NOx: The Nitrogen Oxides That Poison City Air

Forged from thin air: the three ways NOx is born

Nitrogen makes up 78% of the atmosphere yet is almost chemically inert because breaking N≡N costs 945 kJ/mol. Combustion supplies that energy, and there are three named routes to NOx:

• Thermal NOx (the Zeldovich mechanism) — dominant above ~1500 °C. Oxygen atoms from the flame attack N₂ in a two-step chain:
  O· + N₂ → NO + N·
  N· + O₂ → NO + O·
  The first step has a huge activation energy (~318 kJ/mol), so NO output rises exponentially with peak flame temperature — hotter, leaner combustion makes far more of it.
• Prompt NOx (the Fenimore mechanism) — in fuel-rich flame fronts, hydrocarbon radicals attack N₂ directly: CH· + N₂ → HCN + N·, which then oxidises to NO. It is fast and happens even at lower temperatures.
• Fuel NOx — nitrogen chemically bound in the fuel itself (coal, heavy oil) is released and oxidised. It is the biggest source from coal-fired power plants.

Once in the tailpipe or stack, the primary product is overwhelmingly NO (typically >90%), with only a few percent leaving directly as NO₂. The conversion to red-brown NO₂ happens later, in the open air.

The NO–NO₂–O₃ triangle that runs the lower atmosphere

The single most important fact about NOx is that it cycles rapidly between NO and NO₂ while manufacturing ozone. In clean daytime air, three reactions reach a balance called the photostationary state:

• NO₂ + hν (λ < 420 nm) → NO + O(³P) — sunlight splits NO₂
• O(³P) + O₂ + M → O₃ + M — the oxygen atom makes ozone (M = N₂/O₂ carrying off energy)
• O₃ + NO → NO₂ + O₂ — ozone re-oxidises NO, closing the loop

Left alone, this cycle makes and destroys O₃ with no net gain — it just shuttles an oxygen atom around. The trouble starts when something else converts NO back to NO₂ without consuming ozone. That job is done by peroxy radicals (RO₂· and HO₂·) produced when the hydroxyl radical (·OH) attacks volatile organic compounds (VOCs):

• RO₂· + NO → RO· + NO₂
• HO₂· + NO → ·OH + NO₂

Now NO₂ is regenerated for free, the photolysis loop keeps firing, and ozone piles up. This is why smog is a NOx and VOC problem — you need both. A single NO molecule can turn over this cycle dozens of times, catalytically producing many ozone molecules before it is finally removed.

How NOx makes acid rain and washes out

NOx does not stay airborne forever. During the day the dominant sink is oxidation by the hydroxyl radical to nitric acid:

• ·OH + NO₂ + M → HNO₃ + M

HNO₃ is extremely water-soluble and highly acidic (a strong acid, effectively fully dissociated), so it dissolves into cloud and rain droplets: HNO₃ → H⁺ + NO₃⁻. Together with sulfuric acid from SO₂, this drives acid rain, pulling rainfall pH down from its natural ~5.6 (set by dissolved CO₂) to values of 4.2–4.5 across the industrial northeastern US and Europe in the late 20th century — and below 3 in extreme events.

At night, when there is no sunlight and no ·OH, a different pathway takes over. NO₂ is oxidised by ozone to the nitrate radical, which reacts further:

• NO₂ + O₃ → NO₃· + O₂
• NO₃· + NO₂ ⇌ N₂O₅
• N₂O₅ + H₂O (on aerosol surfaces) → 2 HNO₃

This heterogeneous hydrolysis of dinitrogen pentoxide (N₂O₅) is a major overnight scrubbing route. The nitrate deposited this way is also a fertiliser: NOx emissions add reactive nitrogen to soils and coastal waters, feeding eutrophication far downwind of any tailpipe.

Why it matters: lungs, ozone, and a self-cleaning sky

NOx damages Earth systems at three scales at once. For human health, NO₂ itself is a respiratory irritant that inflames airways and worsens asthma; long-term exposure is linked to reduced lung development in children. But the bigger killer is the ground-level ozone NOx produces — a powerful oxidant that scars lung tissue and damages crops. Ozone-driven yield losses in wheat, soy, and rice run into tens of billions of dollars a year globally.

