In this case, it hasn't been happening intentionally at a meaningful scale; you'd be able to look up and see the thin haze from it, and use a spectrometer to figure out that it's not water vapor.

What has happened is that ordinary sulphur mixed with fossil fuels has produced particulates lower in the atmosphere. These turn into sulfuric acid when in contact with water, resulting in acid rain. Policies to sharply lower sulfate particle emissions have resulted in that becoming far less of a problem, but also accelerated warming in recent years.