AP Environmental Science

## Learning Outcomes

- Distinguish between **primary pollutants** and **secondary pollutants** in air pollution
- Identify the main pollutants released by **fossil fuel combustion**
- Explain why **CO**, **NOx**, **SO2**, and **particulate matter** form during combustion
- Compare **mobile sources** and **stationary sources** of air pollution
- Describe major human health and environmental effects of combustion pollutants
- Link combustion pollutants to **ground-level ozone**, **acid deposition**, and **fine particulate pollution**
- Avoid common APES mistakes, especially confusing direct emissions with pollutants formed in the atmosphere

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand fossil fuel combustion pollutants, with a focus on the following syllabus points:

- Know that fossil fuel combustion means burning **coal, oil, gasoline, diesel, or natural gas** for energy, heat, or transport
- Identify the major direct combustion pollutants as **carbon monoxide (CO), nitrogen oxides (NOx), sulfur dioxide (SO2), and particulate matter (PM)**
- Explain the difference between **primary pollutants** emitted directly and **secondary pollutants** formed in the atmosphere
- Understand why **CO** is linked to incomplete combustion, **NOx** to high-temperature combustion, and **SO2** to sulfur in fuel
- Compare **mobile sources** such as cars and trucks with **stationary sources** such as power plants and factories
- Know that **ground-level ozone (O3)** is a **secondary pollutant** formed from **NOx + VOCs + sunlight**
- Understand that **SO2** and **NOx** contribute to **acid deposition** and to secondary fine particles
- Recognise that **CO, NO2, SO2, PM, O3, and Pb** are EPA **criteria pollutants**, but lead is not a major modern combustion product from unleaded fuels

## 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 a primary pollutant and a secondary pollutant?
2. Why does carbon monoxide form during fossil fuel combustion?
3. Which combustion pollutant is most strongly associated with burning sulfur-containing coal and oil?
4. How do mobile sources and stationary sources differ as emitters of fossil fuel pollutants?
5. Why is ground-level ozone not usually classified as a direct combustion emission?

## Introduction

Fossil fuel combustion is one of the most common sources of air pollution tested in AP Environmental Science. When humans burn coal, gasoline, diesel, oil, or natural gas, they release several pollutants directly into the troposphere. These direct emissions then damage human health, reduce visibility, and can also react further in the atmosphere to create new pollutants.

This topic matters because APES questions often ask you to do more than name a pollutant. You may need to identify its source, explain how it forms, classify it as primary or secondary, and connect it to effects such as smog, acid deposition, or respiratory disease. The key is to understand the cause-and-effect chain, not just memorize a list.

> **Key Term: fossil fuel combustion**
> The burning of coal, oil, gasoline, diesel, or natural gas for energy, heat, or motion.

## Introduction to combustion pollutants

### Primary pollutants and secondary pollutants

The first distinction you must know is between pollutants emitted directly and pollutants formed later in the atmosphere.

> **Key Term: primary pollutant**
> A pollutant released directly into the atmosphere from a source such as a tailpipe, smokestack, or fire.

A **primary pollutant** comes straight from combustion or another source. If a car releases carbon monoxide from its exhaust, that CO is a primary pollutant. If a coal plant releases sulfur dioxide, that SO2 is also primary.

> **Key Term: secondary pollutant**
> A pollutant formed in the atmosphere when primary pollutants react with each other or with natural atmospheric components.

Secondary pollutants are different because they are not emitted in their final harmful form. **Ground-level ozone** is the classic example. Cars and factories do not usually emit ozone directly. Instead, they emit precursor pollutants, especially **NOx** and **VOCs**, and those react in sunlight to form ozone.

This is a frequent exam trap. Students often see ozone listed among criteria pollutants and assume it comes straight from tailpipes. It does not. It is a criteria pollutant, but it is usually secondary.

A second common mistake is to think each pollutant fits neatly into only one category forever. In APES, you should treat **CO, SO2, NO, NO2, and soot PM** as primary when directly emitted. But some products derived from them, like **sulfate particles**, **nitrate particles**, and **ozone**, are secondary.

> **Test Tip:** If a question asks which pollutant is formed in sunlight from other emissions, look for **ground-level ozone**, not carbon monoxide or sulfur dioxide.

### The four core fossil fuel combustion pollutants

For this subtopic, focus first on the four most important direct pollutants from fossil fuel burning: **CO**, **NOx**, **SO2**, and **PM**.

A useful way to remember them is by linking each pollutant to why it forms:

- **CO** → incomplete combustion
- **NOx** → high-temperature combustion
- **SO2** → sulfur in fuel
- **PM** → tiny particles from soot, ash, smoke, and incomplete burning

That pattern matters more than rote recall because APES questions often ask you to explain formation.

