London smog: requires humid/foggy, stagnant air have lots of coal burning SO2 + H20 -> H2SO4 LA. smog: requires clear, sunny skies (since L.A. photochemical smog requires sunlight for at one of the key chemical reactions). NOx + ROG + sunlight --> O3 + NO2 ROG are reactive organic gases from unburned gasoline NOx are oxides of nitrogen