Even though, every couple of years the mausoleum is given a clay mask to remove all the grime and restore the sparkling white marble, the grime returns over and over again every few years.
This phenomenon was researched by scientists who published their research in the ACS journal Environmental Science & Technology. They took samples of the grime found on the marble surface of the magnificent dome and minarets. They then concluded that car and factory exhaust fumes release carbon which resides on the marble as black/brown soot, which reflect the sunlight of the marble giving off a yellow colour to the marble.
The scientists hope to utilise this information to detect and lower traffic around the Taj Mahal and to also aid in the reduction of pollution, reduce the yellowing of the mausoleum, and improve the air quality.
 Taj Mahal Image
American Chemical Society. (2015, January 7). What's in the grime tarnishing the Taj Mahal?. ScienceDaily. Retrieved January 9, 2015 from www.sciencedaily.com/releases/2015/01/150107122918.htm