Wet Earth
Isabel Bear and RG Thomas, the researchers who first named the scent petrichor, found that as early as the 1960's it was being captured to sell as a scent called "matti ka attar" in Uttar Pradesh, India. Now, geosmin is becoming more common as a perfume ingredient. "It's a really potent material and it smells just like the concrete when the rain hits it," said perfumer Marina Barcenilla. "There's something very primitive and very primal about the smell. Even when you dilute it down to the parts per billion range, {humans} can still detect it," she added. Yet,we also have an odd relationship with geosmin - while we are drawn to its scent, many of us dislike its taste.
Even though it is not toxic to humans, the tiniest amount can put people off mineral water or wine when it is present. "We do not know why we dislike geosmin," commented Professor Jeppe Lund Nielsen from Aalborg University in Denmark. "It is not toxic to humans in typical found ranges, but somehow we associate it with something negative," he added.
Petrichor: The Term
Coined by scientists Isabel Joy Bear and Richard Thomas in their 1964 article "Nature of Argillaceous Odour", published in the journal Nature. The word was coined from Greek petros, meaning "stone", and ichor, meaning "the fluid that flows in the veins of the gods"
Plants
According to Professor Nielsen, research also indicates that geosmin could be related to terpenes - the source of scent in many plants. Rain could bring these fragrances out, says Professor Philip Stevenson, a research leader at Royal Botanic Gardens, Kew, London, England. "Often the plant chemicals that smell pleasant are produced in leaf hairs... and the rain may damage these, releasing the compounds. "Rain may also break dry plant material releasing chemicals in a similar way to when you crush dried herbs - the smell becomes stronger," he said. Very dry periods may also slow down plant metabolism, with renewed rainfall giving it a kick start and causing plants to release a pleasant scent.
Lightning Bolts
Thunderstorms have their role to play too, creating the clean, sharp scent of ozone - caused by lightning and other electrical discharges in the atmosphere. Professor Maribeth Stolzenburg of the University of Mississippi explains: "Besides the lightning, the thunderstorm and especially the rain will improve the air quality. Much of the dust, aerosols, and other particulates are rained out and the air clears."