AP Environmental Science

## Learning Outcomes

- Distinguish between **primary pollutants** from coal combustion and **secondary pollutants** formed later in the atmosphere
- Identify the major air pollutants released by coal-fired power plants, including **SO2**, **NOx**, **particulate matter**, and **mercury**
- Explain how coal combustion contributes to **industrial smog**, **acid deposition**, and human health problems
- Understand why **PM2.5** is more dangerous than larger particles such as **PM10**
- Compare **fly ash** with **bottom ash** and match major pollutants to the correct control technologies
- Apply coal-pollution cause-and-effect chains in AP Environmental Science questions and FRQs

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand coal combustion pollutants within atmospheric pollution, with a focus on the following syllabus points:

- Know that coal-fired power plants are major **stationary sources** of air pollution
- Identify **sulfur dioxide (SO2)**, **nitrogen oxides (NOx)**, **particulate matter**, **mercury**, and **carbon dioxide** as major coal combustion emissions
- Explain the difference between **primary pollutants** emitted directly and **secondary pollutants** formed in the atmosphere
- Understand how **SO2** and **NOx** contribute to **acid deposition**
- Understand how coal burning is linked to **industrial smog** rather than photochemical smog
- Explain why **PM2.5** poses greater health risks than larger particles
- Identify common coal-plant pollution controls, including **electrostatic precipitators**, **baghouses**, **scrubbers**, and **selective catalytic reduction**
- Recognise that **CO2** is a greenhouse gas, while pollutants such as **SO2** and **PM** are air pollutants with direct health effects

## 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 burning high-sulfur coal increase the risk of acid deposition?
3. How is industrial smog different from photochemical smog?
4. Why is **PM2.5** more harmful to humans than **PM10**?
5. Which pollution-control device removes particulate matter from coal plant flue gas?

## Introduction

Coal combustion is one of the clearest AP Environmental Science examples of how energy use creates air pollution. When coal is burned to make electricity, it does not just release one pollutant. It releases a mixture of gases and particles, and those emissions can then react in the atmosphere to form even more harmful substances.

This matters for the exam because coal-burning power plants connect several Unit 7 ideas at once: air pollutants, industrial smog, acid deposition, particulate pollution, human health, and pollution-control technology. If you can trace the chain from **coal in a boiler** to **pollutants in the air** to **effects on natural systemss and people**, you will be well prepared for both multiple-choice questions and free-response responses.

> **Key Term: coal combustion pollutants**
> Air pollutants released when coal is burned, especially **sulfur dioxide (SO2)**, **nitrogen oxides (NOx)**, **particulate matter**, **mercury**, and **carbon dioxide**.

### What coal burning releases and why it releases it

Coal is not a pure substance. It contains a lot of carbon, but it also contains sulfur, mineral impurities, ash-forming material, and trace metals. When a coal-fired power plant burns coal in a boiler, each of these components matters.

The carbon in coal reacts with oxygen during combustion. Ideally, that produces carbon dioxide. If combustion is incomplete, some carbon monoxide and soot can also form. For AP Environmental Science, the main climate-related product is **CO2**, but the most test-relevant air-pollution products are **SO2**, **NOx**, and particulate matter.

Sulfur dioxide comes from sulfur already present in the coal. This is why high-sulfur coal is more polluting than low-sulfur coal. The sulfur is part of the fuel itself, so when the coal burns, sulfur is oxidized and released as sulfur dioxide. That makes sulfur content a direct predictor of how much **SO2** a plant may emit unless controls are used.

Nitrogen oxides form mainly because combustion temperatures are very high. At those temperatures, nitrogen and oxygen in the air react. This means coal does not need to contain much nitrogen for **NOx** to be produced. Students often miss this: sulfur pollution mainly depends on sulfur in the fuel, but **NOx** depends strongly on combustion conditions.

Particulate matter can be released directly as ash and soot. Coal also leaves behind solid waste. Some of the finest particles are carried upward with flue gas, while heavier material stays in the furnace.

Mercury and other trace metals can also be released when coal burns. These are present in small amounts, but they matter because toxic substances can still be dangerous at low concentrations.

> **Key Term: stationary source**
> A fixed location that emits pollution, such as a factory or coal-fired power plant.
>
> **Key Term: sulfur dioxide (SO2)**
> A primary air pollutant released mainly when sulfur-containing fuels such as coal are burned.

