AP Environmental Science

## Learning Outcomes

- Explain why reducing **NOx** and **VOCs** lowers photochemical smog formation
- Distinguish between **primary pollutants** that are emitted directly and **secondary pollutants** that form in air
- Identify major mobile and stationary sources of smog precursors
- Describe how catalytic converters, vapor recovery systems, and inspection programs reduce emissions
- Compare direct emission-control policies with indirect strategies such as fuel-economy standards
- Apply this content to AP Environmental Science free-response and multiple-choice questions

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand reducing photochemical smog precursors, with a focus on the following syllabus points:

- Know that **ground-level ozone** is a **secondary pollutant** formed from **NOx**, **VOCs**, and sunlight
- Identify major precursor sources, including vehicles, power plants, industry, solvents, consumer products, and gasoline vapors
- Explain how reducing **NOx** and **VOC** emissions reduces formation of **ozone** and **PANs**
- Understand the role of **catalytic converters** in lowering vehicle emissions of NOx and hydrocarbons
- Identify **vapor recovery** methods used during refueling and fuel handling to reduce VOC emissions
- Know that the **Clean Air Act (1963)** authorizes national air pollution control for mobile and stationary sources
- Understand the difference between direct smog-control tools and indirect measures such as **CAFE standards**
- Recognise that sunlight is required for photochemical smog formation, but sunlight is not itself a pollutant

## 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. Why is reducing ozone directly usually not the main strategy for reducing photochemical smog?
2. What are the two main precursor groups that must be controlled to reduce photochemical smog?
3. How does a catalytic converter reduce smog-forming emissions from vehicles?
4. What is the purpose of a vapor recovery system at a gas pump or in a vehicle fuel system?
5. Why are mobile-source controls often central to smog reduction plans?

## Introduction

Photochemical smog is one of the most testable forms of air pollution in AP Environmental Science because it links chemistry, human activities, health effects, and environmental policy. Students often remember that smog contains ozone, but they miss the more important exam point: **smog is reduced by cutting the pollutants that create ozone**, not by trying to "remove ozone" after it has formed.

This matters because AP questions often ask for methods to reduce photochemical smog, and the strongest answers name **nitrogen oxides** and **volatile organic compounds** as the main targets. If you can explain where those pollutants come from and how specific controls reduce them, you can answer both content and application questions well.

> **Key Term: photochemical smog**
> A form of air pollution, often called brown smog, formed when nitrogen oxides and volatile organic compounds react in the presence of sunlight to produce ground-level ozone and other oxidants.

### Why reducing precursors works

Photochemical smog forms through atmospheric reactions. The basic sequence is that nitrogen dioxide absorbs sunlight, splits, and helps create ozone:

- **NO₂ + sunlight → NO + O**
- **O + O₂ → O₃**

That looks simple, but the crucial extra step is the role of VOCs. Without VOC chemistry, ozone production is more limited because nitric oxide can react back with ozone. VOC reactions help convert NO back into NO₂ in ways that allow **net ozone buildup**. So the AP-level shorthand is:

- **NOx + VOCs + sunlight → O₃**

This is why reducing smog means reducing **precursors**. Ozone is not usually emitted from a smokestack or tailpipe as the main pollutant. It forms later in the troposphere.

> **Key Term: ozone precursor**
> A pollutant that contributes to the formation of ground-level ozone, especially nitrogen oxides and volatile organic compounds.
>
> **Key Term: secondary pollutant**
> A pollutant that is not emitted directly but forms in the atmosphere through reactions involving primary pollutants.

A common student error is to write, "use less ozone." That is too vague and chemically wrong. Better wording is: "reduce **NOx** and **VOC** emissions so less ground-level ozone forms."

Cause-and-effect chain:

1. Vehicles, industry, and fuel handling release NOx and VOCs.
2. Sunlight drives reactions among these pollutants.
3. Ground-level ozone and **PANs** form.
4. Smog harms lungs, crops, and visibility.
5. Therefore, lowering precursor emissions lowers smog formation.
   > **Key Term: PANs**
   > Peroxyacyl nitrates, secondary pollutants formed in photochemical smog that irritate eyes and lungs and damage plants.

There is also a distinction students must keep clear:

- **Stratospheric ozone** protects life from UV radiation
- **Tropospheric ozone** is harmful and is a major part of smog

That contrast appears often in multiple-choice questions.

> **Test Tip:** If a question asks how to reduce photochemical smog, name **NOx** and **VOCs** first. If you only say "reduce ozone," you may miss the core science point.

### Major sources of NOx and VOCs

To explain reduction methods well, you need to know where the precursors come from. In APES, these are usually grouped into **mobile sources** and **stationary sources**.

#### Mobile sources

Mobile sources are vehicles such as cars and trucks. They are especially important in urban smog because they emit both major precursor types.

