Acid Rain

Dry deposition occurs in the intervals of time between precipitation events. Dry deposition includes inputs of tiny particulates from the atmosphere, as well as the uptake of gaseous SO₂ and NO_x by plants, soil, and water. Unlike wet deposition, the rates of dry deposition can be much larger close to point-sources of emission, compared with further away.

Once they are dry deposited, certain chemicals can generate important quantities of acidity when they are chemically transformed in the receiving ecosystem. For example, SO₂ gas can dissolve into the water of lakes or streams, or it can be absorbed by the foliage of plants. This dry-deposited SO₂ is then oxidized to SO₄^2-, which is electrochemically balanced by H⁺, so that acidity results. Dry-deposited NO_x gas can similarly be oxidized to NO₃^- and also balanced by H⁺.

In relatively polluted environments close to emissions sources, the total input of acidifying substances (i.e., wet + dry depositions) is dominated by the dry deposition of acidic substances and their acid-forming precursors. The dry deposition is mostly associated with gaseous SO₂ and NO_x, because wet deposition is little influenced by distance from sources of emission.

For example, within a 25 mi (40 km) radius of the large smelter at Sudbury, about 55% of the total input of sulfur from the atmosphere is due to dry deposition, especially SO₂. However, less than 1% of the SO₂emission from the smelter is deposited in that area, because the tall smokestack is so effective at widely dispersing the emissions.

Because they have such a large surface area of foliage and bark, forests are especially effective at absorbing atmospheric gases and particles. Consequently, dry inputs accounted for about 33% of the total sulfur deposition to a hardwood forest in New Hampshire, 56-63% of the inputs of S and N to a hardwood forest in Tennessee, and 55% of their inputs to a conifer forest in Sweden.