Answer:
The usual culprit is burning coal.
Step-by-step explanation:
This produces a lot of carbon dioxide as well as sulphur dioxide. Other industrial processes, and vehicle emissions, produce similar oxides unless they are 'scrubbed' out. They react with atmospheric moisture to produce acids like carbonic, sulphurous and nitrous acids. Which is then precipitated. It should be remembered that a volcanic eruption has this effect too.