Formation
Acid deposition (dry deposition) occurs when NOX (vehicles, burning coal) and sulfur dioxides (burning coal, volcanoes) are released to the atmosphere. These may react with naturally occurring ammonia gas and fall back to the ground as nitric salts and sulfates. This may be in the form of a gas, particulate, or aerosol.
Acid precipitation (acid rain/snow, wet deposition) is when NOX and SO2 undergo chemical reactions with water to form nitric acid (HNO3) and sulfuric acid (H2SO4). The acids dissolve in rain and snow before falling to the ground.
Effects
Often the effects of acid precipitation and deposition are not felt by the areas that created them. Winds often carry the gases downwind creating a problem for other communities. Acid rain affects both the living and the non-living.
Plants that experience acid deposition (dry or wet) suffer leaf damage and possible stunted growth. Damaged leaves reduce photosynthesis and may be more susceptible to disease.
Soils absorb the contaminated rains and have their pH lowered. This may lead to dissolving needed nutrients or releasing toxic metals like aluminum.
The acidification of bodies of water has also occurred. Acid rain lowers the pH of lakes, ponds, and rivers. This, in turn, harms aquatic life (inability to release eggs and negative affecting gills).
You may have seen the effects of acid rain on old statues and gravestones. Some of the loss of features is due to acid dissolving the rock away.
There are regional soil differences that can mitigate the effects of acid rain. Soils that have calcium or limestone are able to neutralize some of the acids. Also if the bedrock is comprised of limestone or marble (both having a more basic pH) the acid may be neutralized.
Mitigation
These acids can be carried over long distances by winds and deposited onto the ground in the form of rain, snow, or other precipitation. Acid rain can have serious environmental and health impacts, including damaging forests and aquatic ecosystems, eroding buildings and statues, and contributing to respiratory problems in humans.
There are several ways to mitigate acid rain:
- Reduce emissions of SO2 and NOx: This can be achieved through the use of cleaner technologies and fuels, such as natural gas instead of coal, and by implementing emission limits and standards.
- Increase the efficiency of fossil fuel use: By using energy more efficiently, we can reduce the amount of fossil fuels that need to be burned, which in turn reduces SO2 and NOx emissions.
- Promote the use of renewable energy sources: Renewable energy sources, such as solar and wind power, do not produce SO2 or NOx emissions.
- Plant trees and other vegetation: Trees and other vegetation absorb CO2 from the air and can help neutralize the effects of acid rain.
- Increase the use of scrubbers and other pollution control technologies: Scrubbers are devices that can remove SO2 and other pollutants from the emissions of power plants and other industrial facilities.
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Frequently Asked Questions
What is acid rain and how does it form?
Acid rain (more broadly acid deposition) is precipitation or dry particles with unusually low pH formed when sulfur oxides (SO2) and nitrogen oxides (NOx) in the atmosphere react with water, oxygen, and other chemicals to make sulfuric and nitric acids. Major human sources are coal-burning power plants (SO2 and NOx) and motor vehicles (NOx); natural sources include volcanoes and lightning (CED: STB-2.H). These gases can travel long distances and fall as wet deposition (rain, snow, fog) or dry deposition (acidic particles/gases), so communities downwind of coal plants are often most affected (STB-2.I). Effects include soil and freshwater acidification, mobilization of toxic aluminum, and corrosion of buildings; regions with limestone bedrock buffer acid rain better. For more review, see the Topic 7.7 study guide (https://library.fiveable.me/ap-environmental-science/unit-7/acid-rain/study-guide/D55YvaAh5s6AUZHOxsER) and Unit 7 resources (https://library.fiveable.me/ap-environmental-science/unit-7). Practice questions: (https://library.fiveable.me/practice/ap-environmental-science).
Why do coal-burning power plants cause acid rain?
When coal burns in power plants it releases sulfur in the coal as sulfur dioxide (SO2) and nitrogen in the fuel/air as nitrogen oxides (NOx). In the atmosphere those gases react with water, oxygen and other chemicals to form sulfuric acid (H2SO4) and nitric acid (HNO3). Those acids fall as wet deposition (acid rain) or dry deposition downwind of the plants because winds carry the pollutants long distances. Acid deposition acidifies soils and freshwater, mobilizes toxic aluminum from soils, and corrodes buildings and statues. Regions with limestone bedrock buffer acidity better than granite soils, so impacts vary by geology. This is exactly what the CED lists for Topic 7.7 (SO2 and NOx from coal plants → acid deposition; downwind effects; soil/lake acidification). For quick review, see the Topic 7.7 study guide (https://library.fiveable.me/ap-environmental-science/unit-7/acid-rain/study-guide/D55YvaAh5s6AUZHOxsER) and try practice questions (https://library.fiveable.me/practice/ap-environmental-science)—acid rain is commonly tested in Unit 7 multiple-choice and short free-response items.
