On his travels to India, environmental scientist Mike Bergin of the Georgia Institute of Technology in Atlanta noticed that workers were applying "facial masks" on the iconic Taj Mahal, the “crown of palaces” built by emperor Shah Jahan in memory of his wife in 1653. Air pollution had turned the palace’s snowy marble dome surface brown, so workers would periodically clean it by putting clay on the dome’s exterior and then peeling it off. Scientists didn't know what exact process caused the discoloration: Perhaps it was fog droplets oxidizing the surface or maybe sulfurous gas in the air. But Bergin had an idea: Because the brown material could be removed only by clay and not by water, the source of pollution must be small, water-insoluble particles in the air. So he and colleagues measured small particles in the air and stuck sample marble squares on the building to collect the pollutants. Just as he thought, the sample marble squares were covered with dust and light-absorbing carbon particles floating in the air. Computer modeling showed that these particles absorb ultraviolet light, thus giving the dome a yellow-brown shade, the team reports online this month in Environmental Science & Technology. Bergin blames vehicle emissions and burning of biomasses such as dung and trash for causing the pollution. Reducing these activities would not only return the Taj Mahal to its former glory, but also improve residents' health, he says.

*Correction, 15 December, 11:23 a.m.: The original photo published with the story showed a mosque at the Taj Mahal site, which was wrongly implied to be the Taj Mahal. It has been replaced with a photo of the actual Taj Mahal building.