A component of the noxious mixture of air pollutants in photochemical smog.

Wikipedia definition:

An aldehyde /ˈældᵻhaɪd/ or alkanal is an organic compound containing a functional group with the structure −CHO, consisting of a carbonyl center (a carbon double-bonded to oxygen) with the carbon atom also bonded to hydrogen and to an R group, which is any generic alkyl or side chain. The group—without R—is the aldehyde group, also known as the formyl group. Aldehydes are common in organic chemistry. Many fragrances are aldehydes.

Source: Wikipedia - Aldehyde