What's the difference between nitrogen oxides and sulfur oxides in acid rain formation?
Short version: both NOx (NO and NO2) and SOx (mainly SO2) react in the atmosphere to form acids, but they differ in sources and the acids they make. Details you should know for APES: - Sources: NOx mainly comes from motor vehicles and from combustion (including coal-fired power plants); SO2 comes mostly from coal-burning power plants (EK STB-2.H.2). - Chemistry: NOx oxidizes to nitric acid (HNO3); SO2 is oxidized to sulfuric acid (H2SO4). Both form via gas-phase and aqueous-phase reactions and cause wet deposition (acid rain) or dry deposition. - Behavior & impacts: SO2/SOx often lead to stronger acidification because H2SO4 is a diprotic strong acid, but both lower pH of soils and freshwater, mobilize aluminum, and corrode structures (EK STB-2.I.2). Effects are greatest downwind of coal plants and depend on regional bedrock buffering (e.g., limestone neutralizes acids—EK STB-2.I.3). For more review, see the Topic 7.7 study guide (https://library.fiveable.me/ap-environmental-science/unit-7/acid-rain/study-guide/D55YvaAh5s6AUZHOxsER), Unit 7 overview (https://library.fiveable.me/ap-environmental-science/unit-7), and practice problems (https://library.fiveable.me/practice/ap-environmental-science).
I'm confused about how car exhaust contributes to acid rain - can someone explain?
Cars emit nitrogen oxides (NO and NO2) when fuel burns. In the atmosphere those NOx react with oxygen and water to form nitric acid (HNO3) and related acidic compounds. Those acids fall as wet deposition (acid rain, snow, fog) or as dry deposition (acidic gases/particles)—both are “acid deposition” (CED: EK STB-2.H.1, STB-2.H.2). Downwind communities and ecosystems get the worst effects: soils and freshwater bodies acidify, toxic aluminum can be released from soils, and buildings corrode (EK STB-2.I.1–I.3). For the AP exam, remember motor vehicles are a primary NOx source and coal plants are major SO2 sources (STB-2.H). Review the Topic 7.7 study guide on Fiveable for concise notes (https://library.fiveable.me/ap-environmental-science/unit-7/acid-rain/study-guide/D55YvaAh5s6AUZHOxsER) and practice with more Unit 7 review (https://library.fiveable.me/ap-environmental-science/unit-7) or the AP practice problems (https://library.fiveable.me/practice/ap-environmental-science).
How does acid rain actually damage buildings and statues?
Acid rain (from SO2 and NOx from coal plants and vehicles) forms sulfuric and nitric acids in the atmosphere, lowering rain pH (often below ~5.6). When that acidic water hits buildings and statues it chemically reacts and physically weakens materials: - Carbonate stones (limestone, marble): acids react with calcium carbonate (CaCO3) to make soluble calcium and bicarbonate, slowly dissolving details and inscriptions. - Metals: acids corrode metal fixtures and bronze statues (forming green/black corrosion products and weakening structure). - Paints/coatings and concrete: acids and deposited particles break down protective coatings and leach cement components, causing surface flaking. - Physical amplification: acid deposition speeds up freeze–thaw damage and salt crystallization in pores, increasing cracking and granular disintegration. Because acid deposition travels long distances, downwind communities near coal-burning power plants suffer more (CED EK STB-2.H/I). For review on mechanisms and effects, see the Topic 7.7 study guide (https://library.fiveable.me/ap-environmental-science/unit-7/acid-rain/study-guide/D55YvaAh5s6AUZHOxsER) and Unit 7 overview (https://library.fiveable.me/ap-environmental-science/unit-7). Practice related questions at (https://library.fiveable.me/practice/ap-environmental-science).
Why are some areas more affected by acid rain than others even if they're the same distance from pollution sources?
Even if two places are the same distance from a pollution source, they can get different acid deposition because of wind patterns, topography, precipitation type, and local geology. Pollutants (SO2, NOx) travel long distances and affect downwind communities more strongly—so prevailing winds can steer more acidic air to one site. Mountains can force air to rise and dump wet deposition (rain/snow) on one side. Areas with lots of rain get more wet deposition; dry areas get more dry deposition (gases/particles). Finally, soils and bedrock matter: limestone or carbonate-rich soils buffer acidity, so lakes/soils stay near neutral; granite or thin soils can’t neutralize acids, so pH drops faster and ecosystems suffer more. These are exactly the CED concepts (sources, downwind impact, wet/dry deposition, and buffering) you need for Topic 7.7—see the topic study guide for a quick review (https://library.fiveable.me/ap-environmental-science/unit-7/acid-rain/study-guide/D55YvaAh5s6AUZHOxsER) and more practice (https://library.fiveable.me/practice/ap-environmental-science).
