Why marble is vulnerable to dirty air
The Taj Mahal’s white surface is made largely of marble, whose main mineral, calcite, can be dulled by grime and chemical change. Fine particles from exhaust may settle on the stone; sulfur- and nitrogen-containing pollution can also contribute to deposits and surface damage when it mixes with moisture. Even a thin film can make pale marble look yellow or gray by changing how light reflects from it.
Keeping one source farther away
Restrictions on flights around the monument are intended to reduce the pollution reaching this especially sensitive site. Aircraft produce exhaust, including fine particles and gases, and keeping flight activity away helps limit one possible contribution to the air around the marble. It does not make the air pollution-free: the monument is also exposed to pollution from sources on the ground, and protecting the stone depends on addressing those sources too.
The point is preventive. Once deposits have accumulated, cleaning delicate historic marble is difficult; reducing what reaches its surface is safer than trying to restore its whiteness later.
