Air pollution has been a serious problem for the forests of the Northeast (especially those at high altitudes), which are downwind of the industrial heartland. The chief agent of environmental damage is acid deposition, or acid rain as it is commonly known. This phenomenon occurs when emissions of sulfur dioxide (SO2) and oxides of nitrogen (NOx) react in the atmosphere with water, oxygen, and oxidants to form various acidic compounds. These compounds then fall to the earth in either dry form (such as gas and particles) or wet form (such as rain, snow, and fog). Thus, polluted air can damage trees directly in the dry form or indirectly through its affects on the chemistry of water and soils and by making trees more vulnerable to other biological and environmental stressors. More specifically, acid rain weaken trees by damaging their leaves, limiting the nutrients available to them, or exposing them to toxic substances slowly released from the soil. Acid rain that flows into streams, lakes, and marshes also has serious ecological effects. In watersheds where soils do not have a buffering capacity, acid rain releases aluminum, which is highly toxic to many species of aquatic organisms, from soils into lakes and streams. NRS scientists are study the problems of pollution at many levels, from cellular biochemistry to landscape-level ecology.