Acid deposition, commonly referred to as acid rain, results from atmospheric pollution. It occurs when sulfur dioxide and nitrogen oxides are released into the atmosphere, primarily from burning fossil fuels. These compounds react chemically with water and oxygen to form sulfuric and nitric acids. These acidic compounds return to the Earth’s surface through wet deposition (rain, snow, fog, and hail) and dry deposition (acidic gases and particles). The environmental consequences affect natural ecosystems and human-made objects, starting with surface water bodies.
Impact on Aquatic Environments
The most visible ecological effects of acid deposition are found in aquatic ecosystems like lakes, rivers, and marshes, where it triggers acidification. Normal rainwater has a pH of about 5.6, but acid rain typically has a pH below 5.0. This consistent lowering of pH places stress on aquatic life, as most organisms are adapted to a narrow, relatively neutral pH range.
“Episodic acidification” is a particularly damaging event. It occurs when accumulated acidic deposits, such as snowpack, rapidly melt and release a sudden pulse of highly acidic water into streams and lakes. Even water bodies that are healthy most of the year can experience a temporary drop in pH during spring snowmelt or heavy rainfall. This abrupt change can injure or kill sensitive organisms unable to adjust quickly to the sudden increase in acidity.
The primary danger to fish is not solely low pH but the mobilization of toxic aluminum. Acidic water leaches naturally occurring aluminum from the surrounding soil particles and sediment into the water body. Once dissolved, this aluminum is highly toxic to fish, interfering with their gill function. It causes the mucus on the gills to become sticky and eventually fuses the delicate gill structures. This mechanical blockage reduces the fish’s ability to absorb oxygen, leading to suffocation.
Lowered pH levels also disrupt the reproductive cycle of fish, even in species that tolerate moderately acidic conditions as adults. For sensitive species like trout and salmon, eggs cannot successfully hatch if the water pH drops to 5 or below. The young (fry) are generally more sensitive to acidic conditions than adults, leading to population failure even if the adult stock survives. The aquatic food web is compromised when acid-sensitive invertebrates, such as freshwater shrimps and snails, are lost, removing the food source for fish and amphibians.
Alteration of Soil Chemistry
Acid deposition extends directly to the terrestrial environment, fundamentally changing the chemistry of the soil that supports plant life. Acidic water flowing through the soil causes nutrient leaching. This process strips essential nutrients—positively charged ions (cations) such as calcium, magnesium, and potassium—from soil particles where they are held for plant uptake.
The hydrogen ions in the acid rain displace these nutrients, which are washed away with runoff, making them unavailable to plant roots. The loss of these base cations also diminishes the soil’s natural ability to neutralize acidity, known as its buffering capacity. This leaves the ecosystem more susceptible to future acid deposition. Soils developed from granite or siliceous bedrock are vulnerable because they lack the natural neutralizing compounds found in soils from limestone or marble.
Increased soil acidity also triggers the mobilization of aluminum. Aluminum is naturally present but typically bound in inert, non-toxic forms. As the pH drops, it converts into a soluble, toxic form readily absorbed by plant roots. This mobilized aluminum damages fine root systems, hindering the tree’s ability to absorb water and remaining nutrients.
Acid deposition can also harm the biological components of the soil. Increased acidity negatively impacts beneficial soil microorganisms and fungi responsible for the critical process of decomposing organic matter. Since these microbes break down decaying material to release nutrients back into the soil, their impairment reduces the nutrient cycling efficiency of the ecosystem.
Damage to Forest Ecosystems
Changes in soil chemistry lead directly to a decline in forest health, linking chemical stress and ecosystem damage. Trees weakened by nutrient depletion and aluminum toxicity become less resilient to other environmental stressors. This creates a downward spiral, making trees more susceptible to insect infestation, fungal diseases, and extreme weather events like drought or severe frost.
Acid deposition also causes direct damage to tree foliage, especially when acid fog or cloud water is involved. This acidic mist strips essential nutrients from leaves and needles, damaging the waxy outer layer and reducing photosynthetic capability. Damaged foliage may appear brown or dead and becomes less able to withstand freezing temperatures, which harms species like red spruce at higher elevations.
High-elevation forests are disproportionately affected because they are frequently exposed to acidic cloud and fog water. Water droplets in high-altitude clouds often contain a much higher concentration of acidic pollutants than normal rainfall, resulting in a greater rate of acid deposition on the foliage and soil. Furthermore, soils in many high-elevation areas are naturally thin and have a low buffering capacity, making them vulnerable to both nutrient leaching and aluminum toxicity.
The overall effect of this combined stress is a decline in forest health and productivity, sometimes causing widespread forest dieback. This decline can change the forest composition, as acid-tolerant plant species may replace sensitive ones, thus reducing biodiversity. The long-term impact is a compromised forest that is less vigorous and more vulnerable to future ecological challenges.
Deterioration of Built Structures
Acid deposition is responsible for the physical deterioration of human-made structures, causing economic and cultural losses. Acidic components in the air and precipitation chemically react with materials used in construction and sculpture. This is particularly true for stone materials composed of calcium carbonate, such as marble and limestone, which are susceptible to chemical dissolution.
The sulfuric and nitric acids react with calcium carbonate in the stone, gradually dissolving the material. This leads to the loss of carved details and roughened surfaces on statues and buildings. On sheltered surfaces, the reaction can form a black crust of gypsum, which eventually flakes off, revealing crumbling stone beneath. This slow erosion transforms intricate historical monuments into featureless forms over decades.
Acid deposition also accelerates the corrosion of various metals, including steel, copper, bronze, and zinc. Increased acidity promotes the oxidation of these materials, leading to rust and pitting that compromises the structural integrity of bridges and railings. Acid rain can also damage paint, protective coatings, and surface finishes, requiring more frequent maintenance and restoration of structures.