The resulting increase in UV exposure at the Earth's surface may cause skin cancer and cataracts in humans as well as vegetation damage. It would be helpful if the excess tropospheric ozone could drift up into the stratosphere and repair the depleted ozone layer. However, the stratosphere and the troposphere are separated by a natural barrier called the tropopause . Here the temperature difference between the atmospheric layers traps the excess ozone, much as temperature inversions sometimes trap smog over cities. The excess ozone cannot move upward through it.
All rainfall is by nature somewhat acidic.
Decomposing organic matter, the movement of the sea, geothermal activity and volcanic eruptions all contribute to the accumulation of acidic chemicals in the atmosphere. The principal factor, however, is atmospheric carbon dioxide, which causes a slightly acidic rainfall even in the most pristine of environments. Distilled water, with a pH of 7.0, is considered to be neutral. Uncontaminated precipitation has a pH of about 5.7. More than 200 lakes in the Adirondack Mountains of New York have reportedly become too acidic to support fish life.