A useful exam chain to remember is:

- coal combustion
- direct release of **SO2**, **NOx**, fly ash, mercury, and **CO2**
- atmospheric reactions forming acids, haze, and fine secondary particles
- damage to lungs, natural habitatss, buildings, and climate systems
  > **Test Tip:** If a question mentions a coal-fired power plant, think first of **SO2**, **NOx**, **particulates**, and **mercury**. Do not jump straight to ozone, because ozone is usually a secondary pollutant formed later.

### Primary pollutants, secondary pollutants, and why that distinction matters

Coal plants release both direct pollutants and the ingredients for later pollution. AP questions often test whether you can separate what leaves the smokestack from what forms in the atmosphere afterward.

A **primary pollutant** is emitted directly from the source. In a coal plant, examples include **SO2**, **NOx**, fly ash, mercury, and **CO2**. These come straight from combustion or from materials carried in the flue gas.

A **secondary pollutant** forms after reactions occur in the air. Coal pollution produces important secondary pollutants because **SO2** and **NOx** are reactive. Sulfur dioxide can be converted into sulfate particles and acids. Nitrogen oxides can become nitrate particles and contribute to acid deposition and ozone formation.

This distinction matters because control technology works differently on primary and secondary pollution. If a plant captures fly ash before it leaves the stack, that directly cuts primary particulate emissions. But if **SO2** and **NOx** are released, harmful **secondary PM2.5** can still form later in the atmosphere, beyond the plant itself.

So even if visible ash seems reduced, the air can still suffer from fine particle pollution if gaseous precursors are not controlled.

> **Key Term: primary pollutant**
> A pollutant emitted directly from a source into the atmosphere.
>
> **Key Term: secondary pollutant**
> A pollutant formed in the atmosphere when primary pollutants react with other substances.

Students often make three common mistakes here.

First, they call **SO2** a secondary pollutant because it helps make acid rain. That is wrong. **SO2** is primary; acid deposition products are secondary.

Second, they assume all particulates from coal are direct emissions. That is incomplete. Some particulate matter is emitted directly, but much harmful fine particulate pollution forms later from **SO2** and **NOx**.

Third, they confuse **CO2** with criteria air pollutants. **CO2** matters greatly for climate change, but it is not the same kind of pollutant as **SO2** or particulate matter in air-quality questions.

> **Test Tip:** On FRQs, write the full chain clearly: “Coal plants emit **SO2** and **NOx** as primary pollutants; these react in the atmosphere to form acids and secondary particulate matter.” That wording often earns more credit than vague statements like “coal causes pollution.”

### Particulate matter, fly ash, and why particle size changes the danger

Particulate matter is one of the most serious coal-related pollutants for human health. It includes tiny solid particles and liquid droplets suspended in the air. Coal combustion produces particulate pollution directly and indirectly.

Direct particulate pollution includes soot and fine ash. **Fly ash** is the fine ash that travels with flue gas. **Bottom ash** is the heavier ash that stays in the boiler or furnace bottom. The key difference is location and particle size: fly ash is more connected to air emissions, while bottom ash is a heavier residual solid.

The atmosphere also gains indirect particulate pollution from coal because **SO2** and **NOx** can become sulfate and nitrate particles. These are often extremely fine and can stay airborne longer than larger particles.

> **Key Term: particulate matter (PM)**
> Tiny solid particles or liquid droplets suspended in air.
>
> **Key Term: PM2.5**
> Particulate matter with diameter of **2.5 micrometers or smaller**, able to penetrate deep into the lungs.
>
> **Key Term: PM10**
> Particulate matter with diameter of **10 micrometers or smaller**, larger than PM2.5 but still inhalable.

Why is **PM2.5** more harmful than **PM10**? Size is the main reason. Larger particles are more likely to be trapped in the nose or upper respiratory tract. Smaller particles can travel deeper into the lungs, reaching the bronchioles and alveoli. Some can even enter the bloodstream. That increases risks of asthma aggravation, cardiovascular disease, respiratory illness, and premature death.

This is why a cloud of invisible fine particles may be more dangerous than a visibly dusty plume. Students sometimes focus too much on what can be seen. In air pollution, the smallest particles are often the most damaging.

