AP Environmental Science

## Learning Outcomes

- Understand how acid deposition harms soils, forests, aquatic systems, and built materials
- Explain the cause-and-effect chain from **SO₂** and **NOₓ** emissions to ecological damage
- Distinguish acid deposition from acid rain, and wet deposition from dry deposition
- Identify why **buffering capacity** and **acid-neutralizing capacity (ANC)** control vulnerability
- Explain how nutrient leaching and aluminum mobilization damage plants and aquatic organisms
- Apply pH evidence, including why early life stages of fish are especially vulnerable around **pH 5**
- Use this topic in AP Environmental Science multiple-choice and free-response questions

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand acid deposition impacts, with a focus on the following syllabus points:

- Know that **acid deposition** includes both wet deposition and dry deposition
- Identify **SO₂** and **NOₓ** as major precursors of sulfuric and nitric acids in the atmosphere
- Explain how acid deposition lowers soil and water pH and why effects depend on **buffering capacity**
- Understand **base-cation leaching**, including the loss of **Ca²⁺**, **Mg²⁺**, and **K⁺** from soils
- Explain how soil acidification increases the solubility of toxic metals, especially aluminum
- Describe impacts on lakes and streams, including fish kills, reproductive failure, and altered food webs
- Recognise that lakes and streams with low **ANC** are more vulnerable than well-buffered waters
- Understand material damage to limestone and marble, including reaction with acid and surface loss

## Test Your Knowledge

_Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision._

1. What is the difference between acid rain and acid deposition?
2. Why are some lakes and streams much more vulnerable to acidification than others?
3. How does acid deposition reduce soil fertility?
4. Why does aluminum become more dangerous when soils and waters become more acidic?
5. Why are fish eggs and larvae often harmed before adult fish?

## Introduction

Acid deposition is one of the clearest examples of how air pollution can become a soil, water, vegetation, and infrastructure problem. Pollutants released from smokestacks, power plants, vehicles, and other sources do not stay in the air. They react in the atmosphere, travel downwind, and then return to Earth in wet or dry forms. Once deposited, they can change soil chemistry, lower the pH of lakes and streams, weaken forests, and damage buildings.

For AP Environmental Science, this topic matters because it links chemistry, ecology, and human impacts. Questions often test not just what acid deposition is, but how it causes specific effects: nutrient loss, aluminum toxicity, fish mortality, forest stress, and corrosion of stone. Strong answers explain the full chain from emissions to impact.

> **Key Term: acid deposition**
> The transfer of acidic materials from the atmosphere to Earth's surface in wet form, such as rain or snow, or dry form, such as acidic gases and particles.

### From emissions to acidic deposition

The starting point is usually **sulfur dioxide (SO₂)** and **nitrogen oxides (NOₓ)**. These are released mainly by burning fossil fuels, especially coal and oil, though natural sources such as volcanoes, forest fires, lightning, and some biological processes also contribute. In the atmosphere, these gases react with water vapor and oxygen to form acids.

For sulfur, the broad pathway is:

- **SO₂** enters the atmosphere
- it reacts with water to form sulfurous acid
- further reaction with oxygen forms sulfuric acid

For nitrogen:

- nitrogen monoxide can oxidize to nitrogen dioxide
- nitrogen dioxide reacts with water vapor
- this forms nitrous and nitric acids

These acids then return to Earth.

> **Key Term: wet deposition**
> The return of acidic materials to Earth in precipitation, including rain, snow, fog, sleet, or hail.
>
> **Key Term: dry deposition**
> The return of acidic particles and gases to surfaces in the absence of precipitation.

A common exam mistake is to treat acid rain as the whole topic. Acid rain is only one part of acid deposition. Dry deposition matters because acidic particles can settle on leaves, soil, buildings, and water surfaces, then later be washed into the environments by rain, increasing acidic runoff.

Another important idea is transport time. Wet deposition may occur several days after emissions, so pollution can travel far from its source. That is why lakes, forests, and buildings downwind of major industrial areas can be damaged even if they are not near the original emissions source.

> **Test Tip:** If a question asks for a distinction, write it directly: acid rain is a form of wet deposition, but acid deposition is the broader term because it includes both wet and dry forms.

### Why some soils and forests are damaged more than others

Acid deposition does not affect all soils equally. The key factor is whether the soil can resist changes in pH.

> **Key Term: buffering capacity**
> The ability of soil or water to resist changes in pH by neutralizing added acids.

