7.7 - Acid Rain

Learning Objectives

I. Formation, Chemistry, and Sources

• Precursors: The precursors, or ingredients, for acid rain are NOx and $\text{SO}_2$. These are primary pollutants because they’re emitted directly from a source.

• Major sources: Coal fired power plants are the major source of $\text{SO}_2$, along with metal factories and vehicles that burn diesel fuel. The number one source of NOx is vehicle emissions.

• Natural sources: volcanoes release $\text{SO}_2$, and lightning and wildfires make NOx. Near active volcanoes these matter a lot (Hawaii's Kilauea causes acidic "vog" and acid rain downwind), but in industrial regions most acid deposition comes from human sources.

• Chemistry: $\text{SO}_2$ and NOx combine with oxygen or water in the atmosphere to form secondary air pollutants, specifically nitric acid and sulfuric acid.
• Sulfuric acid: $\text{S} + \text{O}_2 \rightarrow \text{SO}_2$ (burning coal), then $2\text{SO}_2 + \text{O}_2 \rightarrow 2\text{SO}_3$, then $\text{SO}_3 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{SO}_4$
• Nitric acid: $\text{N}_2 + \text{O}_2 \rightarrow 2\text{NO}$ (hot combustion), then $2\text{NO} + \text{O}_2 \rightarrow 2\text{NO}_2$, then $4\text{NO}_2 + \text{O}_2 + 2\text{H}_2\text{O} \rightarrow 4\text{HNO}_3$
• Wet and dry deposition: the acids fall in rain, snow, or fog (wet deposition) or settle as dry particles and gases that turn acidic when they get wet (dry deposition).

• Ions and acidity: These secondary acids then dissociate (separate) in the presence of water, forming nitrate and sulfate ions along with dissociated hydrogen ions ($\text{H}^+$). Those $\text{H}^+$ ions dissolve into water droplets that fall as rain.
• $\text{H}_2\text{SO}_4 \rightarrow 2\text{H}^+ + \text{SO}_4^{2-}$ and $\text{HNO}_3 \rightarrow \text{H}^+ + \text{NO}_3^-$

• pH scale: The pH scale measures the concentration of $\text{H}^+$ ions. A lower pH, meaning a more acidic environment, results from an increase in the number or concentration of $\text{H}^+$ ions.
• Normal rain is already slightly acidic: $\text{CO}_2$ dissolves in rain to form carbonic acid, giving clean rain a pH of about 5.6. Rain below that counts as acid rain.
• The pH scale is logarithmic: each drop of 1 pH unit means 10 times more $\text{H}^+$. Rain at pH 4.6 has 10 times more $\text{H}^+$ than normal rain (5.6), and rain at pH 3.6 has $10^2 = 100$ times more. For a difference that isn't a whole number, use $10^{\text{difference}}$: rain at pH 4.3 has $10^{5.6 - 4.3} = 10^{1.3} \approx 20$ times more $\text{H}^+$ than normal rain.

• Distribution: Acid deposition is worse in the eastern United States, because prevailing westerly winds drive the emissions from industrial areas, like the “rust belt” where metal producing facilities and power plants are located, eastward.
• Examples: lakes in New York's Adirondack Mountains, downwind of coal plants in the Ohio River Valley, became so acidic by the 1970s and 1980s that many lost their fish. In Europe, lakes in Sweden and Norway lost fish to acid rain from power plants and factories in Britain and central Europe.

II. Environmental Effects

• Acidification: Acid deposition leads to the acidification of soil and water sources.
• Aquatic life: most fish eggs can't hatch once a lake drops to about pH 5, and dissolved aluminum clogs fish gills. Snails and clams, which build calcium carbonate shells, are often the first to disappear (below about pH 6).

• Nutrient leaching: The positively charged $\text{H}^+$ ions displace or leach other positively charged soil nutrients, like calcium, potassium, and magnesium, from negatively charged clay particles. Those nutrients are essential for plant growth.

• Toxic metal solubilization: $\text{H}^+$ ions make toxic metals like aluminum more soluble. Acid deposition bumps the aluminum off its binding sites and introduces it into the soil at concentrations that can be toxic to plants and to organisms in aquatic ecosystems.

• Plant damage: When plants absorb toxic aluminum ions, they experience stunted root growth and potentially death.

• Structural damage: Acid deposition can cause human structures built with limestone, like monuments or steps, to deteriorate and corrode rapidly.
• Limestone and marble are made of calcium carbonate, which reacts with the acid and dissolves: $\text{CaCO}_3 + \text{H}_2\text{SO}_4 \rightarrow \text{CaSO}_4 + \text{H}_2\text{O} + \text{CO}_2$. Acid deposition also corrodes metals, like steel bridges and railroad tracks, and damages car paint.

• Regional differences (buffering): the same acid rain does very different damage depending on the bedrock and soil. Lakes and soils over limestone (calcium carbonate) neutralize the acid, so their pH barely changes. Areas with granite bedrock and thin soils, like the Adirondacks, New England, eastern Canada, and Scandinavia, have almost no buffering, so their lakes acidify quickly.

III. Mitigation and Reduction

• Prevention: The most effective way to mitigate acid rain is preventing it from happening in the first place. That includes using any renewable energy resource that results in less $\text{SO}_2$ and NOx emissions.

• Technological controls:
• Adding dry and wet scrubbers with chemical agents can trap some of the $\text{SO}_2$ before it’s emitted.
• Using fluidized bed combustion, or combusting coal at lower temperatures, helps reduce NOx emissions.

• Neutralization (limestone): Limestone is a base, composed primarily of calcium carbonate, that occurs naturally and can be used to neutralize acidic conditions. When $\text{H}^+$ ions react with calcium carbonate, it forms bicarbonate and helps absorb those excess $\text{H}^+$ ions.
• This process moves the pH closer to seven.
• Humans can simulate this natural buffering by crushing up limestone and spreading it over areas impacted by acid deposition.

• Policy success: The Clean Air Act has led to a dramatic decrease in acid deposition over time, particularly in the eastern U.S.
• The numbers: the 1990 Clean Air Act Amendments created the Acid Rain Program, a cap and trade system for $\text{SO}_2$ from power plants (7.6). From 1995 to 2023, $\text{SO}_2$ emissions from U.S. power plants fell 95% and NOx fell 89%, and many acidified lakes in the Northeast are slowly recovering.

Big picture: Acid rain is a complex environmental problem caused by the atmospheric conversion of $\text{SO}_2$ and NOx into sulfuric and nitric acids, which then release damaging $\text{H}^+$ ions on contact with the environment. That acidification not only leaches essential nutrients from the soil, it also increases the solubility of toxic metals like aluminum, which can stunt root growth and kill aquatic life.