Petrichor Explained

Petrichor is the earthy scent produced when rain falls on dry soil. The word is constructed, the ethereal fluid that is the blood of the gods in Greek mythology.

Origins

On 17 April 1891, a brief note on the phenomenon, by (1833–1908), appeared in The Chemical News^[1] —it was re-published in its entirety, a month later, in The Scientific American^[2] —in which he wrote, "This subject, with which I was occupied more than twenty-five years ago, appears from a paragraph in the last number of the Chemical News^[3] to have recently attracted the attention of Professor Berthelot and [Monsieur G.] Andre."

Phipson was referring to a short paper read by Berthelot and André at the meeting of the French Académie des Sciences on 23 April 1891, and printed in Volume 112 (1891) of Comptes Rendus, entitled "Sur l'odeur propre de la terre" ("On the earth's own smell").^{[4] [5]}

Phipson continues, "I find, on referring to my old notes, which are dated 1865, that it is doubtful whether I ever published the results of these observations; and as the distinguished chemists I have just named have not quite solved the problem, I hasten to give the results I obtained so long ago." He then theorizes that the odour "... was due to the presence of organic substances closely related to the essential oils of plants ..." and that these substances consist of "... the fragrance emitted by thousands of flowers ..." absorbed into the pores of the soil, and only released when displaced by rain. After attempts to isolate it, he found that it "... appeared to be very similar to, if not identical with, bromo-cedren derived from essence of cedar."

The phenomenon was first scientifically described in a March 1964 paper by Australian researchers Isabel Bear and Dick Thomas, published in the journal Nature.^{[6] [7] [8] [9]} Thomas coined the term "petrichor" to refer to what had previously been known as "argillaceous odour".^[10] In the article, the authors describe how the smell derives from an oil exuded by certain plants during dry periods, whereupon it is absorbed by clay-based soils and rocks. During rain, the oil is released into the air along with another compound, geosmin, a metabolic by-product of certain actinobacteria, such as Streptomyces,^[11] which is emitted by wet soil, producing the distinctive scent; ozone may also be present if there is lightning.^[12] In a follow-up paper, Bear and Thomas (1965) showed that the oil slows seed germination and early plant growth.^[13]

Mechanism

When a raindrop lands on a porous surface, air from the pores forms small bubbles, which float to the surface and release aerosols.^[14] Such aerosols carry the scent, as well as bacteria and viruses from the soil. Raindrops that move at a slower rate tend to produce more aerosols; this serves as an explanation for why petrichor is more common after light rains. Members of the Actinomycetes, gram-positive bacteria, are responsible for producing these aerosols.

The human nose is sensitive to geosmin and is able to detect it at concentrations as low as 0.4 parts per billion.^[15] Some scientists believe that humans appreciate the rain scent because ancestors may have relied on rainy weather for survival.^[16] Camels in the desert also rely on petrichor to locate sources of water such as oases.^[17]

See also

• Dimethyl sulfide – One of the molecules responsible for the odour of the sea
• Mitti attar – a perfume that recreates the loamy smell of a first rain

General references

• Bear, I.J. & Thomas, R.G. (September 1966), "Genesis of Petrichor", Geochimica et Cosmochimica Acta, Vol. 30, No. 9, pp. 869–879.

External links

• "Petrichor" at A Word a Day
• From the Oxford English Dictionary
  • • Petrichor, U. K. Met office.
• Why is the Smell After it Rains So Appealing? The Petrichor phenomenon
• Petrichor – Why we Love the Smell of Rain

Notes and References

1. https://upload.wikimedia.org/wikipedia/commons/8/80/The_chemical_news._Volume_63%2C_January_-_June_1891._%28IA_s713id13691660%29.pdf Phipson, T.L., Cause of the Odour Emitted by the Soil of a Garden after a Summer Shower", The Chemical News, Vol.63, No.1638, (17 April 1891, p.179.
2. The Odor of the Soil after a Shower . T.L. . Phipson . Scientific American . 64 . 20 . May 16, 1891 . 308 . 26100386 .
3. https://upload.wikimedia.org/wikipedia/commons/8/80/The_chemical_news._Volume_63%2C_January_-_June_1891._%28IA_s713id13691660%29.pdf Specific Odour of Soil", The Chemical News, Vol.63, No.1637, (10 April 1891), p.179.
4. . Sur l'odeur propre de la terre . On the earth's own smell . January 1891 . 1891/01 (Tome 112) . 598–599 .
5. Web site: Why You Can Smell Rain . A weather expert explains petrichor – that pleasant, earthy scent that accompanies a storm's first raindrops. . . August 27, 2018 . Tim . Logan . July 14, 2020.
6. 201 . 4923 . 993–995 . Isabel Joy . Bear . Richard G. . Thomas . Nature of argillaceous odour . Nature . March 1964 . 10.1038/201993a0 . 1964Natur.201..993B . 4189441 . The diverse nature of the host materials has led us to propose the name 'petrichor' for this apparently unique odour which can be regarded as an 'ichor' or 'tenuous essence' derived from rock or stone […] it does not imply that petrichor is necessarily a fixed chemical entity but rather it denotes an integral odour, variable within a certain easily recognizable osmic latitude..
7. 33 . 2 . 91 . The Smell of Rain . Weatherwise . 1980 . 10.1080/00431672.1980.9931898 . Apparently, the printed text is a copy from CSIRO journal Ecos, issue February 1976, p. 32.
8. Book: Garg , Anu . The Dord, the Diglot, and an Avocado Or Two: The Hidden Lives and Strange Origins of Words. Penguin. 2007. 9780452288614. 399. .
9. The Conversation . The smell of rain: how CSIRO invented a new word . March 31, 2015 . Howard . Poynton .
10. News: Isabel 'Joy' Bear. CSIROpedia. CSIRO. 11 April 2014. Colin. Ward. 9 May 2020. Thomas gave the name 'petrichor' to this odour..
11. 10.1038/s41564-020-0697-x. 2058-5276. 5. 6. 821–829. Becher. Paul G.. Verschut. Vasiliki. Bibb. Maureen J.. Bush. Matthew J.. Molnár. Béla P.. Barane. Elisabeth. Al-Bassam. Mahmoud M.. Chandra. Govind. Song. Lijiang. Challis. Gregory L.. Buttner. Mark J.. Flärdh. Klas. Developmentally regulated volatiles geosmin and 2-methylisoborneol attract a soil arthropod to Streptomyces bacteria promoting spore dispersal. Nature Microbiology. 2022-01-01. 2020-06-01. 32251369. 214808046.
12. News: Storm Scents: It's True, You Can Smell Oncoming Summer Rain: Researchers have teased out the aromas associated with a rainstorm and deciphered the olfactory messages they convey. July 20, 2012. Scientific American. July 18, 2012. Daisy. Yuhas .
13. 207. 5005. 1415–1416. Isabel Joy. Bear. Richard G.. Thomas. Petrichor and plant growth. Nature. September 1965. 10.1038/2071415a0 . 1965Natur.207.1415B . 4174301.
14. Web site: Chu. Jennifer. 14 January 2015. Rainfall can release aerosols, study finds. 17 January 2015. MIT News.
15. Polak . E.H. . Provasi . J. . Odor sensitivity to geosmin enantiomers . Chemical Senses . 17 . 23–26 . 1992 . 10.1093/chemse/17.1.23 .
16. Web site: Palermo. Elizabeth. Why Does Rain Smell Good?. LiveScience.com. Live Science. 21 June 2013 . 17 January 2015 .
17. Web site: Sure can smell the rain . 2023-08-24 . Soil System Sciences . en-GB.