## Major pollutants from fossil fuel combustion

### Carbon monoxide and nitrogen oxides

**Carbon monoxide (CO)** is a colorless, odorless gas produced when fuel burns without enough oxygen for complete combustion. In ideal complete combustion of a carbon-based fuel, most carbon ends up as carbon dioxide. But when oxygen is limited, combustion is incomplete and more **CO** forms.

This is why motor vehicles are a major source. Engines do not always achieve perfectly complete combustion, especially in stop-and-go traffic, poor maintenance conditions, or poorly ventilated burning systems.

CO is dangerous because it binds strongly to hemoglobin in the blood. That reduces the blood's ability to carry oxygen. The body can then suffer oxygen shortage even when a person is still breathing air.

The health effects follow from that mechanism:

- headache
- dizziness
- nausea
- confusion
- loss of consciousness
- death at high concentrations

The key reasoning is simple: if oxygen transport falls, tissues suffer.

> **Key Term: carbon monoxide (CO)**
> A colorless, odorless primary air pollutant produced mainly by incomplete combustion of carbon-based fuels.

**Nitrogen oxides (NOx)** are another major product of combustion, but they form for a different reason. They do not mainly come from nitrogen inside the fuel. Instead, they form when the nitrogen and oxygen already present in air react at very high temperatures.

That means **NOx** is strongly linked to engines, power plants, and industrial furnaces. The hotter the combustion environment, the easier it is for N2 and O2 to react.

> **Key Term: nitrogen oxides (NOx)**
> A group of nitrogen-oxygen gases, mainly NO and NO2, formed largely when nitrogen and oxygen in air react during high-temperature combustion.

Students often reduce NOx to only nitrogen dioxide. On the exam, **NOx** usually means **NO + NO2** together. Nitric oxide may be emitted first, then some of it is converted to nitrogen dioxide in air.

NOx has several important effects:

- respiratory irritation
- worsening asthma and bronchitis
- formation of **ground-level ozone**
- contribution to **acid deposition**
- contribution to **secondary nitrate particulates**

That last point is easy to miss. NOx does not just irritate lungs directly. It also helps create other harmful pollutants.

A simple reaction summary you should know is:

NOx + VOCs + sunlight → O3

That is why hot, sunny cities with lots of vehicle emissions often have photochemical smog.

> **Test Tip:** If the question asks why NOx forms, do not say “because fuel contains nitrogen” unless that is specifically supported. The safer APES explanation is **high-temperature reaction of atmospheric nitrogen and oxygen**.

### Sulfur dioxide and particulate matter

**Sulfur dioxide (SO2)** forms when fuels containing sulfur impurities are burned. This is why SO2 is especially associated with **coal** and **oil**, rather than with modern unleaded gasoline vehicles.

Coal-fired power plants, oil-burning facilities, some industrial processes, and some ships burning high-sulfur fuel are major SO2 sources.

> **Key Term: sulfur dioxide (SO2)**
> A primary air pollutant formed mainly when sulfur-containing fuels, especially coal and oil, are burned.

SO2 is a respiratory irritant by itself, but APES questions often expect more than that. It also reacts in the atmosphere to form **sulfuric acid** and **sulfate particles**. That links it to both **acid deposition** and **fine particulate pollution**.

The simplified chemistry is:

- SO2 reacts with oxygen to form SO3
- SO3 reacts with water to form H2SO4

That sulfuric acid can become part of wet deposition or can help form tiny sulfate aerosols. So a coal plant emitting SO2 is not only causing direct air pollution nearby. It can also contribute to wider regional damage.

**Particulate matter (PM)** is a mixture of tiny solid particles and liquid droplets suspended in air.

> **Key Term: particulate matter (PM)**
> Tiny solid particles or liquid droplets in air, including soot, smoke, ash, dust, and some particles formed secondarily in the atmosphere.

PM is tricky because it can be both **primary** and **secondary**.

Primary PM includes:

- soot from diesel exhaust
- black carbon from incomplete combustion
- ash from burning fuel
- smoke particles

Secondary PM includes particles formed when gases such as SO2 and NOx react in air and later condense into sulfate or nitrate particles.

This dual identity is a classic APES distinction. If the question refers to **soot from exhaust**, that is primary PM. If it refers to **sulfate aerosols** or **nitrate particles**, that is secondary PM.

Another key distinction is particle size:

- **PM10** = particles 10 micrometers or smaller
- **PM2.5** = particles 2.5 micrometers or smaller

PM2.5 is generally more dangerous because it penetrates deeper into the lungs and may enter the bloodstream. Combustion, especially diesel combustion, is strongly associated with fine PM2.5.

Health effects of PM include:

- irritation of eyes, nose, and throat
- reduced lung function
- worsening asthma
- cardiovascular stress
- stroke and heart disease risk
- premature death in sensitive groups

The reason PM2.5 is so dangerous is physical, not chemical alone. Its small size allows deeper penetration.

## Sources, effects, and exam distinctions

### Mobile sources versus stationary sources

APES often asks students to compare where these pollutants come from. The broad source categories are **mobile** and **stationary**.

