AP Environmental Science

## Learning Outcomes

- Understand how photochemical smog forms from **NOx**, **VOCs**, and sunlight
- Identify the difference between **primary pollutants** and **secondary pollutants** in smog formation
- Explain why **ground-level ozone** forms and why it is harmful
- Distinguish **tropospheric ozone** from **stratospheric ozone**
- Describe why photochemical smog is most common in hot, sunny urban areas
- Explain how **temperature inversions** and stagnant air worsen smog events
- Apply this content to AP Environmental Science multiple-choice and FRQ questions

## AP Environmental Science Syllabus

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

- Know that photochemical smog forms when **nitrogen oxides (NOx)** and **volatile organic compounds (VOCs)** react in the presence of sunlight
- Identify **NOx** and **VOCs** as major precursor pollutants and **ground-level ozone** as a secondary pollutant
- Explain the role of sunlight in splitting **NO2** and starting ozone formation
- Understand why **VOCs** help ozone build up rather than simply naming them as reactants
- Distinguish **photochemical smog** from **industrial or gray smog**
- Know why photochemical smog is called **brown smog**
- Understand how **temperature inversions**, stagnant air, and urban topography intensify smog episodes
- Explain health and environmental effects, including respiratory irritation, reduced visibility, and plant damage

## 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. Which primary pollutants are needed for photochemical smog to form?
2. Why is ground-level ozone classified as a secondary pollutant?
3. How does sunlight help produce ozone from nitrogen dioxide?
4. Why are hot, sunny cities more likely to experience photochemical smog?
5. How does a temperature inversion worsen a smog event?

## Introduction

Photochemical smog is one of the most tested forms of air pollution in AP Environmental Science because it brings together atmospheric chemistry, fossil fuel combustion, weather, and human health. Students often remember the shorthand formula **NOx + VOCs + sunlight = smog**, but exams usually reward more than that. You need to explain the steps, the conditions, and the effects.

This topic also tests an important AP skill: following a cause-and-effect chain. Vehicle exhaust and industrial emissions release precursor pollutants. Sunlight drives atmospheric reactions. These reactions create new harmful chemicals, especially ground-level ozone. Weather and topography then determine whether the pollution disperses or builds up.

> **Key Term: photochemical smog**
> A type of air pollution formed when nitrogen oxides and volatile organic compounds react in the presence of sunlight, producing ground-level ozone and other secondary pollutants.

## Introduction to the chemistry of photochemical smog

### Primary pollutants, secondary pollutants, and the basic reaction chain

The first step is to separate what is **emitted directly** from what is **formed in the atmosphere**. This is one of the most common AP mistakes.

> **Key Term: primary pollutant**
> A pollutant released directly from a source, such as vehicle exhaust or smokestacks.
>
> **Key Term: secondary pollutant**
> A pollutant formed in the atmosphere when primary pollutants react chemically.

In photochemical smog, the main primary pollutants are:

- **NOx**, especially **NO** and **NO2**
- **VOCs**, including gasoline vapors and other hydrocarbons

The main secondary pollutants include:

- **ground-level ozone (O3)**
- **PANs** (peroxyacyl nitrates)
- sometimes other oxidants such as aldehydes

A useful AP shorthand is:

- **NOx + VOCs + sunlight → photochemical smog**

But that is only the summary. The real process is multi-step.

A core simplified pathway is:

1. Combustion releases **NO** and **NO2**
2. Sunlight breaks apart **NO2**
3. The freed oxygen atom combines with **O2**
4. **O3** forms

The key reactions are:

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

This is why sunlight matters. Without light energy, the oxygen atom is not produced in this way, so ozone formation is much lower.

> **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.
>
> **Test Tip:** If a question asks for pollutants that _cause_ photochemical smog, the best answers are usually **NOx** and **VOCs**, not ozone. Ozone is a product of the process, not a main direct emission.

### Where the precursors come from

For AP exam answers, do not stop at naming the chemicals. You should connect them to sources.

**NOx sources** include:

- motor vehicles
- trucks and buses
- fossil-fuel power plants
- industrial combustion
- boilers

These sources all involve high temperatures, which allow atmospheric nitrogen and oxygen to react.

