Topic 7.2 Notes – Photochemical Smog

What Photochemical Smog Is

Photochemical smog is a mixture of secondary pollutants that forms in the troposphere when nitrogen oxides and VOCs react in sunlight and heat.

A quick reminder helps here:

• Primary pollutants are emitted directly into the air.
• Secondary pollutants form in the atmosphere after reactions happen.

Photochemical smog matters because it is not emitted fully formed. Cars do not release “smog” or ozone directly. They release the ingredients.

The core relationship is:

$$\text{NO}_x + \text{VOCs} + \text{heat} + \text{sunlight} \rightarrow \text{ground-level ozone} + \text{other photochemical oxidants}$$

You may also hear it called brown smog or Los Angeles-type smog. The brown haze is partly due to nitrogen dioxide, and the photo below shows the kind of urban haze that gave Los Angeles-type smog its name.

Los Angeles smog over an urban basin

Keep it separate from gray industrial smog, which comes more from coal burning, sulfur dioxide, particulates, and cool humid air.

One AP trap shows up all the time. Ground-level ozone is harmful pollution, but stratospheric ozone is the protective ozone layer. Same molecule, different place, different effect.

The Main Precursors and Products

Nitrogen oxides

NOx means nitrogen oxides, especially NO and NO₂.

They form mainly during high-temperature combustion, so big human sources are:

• Motor vehicles
• Fossil-fuel power plants
• Industrial boilers
• Off-road engines

Nitrogen oxides are often highest early in the day, especially during morning traffic.

Volatile organic compounds

VOCs are carbon-containing compounds that evaporate or sublime at room temperature.

Illustrative examples you should know:

• Formaldehyde
• Gasoline

Major human sources include:

• Gasoline vapors
• Vehicle exhaust
• Solvents, paints, and adhesives
• Industrial emissions

A natural source also matters on tests. Trees and other vegetation release VOCs too.

Main products

The atmosphere turns those precursors into:

• Ground-level ozone
• Nitrogen dioxide
• Aldehydes and other oxidized organics
• PANs and other photochemical oxidants

How Smog Forms and Why It Peaks Later

This daily pattern is one of the most tested parts of the topic.

1. Morning traffic releases lots of NOx and VOCs.
2. As the sun gets stronger, reactions involving NO₂ speed up.
3. Oxygen and sunlight help form ozone.
4. VOCs let ozone build up instead of getting used up quickly in the NO-NO₂ cycle.
5. After several hours of reaction time, ozone peaks in the afternoon.

So the key point is simple. Ozone is not emitted directly by cars. It forms later.

Patterns to remember:

• Daily
  • NOx higher early
  • Ozone peaks later
  • Ozone drops after sunset as photochemical production slows
• Seasonal
  • Smog is usually worse in summer because of stronger sunlight, warmer temperatures, and faster VOC evaporation
• Urban and downwind
  • Smog often starts in cities with many vehicles
  • Ozone can be carried downwind, so highest levels may occur away from the original source

What Affects Smog Severity

Several conditions control how bad an episode gets:

• Sunlight increases photochemical reactions.
• Temperature speeds reactions and raises VOC evaporation.
• Precursor concentration matters because more NOx and VOCs mean more smog potential.
• Wind and mixing can disperse pollution, but can also move ozone downwind.
• Topography like mountains and basins traps polluted air.
• Thermal inversions trap pollutants near the ground.
• Precipitation and storm mixing usually reduce smog.

The classic example is Los Angeles. Heavy traffic, sunny warm weather, and basin topography make it a classic smog city.

Effects and Ways to Reduce It

Human health effects include:

• Respiratory problems
• Eye irritation
• Coughing, wheezing, chest discomfort
• Shortness of breath
• Worsened asthma and lower lung function

Environmental effects include:

• Reduced visibility
• Plant damage from ozone
• Lower photosynthesis
• Slower growth
• Reduced crop yields

Reducing smog means cutting both precursor groups.

Reducing nitrogen oxides

• Less vehicle use, public transit, carpooling
• Catalytic converters
• Vehicle emission standards and engine maintenance
• Lower-emission electricity and industrial controls

Reducing VOCs

• Vapor recovery at gas stations
• Sealed fuel storage
• Low-VOC paints, solvents, and products
• Industrial leak detection and emission controls

The prevention idea ties the whole topic together. Lower morning and daytime NOx and VOC emissions, and you lower afternoon ozone peaks.