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This is why it smells so nice after it rains

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A new book. Petrol. Freshly-baked bread. Burning wood. And the fresh smell just after it rains.

These are some smells that are, beyond any argument (and we’ll fight anyone who doesn’t agree on petrol), simply glorious.

Yet the way in which the last of those smells – that lovely earthy aroma after a rain shower – is produced has only been partially understood, until now.

The smell itself is due to an organic compound called geosmin (a bicyclic alcohol with formula C12H22O), with its name appropriately derived from the Greek words for ‘earth’ and ‘smell’ (say what you see guys), which is produced by the bacteria Streptomyces when they die. The human nose is extremely sensitive to geosmin and is able to detect it at concentrations as low as five parts per trillion.

However, the method by which that compound reaches our noses has been the subject of debate. Now, though, a group of mechanical engineers have published a paper in Nature that explains the process.

Using high-speed cameras and fluorescent dye, the researchers filmed drops of water which fell on different types of soil.

When a raindrop hits the soil at the right speed, air bubbles – no wider than the width of a human hair – are trapped underneath. As they rise up to reach the surface, they burst, catapulting hundreds more smaller droplets into the air. Some of these droplets carry both geosmin and bacteria, with the bacteria able to survive in the tiny droplet for around an hour. Once in the air, the bacteria can then be moved further by the wind.

"It's very similar to what happens if you pour a glass of champagne or any carbonated beverage and you see that fizz on top," said Cullen Buie, a mechanical engineer at the Massachusetts Institute of Technology and one of the authors of the paper. In carbonated drinks, the bubbles are obviously carbon dioxide, whereas in this case, the bubbles are air.

However, this is not just a point of interest – sometimes, the bacteria moved into the air by the droplets can be harmful. An infectious, and potentially fatal, disease called melioidosis spreads during the rainy season in Southeast Asia and northern Australia and this research was inspired in part by the desire to understand the link between the rain and the disease.

[Main Image: iStock/Gifs: Joung et al. Nature Communications]

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