**VOCs sources** include:

- gasoline vapors
- solvents
- paints
- industrial emissions
- refineries
- gas stations
- some consumer products

Some VOCs also come from natural vegetation, but in urban smog episodes the key AP focus is usually human sources.

This is why photochemical smog is strongly linked to cities. Cities combine large numbers of vehicles, fuel storage, industrial activity, and warm paved surfaces.

### Why ozone does not always build up automatically

Students often think that if NOx is present, ozone simply rises. That is too simple.

There is an important competing reaction:

- **NO + O3 → NO2 + O2**

This reaction destroys ozone. So if only NO and ozone interact, ozone can be removed as fast as it forms.

This is where **VOCs** matter. VOC chemistry helps convert **NO back to NO2** without using up ozone. That means the atmosphere keeps making more **NO2** available for sunlight to split, while not destroying all the ozone that has formed. The result is **net ozone buildup**.

That is the deeper reason VOCs are important. They do not just “join in.” They help keep the reaction cycle going in a way that lets ozone accumulate.

> **Key Term: volatile organic compounds (VOCs)**
> Carbon-containing compounds that evaporate easily and react in the atmosphere with NOx, helping form photochemical smog.

## How and when photochemical smog develops

### The daily pattern of smog formation

Photochemical smog often follows a daily rhythm in urban areas.

In the **early morning**, traffic increases as people commute. Emissions of **NOx** and **VOCs** rise sharply.

As the day gets brighter, more **NO2** absorbs sunlight and splits:

- **NO2 + sunlight → NO + O**

Then:

- **O + O2 → O3**

By the **afternoon**, ozone concentrations often peak because sunlight is strongest and enough precursor pollutants are already in the air.

Later, when the sun gets lower or sets, ozone production slows because the key light-driven reaction weakens or stops.

This daily pattern explains why photochemical smog is often worst in the afternoon, even though many emissions were released earlier.

The AP course often treats this as a sequence:

- morning emissions rise
- sunlight strengthens
- NO2 breaks apart
- ozone accumulates
- evening reduces ozone production

### Why it is called brown smog

Photochemical smog is also called **brown smog** because **NO2** contributes a brownish haze to the air.

This is a very useful distinction because students sometimes confuse it with **gray smog**, which is more closely linked to sulfur pollution and coal combustion.

Photochemical smog is mainly:

- nitrogen-based
- sunlight-driven
- urban
- common in warm seasons

Industrial or gray smog is mainly:

- sulfur-based
- linked to coal burning
- associated with soot and acidic droplets
  > **Key Term: ground-level ozone**
  > Ozone in the troposphere formed by atmospheric reactions involving NOx and VOCs; it is a harmful secondary pollutant.
  >
  > **Key Term: tropospheric ozone**
  > Ozone found in the lowest layer of the atmosphere, where it acts as an air pollutant rather than a protective shield.
  >
  > **Test Tip:** If an AP question contrasts ozone in two places, remember: **stratospheric ozone helps, tropospheric ozone harms**.

### Why hot, sunny urban areas are most at risk

Photochemical smog is most likely in places with:

- lots of **NOx** and **VOCs**
- strong **sunlight**
- **warm temperatures**
- **stagnant air**

Warm, sunny conditions increase the rate of photochemical reactions. Urban areas also contain many dark surfaces, such as asphalt and concrete, that absorb heat and raise local temperatures. This can increase the speed of atmospheric chemical reactions.

Heavy traffic adds large amounts of precursor pollutants. If winds are weak, those pollutants stay nearby instead of dispersing.

Topography also matters. Cities in basins or valleys can trap polluted air. Los Angeles is the classic example because it combines:

- large vehicle emissions
- warm sunny weather
- basin topography
- frequent stagnant conditions

A city does not need all of these conditions to get smog, but the more it has, the stronger the smog event tends to be.

## Atmospheric conditions that make smog worse

### Temperature inversions and stagnant air

Normally, warm air near the surface rises and mixes with the cooler air above. This vertical movement helps disperse pollutants.

A **temperature inversion** reverses that pattern. A layer of warmer air sits above cooler air near the ground. That upper warm layer acts like a lid, trapping pollutants close to the surface.

> **Key Term: temperature inversion**
> An atmospheric condition in which warm air overlies cooler surface air, preventing vertical mixing and trapping pollutants near the ground.

