AP Environmental Science

## Learning Outcomes

- Distinguish between **primary pollutants** and the secondary pollutant acid rain
- Identify the major human sources of **sulfur dioxide (SO2)** and **nitrogen oxides (NOx)**
- Explain why coal-burning power plants are strongly linked to acid deposition
- Compare stationary and mobile sources of acid rain precursors
- Recognise important natural sources such as volcanoes, lightning, and soil bacteria
- Apply source knowledge to AP Environmental Science multiple-choice and free-response questions

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand the sources of acid rain precursors, with a focus on the following syllabus points:

- Know that acid rain is a **secondary pollutant** formed mainly from **SO2** and **NOx**
- Identify major human sources of **SO2**, especially coal-burning power plants, industrial combustion, and metal smelting
- Identify major human sources of **NOx**, especially motor vehicles, power plants, and other high-temperature combustion sources
- Explain how **stationary sources** differ from **mobile sources**
- Understand that acid deposition includes both **wet deposition** and **dry deposition**
- Recognise that some precursor emissions come from natural sources, including volcanoes, lightning, forest fires, and soil processes
- Distinguish acid rain precursors from greenhouse gases such as **CO2**
- Explain how emitted precursor gases are transformed into **sulfuric acid** and **nitric acid** in the atmosphere

## 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 are the two main precursor pollutants that cause acid rain?
2. Why are coal-burning power plants usually linked more strongly to **SO2** than to **NOx**?
3. Name two major mobile or stationary human sources of **NOx**.
4. What is the difference between a precursor pollutant and the acid deposited on Earth?
5. Give one natural source of **SO2** and one natural source of **NOx**.

## Introduction

Acid rain is one of the clearest examples of how air pollution can become a wider environmental problem. The harmful acids are not usually emitted directly. Instead, gases are released into the atmosphere, chemically changed, and then returned to Earth as wet or dry deposition. That means AP Environmental Science questions often test whether you can identify the original pollutant sources rather than simply describing the damage caused later.

For this subtopic, the most useful idea is simple: **acid rain begins with precursor emissions**. If you know which pollutant is released, which activity produces it, and what type of source it comes from, you can answer most APES questions on acid deposition.

> **Key Term: acid rain precursor**
> A pollutant released into the atmosphere that later reacts to form the acids in acid deposition, mainly **SO2** and **NOx**.

## Introduction to the key precursor pollutants

Acid rain is mainly produced from two groups of primary pollutants: **sulfur dioxide (SO2)** and **nitrogen oxides (NOx)**. These are released directly into the air, then react with water vapor, oxygen, and other atmospheric chemicals. The result is mainly **sulfuric acid (H2SO4)** and **nitric acid (HNO3)**.

This matters because students often confuse the emitted gases with the acids themselves. On the exam, if a question asks for the _source_ of acid rain, the correct answer is usually not “sulfuric acid” or “nitric acid.” Those acids are the end products. The source is the activity that emits **SO2** or **NOx**.

> **Key Term: primary pollutant**
> A pollutant released directly into the atmosphere from a source.
>
> **Key Term: secondary pollutant**
> A pollutant formed in the atmosphere by reactions involving primary pollutants.

Acid rain is a **secondary pollutant**. **SO2** and **NOx** are the main **primary pollutants** behind it.

A simplified reaction sequence for sulfur is:

- **SO2 + H2O → H2SO3**
- **H2SO3 + O2 → H2SO4**

A simplified reaction sequence for nitrogen is:

- **NO + O2 → NO2**
- **NO2 + H2O → acids including HNO3**

You do not need advanced atmospheric chemistry to score well in APES. You do need to know the logic: **combustion releases precursor gases, atmospheric reactions make acids, deposition returns them to Earth**.

> **Test Tip:** If a question asks what “causes acid rain,” name **SO2 and NOx**, not just “pollution” and not **CO2**. AP questions often reward that distinction.

### Major human sources of sulfur dioxide

The dominant human source pattern for **SO2** is strongly tied to **fossil fuels that contain sulfur**, especially **coal**. When coal is burned, sulfur in the fuel combines with oxygen and forms sulfur dioxide. That is why coal-burning power plants are such a classic acid rain source in AP Environmental Science.

The most important **SO2** sources to know are:

- **coal-burning power plants**
- **oil-burning power plants and industrial boilers**
- **oil refineries and large industrial plants**
- **metal smelting and ore processing**

Coal-burning power plants are emphasized so often because they are large **stationary sources**. They burn very large amounts of fuel in one place, so their emissions are concentrated and significant. Even if motor vehicles are numerous, they generally do not match coal plants as major **SO2** sources in modern APES-style comparisons.

