AP Environmental Science

## Learning Outcomes

- Explain how **photochemical smog** forms from **NOx**, **VOCs**, and sunlight
- Identify the environmental conditions that increase or reduce smog formation
- Distinguish between **primary pollutants** and **secondary pollutants** in smog
- Understand why urban areas and certain landforms are especially vulnerable
- Apply the ideas of **temperature inversion**, **urban heat islands**, and wind patterns to exam questions
- Avoid common errors such as confusing **tropospheric ozone** with **stratospheric ozone**

## AP Environmental Science Syllabus

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

- Know that photochemical smog forms when **NOx** and **VOCs** react in the presence of **sunlight**
- Identify **NOx** and **VOCs** as major precursor pollutants and **tropospheric ozone** and **PANs** as key secondary pollutants
- Explain how **strong sunlight**, **high temperature**, **stagnant air**, and **low wind** increase smog formation
- Understand how a **temperature inversion** traps pollutants near the ground and worsens smog episodes
- Explain why **valleys**, **basins**, and cities surrounded by mountains are more vulnerable to smog
- Know that **urban heat islands** raise local temperatures and can speed photochemical reactions
- Distinguish between **photochemical smog** and **industrial smog**
- Understand that **ground-level ozone** is harmful, while **stratospheric ozone** is protective

## 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 three ingredients are needed for photochemical smog to form?
2. Why are hot, sunny summer afternoons often the worst time for ozone pollution?
3. How does a temperature inversion increase smog near the ground?
4. Why are places such as the Los Angeles Basin especially vulnerable to smog?
5. What is the difference between a primary pollutant and a secondary pollutant in photochemical smog?

## Introduction

Photochemical smog is one of the most tested air-pollution topics in AP Environmental Science because it connects chemistry, weather, landforms, human activity, and health effects. To answer questions well, you need more than the basic idea that “cars cause smog.” You need to understand why some days are much worse than others, why some cities struggle more than others, and how atmospheric conditions can turn ordinary emissions into a major pollution event.

Photochemical smog is especially associated with warm, sunny urban environments. Vehicle exhaust, fuel evaporation, industrial emissions, and atmospheric conditions combine to produce a brownish haze rich in **ground-level ozone** and other oxidants. The exam often asks you to trace the cause-and-effect chain from emissions to weather conditions to human and environmental impacts.

> **Key Term: photochemical smog**
> A form of air pollution produced when sunlight drives reactions between nitrogen oxides (NOx) and volatile organic compounds (VOCs), forming ground-level ozone and other secondary pollutants.

### How photochemical smog forms and why weather matters

Photochemical smog is not emitted directly. It must be produced in the atmosphere from precursor pollutants. The main starting materials are **nitrogen oxides** and **volatile organic compounds**. These are released largely by motor vehicles, power generation, industry, fuel evaporation, and some solvents.

> **Key Term: primary pollutant**
> A pollutant emitted directly from a source, such as NOx or VOCs from vehicles and industry.
>
> **Key Term: secondary pollutant**
> A pollutant formed in the atmosphere by chemical reactions, such as ozone or PANs in photochemical smog.

The key APES shorthand is:

NOx + VOCs + sunlight → photochemical smog

A simplified ozone sequence is:

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

This matters because it shows that **sunlight is not just helpful but necessary**. Without enough solar energy, especially UV radiation, nitrogen dioxide is not split efficiently and ozone does not build up in the same way.

Why does this usually peak later in the day rather than during morning rush hour? Morning traffic produces a lot of NOx and VOCs, but ozone needs time and sunlight intensity to build. As the day warms and sunlight becomes stronger, the chemistry speeds up. That is why ozone concentrations are often highest on **summer afternoons**, not early mornings.

VOCs also matter in a way students often miss. They do not simply “sit there” as extra pollutants. They participate in reaction pathways that help ozone accumulate rather than being quickly removed. So if an FRQ asks why vehicle-heavy cities have worse smog, mention not just NOx but also fuel vapors and VOC emissions.

