Also, instead of 5.5, the pH of this rainwater was 4.5. That is, it was ten times more acidic than I had expected if the only dissolved substance that contributed to pH were CO 2 . Obviously, something else was in the rainwater. After an hour, I put the collector outside again. However, this time pH fell to 3.4, another ten-fold increase in acidity.
Air can contain strong acids such as sulfuric and nitric acids.
It can also produced by burning coal, particularly soft or high sulfur coal. Map of the U.S. illustrating the distribution of rainfall pH. Note the plume of acidic rainfall over the northeastern states. Pennsylvania lies in the middle of the most acidic deposition. Anthropogenic sources of atmospheric sulfate and nitrate in the U.S. nitric acid by use of the internal combustion engine. Most of that is due to the release of sulfates from coal-fired power plants and heavy industries in the Ohio valley.