What Is Acidification?

Acidification is the process by which the pH of a body of water, a soil, or another chemical system falls as the concentration of hydrogen ions rises. In environmental science the term refers to a sustained shift rather than a momentary fluctuation, usually driven by the addition of acids or of gases that form acids on contact with water. Because the pH scale is logarithmic, a drop of one unit corresponds to a tenfold increase in hydrogen ion concentration, so changes that look small in pH units represent large changes in chemistry.

Two families of acidification dominate the environmental literature. The first is ocean acidification, caused by seawater uptake of atmospheric carbon dioxide. The second is the acidification of soils, lakes, and streams caused by atmospheric deposition of sulfur and nitrogen compounds, commonly known as acid rain. Both are treated as pollution problems in that the driver is anthropogenic emissions, and both are studied through the same underlying acid-base and buffering chemistry, though their sources, timescales, and mitigation options differ.

The Underlying Chemistry

When an acid-forming gas dissolves in water, it shifts the balance of dissolved carbonate, bicarbonate, sulfate, or nitrate species and releases hydrogen ions. Carbon dioxide reacts with water to form carbonic acid, which dissociates into bicarbonate and a hydrogen ion, while sulfur dioxide and nitrogen oxides oxidize in the atmosphere to sulfuric and nitric acid. What determines the resulting pH change is the buffering capacity of the receiving system, meaning its stock of base cations and alkalinity. Catchments underlain by limestone neutralize incoming acid readily, while thin soils over granite have little acid neutralizing capacity and acidify quickly. Acidification also mobilizes metals: falling pH releases inorganic aluminum from soil minerals into soil water and surface water, where it becomes toxic to fish and interferes with nutrient uptake by tree roots.

Ocean Acidification

The ocean has absorbed a large share of the carbon dioxide released by fossil fuel combustion, land use change, and cement production. According to the NOAA Ocean Acidification Program, average surface ocean pH has fallen by about 0.1 unit since the beginning of the industrial era, roughly a 30 percent increase in acidity, and carbonate ion concentration has declined along with it. The consequence for marine life is chiefly a reduction in the saturation state of calcium carbonate minerals, particularly aragonite, which makes it harder for corals, oysters, clams, sea urchins, and calcareous plankton to build and maintain shells and skeletons. Monitoring relies on moored buoys, repeat hydrographic surveys, and autonomous platforms carrying pH and partial pressure sensors, since detecting a trend of this size requires measurement precision better than 0.01 pH units.

Acid Deposition on Land and Fresh Water

Acid deposition covers both wet deposition, delivered as rain, snow, and fog, and dry deposition of acidic particles and gases. The US Environmental Protection Agency description of acid rain traces the pathway from sulfur dioxide and nitrogen oxide emissions through atmospheric oxidation and long-range transport to deposition far downwind of the source. Decades of deposition across northeastern North America and northern Europe depleted calcium and magnesium from forest soils, acidified thousands of lakes and streams, and reduced fish populations. Emissions controls, notably Title IV of the US Clean Air Act, cut utility sulfur dioxide emissions sharply and measurable recovery has followed, though soil base cation pools rebuild slowly. Assessments of global trends in rainfall acidity and its effects on plants and soil show the problem shifting toward rapidly industrializing regions.

Applications

Acidification research and monitoring connect to a range of fields, including:

• Ocean and coastal observing systems using autonomous pH and carbon sensors
• Air quality regulation and emissions control technology for power plants
• Fisheries and aquaculture management, particularly shellfish hatcheries
• Forestry and agricultural soil management, including liming programs
• Water treatment and corrosion control in distribution networks
• Climate and carbon cycle modeling