When the first light rain falls on the parched soil surface after dry spell of weather, the soil releases an fragrant aroma which is like magic to humans. This magical aroma has been described by many writers. So much for the feeling and aroma. But what is actually happening to create this magic.
Scientists have a explanation for it. Two Australian scientists in 1964 coined the term “PETRICHOR” for it. The word is made by shortening the two words “PETRA” meaning “ROCK” and “ICHOR” the fluid which flows in the veins of gods.
In fact soil which is mainly clay has many chemical compounds which are released by many tiny plants trapped in its lattice. These are volatile oils predominately stearic acid and palmitic acids which are fatty acids with long chain carbon structure with acid group on one end and methyl group on the other.
Also there is a chemical called “GEOSMIN” released by a soil bacteria called “ACTINOMYCETES”. Geosmin is a organic cyclic compounds with sweet smell and human nose can detect its smell down to 5 parts per trillion concentration in the air.
If the rains are accompanied by lightening and thunder, oxygen and nitrogen molecules in the air are broken down to oxygen and nitrogen radicals and recombine to form NITROUS OXIDE and OZONE. Ozone also contributes to the aroma.
So in nutshell the substances which are responsible for the aromatic earthy smell are vegetable volatile oils, geosmin and ozone.
When the rain water seeps through the soil, it enters the lattice spaces and displaces these substances out. The smells are stronger when rain is not heavy.
Note: These images are taken from Wikipedia.