India's iconic Taj Mahal has been yellowing and for several years, workers apply a clay mask to remove layers of grime and reveal the white marble underneath.
Scientists are getting to the bottom to find the types of pollutants, which cause the discolouring. Their findings could help inform efforts to protect the mausoleum and other surfaces from pollution.
Indian officials have tried to reduce the effects of pollution on the Taj Mahal by restricting nearby traffic and limiting local industrial emissions.
Despite regulations and an occasional deep clean, the domes and minarets continue to accrue a layer of stain. So far, no studies on the specific compounds causing it to appear yellow have been published.
Mike H. Bergin, Sachchida Nand Tripathi and colleagues analyzed particles in the air and on marble samples near the main dome.
The team estimated how these specks reflect light and therefore affect the color of the building using a novel method they developed.
Black carbon and brown carbon from the burning of trash, fuels and other materials are among the primary pollutants tarnishing the Taj Mahal.
In the future, their approach could be used to craft strategies to address the chronic yellowing and improve air quality, they say.