Petrichor is the smell of rain when it falls on dry ground. The aroma comes from a mixture of chemicals, including geosmin from algae and bacteria, ozone from the effect of lightning on air, and volatile oils released by plants.
What Is Petrichor?
The term “petrichor” comes from the Greek words petra (stone) and ichor (the ethereal blood of the gods). Australian scientists Isabel Joy Bear and Richard G. Thomas introduced the word in their 1964 Nature article to describe the unique, earthy scent experienced when rain falls on dry soil. Prior to this, scientists referred to the scent as “argillaceous odour”. Chemical analysis of air during rain show it contains three groups of chemicals that are not normally present: geosmin, ozone, and certain plant oils.
Lightning produces ozone well in advance of a thunderstorm, so its scent often precedes rain. When raindrops fall, their impact disturbs geosmin and plant oils on rocks and soil. The water also traps tiny air bubbles that travel upwards through the raindrop and burst out in a fizz of aerosols, spreading the petrichor scent.
Geosmin
Geosmin is the primary compound responsible for the smell of rain. The word stems from the Greek words geo (earth) and osme (smell). Geosmin is a bicyclic alcohol produced by certain blue-green algae (cyanobacteria) and bacteria (mainly Actinomyces, such as Streptomyces). The compound has an earthy, musty, or muddy scent. Humans are highly sensitive to smelling or tasting geosmin. Its odor is detectable at levels as low as 5 parts per trillion. Many animals also smell geosmin. For example, camels use the odor for finding water in deserts.
Cyanobacteria and bacteria that produce geosmin release it when they die. When rain falls, it disrupts the surface and spreads geosmin into air, contributing its part to the petrichor scent.
While most people find the aroma of geosmin pleasant in rain, it also has a flavor. It is the molecule that gives beets and some wines their earthy flavor. However, it also gives water and fish (especially freshwater fish) a muddy or musty taste. Water treatment plants often take steps to remove geosmin from drinking water, typically using activated carbon or advanced oxidation. Fish farms purge the flavor by housing fish in clean water prior to harvesting. Adding lemon, vinegar, or another acid to a fish recipe breaks down the compound so that the dish tastes better.
Ozone in Petrichor
Lightning is the main source of the ozone scent of a thunderstorm. The energy of a lightning strike breaks the chemical bonds that hold nitrogen (N2) and oxygen (O2) molecules together. Some of the oxygen molecules combine and form ozone (O3). While a small amount of ozone in air smells like rain, too much is unhealthy and causes health effects, such as worsening asthma.
Volatile Plant Oils in Petrichor
Plants release oils during dry spells that protect them from loosing too much water and also deposit on the ground and prevent seed germination. This minimizes the chance of seedlings sprouting and then dying from lack of moisture. When rain falls, these oils splash up and enter the air, adding another earthy component to petrichor. The principal oils are stearic acid and palmitic acid. Both of these oils are nearly odorless, but they have a slight waxy, soapy aroma.
References
- Anderson, W.; G.J. Prescott; S. Packham; J. Mullins; M. Brookes; A. Seaton (2001). “Asthma admissions and thunderstorms: a study of pollen, fungal spores, rainfall, and ozone”. QJM: An International Journal of Medicine. 94 (8): 429–433. doi:10.1093/qjmed/94.8.429
- Bear, Isabel Joy; Thomas, Richard G. (March 1964). “Nature of argillaceous odour”. Nature. 201 (4923): 993–995. doi:10.1038/201993a0
- Gerber, N. N.; Lechevalier, H. A. (1965). “Geosmin, an Earthy-Smelling Substance Isolated from Actinomycetes”. Applied Microbiology. 13 (6): 935–938. doi:10.1128/am.13.6.935-938.1965
- Phipson, T.L. (May 16, 1891). “The Odor of the Soil after a Shower”. Scientific American. 64 (20): 308.
- Watson, W.; Juttner, F. (2019). Taste and Odour in Source and Drinking Water: Causes, Controls, and Consequences. IWA Publishing. ISBN 9781780406657.