Question

How is air pollution a threat to stone monument and metallic installation?

Answer 1

Stone monuments can be severely damaged by acids.  Coal plants for example emit large amounts of NOx and Sulphur dioxides (SO2).  These contribute to acid rain*, which dissolve away stone monuments little by little.

Soot (small particles of carbon created during combustion) from cars can also collect on these monuments.  I don't suspect any damage is done, but it'll still cost more to maintain them.

* There seems to be a common myth that acid rain was solved.  While it's true that governments today acknowledge the damage done from acid rain, levels have fallen in certain places (54% decline in SO2 levels between 1990 and 2007 [1]), and the projected cost savings from these measures have been huge ... Acid rain is still a problem, rain water is still more acidic than it would be without anthropogenic emissions, forests/lakes/soils are still exposed to too much acid

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