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Rashmi wanted to visit the Taj Mahal in Agra. She always wanted to see the beautiful white marble structure and dreamt of going there at least once in her lifetime. From her childhood, she had heard stories about the beauty of the Taj Mahal from her grandmother. One fine day her father planned a vacation to Agra to visit the Taj mahal. On seeing the Taj Mahal, she was upset since the white marble was almost yellow, and there were black spots in many places. She asked her father the reason behind this discolouration of the beautiful monument.  
The answer to Rashmi's question is given in the case study given below.
The Taj Mahal
Taj Mahal in Agra
The monument Taj Mahal is one of India's renowned tourist destination which is located in Agra. Many experts have warned that the pollutants in air is the major cause of discoloring of the white marble used in the Taj Mahal. This is mainly because of the industries around Agra which produce harmful pollutants like sulphur dioxide and nitrogen dioxide. When there is rain these gases mix with the rain becoming acidic and this is the cause of acid rain. The acid rain contains sulphuric acid and nitric acid which are highly corrosive and corrodes the white marble. This phenomenon is also called "Marble Cancer". Even more soot particles also known as suspend particulate matter (SPM) from the nearby Mathura refinery has caused yellowing of the marble. In order to conserve the Taj Mahal the Supreme Court has ordered the industries and automobiles in Delhi to switch to CNG (Compressed Natural Gas) and Liquified Petroleum Gas( LPG) to reduce pollution.