Infographic on the aroma of rain. The smell of rain is sometimes referred to as petrichor and has a number of contributors. Oils secreted by plants, compounds secreted by bacteria and the formation of ozone can all play a part. Actinomycetes, soil-dwelling bacteria, secrete geosmin, which has an earthy aroma. Rain disturbs geosmin from the soil. Plants secrete oils in dry periods, and rain causes oxidation products of these oils to be released. In stormy weather, electrical discharges from lightning strikes can split oxygen molecules in the air, and the individual oxygen atoms produces can combine with other oxygen molecules to form ozone, giving rise to the 'pre-rain' smell.
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Everyone’s familiar with the smell after a heavy rain shower in the summer – or, for that matter, the ominous scent of an oncoming thunderstorm. In the third part of the Aroma Chemistry series, this graphic examines the chemical compounds that are the major contributors to these smells, and how they arise.

Different compounds are involved to varying degrees in the smell of rain, depending on whether we’re talking about the post-rain or the pre-rain smell. The three major origins of these compounds are bacteria in the ground and soil, oils released by plants during periods of dry weather, and electrical charge in the vicinity of thunderstorms.

Firstly, let’s consider the bacteria, as the compound they produce is the most widely cited whenever the smell of rain is discussed. It’s a particular class of soil-dwelling bacteria, called actinomycetes, that produce the compound we’re interested in – geosmin. They secrete it into the surrounding soil, and it is then disturbed by rainfall, spreading in the air and allowing us to detect it . Incidentally, there doesn’t need to be much geosmin in the air at all for us to detect it. It’s been estimated humans can detect geosmin at concentrations as low as 5 parts per trillion, which is roughly equivalent to a teaspoon of geosmin in 200 Olympic swimming pools.

Geosmin is also present in beetroot, and responsible for its earth flavour. Additionally, the presence of geosmin in water can cause an unpleasant, muddy taste. However, as far as the smell of rain goes, it isn’t the sole culprit.

Another factor is the oils produced by plants, particularly when there is a relatively long spell of dry weather. Studies have suggested that plants produce a particular mix of oils during dry weather, designed to inhibit growth and minimise competition for water. These oils collect in the soil and in rocks; rain then causes a range of smaller, volatile compounds within them to be released into the air. Their combination with geosmin in the air causes ‘petrichor’ – the name given by scientists in 1964 to the smell after rain.

The smell before rain, however, has a different cause. Particularly, the smell before a thunderstorm is a consequence of the electrical charge present in the atmosphere. This causes the splitting of some oxygen molecules in the atmosphere into individual oxygen atoms, which can then combine with other oxygen molecules in the atmosphere to form ozone, O3. Ozone has a sharp odour, compared by some to that of chlorine, or of burnt wires. It’s unstable in the lower atmosphere, and is usually only found higher up – however, the downdrafts of wind produced by a storm can sweep it down from the higher atmosphere, making it possible for us to detect it, and giving the calm before a storm that ‘pre-rain’ smell.



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