Step 1: Recall the ingredients of photochemical smog.
This type of smog is a secondary pollutant, meaning it does not come directly from a chimney but forms afterward in the atmosphere.
Step 2: Identify what triggers the reaction.
It forms when nitrogen oxides and hydrocarbons released mainly from vehicle exhaust react together, and sunlight provides the energy needed to drive that reaction.
Step 3: Note the products.
The reaction produces ozone and other oxidants that give the smog its brownish haze, so the essential requirement is sunlight along with $NO_x$ and hydrocarbons.
\[ \boxed{\text{Sunlight, } NO_x \text{ and hydrocarbons}} \]