Acid rain (there’s also acid fog and snow) is defined as being significantly more acidic than rain water, which is about pH 5.6 due to the water reacting with CO2 in the atmosphere on it’s way down and forming carbonic acid. So you’re usually looking at rain with a pH lower than 5.0.
You can get short term natural acid rain from HCl released in volcanic eruptions, but the man-made form comes mostly from sulfuric and nitric acid, coming from sulfur dioxide and nitrogen oxides in the gas form reacting with water in the atmosphere. Those come mostly from hydrocarbon fuel combustion, particularly coal which has a fair amount of sulfur in it. There are ways to decrease the sulfur content (and SO2 emmisions), but it costs.
So it’s air pollution that gives rise to anthroprogenic acid rain, and it’s the weather patterns responsible for distributing the pollution to areas where it falls to the ground mixed in with rain, which can be hundreds of miles away or more. No websites with additional info, just stuff coming out of “Environmental Chemistry” - Colin Baird, 2nd ed. 1999.