Photochemical Smog: How Sunlight Cooks Traffic Exhaust Into Ozone

The three ingredients: NOₓ, VOCs, and photons

Photochemical smog needs exactly three things, and remove any one and it collapses:

• Nitrogen oxides (NOₓ = NO + NO₂). Combustion in car engines and power plants burns hot enough (> 1300 °C) to force otherwise inert N₂ and O₂ to react: N₂ + O₂ → 2 NO. This thermal (Zeldovich) NO is emitted mostly as colorless NO, which oxidizes in air to brown NO₂.
• Volatile organic compounds (VOCs). Unburned fuel, evaporated gasoline, solvents, and biogenic isoprene from trees. These are the carbon fuel of the radical chain.
• Sunlight. Specifically photons with wavelength below ~420 nm, energetic enough to photolyze NO₂ and split it into NO and an oxygen atom.

The single reaction that lights the fuse is the photolysis of NO₂:

• NO₂ + hν (λ < 420 nm) → NO + O(³P)
• O(³P) + O₂ + M → O₃ + M  (M = N₂ or O₂, a third body carrying off energy)

That is the only significant way ozone is made in the lower atmosphere — an oxygen atom liberated from NO₂ latching onto O₂. Everything else in smog chemistry exists to keep feeding NO₂ into this step.

The photostationary state — and why it isn't enough

Left alone, the ozone made above would be destroyed almost as fast as it forms, because ozone attacks the NO that photolysis released:

• NO + O₃ → NO₂ + O₂

These three reactions — NO₂ photolysis, O-atom + O₂, and NO + O₃ — form a tight loop called the photostationary state (PSS). In this loop no net ozone accumulates; NO₂ is merely shuttled to NO and back, cycling one O₃ molecule in and out. The steady-state ozone is set by the Leighton relationship:

• [O₃] ≈ (j_NO₂ × [NO₂]) / (k × [NO])

where j_NO₂ is the photolysis rate and k is the rate constant for NO + O₃ (≈ 1.9 × 10⁻¹⁴ cm³·molecule⁻¹·s⁻¹ at 298 K). If that were the whole story, cities would never breach ~30 ppb of ozone. The reason smog builds to 200+ ppb is that something else converts NO back to NO₂ without consuming ozone. That saboteur is the VOC radical chain.

The radical chain that breaks the cycle

The chemistry that lets ozone pile up is a chain reaction carried by the hydroxyl radical (OH•) and the peroxy radicals it spawns. Take a generic hydrocarbon RH (say, propane or a fragment of gasoline):

• Initiation: OH• + RH → R• + H₂O
• O₂ addition: R• + O₂ + M → RO₂• (an organic peroxy radical)
• The crucial NO oxidation: RO₂• + NO → RO• + NO₂
• RO• + O₂ → carbonyl (aldehyde/ketone) + HO₂•
• Regeneration: HO₂• + NO → NO₂ + OH•

Notice what just happened: two molecules of NO were oxidized to NO₂ by peroxy radicals, not by ozone. Each of those NO₂ molecules is then free to photolyze and make a fresh O₃ — and the OH• is handed back to attack another hydrocarbon. The chain turns over dozens of times before terminating. This is why a single VOC molecule can catalyze the formation of several ozone molecules, and why controlling hydrocarbons is often as important as controlling NOₓ.

Chain termination removes the radicals: OH• + NO₂ + M → HNO₃ + M (making nitric acid, a smog contributor to acid deposition), or HO₂• + HO₂• → H₂O₂ + O₂.

PAN, the eye-stinging thermal grenade

One product deserves its own section. When the VOC being oxidized is an aldehyde like acetaldehyde (CH₃CHO), the chain produces the acetylperoxy radical, CH₃C(O)OO•. This radical can add NO₂ directly:

• CH₃C(O)OO• + NO₂ ⇌ CH₃C(O)OONO₂  (peroxyacetyl nitrate, PAN)

PAN is the compound most responsible for the eye irritation that defines an LA smog day, and it is a potent phytotoxin that damages crops at just a few ppb. Its most important property is that the reaction is an equilibrium. PAN is thermally unstable: warm it and it releases NO₂ and the peroxy radical again. Its lifetime is minutes at 25 °C but stretches to months at −20 °C in the cold upper troposphere.

