Fragrant materials are listed by Poucher[1] in order of volatility and are grouped under respective evaporation coefficients (perfume notes) that range from 1 to 100.
Perceived immediately upon application of a perfume, top notes consist of small, light molecules that evaporate quickly.
They serve to mask the often unpleasant initial impression of base notes, which become more pleasant with time.
Scents from this note class disappear anywhere from twenty minutes to one hour after the application of a perfume.
Compounds of this class are often the fixatives used to hold and boost the strength of the lighter top and middle notes.