NEW DELHI: India’s white marvel, the Taj Mahal , is slowly turning brownish-yellow because of air pollution, says an Indo-US study which also identifies the pollutants responsible for the effect.It says Taj is changing colour due to deposition of dust and carbon-containing particles emitted in the burning of fossil fuels, biomass and garbage. The study confirms what has been suspected for long — that Agra’s poor air quality is impacting India’s most celebrated monument.The research was conducted by experts from US universities — Georgia Institute of Technology and University of Wisconsin — as well as Indian Institute of Technology, Kanpur and Archaeological Survey of India. The paper was published in the Environmental Science & Technology journal in December.The findings can lead to targeted strategies to curb air pollution in and around Agra and more effective ways to cleanse the marble surface of the 366-year-old mausoleum, which remains by far the most visited man-made structure in the country with footfalls of more than 6 million in 2013.The researchers first analysed air samples at the site for roughly a year using filters and found high concentrations of suspended particles that could potentially discolour Taj’s surface.Clean marble samples were then placed at various points on the monument accessible only to ASI staff. After two months of exposure, the samples were analysed using electron microscope and X-ray spectroscope.The pollutants deposited on the marble were identified through these investigations. Researchers found 3% of the deposits to be black carbon, around 30% organic carbon (or brown carbon) and most of the rest dust. Black carbon is emitted by vehicles and other machines that burn fossil fuels. Brown carbon is typically released through burning of biomass and garbage, a common practice in the region.S N Tripathi of IIT Kanpur, one of the authors, said the team then used a novel approach to estimate how these deposited particles would impact light reflecting off the marble surface. “We found that black carbon gives a greyish colour to the surface while the presence of brown carbon and dust results in yellowish-brown hues,” he said.“Results indicate that deposited light absorbing dust and carbonaceous particles are responsible for the surface discolouration of the Taj Mahal,” the study concludes.Since 2008, ASI has been trying to fight the yellowing of the monument by giving it a clay pack treatment using the lime-rich Fuller’s earth (Multani mitti) to clean the marble surface. Researchers are now keen on studying the efficacy of this method and finding ways of improving it.“Now that we know what’s causing the yellowing, the focus should now shift to undoing the effect,” Tripathi said.