## Learning Outcomes

- Explain why ground-level ozone is usually lowest in the early morning and highest later in the day
- Identify the roles of **NOx**, **VOCs**, and sunlight in the daily ozone cycle
- Distinguish between primary pollutants and the secondary pollutant ozone
- Describe how morning traffic can both contribute to and temporarily reduce ozone levels
- Interpret an hourly ozone graph using photochemical smog chemistry
- Predict how weather conditions such as heat, sunlight, and stagnant air affect ozone buildup

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand the daily ozone concentration pattern within photochemical smog, with a focus on the following syllabus points:

- Know that ground-level ozone is a **secondary pollutant** formed from **NOx** and **VOCs** in sunlight
- Identify the typical daily pattern: low near dawn, rising after sunrise, afternoon peak, and nighttime decline
- Explain how fresh **NO** from morning traffic can lower ozone by **titration**
- Understand that hot, sunny, and stagnant conditions increase ozone formation
- Distinguish between harmful **tropospheric ozone** and protective **stratospheric ozone**
- Interpret graphs showing hourly ozone concentration changes across a day
- Explain why the highest ozone may occur later in the day and sometimes downwind of the main emission source

## 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 ground-level ozone often low during the morning rush hour even though traffic releases ozone precursors?
2. What three ingredients are needed for strong photochemical smog formation?
3. Why does ozone concentration usually peak in the afternoon rather than before sunrise?
4. How does ozone concentration usually change after sunset?
5. What is the difference between tropospheric ozone and stratospheric ozone?

## Introduction

Photochemical smog is one of the most tested forms of air pollution in AP Environmental Science because it links chemistry, weather, human activity, and health effects. The daily ozone concentration pattern is especially high-yield because exam questions often ask you to explain why ozone changes over the course of a day, not just how it forms in general.

The key idea is that ground-level ozone does not come straight from a tailpipe or smokestack. Instead, it forms in the air as sunlight acts on nitrogen oxides and volatile organic compounds. Because sunlight changes through the day, ozone also follows a daily cycle.

> **Key Term: ground-level ozone**
> Ozone in the troposphere that forms near Earth’s surface and acts as a harmful air pollutant.

### Ozone as a secondary pollutant

To understand the daily pattern, start with a distinction students often miss: the pollutants emitted in the morning are not the same as the pollutant measured later in the day.

Cars, trucks, power plants, and some industrial sources directly emit **nitrogen oxides (NOx)** and **volatile organic compounds (VOCs)**. These are **primary pollutants** because they are released directly. Ozone is different. It is produced later by atmospheric reactions, so it is a **secondary pollutant**.

> **Key Term: secondary pollutant**
> A pollutant that is not emitted directly but forms in the atmosphere from reactions involving primary pollutants.
>
> **Key Term: nitrogen oxides (NOx)**
> A group of nitrogen-oxygen gases, mainly nitric oxide (NO) and nitrogen dioxide (NO₂), produced largely by high-temperature combustion.
>
> **Key Term: volatile organic compounds (VOCs)**
> Carbon-based compounds that evaporate easily and react in the atmosphere, helping form ozone and photochemical smog.

This matters for exam reasoning. If a question asks what is emitted during the morning commute, the answer is not ozone. The correct answer is usually **NOx** and **VOCs**. If the question asks what builds up later as a result of those emissions plus sunlight, the answer is ozone.

A useful APES shorthand is:

- **NOx + VOCs + sunlight → ozone**

That shorthand is accurate enough for many exam questions, but you also need to know why the pattern changes hour by hour.

### The daily ozone cycle: from dawn to night

The daily ozone concentration pattern is called a **diurnal ozone cycle**. In most urban settings, ozone is:

- relatively low around dawn and early morning
- increasing through late morning
- highest in the afternoon or late afternoon
- decreasing during the evening and overnight
  > **Key Term: diurnal ozone cycle**
  > The daily rise and fall of surface ozone concentration over a 24-hour period.

This pattern is strongly tied to sunlight. Sunlight is weakest at night and strongest during the middle of the day, so the atmospheric chemistry that makes ozone is also weakest at night and strongest in daytime.