What happens to lakes and soil when acid rain falls on them?
When acid deposition (from SOx and NOx—often from coal plants and cars) falls on lakes and soil it lowers pH and causes a chain of harmful effects. In lakes/ponds acidity increases, which can mobilize toxic aluminum from sediments, killing sensitive fish and aquatic invertebrates and reducing biodiversity. Some species tolerate it, but communities downwind of coal-burning power plants are most affected (CED EK STB-2.I.1–2). In soils acid rain leaches away base cations (Ca2+, Mg2+, K+) that plants need, damages root systems, and increases soluble aluminum that’s toxic to roots and microbes. Regional buffering matters: limestone/alkaline bedrock neutralizes acids and protects lakes/soils, while granite/poor soils offer little buffering (CED EK STB-2.I.3). For AP review, see the Topic 7.7 study guide (https://library.fiveable.me/ap-environmental-science/unit-7/acid-rain/study-guide/D55YvaAh5s6AUZHOxsER) and practice questions (https://library.fiveable.me/practice/ap-environmental-science).
How does limestone bedrock help protect against acid rain damage?
Limestone (calcium carbonate, CaCO3) helps neutralize acid rain because its carbonate ions react with incoming H+ (acid) to form bicarbonate and water, removing acidity from runoff and lakes. Chemically: CaCO3 + 2H+ → Ca2+ + H2O + CO2 (or HCO3–). That raises pH, preventing soil and freshwater acidification, limiting toxic aluminum mobilization, and reducing harm to fish and aquatic organisms. Regions underlain by limestone therefore show less lake acidification and slower corrosion of some structures than areas with granite or sandy bedrock (CED EK STB-2.I.3). Buffering isn’t infinite—if acid inputs exceed the rock’s capacity, acidification still occurs. For more AP-aligned review on acid rain and buffering, see the Topic 7.7 study guide (https://library.fiveable.me/ap-environmental-science/unit-7/acid-rain/study-guide/D55YvaAh5s6AUZHOxsER) and practice questions (https://library.fiveable.me/practice/ap-environmental-science).
Can acid rain kill fish and plants, and if so how?
Yes—acid deposition can cause fish and plant mortality. SO2 and NOx from vehicles and coal-burning power plants react in the atmosphere to form sulfuric and nitric acids that fall as wet or dry deposition (CED EK STB-2.H.1–2). When acid rain lowers pH in soils and freshwater (often below ~5), it (1) leaches calcium and other nutrients plants need, stunts growth, and damages root function; and (2) mobilizes toxic aluminum from bedrock into lakes and soils. Elevated Al3+ and low pH disrupt gill function, reproduction, and egg survival in fish, leading to population declines and direct mortality. Regional impacts depend on bedrock buffering—limestone areas neutralize acids, while granitic/peaty regions are vulnerable (EK STB-2.I.3). For AP exam review, study the mechanisms and sources (motor vehicles, coal plants) and regional buffering (see the Topic 7.7 study guide) (https://library.fiveable.me/ap-environmental-science/unit-7/acid-rain/study-guide/D55YvaAh5s6AUZHOxsER). Practice problems are at (https://library.fiveable.me/practice/ap-environmental-science).
What are the main sources of sulfur dioxide that cause acid deposition?
The primary sources of sulfur dioxide (SO2) that drive acid deposition are mostly anthropogenic—especially coal-burning power plants, which emit large amounts of SO2 when coal’s sulfur is oxidized during combustion (EK STB-2.H.2). Other human sources include industrial smelting/refineries, oil and heavy-fuel combustion, and some older industrial boilers. There are also natural sources (volcanoes, wildfires, and marine/biogenic emissions), but on the AP exam you should emphasize coal-fired electricity generation as the main contributor to acid rain (EK STB-2.H.1). Remember SO2 can travel long distances as a gas or convert to sulfate aerosols and fall as wet or dry deposition, so impacts show up in downwind communities (EK STB-2.I.1). For a concise review tied to Topic 7.7, see the Fiveable study guide (https://library.fiveable.me/ap-environmental-science/unit-7/acid-rain/study-guide/D55YvaAh5s6AUZHOxsER).
I don't understand why communities downwind from power plants get hit harder by acid rain?
Short answer: because wind carries the sulfur oxides (SO2) and nitrogen oxides (NOx) from coal-burning power plants downwind, where they react in the atmosphere to form sulfuric and nitric acids that then fall as wet or dry deposition. Power plants are big, concentrated anthropogenic sources, so their emissions create plumes that travel long distances (long-range atmospheric transport). Those gases oxidize to H2SO4 and HNO3, get scavenged by clouds (wet deposition) or settle out (dry deposition), andso communities downwind get higher acid inputs. Local damage also depends on soil/bedrock buffering—limestone can neutralize acids, granite can’t—so identical rainfall causes different effects (EK STB-2.H/I). This is a common AP-style concept—review the Topic 7.7 study guide (https://library.fiveable.me/ap-environmental-science/unit-7/acid-rain/study-guide/D55YvaAh5s6AUZHOxsER) and try practice questions (https://library.fiveable.me/practice/ap-environmental-science) to prep for related exam items.
