Every several years, the Archeological Survey of India apply a clay mask to India's iconic but yellowing Taj Mahal to remove layers of grime and reveal the white marble underneath.
Now scientists are getting to the bottom of what kinds of pollutants are discolouring one of the world's celebrated wonders.
In what is the first published study that has looked closely at what specific compounds are causing it to appear yellow, scientists have conclude that black carbon and brown carbon from the burning of trash and fuels are among the primary pollutants tarnishing the monument of love.
The latest findings will now help inform efforts to protect the mausoleum and other surfaces from pollution.
Mike H Bergin and Sachchida Nand Tripathi from the American Chemical Society note that Indian officials have tried to reduce the effects of pollution on the Taj Mahal by restricting nearby traffic and limiting local industrial emissions.
But despite regulations and an occasional deep clean, the domes and minarets continue to accrue a layer of soot.
Researchers analyzed particles in the air and on marble samples near the main dome over several months.
Using a novel method they developed, the team estimated how these specks reflect light and therefore affect the color of the building.
In the future, their approach could be used to craft strategies to address the chronic yellowing and improve air quality, they say.
