AP Environmental Science

## Learning Outcomes

- Understand why acid deposition is a regional and transboundary pollution problem, not just a local one
- Distinguish clearly between **acid rain** and **acid deposition**
- Explain how **SO2** and **NOx** are emitted, transformed, transported, and deposited downwind
- Compare **wet deposition** and **dry deposition** and explain how each causes damage
- Identify why some downwind environmentss are more vulnerable because of low buffering capacity
- Describe major ecological and human-made impacts, including acidified lakes, nutrient leaching, and corrosion of buildings
- Apply this topic to AP Environmental Science multiple-choice and FRQ questions using process-based reasoning

## AP Environmental Science Syllabus

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

- Know that **acid deposition** includes both **wet deposition** and **dry deposition**, while **acid rain** refers only to the wet form
- Identify **sulfur dioxide (SO2)** and **nitrogen oxides (NOx)** as the main precursor pollutants
- Explain how these pollutants are converted in the atmosphere into **sulfuric acid (H2SO4)** and **nitric acid (HNO3)**
- Understand how **prevailing winds** can transport pollutants hundreds of miles from upwind sources to downwind regions
- Explain why areas with **thin soils**, **granite bedrock**, or low calcium content are more sensitive to acidification
- Describe major effects on lakes, streams, soils, forests, and buildings
- Recognise the importance of the **Clean Air Act amendments**, the **Acid Rain Program**, and the **Canada–United States Air Quality Agreement (1991)**
- Identify the role of **NADP** in wet deposition monitoring and **CASTNET** in dry deposition estimates

## 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. How can pollution from one state or country cause acidification in another?
3. Why are the Adirondacks and other downwind regions especially vulnerable to acid deposition?
4. What is the difference between wet deposition and dry deposition?
5. Name the two main gases that produce acid deposition and the acids they form.

## Introduction

Downwind acid deposition is one of the best examples in AP Environmental Science of how pollution does not stay where it is produced. A power plant, refinery, or vehicle fleet may release gases in one location, but winds and atmospheric chemistry can shift the environmental damage far away from the original source. That is why acid deposition is often tested as a regional systems problem rather than a simple local pollution problem.

For the exam, you need more than a definition. You need to understand the full chain: emissions from fossil fuel combustion, atmospheric conversion into acids, long-range transport by prevailing winds, and deposition onto natural habitatss that may be many miles downwind. You also need to explain why some places are damaged much more severely than others.

> **Key Term: acid deposition**
> The transfer of acidic material from the atmosphere to Earth in wet forms such as rain or snow and dry forms such as gases or particles.

### The transport pathway: from upwind emissions to downwind damage

The starting point is the release of **primary pollutants** into the atmosphere, especially **sulfur dioxide (SO2)** and **nitrogen oxides (NOx)**. These come mainly from burning fossil fuels, especially coal and oil in power plants and industry, and also from vehicles and some natural sources such as volcanoes, forest fires, and lightning.

> **Key Term: primary pollutant**
> A pollutant released directly into the atmosphere from a source rather than formed later by atmospheric reactions.

Acid deposition itself is a **secondary pollution** problem. The gases released at the source are not yet acid rain. They must first react in the atmosphere.

Sulfur dioxide follows a pathway like this:

- **SO2 + H2O → H2SO3**
- sulfurous acid is then oxidised to **H2SO4**

Nitrogen oxides follow a pathway like this:

- **NO + O2 → NO2**
- nitrogen dioxide reacts with water vapor to form **nitrous acid** and **nitric acid**
- the main strong acid of exam importance is **HNO3**

This matters because it means the pollution often becomes more damaging after release, not at the smokestack itself.

> **Key Term: secondary pollutant**
> A pollutant formed in the atmosphere by reactions involving primary pollutants.

Once formed, these acidic compounds or related sulfate and nitrate particles can remain in the atmosphere long enough to travel substantial distances. In the mid-latitudes, **prevailing westerlies** often move air masses from west to east. In the United States, that historically meant emissions from the **Midwest and Ohio Valley** were carried toward the **Northeast** and **Mid-Atlantic**.

This is the core idea of downwind deposition: the place harmed may not be the place that emitted most of the pollution.

> **Key Term: downwind deposition**
> Acid deposition that occurs in regions located downwind of the original emission source after pollutants are transported by air currents.

