during the premonsoon season of2012. The surface area concentration (surface area of particlesper unit marble surface area per micron) distribution ofparticles as a function of particle size deposited to the marbletarget (Figure 3b) shows peaks in particle size at roughly 1−2μm, and an additional mode at 4−5 μm. The fraction of thesurface covered by particles is estimated to be ∼30%.Approximately 70% of the deposited particle surface area isfor particles having diameters greater than 2 μm, indicating thata large fraction of the particle surface area concentration is dueto the deposition of coarse particles. The dominance of coarseparticles is due to both the relatively high concentration of dustparticles measured in Agra, as well as the fact that the drydeposition velocity of coarse (∼5 μm) particles is roughly 100times greater than that of accumulation mode (∼1.0 μm)

Figure 2. General chemical composition of (a) total suspendedparticulate matter (TSP) and fine particulate matter (PM2.5), and (b)source apportionment of PM2.5 organic mass (OM) based on filtersampling at the Taj Mahal. Figure 3. (a) SEM image of marble target from Taj Mahal indicating

deposited particles and (b) surface area concentration of depositedparticles on the marble target as a function of particle diameter.

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aerosol particles.15 It is likely that dry deposition is thedominant mode of particle transport to the Taj surface giventhe relatively low amount of precipitation in Agra during theFall through Spring, when particle loadings are high and thesummer monsoon rainfall is not occurring. It is also importantto note that water insoluble particles (such as dust, BC, and afraction of OM) are likely not easily removed from the TajMahal surface by precipitation wash-off once deposited. This isbased on similar observations of the buildup of water insolubleparticles on leaf surfaces observed in the Yangtze delta region ofChina, an area that also experiences high PM loadings.11

The EDX analyses indicate that more than 70% of particlesare primarily crustal in origin, with spectra dominated by crustalelements. The crustal particles were typically in the coarse(having diameters greater than ∼2−3 μm) size fraction.Particles having diameters less than 2 μm, which account forroughly 30% of the deposited particle surface area, alsocontained significant amounts of carbon likely from the sourcesof OM highlighted in CMB results in Figure 2b. More detailedinformation on the EDX analyses, including example spectra ofparticle EDX analyses are given in SI section S2.Deposited Particles and Perceived Color. To estimate

τλ we assume that particles having diameters less than 3 μm arecomposed primarily of light absorbing organic carbon (BrC)with wavelength dependent refractive indices reported by Liu etal.16 This assumption is based on the fact that ambient filtersindicated that roughly half of the PM2.5 was carbonaceous innature, combined with the fact that we did not observe thepresence of major ion related elements (i.e., S) deposted to themarble targets, but did see a dominance of carbon particles inthe less than 3 μm particle sizes. We also assume that 10% ofthe particles less than 3 μm are black carbon (BC) particleswith a refractive index commonly used for soot particles.13 It islikely that we overestimate the influence light absorption byBrC and BC since we assume that all of the particles less than 3μm are carbonaceous and not elemental or ionic in nature.Aerosol optical depth contributions at 400 nm for dust, BrCand BC are estimated to be 0.222, 0.144, and 0.016 respectively,highlighting the importance of light extinction by all threecomponents. The single scattering albedo at 400 and 700 nm isestimated to be 0.64 and 0.95, indicating that a significantamount of light absorption occurs at near-ultraviolet wave-lengths preferentially to that at the longer 700 nm wavelength.Figure 4a shows the surface reflectance of a pure marble

surface as well as the estimated surface reflectance for severalcases including the influence from each light absorbingparticulate component separately (BC, BrC and dust), andthe case when all components are combined. As seen in theplot, BC absorbs across all wavelengths evenly, with browncarbon showing preferential absorption at shorter wavelengthsnear 400 nm. Dust substantially reduces the surface reflectanceat all wavelengths, and preferentially so at shorter wavelengthsdue to the presence of hematite, which absorbs at bluewavelengths. When combined, dust, BrC, and BC are estimatedto substantially alter the surface reflectance with enhancedabsorption (i.e., lower values of surface reflectance) at shorterwavelengths.Figure 4b shows the estimated change in color of the white

surface for deposition of the light absorbing particles measuredin Figure 3b over the two-month period the targets wereexposed. Results indicate that separately each componentcontributes to the color change of the Taj Mahal white marblesurfaces. For BC alone (which we estimate to account for 3% of

the total particle surface area) the color change results in agreyish color given that the change in surface reflectance isproportionally similar at each wavelength. Both BrC (∼30% ofthe total particle surface area) and dust influence color withpreferential absorption in the UV resulting in yellowish-brownhues. When combined, the perceived color of the surface shiftstoward darker shades of yellow-brown. It should be pointed outthat our sample targets were mounted for a relatively short time(∼2 months) compared to the typical time between cleaningsof the outer Taj Mahal surfaces (several years), and therefore itmay be expected that the perceived color of the marble targetwould be attenuated compared to that of the white surfaces ofthe Taj Mahal. Indeed the marble target surface did appearsomewhat lighter in color as compared to the color estimates inFigure 4b, although qualitatively were similar. There are severaluncertainties in estimating perceived color including theloading, particle size, and optical properties. Analyses (includedin SI section S3) suggest that results are moderately sensitive toboth particle loading and size. For example, assuming anuncertainty of 50% for aerosol loading only moderatelyinfluences the perceived color and does not change theconclusions that both dust and carbonaceous particlescontribute to the perceived color change of the Taj Mahal.Overall, the results indicate that light absorbing particles play animportant role in the discoloration of the Taj Mahal surface andthat dust, as well as BC and BrC that are primarily frombiomass combustion, trash/refuse burning, and mobile sources,all make significant contributions to the discoloration.This work further suggests that the deposition of light

absorbing particulate matter to both natural and human-madesurfaces results in a substantial discoloration in regions of highaerosol loading. The discoloration impacts not only cultural

Figure 4. (a) Estimated marble target surface reflectance for a cleansurface and surface area coverage of particles based on Figure 3 forblack carbon (BC), brown carbon (BrC), dust and all particles (BC+BrC+dust) (b) Change in color of white marble surface for dust,BrC, BC separately and combined. Values in parentheses representfraction of total surface area concentration contributed by eachcomponent with AOD values estimated by eq 1 at 400 nm.

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artifacts but also the aesthetics of the environment through themodification of surface albedo, and hence perceived color. Themeasurement/modeling approach developed in this paperallows surface color changes to be estimated based on therelative amounts of light absorbing particles deposited tosurfaces, and can be used to develop future control strategies toprevent the discoloration of the environment by particledeposition, that will also improve air quality.

■ ASSOCIATED CONTENT*S Supporting InformationSupporting Information includes details related to filter andmarble surrogate surface sampling (S1), chemical analyses onboth the filters and marble surrogate surfaces (S2), as well asthe approach to estimate perceived color based on surfacereflectance (S3). This material is available free of charge via theInternet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Authors*Phone: 404-894-9723; e-mail: mike.bergin@ce.gatech.edu.*Phone: 91-512-259-7845; e-mail: snt@iitk.ic.in.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was supported by grants from the Indo US Scienceand Technology Forum (IUSSTF) as well as from the U.S.EPA (Grant No. RD83479901), and the NSF PIRE Grant No.1243535. In addition, chemical analyses at IIT Kanpur wereconducted by N. Rastogi and C.M. Shukla. We are also gratefulfor the efforts by the ASI Staff at the Taj Mahal.

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