Effect of Acid Rain on the Taj Mahal

http://www.adventurewomen.com/blog/article/11-places-to-see-before-the-disappear-taj-mahal/http://blog.travelworldpassport.com/taj-mahal-a-marble-mausoleum-for-the-love-of-life/

I found the video on the impact of acid-base reactions particularly on statues very interesting. It was surprising to hear that water in it’s neutrality can act as an acid or a base, by either donating or accepting a proton. This is what forms the base of acid rain reactions that then affect multiple historic monuments and statues.
One example that I have always heard about it the transformation of the Taj Mahal over the years due to acid rain. As we now acid rain is formed by the reaction of sulfur dioxide, nitric oxide and carbon dioxide with pure water (pH 7) to form acid rain (pH < 5.6). The effects on acid rain on monuments made of marble and limestone are detrimental. The Taj Mahal composed entirely out of marble (a sign of affection and purity by Emperor Shah Jahan for his wife Mumtaz) has been a major victim to this naturally occurring phenomenon.
As we can see from the pictures above, over the years the pristine bone white structure has been tainted a nasty yellow due to the occurance of acid rain. Marble, just like limestone, consists of calcium carbonate (CaCO3) which is highly reactive when it comes with contact with sulfuric acid (the primary component of acid rain).

CaCO3 + H2SO4(aq) = Ca2+(aq) + SO2-4(aq) + H2O + CO2

Marble which is made up off large crystals and porous in nature is highly responsive to the sulfuric acid ions. When a solution of sulfuric acid and water falls on marble, it forms a solution containing calcium and sulfate ions. When this solution dries up, the ions crystalize as CaSO4.2H2O, which is gypsum. Gypsum is soluble in water so when it is washed away, it leaves behind crumbling stone. On the other hand, when this gypsum accumulates, like it has in the case of the Taj Mahal, it attracts dust, carbon particles, dry-ash and other dark pollutants resulting in the blackening of the surface of the structure. As the result, over the years the gorgeous white monument has gradually yellowed as a result of this acid-base reaction.