> **Key Term: tropospheric ozone**
> Ground-level ozone in the lower atmosphere; a harmful secondary pollutant formed from NOx, VOCs, and sunlight.

A common mistake is to say ozone itself is emitted by cars. It is not. Cars emit precursor pollutants. Ozone forms later in the air.

> **Test Tip:** If a multiple-choice question asks which pollutant in smog is secondary, the safe answer is usually **ozone** or **PANs**, not NOx or VOCs.

### Sunlight, temperature, and the urban heat island effect

The two most important environmental drivers of photochemical smog are **sunlight** and **temperature**. These work together, but they are not identical factors.

First, **strong sunlight** powers the chemical reactions that create ozone and related oxidants. This is why photochemical smog is often called **summer smog** or **brown smog**. Clear skies matter because clouds reduce incoming solar radiation. Under stable high-pressure conditions, skies are often clearer, so more sunlight reaches the polluted air mass.

Second, **high temperature** speeds reaction rates. In general, many chemical reactions proceed faster when molecules are moving faster, which happens at higher temperatures. Hot days therefore help ozone form more quickly and in larger amounts. Temperature can also increase VOC emissions, especially from fuel evaporation, solvents, and heated urban surfaces. In other words, hot weather does not only speed the reactions; it can also increase the supply of raw materials.

This is where the **urban heat island** effect becomes important. Cities are often warmer than surrounding rural areas because concrete, asphalt, and buildings absorb and reradiate heat. There is also less vegetation to cool the area by shade and evapotranspiration.

> **Key Term: urban heat island**
> A condition in which urban areas are warmer than nearby rural areas because built surfaces absorb and reradiate heat.

Why does this worsen smog? The heat island raises local temperature, which increases photochemical reaction rates and can support stronger ozone formation. It can also increase energy demand for air conditioning, which may indirectly increase emissions if electricity comes from fossil-fuel combustion. So the city becomes both a stronger emitter and a better chemical setting for smog.

Students sometimes overemphasise humidity. Humidity can affect haze and visibility, but for APES photochemical smog, it is not the main driver. The strongest factors are sunlight, heat, precursor concentration, and poor mixing.

A useful comparison:

- **Photochemical smog**: warm, sunny, ozone-rich, NOx + VOC driven
- **Industrial smog**: more associated with coal burning, sulfur dioxide, particulates, and cooler, humid conditions

That distinction appears often in multiple-choice questions.

### Wind, stagnant air, inversions, and topography

Even if a city emits lots of NOx and VOCs, severe smog does not automatically form. The pollutants must remain concentrated long enough for the chemistry to occur. That is why **stagnant air** and **low wind speed** are major environmental factors.

> **Key Term: stagnant air**
> Air conditions with little movement or mixing, allowing pollutants to accumulate near the surface.

Wind has a double effect. If winds are strong enough, they disperse pollutants and reduce local smog buildup. But this does not make the pollution disappear. It can move ozone and precursor pollutants downwind, spreading the problem elsewhere. That is why rural or suburban areas downwind of cities can still experience ozone pollution.

Low wind, by contrast, lets pollutants remain in place. Concentrations of NOx and VOCs stay high, so ozone formation continues. APES questions may describe “clear skies, hot temperatures, and little wind” as the classic setup for a smog event.

Vertical mixing matters too. Under normal conditions, warm surface air rises and mixes with cooler air above, helping disperse pollutants. But a **temperature inversion** reverses this pattern.

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

A temperature inversion acts like a lid over the city. Polluted air near the surface cannot rise and disperse, so NOx, VOCs, particulates, and ozone remain trapped where people breathe them. This can greatly increase smog intensity and health risks.

