Photochemical smog forms when vehicle combustion gases (NOx, VOCs) react with solar radiation to produce ozone, PAN (peroxyacetyl nitrate), and other oxidants. The result is a visible brown haze over cities.
Favourable conditions
It worsens with temperature inversions (trapping pollutants), high solar radiation (summer), and light winds. Madrid and Barcelona experience smog episodes, though less severe than Los Angeles or Mexico City. Anticyclonic subsidence aggravates the problem by stabilising the mixing layer.