Coal combustion also contributes to haze because fine particles scatter light and reduce visibility. So particulate pollution affects both health and environmental quality.

> **Key Term: fly ash**
> Fine ash particles carried with flue gas during coal combustion; a form of primary particulate pollution.
>
> **Key Term: bottom ash**
> Heavier ash left behind in the furnace or boiler during coal combustion.

The EPA strengthened the annual **PM2.5** standard to **9.0 micrograms per cubic meter** on February 7, 2024. You do not need to build whole answers around that figure, but it shows how closely fine particulate pollution is regulated.

### Industrial smog, acid deposition, and major control technologies

Coal combustion is strongly associated with **industrial smog**, also called **gray smog**, **sulfurous smog**, or **London smog**. This type of smog is driven mainly by **SO2** and particulates. It is different from photochemical smog, which is tied more closely to **NOx**, VOCs, sunlight, and ozone.

Industrial smog forms most readily where coal burning is heavy and weather conditions trap pollutants near the ground. Cool, humid air and temperature inversions make it worse. The classic case is the Great London Smog of **1952**, when coal smoke, sulfur dioxide, particulates, fog, and an inversion combined into a deadly event. Immediate deaths were widely estimated around **4,000**, with later estimates much higher.

> **Key Term: industrial smog**
> A sulfur-rich form of smog linked mainly to coal burning, high **SO2** levels, and particulate matter.

Coal combustion also drives **acid deposition**. Sulfur dioxide and nitrogen oxides react in the atmosphere to form sulfuric and nitric acids. These acids return to Earth in rain, snow, fog, or dry particles.

> **Key Term: acid deposition**
> Acidic material deposited from the atmosphere as wet deposition, such as rain or snow, or as dry particles and gases.

The impacts are broad:

- lower pH in lakes and streams
- damage to plant leaves and reduced crop yields
- leaching of important soil nutrients
- greater solubility of toxic metals
- weathering of limestone and other building materials

Coal plants use several major technologies to reduce emissions, and APES often asks you to match each device to the correct pollutant.

**Electrostatic precipitators (ESPs)** remove particulate matter, especially fly ash. They electrically charge particles and attract them to collection plates.

**Baghouse filters** also remove particulates by trapping them in fabric filters.

**Flue gas desulfurization**, commonly called a **scrubber**, removes **SO2**, often using limestone or lime-based materials.

**Selective catalytic reduction (SCR)** reduces **NOx**, usually by converting it to nitrogen and water with ammonia and a reductant.

Students often confuse these devices. Remember the basic matching:

- **ESP / baghouse** → particulates
- **scrubber** → **SO2**
- **SCR** → **NOx**

Also remember the limit of particle-capture devices: they remove direct particles such as fly ash, but they do not directly prevent secondary sulfate or nitrate particles from forming later in the atmosphere unless **SO2** and **NOx** are also reduced.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Coal-fired power plants are major **stationary sources** of air pollution
- Major coal combustion pollutants include **SO2**, **NOx**, **particulate matter**, **mercury**, and **CO2**
- **SO2**, **NOx**, fly ash, and mercury are **primary pollutants** because they are emitted directly
- Sulfates, nitrates, and the acids involved in **acid deposition** are **secondary pollutants**
- High-sulfur coal produces more **SO2** when burned
- **NOx** forms mainly because high-temperature combustion makes nitrogen and oxygen in air react
- Coal combustion contributes to **industrial smog**, which is associated with **SO2** and particulates
- **PM2.5** is more dangerous than **PM10** because smaller particles penetrate deeper into the lungs
- **Fly ash** is fine ash carried in flue gas, while **bottom ash** is heavier ash left in the furnace
- **SO2** and **NOx** contribute to **acid deposition** and secondary particulate formation
- **Electrostatic precipitators** and **baghouses** remove particulates, **scrubbers** remove **SO2**, and **SCR** reduces **NOx**
- **CO2** is a greenhouse gas from coal combustion, but it should not be confused with criteria air pollutants such as **SO2** and particulate matter

## Key Terms and Concepts

- coal combustion pollutants
- stationary source
- sulfur dioxide (SO2)
- primary pollutant
- secondary pollutant
- particulate matter (PM)
- PM2.5
- PM10
- fly ash
- bottom ash
- industrial smog
- acid deposition


Introduction to Air Pollution - Coal combustion pollutants