Soils with more carbonate or bicarbonate material, often linked to limestone-rich areas, can neutralize incoming acid more effectively. Soils derived from granite, or soils already depleted of calcium, are much less protected. That is why some regions become acidified quickly while others show less change.

One of the main effects is **base-cation leaching**. Hydrogen ions from acids displace nutrient ions attached to soil particles. These nutrients are then washed away.

> **Key Term: base-cation leaching**
> The removal from soil of positively charged nutrient ions such as **Ca²⁺**, **Mg²⁺**, **K⁺**, and **Na⁺** by acidic inputs.

This matters because those ions do two jobs:

- they are essential nutrients for plant growth
- they help the soil buffer further acid inputs

So acid deposition does not just lower pH once. It also strips away the very chemicals that could have softened future impacts. That creates a worsening cycle:

1. acid enters soil
2. nutrient cations are displaced and leached
3. soil fertility declines
4. buffering capacity falls
5. future acid inputs cause even larger pH changes

Acidified soils also change the soil biological community. Decomposers and mycorrhizal fungi can be harmed by acidic conditions. That slows nutrient cycling and reduces plant access to water and minerals. So forest decline is not just a direct "acid burns leaves" story. It is also a below-ground chemistry and biology story.

Another major effect is aluminum mobilization.

> **Key Term: aluminum mobilization**
> The release of aluminum from soil minerals into soluble forms under acidic conditions, making it more available and more toxic to plants and aquatic life.

In less acidic soil, much of the aluminum stays bound in minerals and is not a major immediate threat. As pH falls, more aluminum enters the soil solution. This damages fine roots and interferes with plant uptake of **Ca²⁺** and **Mg²⁺**. So plants can suffer both direct toxicity and nutrient deficiency at the same time.

Forest effects often appear as weakened trees rather than instant death. Trees may show:

- reduced vigor
- greater susceptibility to pests and disease
- slower growth
- lower cold tolerance
- greater risk from drought stress

A classic sensitive example is **red spruce**. Calcium depletion is linked to reduced cold tolerance in needles, which increases the risk of winter injury. This is a good AP example because it shows that acid deposition can weaken trees indirectly, making other stressors more harmful.

### Why lakes and streams become acid-sensitive

Acid deposition often causes its most visible ecological effects in lakes and streams. Water bodies can become more acidic directly from rainfall and snowmelt, but also indirectly when acidic inputs alter surrounding soils and release toxic materials into runoff.

The key idea here is that not all water bodies can neutralize acid equally well.

> **Key Term: acid-neutralizing capacity (ANC)**
> A measure of how well water can neutralize added acid; low ANC means high vulnerability to acidification.

Waters with low ANC are often found in areas with poorly buffered bedrock and thin soils. Many lakes and streams in the Northeast and Appalachian regions fit this pattern. If two lakes receive the same acidic input, the one with lower ANC will show a larger pH drop.

This is a strong AP cause-and-effect chain:

- **SO₂** and **NOₓ** emissions form acids
- acids enter watershed
- low-buffer soil and low-ANC water cannot neutralize them well
- water pH falls
- aluminum enters streams and lakes from acidified soils
- fish and invertebrates are stressed or die
- food webs change

The role of aluminum is often missed. Students sometimes write only "low pH kills fish." That is incomplete. Lower pH harms aquatic life directly, but dissolved aluminum can be just as serious or worse because it damages fish gills and disrupts ion regulation.

Fish do not respond equally at all life stages. Early life stages are the most sensitive. Eggs and larvae have narrow tolerance ranges and less developed ability to regulate body chemistry.

Around **pH 5**, many fish eggs may be damaged or fail to hatch. This is a useful benchmark, but not a universal cutoff for all species. Trout, salmon, and other cold-water fish are commonly used as examples of acid-sensitive organisms.

Acidification also affects:

- aquatic insects and macroinvertebrates
- amphibians
- plankton and other small prey organisms

This creates both direct and indirect effects. A fish population can decline because:

- eggs fail to hatch
- juveniles suffer toxicity
- adults experience gill damage
- prey organisms decline, reducing food supply

That final point is important in exam answers. Food-web effects matter. Even when adult fish can survive chemically, their populations may still shrink if acid-sensitive invertebrates disappear first.

One especially severe event is **acid shock**.

> **Key Term: acid shock**
> A sudden surge in acidity in lakes and streams, often caused by rapid snowmelt releasing stored acidic particles.