A useful APES chain to memorise is:

- upwind fossil fuel combustion
- release of **SO2** and **NOx**
- atmospheric conversion to sulfuric and nitric acids
- transport by prevailing winds
- wet or dry deposition in downwind areass
- acidification and ecological damage
  > **Test Tip:** In FRQs, score points by writing the process in sequence. Examiners reward linked reasoning more than isolated facts.

### Wet deposition and dry deposition: similar source, different route

Many students use “acid rain” and “acid deposition” as if they mean exactly the same thing. They do not.

> **Key Term: acid rain**
> The wet form of acid deposition, including acidic rain, snow, sleet, or fog.

Acid rain is only one part of the broader process. **Wet deposition** includes rain, fog, and snow carrying acidic material to the surface. **Dry deposition** happens when acidic gases and particles settle directly on surfaces without precipitation.

> **Key Term: wet deposition**
> The return of acidic material to Earth in precipitation such as rain, snow, sleet, or fog.
>
> **Key Term: dry deposition**
> The settling of acidic gases and particles directly onto land, water, plants, or buildings without precipitation.

This distinction matters because dry deposition can look less dramatic but still causes major harm. Textbook material states that about **half of atmospheric acidity** can return to Earth through dry deposition. In dry conditions, acidic particles may stick to buildings, cars, tree leaves, and soil surfaces. Later, rain can wash them into streams and lakes, increasing acidic runoff.

So dry deposition is not “harmless until it rains.” It often becomes a delayed input to aquatic systems.

Another key distinction is time and distance. Dry deposition may occur within a few days after emission, while wet deposition can be delayed longer, often allowing pollutants to travel farther downwind before being deposited. That helps explain why acid rain is considered a cross-state and even cross-border issue.

Normal rain is already slightly acidic, about **pH 5.6**, because dissolved **CO2** forms weak carbonic acid. Acid deposition is typically defined as precipitation **below pH 5.6**. Acid rain can be much more acidic, with some textbook examples as low as **pH 2.3**.

Because the pH scale is logarithmic, a fall of one pH unit means a tenfold increase in acidity. Students often forget how large that change is.

> **Test Tip:** If a multiple-choice question asks which statement is false, watch for “normal rain has pH 7.” That is incorrect. Normal rain is slightly acidic because of dissolved CO2.

### Why some downwind regions are hit harder than others

Not all downwind places are equally sensitive. This is one of the most tested ideas in acid deposition questions.

The intensity of damage depends on:

- how acidic the deposition is
- how often deposition occurs
- the **buffering capacity** of the soil or bedrock
- the organisms present in the environment
  > **Key Term: buffering capacity**
  > The ability of soils or waters to resist changes in pH by neutralising added acids.

Areas with soils derived from **granite** are often vulnerable because granite-based soils tend to have low acid-neutralising capacity. Areas that have already been **leached** of calcium are also highly sensitive, because calcium helps buffer acidity.

> **Key Term: leaching**
> The removal of dissolved nutrients or minerals as water moves through soil.

This is why the **Adirondacks**, parts of the **Appalachians**, and some regions of **eastern Canada** were historically damaged by acid deposition from upwind U.S. sources. Their thin soils and low buffering capacity made them less able to neutralise incoming acids.

By contrast, regions with more limestone-derived material may better neutralise acid because carbonate minerals can buffer hydrogen ions.

A good exam explanation is not just “acid rain damages lakes.” A stronger answer is: “Acid deposition causes greater damage in areas with low buffering capacity, such as granite-based soils, because there are fewer base cations available to neutralise incoming acids.”

### Ecological and material effects in downwind areas

The effects of downwind acid deposition are broad because acids alter water chemistry, soil chemistry, and biological processes.

In **lakes and streams**, acid deposition lowers pH and can lead to fish stress or fish kills. One especially harmful mechanism is the mobilisation of **aluminum**, which becomes more soluble in acidic conditions and can be toxic to aquatic life.

> **Key Term: acidification**
> The lowering of pH in water or soil caused by the addition of acidic substances.

Fish, amphibians, and macroinvertebrates can all be affected. Early life stages are often especially vulnerable.

In **soils**, acid deposition causes nutrient loss through leaching. Important cations such as **Ca2+**, **K+**, and **Mg2+** are washed out of soils. At the same time, toxic metals may become more soluble, including **lead**, **cadmium**, and **mercury**. This weakens plant growth and also affects decomposers and mycorrhizal fungi.

