Why do a perfume's top notes arrive first and fade before its deeper notes?

Answer: The more volatile molecules evaporate sooner, so they arrive first and fade first

The more volatile molecules evaporate sooner, so they arrive first and fade first ✓ — The top notes are the fast-evaporating part of the mixture. Their molecules leave the skin and enter the air sooner, so they reach the nose before the slower, heavier molecules do. As the fast molecules thin out, the less volatile deeper notes become easier to notice; they were often present from the start but masked. This is a timing pattern in the mixture, not simply a change in how strong the perfume is.

The opening blend is more concentrated, so it thins out first as it spreads — A stronger opening can feel more prominent, so concentration is a reasonable first guess. But the top-to-deeper sequence is mainly a timing effect, not a volume knob. A small amount of a very volatile citrus material can dominate the first minutes while heavier woody or musky materials are still leaving the skin slowly. That is why two perfumes of similar strength can still unfold in different orders.

The sense of smell adapts to the opening, so deeper notes seem new — Smell adaptation is real: after you smell one odor for a while, you may notice it less. It still does not explain why citrus-like openings are usually obvious right away while woods and musks dominate later. The physical mixture above the skin is changing as molecules evaporate at different rates. Your brain is involved, but the bottle is also running a chemistry clock.