> **Key Term: mobile sources**
> Moving sources of pollution such as cars, trucks, buses, ships, trains, and airplanes.
>
> **Key Term: stationary sources**
> Fixed-location sources of pollution such as power plants, industrial boilers, refineries, and factories.

A helpful pattern is:

- **Mobile sources** mainly emit **CO, NOx, and PM**
- **Stationary sources** mainly emit **SO2, NOx, and PM**

This pattern is not absolute, but it is very useful for APES.

Why this pattern works:

- Vehicles often produce lots of **CO** because incomplete combustion in engines is common
- Vehicle engines also run hot, so they produce **NOx**
- Diesel engines are major **PM** emitters
- Coal- and oil-burning power plants often emit much more **SO2** because those fuels contain sulfur

So if a question asks which pollutant is most linked to cars, **CO** is often the best answer. If it asks which pollutant is most linked to coal-burning power plants, **SO2** is usually best.

Ships deserve a quick note. They are mobile sources, but they may emit more sulfur pollution than most road vehicles if they use high-sulfur fuel.

### From combustion pollutants to smog, acid deposition, and haze

Combustion pollutants matter not only because of their direct effects, but because they drive later atmospheric changes.

**Ground-level ozone** forms secondarily when **NOx** and **VOCs** react in sunlight. This is a major part of photochemical smog and is especially common on hot sunny days.

> **Key Term: ground-level ozone (O3)**
> A harmful secondary pollutant formed in the troposphere when NOx and VOCs react in sunlight.

Ground-level ozone harms lungs, irritates airways, and can reduce plant productivity. This is one place where APES loves contrast:

- **stratospheric ozone** protects life from UV
- **tropospheric ozone** harms life

**Acid deposition** forms mainly when **SO2** and **NOx** react in the atmosphere to produce sulfuric and nitric acids.

> **Key Term: acid deposition**
> The return of acidic particles, gases, or precipitation to Earth, mainly caused by atmospheric reactions involving SO2 and NOx.

Acid deposition includes:

- **wet deposition**: acid rain, snow, fog
- **dry deposition**: acidic gases and particles settling onto surfaces

Its environmental effects include:

- acidification of lakes and streams
- nutrient leaching from soils
- forest damage
- harm to aquatic organisms
- corrosion of buildings and monuments

Combustion pollutants also contribute to **haze** and visibility reduction. Fine sulfate and nitrate particles scatter light efficiently, so polluted air often looks hazy not just because “there is smoke,” but because tiny particles alter how light moves through the atmosphere.

### Common APES mistakes to avoid

Students often lose marks through small classification errors.

Mistake 1: saying ozone is emitted directly from cars  
Correction: cars emit NOx and VOCs; ozone forms later in sunlight.

Mistake 2: saying SO2 is a secondary pollutant  
Correction: SO2 is primary when released from combustion. Sulfuric acid and sulfate particles are secondary.

Mistake 3: treating NOx as only NO2  
Correction: NOx usually means a group, mainly NO and NO2.

Mistake 4: forgetting PM can be both primary and secondary  
Correction: soot is primary; sulfate/nitrate PM is secondary.

Mistake 5: mixing climate pollutants with criteria pollutants without care  
Correction: **CO2** matters greatly for climate change, but in APES air-pollution questions about criteria pollutants, the main fossil fuel list is usually **CO, NOx, SO2, and PM**.

That last point is especially useful. Carbon dioxide is absolutely important, but for this subtopic it is usually treated separately from the health-focused combustion pollutant list.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Fossil fuel combustion means burning coal, oil, gasoline, diesel, or natural gas
- The main direct combustion pollutants are **CO, NOx, SO2, and PM**
- **Primary pollutants** are emitted directly, while **secondary pollutants** form in the atmosphere
- **CO** is mainly produced by incomplete combustion
- **NOx** forms mainly during high-temperature combustion from nitrogen and oxygen in air
- **SO2** forms mainly when sulfur-containing fuels, especially coal and oil, are burned
- **PM** includes soot, ash, smoke, and some particles formed secondarily in air
- **PM2.5** is generally more dangerous than PM10 because it penetrates deeper into the lungs
- **Mobile sources** mainly emit CO, NOx, and PM, while **stationary sources** mainly emit SO2, NOx, and PM
- **Ground-level ozone** is a secondary pollutant formed from **NOx + VOCs + sunlight**
- **SO2** and **NOx** contribute to **acid deposition**
- A common exam mistake is confusing **direct combustion pollutants** with **secondary pollutants** formed later in the atmosphere

## Key Terms and Concepts

- fossil fuel combustion
- primary pollutant
- secondary pollutant
- carbon monoxide (CO)
- nitrogen oxides (NOx)
- sulfur dioxide (SO2)
- particulate matter (PM)
- mobile sources
- stationary sources
- ground-level ozone (O3)
- acid deposition


Introduction to Air Pollution - Fossil fuel combustion pollutants