Photochemical smog is a sunlight-driven air pollution problem created by chemical reactions among combustion emissions. Understanding what reacts, what forms, and how reactions propagate explains why smog contains several harmful secondary pollutants.

Core idea: a secondary pollution mixture

What “photochemical smog” means

Photochemical smog is not one chemical. It is a dynamic chemical soup whose composition changes as reactions create and consume pollutants.

Required ingredients

Photochemical smog formation requires these inputs:

EPA schematic showing how ground-level ozone (a key component of photochemical smog) forms secondarily from NOx and VOC emissions in the presence of sunlight. The diagram helps connect primary emission sources (e.g., vehicles/industry) to the atmospheric chemistry that produces ozone downwind. Source

• NOx (mainly NO and NO₂) emitted during fossil-fuel combustion (especially engines and power generation)

• VOCs (volatile organic hydrocarbons) that readily evaporate and react in air

• Sunlight (provides energy to break chemical bonds, especially in NO₂)

• Heat (generally speeds reaction rates and can increase VOC evaporation)

Stepwise formation (simplified mechanism)

1) Initiation: sunlight splits nitrogen dioxide

A key initiation step is the photolysis of nitrogen dioxide:

• NO₂ + sunlight → NO + O

• The freed O atom rapidly reacts with oxygen:

  • O + O₂ → O₃ (ozone)

This is why sunlight is central: it starts a chain that produces ground-level ozone, one of the most important smog pollutants.

2) Propagation: VOC chemistry helps ozone accumulate

If only NOx were present, ozone would often be removed by reacting with nitric oxide:

• NO + O₃ → NO₂ + O₂ (this consumes ozone)

VOCs change the system by generating highly reactive intermediates (especially peroxy radicals, often written conceptually as RO₂• and HO₂•) that:

• Convert NO → NO₂ without consuming O₃

• Allow more NO₂ to be available for photolysis, which leads to additional O₃ formation

In effect, VOC-driven radical chemistry reduces ozone loss pathways and supports net ozone buildup, turning a short-lived ozone cycle into a smog-forming process.

Ozone isopleth plot showing peak $O_3$ as a function of NOx and VOC emissions, with contours indicating predicted ozone levels. The ridge/transition behavior illustrates why some conditions are NOx-limited while others are VOC-limited—an important framework for understanding when radical-driven chemistry leads to net ozone accumulation. Source

3) Product formation: a “mixture of pollutants”

As reactions continue, the air develops multiple secondary pollutants, including:

Structural depiction of peroxyacetyl nitrate (PAN), a peroxyacyl nitrate formed secondarily in photochemical smog chemistry. Including this structure helps connect the term “PANs” to an actual nitrogen-containing oxidized organic product of VOC–NOx reactions. Source

• Ozone (O₃): a strong oxidant and major component of photochemical smog

• Aldehydes (from VOC oxidation): irritants and reactive intermediates

• Peroxyacyl nitrates (PANs): formed from VOC oxidation products reacting with NOx; important eye and lung irritants

• Nitric acid (HNO₃) and organic nitrates: contribute to particle formation and deposition

• Secondary particulate matter (fine particles) formed when gases (like nitric acid and oxidised organics) convert into or condense onto aerosols

These substances coexist, so “smog” reflects combined chemistry rather than a single emission.

Primary vs secondary pollutants (only as needed for formation)

How to classify what you see in smog

Smog episodes typically start with primary emissions and end with secondary products.

In photochemical smog:

• Primary pollutants: NOx and VOCs

• Secondary pollutants: O₃, PANs, aldehydes, nitric acid, and secondary particulates

Why “heat and sunlight” are explicitly included

Reaction speed and pollutant buildup

Heat and sunlight matter because they:

• Provide energy for photolysis (especially NO₂ → NO + O)

• Increase the rate of oxidation reactions that turn VOCs into radicals and downstream products

• Promote VOC evaporation, increasing the “fuel” available for radical-driven chemistry

Photochemical smog formation, therefore, is best understood as a sunlight-initiated radical chain reaction system linking NOx and VOCs to a complex pollutant mixture.