This makes PAN a reservoir and courier for NOₓ. Air masses lift PAN out of a polluted city, carry it thousands of kilometers, and then — as the air warms while descending elsewhere — release NO₂ far from the original source, seeding ozone formation in supposedly clean regions. PAN is a major reason ozone pollution is a hemispheric, not just a local, problem.

The controlling variables: the NOₓ–VOC seesaw

Ozone production does not respond simply to its precursors. Plotted against NOₓ and VOC concentrations, the ozone yield forms a ridge (an "ozone isopleth"), and cities fall on one of two sides:

• VOC-limited (NOₓ-saturated) regime: Dense urban cores and fresh traffic plumes. Here NOₓ is so abundant that OH• is consumed making HNO₃ (OH• + NO₂), starving the chain. Counterintuitively, cutting NOₓ can raise ozone because it frees up OH• — the reason weekend traffic drops sometimes spike ozone (the "weekend effect").
• NOₓ-limited regime: Suburbs, downwind rural areas, and most of the atmosphere. Here VOCs (often biogenic isoprene) are plentiful and ozone rises with NOₓ. Cutting NOₓ works.

Other levers matter too:

• Temperature: higher T speeds the chain and shifts PAN toward decomposition, releasing NO₂. Ozone episodes correlate tightly with heat waves — a worrying climate feedback.
• Sunlight intensity: j_NO₂ peaks at solar noon, so ozone lags emissions by hours, peaking mid-afternoon.
• Stagnation: a temperature inversion caps the mixing layer, trapping precursors in a shallow, sunlit box. LA's basin, ringed by mountains, is the textbook trap.

Why it matters: health, crops, and climate

Ground-level ozone is a powerful oxidant that attacks the lung lining, triggering inflammation, asthma attacks, and reduced lung function. The WHO's air-quality guideline sets a peak-season 8-hour ozone target near 50 ppb (100 µg/m³); polluted megacities routinely exceed 100–150 ppb, and historic LA episodes hit 500+ ppb. Globally, ozone is associated with hundreds of thousands of premature deaths per year.

The damage is not only human:

• Crops: Ozone and PAN oxidize plant tissue and impair photosynthesis. Ground-level ozone is estimated to cut global staple-crop yields (wheat, soybean, maize) by 5–15%, worth tens of billions of dollars annually.
• Climate: Tropospheric ozone is itself a greenhouse gas — the third-largest anthropogenic contributor after CO₂ and methane, with a radiative forcing of roughly +0.4 W/m². Methane feeds the ozone chain, linking smog to global warming.
• Materials: ozone cracks rubber and degrades textiles, dyes, and paint.

What actually works: the intervention record

Photochemical smog is one of the great success stories of environmental chemistry — where the chemistry was understood and then acted on. The key intervention was the three-way catalytic converter, mandated on US cars from the mid-1970s. It uses platinum, palladium, and rhodium to simultaneously:

• reduce NOₓ: 2 NO + 2 CO → N₂ + 2 CO₂ (over rhodium)
• oxidize CO and unburned hydrocarbons: 2 CO + O₂ → 2 CO₂; and C_xH_y + O₂ → CO₂ + H₂O (over platinum/palladium)

Because the converter attacks both NOₓ and VOCs at the tailpipe, it drains two legs of the smog triangle at once. Combined with reformulated low-volatility gasoline, vapor-recovery nozzles, and solvent regulations, Los Angeles cut its number of ozone-exceedance days by more than 75% even as its vehicle fleet grew.

The remaining frontier is different: much modern smog is NOₓ-limited and diesel-driven, and biogenic VOCs can't be regulated away. The shift to electric vehicles removes tailpipe NOₓ entirely, and that — more than any catalyst — is what will finally take sunlight's toxic brew off the menu in the world's most crowded, sun-baked cities.

Photochemical (Los Angeles-type) smog versus classic reducing (London-type) smog.

Property	Photochemical smog	Sulfurous (London) smog
Chemical character	Oxidizing	Reducing
Driving energy	Sunlight / UV photons	None (thermal, dark)
Primary sources	Vehicle NOₓ + VOCs	Coal SO₂ + soot
Signature species	O₃, PAN, NO₂, HNO₃, aldehydes	SO₂, H₂SO₄ aerosol, smoke
Worst conditions	Hot, sunny, stagnant (afternoon)	Cold, damp, foggy (winter night)
Color / effect	Brown haze, eye irritation	Grey-black fog, respiratory