Acid deposition, more commonly known as acid rain, occurs when emissions of sulfur dioxide (SO2) and nitrogen oxides (NOx) react in the atmosphere (with water, oxygen, and oxidants) to form various acidic compounds. These acidic compounds then fall to earth in either a wet form (rain, snow, and fog) or a dry form (gases, aerosols, and particles). Prevailing winds transport the acidic compounds hundreds of miles, often across state and national borders. At certain levels the acidic compounds, including small particles such as sulfates and nitrates can cause many negative human health and environmental effects. While ecosystems are subject to many stresses, including land-use changes, climate change, and variations in hydrologic and meteorologic cycles, the scientific literature has clearly demonstrated that these pollutants can:
• Degrade air quality,
• Impair visibility,
• Damage public health,
• Acidify lakes and streams,
• Harm sensitive forests,
• Harm sensitive coastal ecosystems, and
• Accelerate the decay of building materials, paints, and cultural artifacts such as buildings, statues, and sculptures.

Title IV was passed by Congress as part of the 1990 Clean Air Act Amendments to reduce emissions of SO₂ and NO_x from fossil fuel-burning power plants in order to protect
ecosystems suffering damage from acid deposition and to improve air quality. At the same time, the National Acid Precipitation Assessment Program (NAPAP) was asked to periodically assess and report to Congress on the implementation of the Acid Rain Program, recent scientific knowledge surrounding acid deposition and its effects, and
the reduction in acid deposition necessary to prevent adverse ecological effects. This NAPAP Report focuses primarily on emission reductions from power plants, both in terms of assessing past reductions under the Acid Rain Program and in projecting the ecological effects of additional reductions of SO₂ and NO_x.

It should be noted that power generation currently contributes approximately 69% of the SO₂ emissions and 22% of the NO_x emissions nationwide. This contribution is decreasing as emissions from power generation continue to decrease, making the other sources of these pollutants more prominent. Modeling suggests that even if SO₂
emissions from power generation were reduced to zero, some lakes and streams would remain acidic due to acid deposition. However, there are several other regulations that reduce emissions of SO₂ and NO_x from these non-power generation sources, such as the Tier II mobile source standards, the Heavy Duty Diesel standards, and the Clean Air Non-Road Diesel Rule, that have also been promulgated since 1990. These regulations, primarily designed to bring counties into attainment with fine particle and ozone air quality standards, also incidentally reduce emissions that contribute to acid deposition.

Implementation of Title IV has successfully and substantially reduced emissions of SO₂ and NO_x from power generation at a significantly lower cost than expected:
• In 2002, SO₂ emissions were 10.2 million tons, 35% lower than 1990 emissions and 40% lower than 1980 emissions.*
• In 2002, NO_x emissions were 4.5 million tons, 33% lower than 1990 emissions.

In addition, SO₂ emissions from all sources have decreased by 32% since 1990 and emissions of NO_x from all sources have decreased by 12% since 1990. Power generating sources continue to close in on the goal of reducing power plant SO₂ emissions from 1980 levels by 50% (to 8.95 million tons) as required by the 1990 Clean Air Act. Power generating sources have also exceed­ed the goal of a two million ton reduction in NO_x emissions from projected 2000 levels as required by the 1990 Clean Air Act.

These emission reductions have contributed to measurable improvements in air quality, reductions in acid deposition, and the beginnings of recovery of acid-sensitive waters in some areas:
• SO2 concentrations in the atmosphere (a precursor to fine particles and acid deposition) have decreased since 1990. Average annual SO2 concentrations in the Northeast in 2000–2002 were 40% lower than they were in 1989–1991, concentrations in the mid-Atlantic were 30% lower, concentrations in the Southeast were 35% lower, and concentrations in the Midwest were 45% lower.
• Sulfate concentrations in the atmosphere (a major component of fine particles, especially in the East) have decreased since 1990 as well. Average annual sulfate concentrations in the Northeast and Midwest in 2000–2002 were approximately 30% lower than they were in 1989–1991, and concentrations in the mid-Atlantic and Southeast were 25% lower.
• Wet sulfate deposition, a major component of acid rain, has also decreased since 1990. Average annual sulfate deposition in the Northeast in 2000–2002 was 40% lower than it was in 1989–1991, deposition in the mid-Atlantic and Midwest was 35% lower, and deposition in the Southeast was 25% lower.
• Wet nitrate deposition has not decreased regionally from historical levels because of the relatively moderate NO_x reduction from power plants and the continuing large contribution (over 50% of total NOx emissions) from other sources of NO_x such as vehicles and nonroad vehicles.
• Although visibility has begun to improve in some parts of the U.S., there is still significant impairment of visibility in many national parks and other Class I areas throughout the U.S.
• Acid neutralizing capacity is beginning to rise in some surface waters in the Northeast, including lakes in the Adirondack Mountains (see graphic below). This is an indication that recovery from acidification is occurring in those areas.