With a temperature inversion:

- NOx stays near the surface
- VOCs stay near the surface
- ozone precursors remain concentrated
- smog gets worse because pollutants are not dispersed

Stagnant air has a similar effect. Even without a strong inversion, if winds are light and the atmosphere is stable, pollutants accumulate in place.

The key exam idea is this: **high emissions alone do not guarantee the worst smog**. Pollutants often become most dangerous when they are trapped.

### Why sunlight, temperature, and trapped pollution work together

It helps to think of smog formation as requiring three parts:

1. **ingredients** — NOx and VOCs
2. **energy source** — sunlight
3. **favorable atmospheric conditions** — warm, stagnant, trapped air

If one is missing, smog is less severe. For example:

- lots of emissions but heavy wind = more dispersal
- lots of sunlight but few emissions = little ozone production
- lots of NOx but little sunlight = weaker photochemical reactions

This is why AP free-response questions often ask for both pollutants and conditions.

A strong answer uses a full chain:

- vehicles and industry emit NOx and VOCs
- sunlight photolyzes NO2
- oxygen atoms form ozone
- VOC chemistry helps ozone accumulate
- stagnant air or inversion traps the pollution
- ozone and other oxidants build into photochemical smog

## Effects of photochemical smog on health and the environments

### Human health effects

Ground-level ozone is the most important harmful component to know. It is a strong respiratory irritant.

It can:

- irritate airways
- worsen asthma
- reduce lung function
- increase coughing
- aggravate bronchitis

**PANs** are also important. They can cause:

- eye irritation
- respiratory discomfort

If an AP question asks for one component of photochemical smog that harms people, **ozone** is usually the safest answer, provided you explain the effect clearly.

### Environmental effects

Photochemical smog affects more than humans.

Ground-level ozone can damage vegetation by:

- reducing photosynthesis
- stressing plants
- lowering productivity
- making plants more vulnerable to pests and disease

PANs can also damage plant tissues and lower crop yields.

Smog also reduces visibility, creating urban haze. That matters environmentally and economically, especially in large cities and tourism areas.

A strong AP answer should connect plant damage to wider ecological effects:

- lower plant productivity means less energy entering food webs
- stressed crops mean lower agricultural yields
- damaged vegetation can alter habitat quality

### A worked FRQ-style explanation

Suppose an FRQ asks: “Explain why photochemical smog forms in a city on a hot sunny day.”

A good response would include all of these steps:

> Vehicles and industry release NOx and VOCs into the air. Sunlight breaks apart NO2 into NO and O. The oxygen atom then combines with O2 to form ozone. VOCs help convert NO back into NO2 without consuming ozone, so ozone builds up. Hot sunny conditions speed these photochemical reactions, and if air is stagnant or trapped by an inversion, the pollutants accumulate near the ground, forming photochemical smog.

That answer is better than simply writing “NOx + VOCs + sunlight make ozone” because it shows mechanism, conditions, and cause-and-effect.

## 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
- The main precursor pollutants are **nitrogen oxides** and **volatile organic compounds**
- **Ground-level ozone** is a **secondary pollutant**, not a direct emission
- Sunlight splits **NO2** into **NO** and **O**, helping start ozone formation
- The key ozone-forming step is **O + O2 → O3**
- **VOCs** help ozone accumulate by converting **NO back to NO2** without using up ozone
- Photochemical smog is called **brown smog** because **NO2** contributes a brown haze
- Smog is most common in **hot, sunny, urban** conditions with many vehicle and industrial emissions
- **Temperature inversions** and stagnant air trap pollutants near the ground and intensify smog episodes
- **Tropospheric ozone** harms health, while **stratospheric ozone** protects life from UV radiation
- Photochemical smog can worsen asthma, irritate eyes and lungs, reduce visibility, and damage crops
- Strong AP answers explain both the chemistry and the atmospheric conditions that let smog build up

## Key Terms and Concepts

- photochemical smog
- primary pollutant
- secondary pollutant
- nitrogen oxides (NOx)
- volatile organic compounds (VOCs)
- ground-level ozone
- tropospheric ozone
- temperature inversion


Photochemical Smog - Photochemical smog formation