The molecules of each ingredient in a fragrance evaporate from the skin at different speeds, according to their size and volatility - all the while interacting both with the fragrance's other ingredients and with the wearer's skin chemistry. In perfumes like Ffern's that use natural ingredients, ingredients may contain many hundreds of different molecules, all of which contribute to the overall scent profile of a fragrance.
Top notes are the fastest to evaporate, meaning essentially that their molecules are the lightest and most reactive. While it may feel as though the top notes, which usually 'last' no longer than an hour or two once the fragrance has settled, are somehow wasted, that all-important first impression of the fragrance is entirely dependent on them. Furthermore, the continued development of the fragrance through the day is determined by how those fast-fading ingredients have been blended with the mid and base notes to come. Top notes include citruses such as lemon and green mandarin, or sharper herbs like rosemary and peppermint.