The gradual yellowness and discoloration of the marble of the Taj Mahal is an effect of which environmental phenomenon?

The yellowness of the Taj Mahal is primarily driven by Acid rain, which occurs when air pollutants like sulfur dioxide (${\text{SO}}_2$) and nitrogen oxides (${\text{NO}}_x$) react with water vapor in the atmosphere to form acids. These acids react chemically with the calcium carbonate (${\text{CaCO}}_3$) of the marble, causing corrosion and discoloration. Particulate air pollution (soot and carbon particles) also contributes to the staining.