The formation of photochemical smog (Figure 18-9) begins when exhaust from morning commuter vehicles releases large amounts of NO and VOCs into the air over a city. The NO is converted to reddish-brown NO2, explaining why photochemical smog is sometimes called brown-air smog. When exposed to ultraviolet radiation from the sun, some of the NO2 reacts in complex ways with VOCs released by certain trees (such as some oak species, sweet gums, and poplars), motor vehicles, and businesses (such as bakeries and dry cleaners).