For climate, NOx is a double agent. By making tropospheric ozone (a greenhouse gas) it warms; but by boosting ·OH concentrations it shortens the lifetime of methane, a potent greenhouse gas, which cools. It also produces cooling nitrate aerosols. The net effect is modest but genuinely uncertain — a rare case where a pollutant's climate sign is debated.

For atmospheric chemistry as a whole, NOx sets the concentration of the hydroxyl radical, the atmosphere's chief oxidant and 'detergent.' ·OH scrubs out methane, carbon monoxide, and countless pollutants. Because NOx recycles HO₂· back to ·OH (HO₂· + NO → ·OH + NO₂), the amount of NOx present quietly governs how fast the entire lower atmosphere cleanses itself.

The controlling variables: temperature, sunlight, and the VOC/NOx ratio

NOx chemistry is exquisitely sensitive to a handful of knobs:

• Peak combustion temperature — thermal NOx scales exponentially with it. Lowering flame temperature (via exhaust gas recirculation, staged burning, or lean-premixed gas turbines) is the front-line control at the source.
• Sunlight (photon flux) — NO₂ photolysis and ·OH production both require UV, so ozone builds through the afternoon and collapses at night. Smog is a daytime, sunny-climate phenomenon; hence Los Angeles, Mexico City, and Delhi.
• The VOC-to-NOx ratio — this decides which lever works. In VOC-limited city cores (high NOx), cutting NOx alone can perversely raise ozone because NO normally titrates it away (O₃ + NO → NO₂ + O₂); you must cut VOCs. In NOx-limited rural and downwind air, cutting NOx directly lowers ozone. Getting this diagnosis wrong wastes billions in the wrong controls.
• Temperature and humidity — heat accelerates the whole radical chemistry, which is why ozone-alert days cluster in summer heatwaves.

Fighting back: catalytic converters and SCR

The chemistry that makes NOx also lets us destroy it — by running the reactions backward toward harmless N₂. Two technologies dominate:

• The three-way catalytic converter (petrol cars, since the 1970s) uses platinum, palladium, and rhodium on a ceramic honeycomb. Held at a precise stoichiometric air-fuel ratio, rhodium reduces NOx while CO acts as the reductant:
  2 NO + 2 CO → N₂ + 2 CO₂
  and simultaneously CO and unburnt hydrocarbons are oxidised to CO₂ and H₂O. A modern converter removes >90% of NOx.
• Selective catalytic reduction (SCR) — used on diesel trucks, ships, and power plants, which run lean (excess oxygen) where a three-way catalyst cannot work. Ammonia (dosed as urea, the 'AdBlue' or 'DEF' fluid) reduces NOx over a vanadia or zeolite catalyst:
  4 NO + 4 NH₃ + O₂ → 4 N₂ + 6 H₂O
  2 NO₂ + 4 NH₃ + O₂ → 3 N₂ + 6 H₂O

These controls worked spectacularly. US NOx emissions fell roughly 60% from their late-1990s peak even as driving increased, and satellite NO₂ columns over Western cities dropped visibly. The 2015 'Dieselgate' scandal — cars that switched off their NOx controls on the road — showed both how effective the chemistry is and how tempting it is to cheat it for fuel economy.

The two faces of NOx: nitric oxide versus nitrogen dioxide

Property	NO (nitric oxide)	NO₂ (nitrogen dioxide)
Oxidation state of N	+2	+4
Colour / smell	Colourless, faint	Red-brown, sharp (bleach-like)
Role near source	Emitted directly from flame	Formed by oxidation of NO
Radical / spin	Odd-electron radical (·NO)	Odd-electron radical (·NO₂)
Atmospheric fate	Oxidised to NO₂ in minutes	Photolysed to O₃ or oxidised to HNO₃
Main hazard	Precursor; ozone maker	Toxic itself; acid rain source