A simplified APES-style timeline looks like this:

- **Early morning:** ozone low
- **Morning to late morning:** ozone rises
- **Afternoon:** ozone peaks
- **Night:** ozone falls

The textbook sequence supports this. Morning traffic increases NOx and VOC emissions. As sunlight intensifies, nitrogen dioxide breaks down and ozone forms. By late afternoon, ozone production stops once sunlight fades.

The chemistry behind the rise includes these reactions:

- **NO₂ + UV light → NO + O**
- **O + O₂ → O₃**

These reactions explain why strong sunlight is necessary. Without light energy, nitrogen dioxide does not split to release the oxygen atom needed for ozone formation.

> **Test Tip:** On graph questions, first identify the time of day. If the line peaks in the afternoon, that strongly suggests photochemical ozone formation rather than direct pollutant release.

### Why morning ozone can be low even during heavy traffic

This is one of the most tested ideas in this subtopic because it seems counterintuitive. Students often think ozone should be highest during rush hour since that is when emissions are highest. But local ozone is often relatively low in early morning urban air.

The reason is **ozone titration**.

Freshly emitted **nitric oxide (NO)** from vehicle exhaust can react with ozone and remove it:

- **NO + O₃ → NO₂ + O₂**

This reaction lowers ozone concentration near roads and dense traffic corridors. So the same traffic that supplies the raw materials for later ozone production can temporarily reduce local ozone in the short term.

> **Key Term: ozone titration**
> The removal of ozone by reaction with freshly emitted nitric oxide, especially in early morning urban air.

This is a strong example of cause and effect:

1. Morning traffic emits a lot of **NO** and **VOCs**
2. Fresh **NO** reacts with existing ozone
3. Ozone concentration near the source stays low or even drops
4. Later, as sunlight becomes stronger, photochemical reactions produce net ozone
5. Ozone then rises through the day

This is why APES questions may say that traffic both contributes to smog and suppresses ozone at first. Both can be true, depending on timing.

A common mistake is to treat all nitrogen oxides as doing exactly the same thing at every moment. In reality, **NO** can destroy ozone directly, while **NO₂** in sunlight can help generate ozone.

> **Test Tip:** If an exam question asks why ozone is not highest during rush hour, mention both weaker early sunlight and ozone titration by fresh NO.

### Why ozone peaks in the afternoon

By late morning and afternoon, several things come together to raise ozone levels.

First, sunlight is much stronger, so the reactions that split **NO₂** and form ozone proceed faster. Second, morning emissions of NOx and VOCs have had time to mix, react, and accumulate. Third, warmer temperatures often speed the chemistry involved in photochemical smog. Fourth, stagnant air can prevent pollutants from dispersing.

This means the afternoon ozone peak is not caused by one single factor. It is the result of:

- precursor emissions earlier in the day
- several hours of sunlight-driven chemistry
- heat that favors reaction rates
- weak air movement that allows buildup

The result is a late-day maximum in surface ozone concentration. In some locations, especially places downwind from city centers, the peak may occur even later because the polluted air mass has to travel and age before ozone concentration reaches its maximum.

This is an important distinction. The greatest emissions do not always match the greatest ozone concentration. Near the source, fresh NO may suppress ozone. Farther downwind, after time in sunlight, ozone can become higher.

> **Key Term: ozone precursors**
> Pollutants such as NOx and VOCs that react in the atmosphere to produce ozone.

NOx and VOCs also help produce other smog components. For example, reactions involving VOCs can form **peroxyacyl nitrates (PANs)**, which are eye and respiratory irritants. That is why photochemical smog is not just “ozone only,” even though ozone is its major ingredient.

### Nighttime decline and the role of weather

After sunset, ozone production slows sharply because there is no longer enough sunlight to drive the key photochemical reactions. That alone causes ozone to stop increasing, but nighttime decline happens for more than one reason.

Ozone can be removed at night by:

- continued reaction with **NO**
- deposition onto surfaces such as soil, plants, and buildings
- lack of new photochemical production

This means nighttime ozone concentration usually falls until near dawn, when the next cycle begins.

