Taj: the pollutants causing discolouration identified
Garima Singh- Finally, the specific pollutants in the air that are responsible for the discolouration of the white marble of Taj Mahal have been identified. Particulate carbon and fine dust particles that are deposited on the marble are responsible for its browning. Carbon is of two types — black carbon and light absorbing organic carbon or brown carbon. The results from a study were published a few days ago in the journal Environmental Science & Technology. Besides studying air samples collected from the area, the authors used marble samples on the building to collect the pollutants. They also undertook computer modelling to study the colour change brought about by reflectance of the particles. Both organic carbon and dust particles have the ability to preferentially absorb light in the blue region of the spectrum. The absorption of blue light by these pollutants in turn gives the marble surface a brown hue. The ability of fine dust particles to produce the brown hue is a well known in North India. Though the absorption of blue light by individual dust particles may be smaller than that by brown carbon, the copious amount of dust of two micron size found in the particulate matter makes the overall absorption much higher than that by brown carbon.
The study revealed that particles larger than two micron in size accounted for nearly 70 per cent of the deposited particle surface area. These relatively coarse particles are by default the dust particles. On studying the marble samples, the researchers found that black carbon produces a greyish discolouration, while brown carbon and dust produce yellowish-brown hues. A combination of these two result in darker shades of yellow-brown. The sample targets were in place only for a brief period of two months.
 R. Prasad, Taj: the pollutants causing discolouration identified , The Hindu, 17 (December 18, 2014).