Disappointed as I was about having to cancel my end-of-a-weird-term garden party (taking place at a social distance, of course), I do love a good rainy day. I was out walking my dog when I smelt that distinctive odour of fresh rain… this got me thinking. Where does that all-too-familiar rain smell come from?
It would be frankly ridiculous to assume that water falling from the sky, the same H2O that comes out of my kitchen tap, has any kind of scent. And yet, a certain ‘earthy’ and fresh smell is evident in the air when it rains. The name of this smell is petrichor.
Definition: a pleasant smell that frequently accompanies the first rain after a long period of warm, dry weather.
This comes from the Greek word, meaning ‘the blood of stones’ and was coined by two Australian scientists in the 1960s, Isabel Bear and Richard Thomas, who were searching for the origin of the scent. Its sources were identified in two places, plants and bacteria, living in the soil.
The scientists found they could extract a yellowy oil from warm, dry rocks, clay and soil. This oil contained fatty acids from plants, namely palmitic acid and stearic acid, as well as other smaller compounds. Alone these compounds aren’t particularly odorous, but after some time in the soil, it was found they get broken down into smaller, much smellier molecules.
The other key contributor are bacteria in the soil called actinomycetes, tiny microorganisms that can be found in rural and urban areas as well as in marine environments. They decompose dead organic matter into simple chemical compounds which can then become nutrients for developing plants and other organisms. A by-product of this activity is geosmin, meaning ‘the smell of earth’, an organic compound that, when combined with the broken-down fatty acids, adds to the petrichor scent. Geosmin is a type of alcohol, and like most alcohol molecules, tends to have a strong scent, but the complex chemical structure of geosmin means the human nose can detect it at concentrations less than 10 parts per trillion – in context, around a teaspoon in 200 Olympic sized swimming pools!
So why is that we only associate the distinctive smell with rain? Why can’t we smell it all the time since it is always present in the soil?
The decomposition activity rate of actinomycetes slows considerably during a prolonged period of dryness. The air becomes more humid, or as many refer to it ‘muggy’, just before a rain event and the ground begins to moisten. This speeds up the activity rate of the bacteria, leading to increased production of that odorous geosmin.
In 2015, research was carried out into how the smell actually reaches your nose. High-speed videos of water droplets hitting a porous surface, showed air bubbles getting trapped beneath the rain drop. Those air bubbles then burst, spewing tiny jets of water, effectively creating a natural aerosol of water from the raindrops and chemicals from the soil. These small water drops can travel on the wind much more easily than raindrops and hence carry the smell of petrichor a considerable distance both geographically across an area but also from the ground to our noses. Only a light rain will have this effect as heavy rain doesn’t create these bubbles and this scent eventually goes away after the rain has passed and the ground begins to dry.
P.S. for those equally disappointed about my lack of garden party fun, I rearranged to Friday when the weather is supposed to be delightful. Please cross your fingers and toes that this is the case!
References:
· https://www.sciencenewsforstudents.org/article/scientists-say-petrichor
· https://www.youtube.com/watch?v=2txpbrjnLiY&feature=emb_title
· https://earthsky.org/earth/what-is-smell-of-rain-petrichor
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