2/3 proportion of acid rain is attributable to humans.
Step-by-step explanation:
Acid rain occurs due to the presence of nitrogen oxides and sulphur dioxides in atmosphere, which on reacting with water produces sulphuric and nitric acids. The automobiles, burning of fossil fuels, industrial smoke, coal-burning are all man-made and contribute the release of sulphur dioxides and nitrogen oxides in the atmosphere causing acid rain.