Acid rain on Taj Mahal is mainly due to a near-by

 We know that normal rain has a pH value of around 5.0 - 5.5, which is slightly acidic. However, when rain combines with sulphur dioxide or nitrogen oxides then it is called as acid rain which is usually produced from power plants and automobiles. Typical acid rain has a pH value of 4.6 which is more acidic. 

 ${\mathrm{SO}}_2$ and ${\mathrm{NO}}_2$ are the pollutant gases largely responsible for the discoloured and lustreless nature of marble of the Taj Mahal.

Mathura refinery releases oxides of Sulphur which causes acid rain.

Thus Acid rain on Taj Mahal is mainly due to a near-by refinery.