12.1.4—Atmospheric NOx and photochemical smog

Nitrogen oxides in air can react with unburned hydrocarbons under sunlight to produce secondary pollutants, including peroxyacetyl nitrate (PAN), a component of photochemical smog.

NO₂ absorbs light and generates reactive species that help oxidise hydrocarbons. Smog formation therefore depends on sunlight, NOx and volatile organic compounds; it is not simply “smoke from engines”.

A sunny urban basin with heavy traffic can accumulate ozone and PAN when weak winds limit dispersion. Reducing either NOx or hydrocarbon emissions can alter the reaction chain.

Photochemical smog is not the same as sulfurous smog or acid rain. Keep the reactants, sunlight requirement and secondary products distinct.