During winter, acidic particles can accumulate in snowpack. When snow melts quickly, those acids can be released in a short burst. The resulting acidity in streams and lakes may become several times higher than that of ordinary acidic rainfall. Organisms that could survive gradual change may not survive a sudden pulse.

> **Test Tip:** In FRQs, do not stop at "water becomes acidic." To earn stronger points, add the mechanism: low pH plus dissolved aluminum harms gills, eggs, and invertebrates, leading to lower biodiversity and altered food webs.

### Impacts on materials, monuments, and human systems

Acid deposition is also important because it damages nonliving materials. Limestone and marble are especially vulnerable because they are rich in **calcium carbonate (CaCO₃)**.

Sulfuric acid can react with calcium carbonate:

- **CaCO₃ + H₂SO₄ → CaSO₄ + CO₂ + H₂O**

This reaction removes material from the stone surface. Fine carved details on statues and monuments can be worn away over time. On sheltered surfaces, calcium sulfate may hydrate to form gypsum crusts, which can trap soot, darken, crack, and flake away.

> **Key Term: material corrosion**
> The chemical breakdown of building materials by acidic pollutants, especially damage to limestone, marble, metals, and paint.

This is why acid deposition damages:

- statues and sculptures
- limestone and marble buildings
- painted surfaces
- infrastructure exposed to acidic runoff or particles

For APES, the key point is not memorizing every reaction product, but understanding that acidic compounds chemically weather carbonate stone and accelerate material loss.

Acid deposition also has human health relevance, though the main issue is not usually direct skin contact with rain. Acid rain is too diluted to burn skin. The more serious human concern is the air pollution that causes it: **SO₂**, **NOₓ**, and associated particulates can irritate the respiratory system.

A subtle but useful extension is that soil and water acidification can increase the mobility of harmful metals, including lead, cadmium, mercury, and methyl mercury. Once these move into aquatic systems, they may enter food chains. So acid deposition can connect atmospheric pollution to later aquatic toxicity.

### How to explain acid deposition impacts in an FRQ

Strong AP Environmental Science answers on this topic usually follow a clear chain. Instead of listing random harms, organise by receptor:

- soils and forests
- lakes and streams
- buildings and materials

A strong structure would be:

1. identify the pollutant source: **SO₂** or **NOₓ**
2. state acid formation in the atmosphere
3. explain wet or dry deposition
4. show what happens after deposition:
   - lower pH
   - nutrient leaching
   - aluminum mobilization
   - reduced buffering
5. connect chemistry to biological or material effects

For example, if asked why one watershed is more damaged than another, mention geology and buffering:

- granite-derived or low-calcium soils have lower buffering capacity
- low ANC waters acidify faster
- carbonate-rich systems resist pH change better

If asked about biotic impacts, separate direct and indirect effects:

- direct: low pH harms eggs, gills, roots
- indirect: prey loss, nutrient cycling disruption, weaker forest health

If asked for a solution, the strongest long-term answer is emissions reduction, not just treating symptoms. Liming lakes or soils can help temporarily, but it does not remove the atmospheric source.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Acid deposition includes both wet deposition and dry deposition
- Major precursors are **SO₂** and **NOₓ**, which form sulfuric and nitric acids in the atmosphere
- Effects depend not only on acid input, but also on soil and water **buffering capacity**
- Acid deposition causes **base-cation leaching**, removing nutrients such as **Ca²⁺**, **Mg²⁺**, and **K⁺**
- Nutrient loss lowers both soil fertility and the soil's future ability to buffer acids
- Acidified soils can release soluble aluminum, which is toxic to roots and aquatic organisms
- Forest impacts include weaker trees, reduced cold tolerance, and greater vulnerability to pests, disease, and drought
- Low-ANC lakes and streams are especially vulnerable to acidification
- Fish eggs and early life stages are often harmed around **pH 5**, though sensitivity varies by species
- Acid shock can occur during rapid snowmelt, causing sudden severe acidity in streams and lakes
- Acid deposition can damage limestone and marble through reactions with **calcium carbonate**
- The best long-term fix is reducing **SO₂** and **NOₓ** emissions rather than only treating symptoms

## Key Terms and Concepts

- acid deposition
- wet deposition
- dry deposition
- buffering capacity
- base-cation leaching
- aluminum mobilization
- acid-neutralizing capacity (ANC)
- acid shock
- material corrosion


Acid Rain - Acid deposition impacts