“Scent of earth, sweet with the evening rain”, wrote Edith and Saretta Nesbit in All Round The Year (1888). A spell of fine, dry weather was suddenly punctuated by a short, sharp burst of rain. As I walked in the garden, I was conscious of an intensely earthy, fresh, almost sweet aroma, as if the shower had woken the earth and the plants from their slumbers and they were rejoicing by giving off this distinctive fragrance. It is one of the most evocative and invigorating smells of summer and so alluring is it that you can even buy it in a bottle.
For several generations enterprising perfumiers in Kannauj in India’s Uttar Pradesh have captured and absorbed the scent in sandalwood oil in a process which takes around fifteen days to complete. Having baked clay in a kiln, they immerse it in water held in copper cauldrons called degs, sealed with earth. A cow dung fire is lit under the cauldron and the resultant vapour travels through bamboo pipes to condense in receivers, over a base of oil, to form what they call matti ka attar or “earth perfume”, an essence released by the interaction between earth and water. It is used as a perfume, in air fresheners and, because of its soothing properties, in aromatherapy.
Although James Joyce did not seem the sort of chap who would splash a bit of perfume behind his ears, he too recognised that the fresh smell after a shower of rain was due to some reaction with the earth. In 1916, he wrote in A Portrait of the Artist as a Young Man, how “the trees in Stephen’s Green were fragrant of rain and the rain-sodden earth gave forth its mortal odour, a faint incense rising upward through the mould from many hearts”. Curiously though, it took scientists until 1964 to understand quite what was going on.
That many dry clays and soils gave off a peculiar and characteristic odour when moistened with water was a phenomenon recognised in all standard mineralogy textbooks at the time, but Joy Bear and Richard Thomas, working for the CSIRO Division of Mineral Chemistry in Melbourne, were intrigued to understand why and how. They set about steam distilling rocks that had been exposed to warm, dry conditions.
What they found, and documented in their ground-breaking paper, Nature of Argillaceous Odour (Nature, March 7, 1964), was a yellowish oil trapped in the rocks. It took an interaction with moisture to release it. For want of a better word, they called the oil petrichor, a compound word made from two Greek words, petra, meaning rock, and ichor, which, in mythology, was used to describe the ethereal fluid which flowed through the veins of the gods instead of blood.
Even a modest increase in humidity is sufficient to fill the pores in rocks and soil with tiny amounts of water, which flush out the oil and release the petrichor into the air. When it begins to rain, the process is accelerated, and the wind helps to disperse the aroma.
Bear and Thomas may have explained why petrichor is produced, but it not until 2015 that two scientists from the Massachusetts Institute of Technology, Young Soo Joung and Cullen Buie, explained the mechanics of the process in a paper published in Nature Communications . Using high-speed cameras to film what happened when raindrops hit the ground, they discovered that on impact they started to flatten, trapping tiny air bubbles. These bubbles then shot upwards, rather like in a glass of champagne, pushing through the surface of the droplet, before bursting out into the air in a fizz of aerosols.
The number of aerosol droplets generated was dependent upon not only the speed at which the droplets hit the surface and on the properties of the surface itself but also the intensity of the rainfall. Perhaps counter-intuitively, they found that light and moderate rain showers generated more aerosol droplets than did prolonged, heavy downpours.
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