Can You Smell Rain? Geosmin and Petrichor

Do you know the smell of the air before or after it rains? It isn't the water that you smell, but a mixture of other chemicals. The odor you smell before the rain comes from ozone, a form of oxygen which is produced by lightning, and ionized gases in the atmosphere. The name given to the characteristic odor of rain after it rains, especially following a dry spell, is petrichor. The word petrichor comes from the from Greek, Petros, meaning ‘stone’ + ichor, the fluid flowing in the veins of the gods in Greek mythology. Petrichor is caused primarily by a molecule called geosmin.

About Geosmin

Geosmin (meaning earth smell in Greek) is produced by Streptomyces, a Gram-positive type of Actinobacteria. The chemical is released by the bacteria when they die. It is a bicyclic alcohol with the chemical formula C₁₂H₂₂O. Humans are very sensitive to geosmin and can detect it at levels as low as 5 parts per trillion.

Geosmin in Food—a Cooking Tip

Geosmin contributes an earthy, sometimes unpleasant flavor to foods. Geosmin is found in beets and also freshwater fish, such as catfish and carp, where it concentrates in the fatty skin and dark muscle tissues. Cooking these foods together with an acidic ingredient renders the geosmin odorless. Common ingredients you can use include vinegar and citrus juices.

Plant Oils

Geosmin isn't the only molecule that you smell after it rains. In a 1964 Nature article, researchers Bear and Thomas analyzed air from rainstorms and found ozone, geosmin, and also aromatic plant oils. During dry spells, some plants release the oil, which is absorbed into clay and soil around the plant. The purpose of the oil is to slow seed germination and growth since it would be unlikely for the seedlings to prosper with insufficient water.

Sources

• _{Bear, I.J.; R.G. Thomas (March 1964). "Nature of argillaceous odor". Nature 201 (4923): 993–995.}