In **forests**, the result is not usually that acid directly “burns” every tree. The more common pathway is slower and more system-based:

- acidic deposition changes soil pH
- nutrients are leached away
- toxic metals become more available
- roots and soil microorganisms function less well
- trees become weaker and more vulnerable to cold, drought, insects, and disease

High-elevation forests can be especially affected because acidic fog and cloud deposition may increase total acid input.

A further effect is **acid shock**. When snowpack contains stored acidic particles and then melts rapidly, lakes and streams can receive a sudden pulse of acidity far stronger than ordinary acidic rainfall, in some cases **five to ten times** the concentration.

> **Key Term: acid shock**
> A sudden increase in acidity in lakes or streams caused by rapid snowmelt releasing accumulated acidic particles.

This is a good example of why winter deposition can produce spring ecological damage.

Acid deposition also affects the built environment. It accelerates the decay of **limestone**, **marble**, metals, paints, and historic structures. This matters in APES because the impacts are not only ecological but also economic and cultural.

One extra regional example worth remembering is **Florida**, where acidic precipitation can help dissolve limestone, contributing to sinkhole formation.

### Monitoring, policy, and why acid deposition became a regional law issue

Because downwind acid deposition crosses political boundaries, local regulation alone is not enough. This is why the topic connects strongly to environmental policy.

In the United States, the **Clean Air Act** and especially its **1990 amendments** led to major reductions in emissions of the precursor pollutants. The **Acid Rain Program** is best known for using a cap-and-trade system for **SO2** from power plants. Power plants also reduced emissions by switching to lower-sulfur coal, installing scrubbers, and shifting toward fuels such as natural gas.

This policy success can be seen in monitoring data. EPA summaries report that **wet sulfate deposition** declined by more than **70%** between **1989–1991** and **2020–2022** nationwide. That does not mean the issue has disappeared, but it does show that emission control can greatly reduce deposition.

Monitoring systems matter in exam questions too:

> **Key Term: NADP**
> The National Atmospheric Deposition Program, which monitors precipitation chemistry and is mainly used to track wet deposition.
>
> **Key Term: CASTNET**
> A monitoring network used to estimate dry deposition and track sulfur and nitrogen pollutants in the atmosphere.

NADP data are especially useful for wet deposition trends such as sulfate, nitrate, and precipitation pH. CASTNET helps estimate dry deposition of sulfur dioxide, nitric acid vapor, and sulfate or nitrate particles.

Downwind deposition is also an international issue. The **Canada–United States Air Quality Agreement (1991)** addressed transboundary acid rain by committing both countries to reduce **SO2** and **NOx**, share monitoring data, and cooperate on control measures. This agreement matters because eastern Canada historically received significant acid deposition from U.S. emissions transported across the border.

A good APES policy explanation links the law to the process: if pollution is transported across states or countries, then interstate or international regulation is needed.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- **Acid deposition** includes both wet and dry forms, while **acid rain** refers only to the wet form
- The main precursor pollutants are **sulfur dioxide (SO2)** and **nitrogen oxides (NOx)**
- These pollutants are converted in the atmosphere into **sulfuric acid (H2SO4)** and **nitric acid (HNO3)**
- Downwind acid deposition happens because pollutants can be transported long distances by **prevailing winds**
- In the United States, emissions from the **Midwest and Ohio Valley** historically affected the **Northeast** and **Mid-Atlantic**
- **Wet deposition** includes rain, snow, sleet, and fog, while **dry deposition** involves acidic gases and particles settling directly
- Normal rain is about **pH 5.6** because of dissolved **CO2**, so acid deposition is generally below **pH 5.6**
- Downwind regions with **thin soils**, **granite bedrock**, or low calcium content are more vulnerable because they have low **buffering capacity**
- Major impacts include acidified lakes and streams, nutrient leaching, aluminum toxicity, forest damage, and corrosion of buildings
- **Acid shock** can occur during rapid snowmelt when stored acidic particles are released into waters
- **NADP** is mainly used to monitor wet deposition, while **CASTNET** is used for dry deposition estimates
- The **Clean Air Act**, **Acid Rain Program**, and **Canada–United States Air Quality Agreement (1991)** were major policy responses to this regional pollution problem

## Key Terms and Concepts

- acid deposition
- primary pollutant
- secondary pollutant
- downwind deposition
- acid rain
- wet deposition
- dry deposition
- buffering capacity
- leaching
- acidification
- acid shock
- NADP
- CASTNET


Acid Rain - Downwind acid deposition