- **NOx** forms in engines when nitrogen and oxygen in air react at high combustion temperatures
- **VOCs** come from unburned hydrocarbons in exhaust and from gasoline vapors escaping during refueling, storage, and transport

This is why photochemical smog is strongly associated with large cities, warm sunny days, and heavy traffic.

#### Stationary sources

Stationary sources include power plants, refineries, and industrial facilities.

- Combustion at power plants and industrial sites produces **NOx**
- Refineries, chemical plants, solvents, paints, and industrial products release **VOCs**
- Fuel storage tanks and transfer systems can also leak VOCs

Students sometimes think VOCs are only from car exhaust. That is incomplete. A stronger answer includes:

- gasoline vapors
- solvents
- paints
- consumer products
- industrial emissions

Some VOCs are natural, including compounds released by trees, but AP exam questions on smog reduction usually focus on **anthropogenic** sources that can be controlled.

> **Key Term: nitrogen oxides (NOx)**
> A group of nitrogen-oxygen gases, especially NO and NO₂, produced mainly by high-temperature combustion and important in smog formation.
>
> **Key Term: volatile organic compounds (VOCs)**
> Carbon-containing compounds that evaporate easily and react in the atmosphere to help form ground-level ozone and other smog pollutants.

#### Why cities are especially vulnerable

Urban areas often have:

- dense traffic
- many fuel stations and storage systems
- industrial emissions
- heat-absorbing surfaces that raise temperature
- weather conditions that can trap pollution

Higher temperatures speed reaction rates. Light winds and inversions can keep pollutants concentrated near the ground. So even though sunlight is required, the real policy target remains precursor emissions.

### Reducing mobile-source precursors

Because motor vehicles are a major source of both NOx and VOCs, many smog-control strategies focus on transport.

#### Catalytic converters

A catalytic converter is one of the highest-yield answers for APES. It reduces harmful emissions from vehicle exhaust by promoting oxidation and reduction reactions.

A modern three-way converter helps do three things:

1. Convert **CO** to **CO₂**
2. Convert unburned hydrocarbons to **CO₂** and **H₂O**
3. Reduce **NOx** to **N₂** and **O₂**

For smog reduction, the most important parts are that catalytic converters lower:

- **hydrocarbons/VOCs**
- **NOx**

That directly reduces ozone precursor emissions.

> **Key Term: catalytic converter**
> A vehicle emission-control device that reduces harmful exhaust pollutants, including nitrogen oxides and hydrocarbons, through catalyzed chemical reactions.

Why this matters for smog:

- less NOx leaves the tailpipe
- less unburned hydrocarbon leaves the tailpipe
- therefore less ozone can form later in the atmosphere

A subtle point: catalytic converters do **not** solve every pollution problem. They do not eliminate CO₂ from fuel combustion. So if an FRQ asks for a limitation, you can say they reduce smog precursors but do not remove the greenhouse gas issue.

#### Inspection and maintenance programs

Some vehicles emit far more pollution than others because of damaged engines, faulty exhaust controls, or poor maintenance. **Inspection and maintenance (I/M) programs** identify these high-emitting vehicles and require repairs.

This reduces smog because a small fraction of badly functioning vehicles can contribute a large share of emissions. In policy terms, this is efficient because it targets the dirtiest vehicles first.

#### Fuel economy and driving behavior

Higher fuel efficiency generally means less fuel is burned for the same distance travelled. Burning less fuel tends to lower total precursor emissions.

This is where **CAFE standards** come in. They are not the main direct smog-control rule, but they help indirectly.

> **Key Term: Corporate Average Fuel Economy (CAFE) standards**
> U.S. fuel-economy standards for vehicles that can indirectly reduce air pollution by lowering fuel use.

Be careful with wording. CAFE standards are mainly an energy and fuel-efficiency policy, not the main direct ozone-control system. A precise answer would say:

- direct smog reduction: emission standards, catalytic converters, I/M programs
- indirect smog reduction: better fuel economy, less driving, less idling

Other practical mobile-source measures include:

- carpooling
- public transport
- reducing idling
- maintaining engines properly
- switching to cleaner vehicle technologies where appropriate
  > **Test Tip:** On FRQs, the strongest transport answer is often "catalytic converters reduce NOx and hydrocarbons from vehicle exhaust." That is more precise than just saying "cleaner cars."

### Reducing VOC emissions from fuel handling and products

Students often focus only on tailpipes and forget that VOCs escape when gasoline evaporates. That matters because VOC control is essential in smog reduction.

#### Vapor recovery systems

When gasoline is pumped into a car, vapors can escape into the air. These vapors are VOCs. Vapor recovery systems capture them before they disperse.

