Acid Formation in the Atmosphere

    First, let us review some basic chemistry as it applies to acid precipitation. Carbonic acid forms naturally in the atmosphere due to the reaction of water (H2O) and carbon dioxide (CO2),
H2O + CO2 -> H2CO3
   while the burning of coal and other organics adds sulfur dioxide (SO2) and Nitrous oxides (NOx) to the atmosphere where they react to form sulfuric acid and nitric acid,
2SO2 + H2O + O2 -> 2H2SO4
4NO2 + 2H2O + O2 -> 4HNO3
    All of these acids will be buffered by reacting with rocks, minerals, etc. on the earth's surface. The most important (and fastest) buffering comes from the reaction with (weathering of) calcite in the form of limestone, dolomite or marble.
H2CO3 + CaCO3 -> 2HCO3- + Ca+2
   When this reaction occurs, the acid is neutralized and the calcite dissolved. While the reaction with calcite is very fast (the standard test for calcite in introductory geology labs is to put very dilute acid on a sample to see if it bubbles (reacts)), the reaction with other rocks is very slow, so most of the acid is not affected. This is why ponds in the Adirondacks became acidified (non-calcite rock in those areas), while Lake Champlain (abundant calcitic bedrock) did not.
   The degree of acidification is the pH of the water, which is defined as the negative logarithm of the concentration of hydrogen ion (H+), or
pH = -log [H+].
(This to a certain degree comes from the old definition of an acid as a proton donor. A hydrogen ion is little more than a proton, so think of it as the amount of free protons floating around).
A pH of 7 is considered neutral, while a pH less than 7 is considered acidic. For example, wine has a pH of about 3.5 and your stomach digestive fluids have a pH of about 1.9.
We should also be aware that increased acidity does not have to be constant, but instead can be episodic. High surface water discharge events (storms, snowmelts) can increase the pH of streams and ponds to dangerous levels for short times.