Why is the beauty of TajMahal threatened?

Dear student, It is caused due to Acid Rain. Acid rain consist of certain acid pollutants which are present in rain water in the form of clouds such as sulphur and nitrogen. These pollutant nitrogen oxides and sulphur oxide fall on the surface of the marble of Taj Mahal.The marble contain limestone consist of certain minerals react with substances water to form yellow colour on the surface and destroy the marble to for a powdery substance. Regards