Petrichor is the fresh scent you smell after a rainfall and the strongest smell is when there has been a period of long, warm weather with no rain.  The mechanism for how that smell is released into the air has not been understood until now…

Researchers at MIT have discovered that as rain drops fall, each raindrop traps tiny air bubbles at the point of contact between the raindrop and what is landed on.  The bubbles then shoot upward like the bubbles in a glass of champagne.  The pleasant smells are from essential oils and fragrances present on the blades of grades and foliage and it is these aromas that arise with the bubbles shooting up as a result of the rain drops.

Source: MITpetrichor