Acid rain is a major threat to the beauty of the Taj Mahal.
Rainwater is the purest form of water but when it passes through the air to reach the ground, many dust particles and harmful gases like oxides of nitrogen, sulphur dioxide etc., mixes with the rainwater, thus making it acidic in nature.
When acid rain falls on the monument which is completely made up of marble, they react with the components of marbles to form a powder-like substance and this is then washed away by the rain. This phenomenon is known as marble cancer.
The soot particles emitted from the Mathura oil refinery located near Agra is leading to the yellowing of the marble.