Causes of Acid Rain
Acid rain refers to any form of precipitation—rain, snow, fog, or hail—with a pH level below 5.6, making it more acidic than normal rainwater. It is primarily caused by the release of sulfur dioxide (SO2) and nitrogen oxides (NOx) into the atmosphere from human activities such as burning fossil fuels in power plants, factories, and vehicles. These pollutants react with water vapor, oxygen, and other chemicals to form sulfuric acid (H2SO4) and nitric acid (HNO3), which then fall to Earth as acid rain.
Key Chemical Processes Involved
The formation of acid rain involves atmospheric reactions where SO2 oxidizes to sulfur trioxide (SO3), which combines with water to produce sulfuric acid, while NOx forms nitric acid through similar oxidation processes. These reactions can occur in clouds or as dry deposition directly onto surfaces. Natural sources like volcanic eruptions contribute minimally compared to anthropogenic emissions, which account for over 90% of acid rain precursors in industrialized regions.
Practical Example: Impacts on Forests and Lakes
In the northeastern United States and Scandinavia, acid rain from Midwest coal plants has acidified lakes and soils, leading to the decline of fish populations in over 1,000 lakes in the Adirondacks alone. For instance, species like trout cannot survive in waters with pH below 5.0, and aluminum leached from acidified soils damages tree roots, causing widespread forest dieback in regions like Germany's Black Forest during the 1980s.
Broader Environmental and Human Effects
Acid rain disrupts ecosystems by lowering soil pH, which reduces nutrient availability and harms plant growth, leading to biodiversity loss. It also contaminates water bodies, affecting aquatic life and drinking water sources. Additionally, it accelerates the corrosion of buildings and monuments made of limestone or marble. On a human scale, inhaled acidic particles exacerbate respiratory conditions like asthma, underscoring the need for emission controls to mitigate these cascading effects.