Nitrogen is another problem. As with sulfur, it enters the air after fossil fuel combustion. Unfortunately, few policies address nitrogen pollutants. Along with sulfur it provides the basis for acid deposition, better known as acid rain. These acidic compounds can threaten ecological balances.
Some fish kills correlate with major acid rain events.
To date there is no definitive proof of acid rain damaging the Park's forests, but changing soil conditions raise concern. Still, using the natural world as an uncontrolled, possibly irreversible, experiment does not benefit the ecosystems. Ozone results when nitrogen oxides and hydrocarbons react in sunlight. Park ozone levels occasionally reach unsafe levels. Ozone can make breathing difficult, especially for people with respiratory problems.