> **Key Term: deposition**
> The process by which a pollutant is removed from the air by settling or reacting onto surfaces such as vegetation, water, soil, or buildings.

Weather controls how strong the daily pattern becomes. The most severe ozone days usually happen when conditions are:

- **hot**
- **sunny**
- **stagnant or calm**

Heat and sunlight favor ozone chemistry, while stagnant air allows pollutants to remain concentrated instead of dispersing.

By contrast, cloudy, cool, or windy conditions usually reduce ozone formation or spread pollutants out enough to lower local concentration.

A useful comparison is:

- **Hot, sunny, stagnant day:** strong afternoon ozone peak
- **Cool, cloudy, windy day:** weaker ozone formation and lower peak

Students should also avoid confusing local smog chemistry with global climate patterns. Tropospheric ozone is a short-lived greenhouse gas, but its main APES relevance here is as a local pollutant and part of urban smog.

### How to interpret an ozone graph and answer FRQs

A common APES graph for this topic shows time on the x-axis and ozone concentration on the y-axis. To interpret it well, describe the pattern first, then explain it with science.

For example, suppose a graph shows:

- low ozone at 6 a.m.
- rising ozone from 9 a.m. to 2 p.m.
- peak at 4 p.m.
- declining ozone after sunset

A good explanation would be:

1. Early morning ozone is low because sunlight is weak and fresh NO from traffic removes ozone by titration.
2. As sunlight becomes stronger, NO₂ breaks down and ozone forms.
3. Morning emissions of NOx and VOCs continue reacting through the day.
4. Ozone peaks in the afternoon after hours of sunlight-driven chemistry and buildup.
5. Ozone declines at night because sunlight is absent and removal continues through deposition and reaction.

That structure turns description into analysis.

Here is a worked exam-style example.

A city records the following surface ozone concentrations:

- 6 a.m.: 18 ppb
- 10 a.m.: 35 ppb
- 2 p.m.: 62 ppb
- 5 p.m.: 70 ppb
- 10 p.m.: 28 ppb

How should you explain the 5 p.m. peak?

- Morning traffic emitted **NOx** and **VOCs**
- Early in the day, fresh **NO** reduced ozone by titration
- Strong afternoon sunlight increased photochemical ozone production
- Ozone and its precursors accumulated through the day
- By 5 p.m., the air mass had experienced enough sunlight-driven chemistry for ozone to reach its maximum

This is the kind of chain of reasoning APES rewards.

Another major distinction to mention when relevant:

- **Stratospheric ozone:** protective, absorbs harmful UV
- **Tropospheric ozone:** harmful, part of photochemical smog
  > **Key Term: stratospheric ozone**
  > Ozone high in the stratosphere that absorbs ultraviolet radiation and protects life on Earth.
  >
  > **Key Term: tropospheric ozone**
  > Ozone in the lower atmosphere that acts as a pollutant and major component of photochemical smog.

If a multiple-choice option says smog forms from stratospheric ozone, eliminate it. Smog involves **tropospheric** ozone.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Ground-level ozone is a **secondary pollutant**, not a directly emitted one
- The main ozone precursors are **NOx** and **VOCs**
- Sunlight is essential for strong photochemical ozone formation
- Daily ozone concentration is usually lowest in the early morning
- Fresh **NO** from traffic can reduce ozone by **titration**
- Ozone usually rises through late morning and peaks in the afternoon
- Hot, sunny, stagnant conditions favor the highest ozone concentrations
- Ozone production slows after sunset because sunlight-driven reactions stop
- Nighttime ozone declines because of reduced production, deposition, and reaction with NO
- The highest ozone is not always at the main emission source; it may occur downwind
- **Tropospheric ozone** is harmful air pollution, while **stratospheric ozone** is protective
- Graph questions should be answered with both pattern description and chemical explanation

## Key Terms and Concepts

- ground-level ozone
- secondary pollutant
- nitrogen oxides (NOx)
- volatile organic compounds (VOCs)
- diurnal ozone cycle
- ozone titration
- ozone precursors
- deposition
- stratospheric ozone
- tropospheric ozone


Photochemical Smog - Daily ozone concentration pattern