The pH scale is a fundamental measurement tool in chemistry that specifies the acidity or basicity of an aqueous solution. This scale ranges from 0 to 14, where a value of 7 is considered neutral. The measurement is a logarithmic expression of the concentration of hydrogen ions ($H^+$). A lower pH number indicates a higher concentration of these ions, signaling greater acidity. Precipitation, including rain, snow, and fog, is a water-based solution with a measurable pH value.
Understanding Normal Rain pH
Normal, unpolluted rain is not chemically neutral, despite pure water having a pH of 7. The natural acidity results from a continuous atmospheric process. Carbon dioxide ($\text{CO}_2$) is naturally present in the atmosphere, and as raindrops fall, they absorb and dissolve this gas. The dissolved $\text{CO}_2$ reacts with water ($\text{H}_2\text{O}$) to form carbonic acid ($\text{H}_2\text{CO}_3$). This weak acid slightly elevates the concentration of hydrogen ions. Due to this natural carbonation, the pH of typical, unpolluted rain is approximately 5.6, making it mildly acidic.
Sources of Acid Rain
Acid rain occurs when the pH of precipitation drops substantially below the natural baseline of 5.6, typically falling to 5.0 or lower. This increased acidity is primarily caused by two airborne pollutants: sulfur dioxide ($\text{SO}_2$) and nitrogen oxides ($\text{NOx}$). These gases are released mainly through human activities, with the combustion of fossil fuels being the major source. Electric power generation, vehicle exhaust, and industrial manufacturing processes contribute large portions of these emissions.
Once released, $\text{SO}_2$ and $\text{NOx}$ travel long distances, reacting with water, oxygen, and other chemicals in the atmosphere. These reactions transform the pollutants into stronger acids, specifically sulfuric acid ($\text{H}_2\text{SO}_4$) and nitric acid ($\text{HNO}_3$). These powerful acids mix with water droplets in clouds, lowering their pH significantly before falling back to Earth as acid rain, snow, or dry acidic particles.
Impacts of Acid Precipitation
Aquatic Ecosystems
The deposition of precipitation with a pH below 5.0 causes extensive damage across natural and man-made environments. Ecological systems, particularly aquatic ones, show the most distinct symptoms of damage. As acid rain flows into lakes and streams, it lowers the water’s pH, harming or killing sensitive aquatic life, including fish, insects, and amphibians. The acidic water also leaches aluminum from surrounding soil and sediment. This mobilization of aluminum is toxic to fish, causing them to produce excessive mucus that ultimately suffocates them.
Terrestrial Ecosystems
Terrestrial ecosystems also suffer significant consequences from acid precipitation. Acid rain strips important nutrients like calcium and magnesium from the soil, making them unavailable to plants and trees. This nutrient depletion weakens trees, making them less able to withstand cold temperatures, disease, or insect infestations. Increased levels of leached aluminum in the soil can also damage tree roots, inhibiting their ability to absorb water and necessary nutrients. Forests at higher elevations are susceptible because they are frequently bathed in acidic fog and clouds that directly strip nutrients from their foliage.
Infrastructure and Heritage
The effects of acid deposition are visible on human infrastructure and cultural heritage. The sulfuric and nitric acids in the rain corrode metal structures, accelerating the deterioration of bridges, railings, and vehicles. Acid rain also dissolves calcium-based materials, such as limestone, marble, and concrete. This chemical weathering damages buildings, monuments, and statues, visibly eroding their surfaces. The need for costly repairs and replacement demonstrates the economic burden of acid precipitation.