Why is coal especially important? Because it combines two features:

1. it is used in very large quantities for electricity generation
2. it often contains appreciable sulfur impurities

That cause-and-effect chain is worth learning clearly:

- coal contains sulfur
- combustion oxidises that sulfur
- **SO2** enters the atmosphere
- atmospheric reactions form sulfuric acid
- acid deposition affects soils, lakes, forests, and buildings

Industrial facilities also matter. Large boilers that burn oil or coal produce **SO2** for the same basic reason: sulfur in the fuel becomes sulfur dioxide during combustion. But not all industrial **SO2** comes from fuel burning. **Metal smelting** is another important source because ores often contain sulfur-bearing minerals. Processing those ores can release sulfur dioxide even without the exact same pathway as a power station smokestack.

> **Key Term: stationary source**
> A fixed-location pollution source such as a power plant, factory, industrial boiler, or smelter.

A common exam mistake is to say “vehicles are the main source of sulfur dioxide.” They may contribute some sulfur pollution, but in APES the stronger association is **SO2 → coal-burning power plants and smelters**.

> **Test Tip:** Build a memory pair: **SO2 = smokestacks**. Think coal plants, industrial boilers, and smelters.

### Major human sources of nitrogen oxides

The major pattern for **NOx** is different. Nitrogen oxides are formed mainly during **high-temperature combustion**, when nitrogen and oxygen in the air react. This means **NOx** is especially associated with **engines** and other hot combustion systems.

The major **NOx** sources to know are:

- **motor vehicles**
- **electric utilities and power plants**
- **industrial boilers**
- **other high-temperature combustion sources**
  > **Key Term: nitrogen oxides (NOx)**
  > A group of nitrogen-oxygen gases, mainly **NO** and **NO2**, formed especially during high-temperature combustion.

Motor vehicles are a major **mobile source** of **NOx** because internal combustion engines reach temperatures high enough to drive the nitrogen-oxygen reaction. Cars, trucks, and buses therefore contribute heavily to acid rain precursor emissions, especially in urban and suburban areas.

Power plants also emit **NOx**, even though they are more famously linked to **SO2**. This is because any large fossil-fuel combustion system can generate nitrogen oxides if temperatures are high enough. So coal plants contribute to both major precursor categories:

- **SO2** from sulfur in the fuel
- **NOx** from high-temperature combustion

This is one reason coal-burning utilities are so central in acid deposition lessons: they can intensify the problem through two separate pollutant pathways.

Industrial boilers and furnaces work in the same way. If they burn coal, oil, or natural gas at high temperatures, they can produce **NOx**. So if an FRQ asks for a stationary source of nitrogen oxides, a correct answer could be a power plant or industrial combustion source.

> **Key Term: mobile source**
> A moving pollution source such as a car, truck, bus, or ship.

Students often mix up the source pattern by thinking that because vehicles burn fuel, they must also be major **SO2** sources. The better APES distinction is this:

- **vehicles are especially important for NOx**
- **coal plants are especially important for SO2**

That does not mean vehicles emit zero sulfur compounds or that power plants emit zero **NOx**. It means the exam usually wants you to identify the **main association**.

### Stationary vs mobile sources and why the distinction matters

AP Environmental Science often expects you to classify pollution by source type. Acid rain precursor questions become easier if you sort them into **stationary** and **mobile** categories.

**Stationary sources** include:

- coal-burning power plants
- oil refineries
- industrial boilers
- factories
- metal smelters

These are usually major **SO2** sources and can also emit **NOx**.

**Mobile sources** include:

- cars
- trucks
- buses
- ships

These are especially important **NOx** sources.

Why does this distinction matter? Because it shapes pollution control. Stationary sources can often be fitted with technologies such as scrubbers or other emission controls at one large site. Mobile sources are more diffuse because millions of separate engines contribute pollution across a region.

This also helps with data interpretation. If an AP question shows a map with acidified soils downwind of a power plant, the likely source is a **stationary** one, probably linked to **SO2**. If the context is a congested urban area with lots of vehicles, **NOx** is usually central.

There is also an atmospheric transport issue. Acid deposition often occurs far from the emission source. Precursor gases can remain in the air long enough to move many miles downwind before being deposited. So a rural lake may become acidified even if the main source is an urban or industrial region elsewhere.

That is why source and impact are often geographically separated. Students sometimes expect the effects to appear only right next to the smokestack or highway. APES questions may instead describe **downwind** or **downstream** effects.