How do you measure the pH of acid rain and what makes it considered "acidic"?
Measure pH by collecting wet-deposition samples (rain) in a clean plastic or glass container, keep them sealed, and test ASAP. In the field or lab use a calibrated pH meter/electrode for the most accurate reading; pH paper or colorimetric test kits work for quick checks but are less precise. Calibrate the meter with standard buffers (pH 4, 7, 10) and rinse the electrode between samples. Rain is considered “acid rain” when its pH is noticeably below natural background (~5.6). That acidity comes mainly from sulfur oxides (SO2) and nitrogen oxides (NOx) in the atmosphere (from coal-burning power plants, vehicles, and natural sources) forming sulfuric and nitric acids. For APES, note sampling is wet deposition, use of instruments, and linking low pH to SOx/NOx emissions (see Topic 7.7 study guide for review: https://library.fiveable.me/ap-environmental-science/unit-7/acid-rain/study-guide/D55YvaAh5s6AUZHOxsER). For extra practice, try problems at (https://library.fiveable.me/practice/ap-environmental-science).
What's the connection between fossil fuel burning and acid rain formation?
Burning fossil fuels (especially coal in power plants and gasoline/diesel in motor vehicles) releases sulfur dioxide (SO2) and nitrogen oxides (NOx) into the atmosphere. Those gases react with water vapor and oxygen to form sulfuric and nitric acids, which fall as wet deposition (acid rain) or as dry deposition (acidic particles/gases). Acid deposition travels long distances, so communities downwind from coal-burning power plants are most affected. Environmentally, acid rain acidifies soils and freshwater, mobilizes toxic aluminum, and corrodes buildings; regions with limestone bedrock buffer lakes better than areas with granite. This is exactly what the AP CED expects you to know for Topic 7.7 (EK STB-2.H.1–2 and EK STB-2.I.1–3). For a focused review, see the Topic 7.7 study guide (https://library.fiveable.me/ap-environmental-science/unit-7/acid-rain/study-guide/D55YvaAh5s6AUZHOxsER) and more Unit 7 resources (https://library.fiveable.me/ap-environmental-science/unit-7). Practice related questions at (https://library.fiveable.me/practice/ap-environmental-science).
Why doesn't acid rain affect all regions equally - what makes some places more vulnerable?
Acid rain doesn’t hit everywhere the same because of differences in where pollutants come from, how they travel, and how the landscape reacts. Regions downwind of big sources (like coal-burning power plants and heavy traffic that emit SO2 and NOx) get more acid deposition because of long-range atmospheric transport and prevailing winds. Local climate matters too: areas with lots of precipitation get more wet deposition, while dry regions get more dry deposition. Geology and soils are huge—limestone or carbonate bedrock buffers acid, so lakes/soils stay neutral longer, whereas granite or thin soils can’t neutralize acid, so pH drops and aluminum is mobilized, harming aquatic life and plants. Topography and vegetation (mountainous valleys trap pollutants) also change exposure. For AP review, this matches EK STB-2.H and STB-2.I—see the Topic 7.7 study guide (https://library.fiveable.me/ap-environmental-science/unit-7/acid-rain/study-guide/D55YvaAh5s6AUZHOxsER) and more Unit 7 resources (https://library.fiveable.me/ap-environmental-science/unit-7).
How can we prevent or reduce acid rain from happening?
Preventing/reducing acid rain means cutting the SO2 and NOx that form acidic deposition (from coal plants, vehicles) and limiting long-range transport to downwind communities. Key solutions: install flue-gas desulfurization (scrubbers) and low-NOx burners at coal plants; switch to low-sulfur coal or natural gas and, better, renewables; use catalytic converters and cleaner-burning engines, improve public transit and reduce driving; and enforce emissions limits or market tools (cap-and-trade) for SO2/NOx. Short-term mitigation includes liming acidified lakes/soils to raise pH and reduce aluminum mobilization. These align with AP essentials (EK STB-2.H.1/2 and EK STB-2.I.1–3) and are the kind of “propose a solution” content tested in Free-Response Question 2 (Practice 7). For a focused review on mechanisms and solutions, see the Topic 7.7 study guide (https://library.fiveable.me/ap-environmental-science/unit-7/acid-rain/study-guide/D55YvaAh5s6AUZHOxsER). More unit review and practice questions are at (https://library.fiveable.me/ap-environmental-science/unit-7) and (https://library.fiveable.me/practice/ap-environmental-science).