There are two major forms students should know:

- **Stage II vapor recovery** at gas stations
- **ORVR** in vehicles
  > **Key Term: vapor recovery system**
  > A system that captures gasoline vapors during storage or refueling, reducing VOC emissions.
  >
  > **Key Term: onboard refueling vapor recovery (ORVR)**
  > A vehicle-based system that captures gasoline vapors during refueling to reduce VOC emissions.

Historically, many gas stations used **Stage II vapor recovery nozzles**. Modern vehicles increasingly use **ORVR**, which captures vapors in the vehicle itself. In **2012**, EPA determined ORVR was in widespread use, allowing the federal Stage II requirement to be waived in many places, though states still had to revise their implementation plans before removing those systems.

This detail matters because it shows control methods can change with technology. The exam is unlikely to demand every regulatory detail, but it is useful to know that:

- older systems often captured vapors at the pump
- newer systems commonly capture vapors in the car

#### Low-VOC products and leak prevention

VOC emissions also come from:

- solvents
- paints
- chemical products
- fuel storage and transfer leaks

So reduction methods include:

- using low-VOC formulations
- sealing storage tanks
- repairing leaks
- controlling evaporative losses in industrial systems

This is a good way to strengthen an FRQ answer beyond "cars cause smog." It shows that VOCs come from more than exhaust.

### Comparing direct and indirect smog-control strategies

A strong APES answer often compares methods.

| Strategy                    | Main target            | Direct or indirect effect on smog? |
| --------------------------- | ---------------------- | ---------------------------------- |
| Catalytic converter         | NOx and hydrocarbons   | Direct                             |
| I/M program                 | High-emitting vehicles | Direct                             |
| Vapor recovery              | Gasoline vapors/VOCs   | Direct                             |
| Emission limits on industry | NOx and VOCs           | Direct                             |
| CAFE standards              | Fuel use               | Indirect                           |
| Public transit/carpooling   | Vehicle use            | Indirect                           |

This comparison matters because students often over-credit indirect policies. If asked for the **best direct way** to reduce photochemical smog, cutting **NOx** and **VOCs** at emission points is the clearest answer.

### Laws, standards, and exam application

The major law to connect to precursor control is the **Clean Air Act (1963)**.

It allows regulation of:

- mobile sources such as cars and trucks
- stationary sources such as power plants and industry
- national air quality standards
- enforcement of emission reductions

You should also know that EPA’s current **ozone NAAQS** is **0.070 ppm** as an 8-hour standard. That standard concerns ambient ozone concentration, but control efforts still focus on reducing precursor emissions.

A sophisticated distinction is that some locations are more **VOC-limited** and others more **NOx-limited**, so the most effective control can vary by place. For APES, you usually do not need full atmospheric modeling, but you can state cautiously that precursor control effectiveness may vary by local conditions.

Worked application example:

A city has frequent summer smog. Major sources are:

- heavy vehicle traffic
- fuel vapors at service stations
- nearby industrial solvent use

Best reduction plan:

1. require catalytic converters and I/M checks for vehicles
2. reduce idling and promote public transit
3. capture gasoline vapors through ORVR and vapor recovery systems
4. require low-VOC solvents and leak control in industry

Why this works:

- steps 1 and 2 reduce NOx and exhaust hydrocarbons
- step 3 reduces evaporative VOCs
- step 4 lowers non-vehicle VOC sources
- all four reduce ozone precursor supply, so less smog forms in sunlight

That chain of reasoning is what AP graders want to see.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Photochemical smog forms when **NOx** and **VOCs** react in the presence of sunlight
- Ground-level ozone is a **secondary pollutant** and is not emitted directly
- Reducing smog means reducing **precursor emissions**, especially NOx and VOCs
- Major mobile-source precursors come from vehicle exhaust and gasoline vapors
- Major stationary-source precursors come from combustion, industry, solvents, and fuel storage
- Catalytic converters reduce **NOx** and hydrocarbons from vehicle exhaust
- Inspection and maintenance programs reduce emissions from malfunctioning high-emitting vehicles
- Vapor recovery systems and **ORVR** reduce VOC emissions during refueling
- **CAFE standards** reduce precursor emissions indirectly by reducing fuel use
- The **Clean Air Act (1963)** provides the main national framework for controlling smog precursors
- **PANs** are secondary pollutants formed in photochemical smog and can irritate eyes and lungs
- Strong AP answers identify specific precursor controls rather than vaguely saying "reduce ozone"

## Key Terms and Concepts

- photochemical smog
- ozone precursor
- secondary pollutant
- PANs
- nitrogen oxides (NOx)
- volatile organic compounds (VOCs)
- catalytic converter
- Corporate Average Fuel Economy (CAFE) standards
- vapor recovery system
- onboard refueling vapor recovery (ORVR)


Photochemical Smog - Reducing smog precursors