The cause-and-effect chain is important:

1. Surface air becomes cooler than air above
2. Warm air sits over the cooler air
3. Vertical air mixing is reduced
4. Pollutants accumulate near the ground
5. Smog concentrations increase

Topography can make this even worse. Cities in **valleys**, **basins**, or locations surrounded by mountains often have reduced air circulation. Mountains can block horizontal airflow and allow polluted air to collect. That is why basin cities are classically vulnerable.

Los Angeles is the most common example. It combines:

- high vehicle dependence
- abundant sunlight
- warm temperatures
- basin topography
- frequent stagnant-air episodes
- temperature inversions
- urban heat island effects

All of these reinforce each other. The same emissions in a cooler, windier, flatter place would usually produce less severe smog.

> **Test Tip:** For FRQs, build a chain: **vehicle emissions → NOx and VOCs accumulate → hot sunny weather speeds reactions → inversion/topography traps air → ozone rises → respiratory harm and plant damage**.

### Why smog peaks in cities and what it does to health and natures

Urban areas are especially vulnerable because they combine the two things photochemical smog needs most: lots of precursor emissions and favorable environmental conditions. Large numbers of cars and trucks release NOx. Fuel handling, solvents, paints, industrial emissions, and gasoline evaporation release VOCs. Add strong sunlight and high temperatures, and the chemistry can proceed rapidly.

But severe urban smog is not only a “city center” issue. In some cases, fresh NO emissions near the immediate source can also react with ozone. Farther downwind, where the pollution plume has aged and mixed in sunlight, ozone may peak. That is one reason regional smog problems can spread beyond the downtown core.

Another major secondary pollutant in photochemical smog is **PANs**.

> **Key Term: PANs (peroxyacyl nitrates)**
> Secondary pollutants formed from reactions involving NOx and VOCs in sunlight; they are strong eye and respiratory irritants and can damage plants.

PANs matter for the exam because they link air pollution to both human health and crop damage. They can persist long enough to travel away from their source area, which means pollution can affect downwind environmentss.

Human health effects of photochemical smog include:

- eye irritation
- throat irritation
- coughing
- chest tightness
- worsened asthma
- reduced lung function
- increased risk for people with existing respiratory disease

Ground-level ozone is especially important because it inflames lung tissue. It does not protect us like stratospheric ozone. Near the surface, it is harmful.

System-wide effects include:

- reduced photosynthesis
- plant tissue damage
- lower crop yields
- forest stress
- weaker resistance to disease and pests

This is why ozone pollution is both a public-health issue and an agricultural issue. A student who only discusses asthma is missing half the picture.

The AP exam may also ask why some mitigation steps work. Limiting vehicle emissions, reducing VOC losses from fuels and solvents, and reducing urban heat island intensity can all help. So can traffic restrictions, better public transport, and measures that reduce temperature buildup such as planting trees and using reflective surfaces.

## 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**, not a direct emission
- The simplified ozone pathway is **NO2 + sunlight → NO + O**, followed by **O + O2 → O3**
- **Strong sunlight** is essential because it drives the reactions that form ozone
- **High temperatures** speed photochemical reactions and can increase VOC emissions
- **Stagnant air** and **low wind** allow precursor pollutants to accumulate and smog to build
- A **temperature inversion** traps polluted air near the ground by preventing vertical mixing
- **Valleys**, **basins**, and mountain-surrounded cities are more prone to smog because air circulation is limited
- **Urban heat islands** worsen smog by raising local temperatures and supporting faster ozone formation
- **PANs** are secondary pollutants in photochemical smog and can irritate eyes and damage vegetation
- Photochemical smog is linked with **warm, sunny conditions**, unlike **industrial smog**, which is more associated with **SO2**, particulates, and cooler humid air
- **Tropospheric ozone** is harmful, while **stratospheric ozone** is protective

## Key Terms and Concepts

- photochemical smog
- primary pollutant
- secondary pollutant
- tropospheric ozone
- urban heat island
- stagnant air
- temperature inversion
- PANs (peroxyacyl nitrates)


Photochemical Smog - Environmental factors affecting smog