Some particles will settle to the ground (in the form of acid
             deposition) or vegetation can absorb some of the SO2 gas
             directly from the atmosphere. When sulfur dioxide comes in
             contact with the atmosphere, it oxidizes and forms a sulfate
             ion. It becomes sulfuric acid as it joins with hydrogen atoms in
             the air and falls down to earth. Oxidation occurs most in clouds,
             especially in heavily polluted air, where other compounds such
             as ammonia and ozone help to catalyze the reaction, increasing
             the amount of sulfur dioxide changing to sulfuric acid. Not all
             of the sulfur dioxide is converted to sulfuric acid, and it is not
             uncommon for a substantial amount to float up into the
             atmosphere, move to another area, and return to earth as sulfur
             dioxide, unconverted. S (in fossil fuels) + O2 =* SO2 2 SO2
             ...