Airborne carbon particles and dust are giving the Taj Mahal’s gleaming white marble dome a dingy brownish cast. But researchers say knowing what’s to blame for the discoloration is only the first step in solving the problem.
Now they have to figure out where the particles are coming from in order to control them.
“Our team was able to show that the pollutants discoloring the Taj Mahal are particulate matter: carbon from burning biomass and refuse, fossil fuels, and dust–possibly from agriculture and road traffic,” says Michael Bergin, professor in the School of Earth and Atmospheric Sciences at Georgia Institute of Technology.
“We have also been able to show how these particles could be responsible for the brownish discoloration observed.”
Built in the 1600s by Mughal emperor Shah Jahan in memory of his third wife, Mumtaz Mahal, the structure is a mausoleum that includes a massive marble dome 115 feet high and minarets that reach 130 feet. Attracting millions of visitors each year, the Taj Mahal became a UNESCO World Heritage Site in 1983.
Beginning in the 1970s, a brownish cast on the white marble that makes up the structures started to appear. Today, routine cleaning, including the painstaking application and removal of a clay material, maintains the brightness of the marble.
Air pollution had been suspected as the culprit responsible for the discoloration, but no systematic study had been done and the specific components of the air pollutants responsible for the discoloration and the mechanisms by which they discolor the surface had remained unknown.
To find out what was causing the color change, researchers used air sampling equipment to measure what was in the air in the Taj Mahal complex from November 2011 through June 2012. Filters from the air-sampling equipment were analyzed for both fine particulate matter (smaller than 2.5 microns in diameter) and total suspended particulate matter. The analysis was done by scientists in both India and the United States.
In addition, researchers placed small samples of pristine marble onto the Taj Mahal at various locations near the main dome. After exposure to air pollutants over a two-month period, the samples were analyzed using an electron microscope to measure the size and the number of particles deposited on their surfaces as well as their elemental signatures. This information allowed the researchers to determine the likely composition of the particles.
Particles of dust, brown organic carbon, and black carbon were found in the filters and on the marble samples, Bergin says. The carbon particles come from a variety of sources, including fuel combustion, cooking and brick-making, trash and refuse burning, and vehicle exhaust. The dust may come from local agricultural activities and vehicular traffic – or from distant sources.
To check their analysis, the researchers refined a model for showing how the surface reflectance of the building’s marble should change with the application of brown and black carbon particles, along with dust. The predictions of the model matched what was being observed on the Taj Mahal.
“We fundamentally showed how these particles change the color of the surface,” says Bergin, who is also associated with Georgia Tech’s School of Civil and Environmental Engineering. “We hope to share this model with others who could use it to determine how urban and natural environments are changing colors due to particulate pollution.”
Now that researchers know what’s discoloring the Taj Mahal, the next step will be to identify the sources of the particles and plan control strategies. The sources could be local—and the government has already taken steps to reduce vehicle and industrial emissions in the area—or the particles could be coming from longer distances away from the region.
While the research focused only on the Taj Mahal itself, reducing particulate matter in the Agra region around the landmark would have additional benefits.
“Some of these particles are really bad for human health, so cleaning up the Taj Mahal could have a huge health benefit for people in the entire region,” Bergin says. “The health of humans and the health of the Taj Mahal are intertwined.”
Researchers from the Indian Institute of Technology at Kanpur, Archaeological Survey of India, and University of Wisconsin collaborated on the research. The Indo US Science and Technology Forum, the US Environmental Protection Agency, and the National Science Foundation provided funding.
Source: Georgia Tech