### Natural sources of acid rain precursors

Although APES usually emphasises human causes, acid rain precursors also have **natural sources**. This is useful because strong answers often acknowledge that not all atmospheric sulfur and nitrogen come from human activity.

Major natural sources include:

- **volcanoes** for **SO2**
- **lightning** for **NOx**
- **forest fires** for both sulfur- and nitrogen-containing emissions
- **soil bacteria** and other soil processes for nitrogen compounds
- **organic decay** and **ocean spray** as smaller sulfur-related contributors in some summaries

Volcanoes are the clearest natural **SO2** source. A volcanic eruption can release large amounts of sulfur dioxide, which may then form acid deposition locally or regionally.

Lightning is a natural **NOx** source because the intense heat of a lightning strike allows atmospheric nitrogen and oxygen to react.

Soil bacteria are also worth remembering. In natural nitrogen cycling, microbial activity in soils can contribute nitrogen compounds to the atmosphere. APES materials often simplify this as a natural source tied to the nitrogen side of acid deposition.

Natural sources matter, but most APES questions on acid rain still focus on **human-caused fossil fuel combustion** as the major driver, especially in modern industrial settings.

> **Key Term: acid deposition**
> The transfer of acidic particles, gases, and precipitation from the atmosphere to Earth as wet deposition or dry deposition.

### Common misconceptions and how to avoid them

A frequent mistake is to confuse acid rain with **greenhouse warming**. **CO2** can make normal rain slightly acidic through carbonic acid, but AP Environmental Science treats **acid rain** mainly as a problem caused by **SO2** and **NOx**, not by carbon dioxide.

Another mistake is to say that acids are emitted directly from smokestacks or tailpipes. The better sequence is:

- sources emit **precursor gases**
- atmospheric reactions convert them
- acids form later
- deposition follows

A third mistake is to write **NO3** as the emitted pollutant. In APES, the source pollutant category is **NOx**, not nitrate. Nitrate is more relevant as a product or dissolved ion later in the process.

A fourth mistake is forgetting **dry deposition**. Acid deposition is not just rain. Acidic gases and particles can settle directly on soil, water, leaves, or buildings, then later be washed into runoff.

Finally, students sometimes answer too vaguely. “Factories and cars” is not wrong, but it is weaker than:

- **SO2 from coal-burning power plants and smelters**
- **NOx from motor vehicles and power plants**

That level of precision helps in both MCQs and FRQs.

### How to apply this in APES questions

On multiple-choice questions, the examiner often tests pairings. The most reliable pairings are:

- **SO2 → coal-fired power plants, industrial combustion, smelters**
- **NOx → motor vehicles, power plants, industrial boilers**

On free-response questions, strong answers usually show a short cause-and-effect chain. For example:

- coal is burned in a power plant
- sulfur in the fuel forms **SO2**
- **SO2** reacts in the atmosphere with water and oxygen
- sulfuric acid forms
- wet or dry deposition acidifies soil or water downwind

That chain shows source, process, and consequence.

If asked to compare sources, explain _why_ they differ:

- power plants are major **SO2** sources because they burn large quantities of sulfur-containing coal
- vehicles are major **NOx** sources because engines generate high combustion temperatures

That kind of reasoning goes beyond recall and matches the style of APES free-response marking.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Acid rain is a **secondary pollutant**, not something usually emitted directly
- The main precursor pollutants are **sulfur dioxide (SO2)** and **nitrogen oxides (NOx)**
- **SO2** is strongly linked to **coal-burning power plants**, industrial combustion, and metal smelting
- **NOx** is strongly linked to **motor vehicles**, power plants, and other high-temperature combustion sources
- Coal-burning utilities are major acid rain sources because they can emit both **SO2** and **NOx**
- **Stationary sources** include power plants, boilers, factories, and smelters
- **Mobile sources** include cars, trucks, buses, and ships
- Important natural **SO2** sources include **volcanoes**
- Important natural **NOx** sources include **lightning**, forest fires, and soil processes
- Acid deposition includes both **wet deposition** and **dry deposition**
- Students should not confuse acid rain precursors with **CO2** or with acids emitted directly
- A strong APES answer links **source → precursor gas → atmospheric reaction → acid deposition**

## Key Terms and Concepts

- acid rain precursor
- primary pollutant
- secondary pollutant
- stationary source
- nitrogen oxides (NOx)
- mobile source
- acid deposition


Acid Rain - Sources of acid rain precursors