Most of these substances could cause soil imbalances, either directly or indirectly, so that doesn't help us decide very much.
The staining of statues implies some sort of airborne contaminant, which basically rules out lead; because of its high density, lead and its compounds are hardly ever airborne.
The reduced visibility further implies that the contaminant is visible, at least in some of its compound forms; this makes ozone, carbon monoxide, and nitrogen oxides all quite unlikely, as they are themselves invisible and rarely form compounds that are visible.
That leaves only particulate matter, so that must be our contaminant. The most likely candidate is smoke from burning coal or wood, but anything that throws fine particles up into the atmosphere could contribute. Many industrial processes release various particulates into the air.
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