Whoosh! That’s the zesty, lemony burst of citral, a natural aldehyde which is present in the oil of quite a few plants, including lemon myrtle, lemongrass, lemon tea tree, lemon verbena, lemons themselves, limes, as well as orange and petitgrain (the flower of the bitter orange). Citral is pure ‘freshness’, and used for that effect. (It also helps to develop rose notes, in soap-making.)
Unfortunately, the downside of citral is that it can lead to sensivities and allergies – so IFRA (the International Fragrance Association) regulates that it has to be used only with other ingredients that prevent a sensitising effect, and insist it’s labelled. (Hence citral is one of the few perfume ingredients you may see listed on the box your scent’s packaged in). Another note of warning: in its purest commercial form, citral can paralyse the nose for several hours, making it impossible to smell anything at all.