The Science of Petrichor

 by  Atalanta Nelson-Smith

Petrichor Noun A pleasant smell that frequently accompanies the first rain after a long period of warm, dry weather. (Oxford Dictionary) 

The smell of petrichor is extremely fascinating as it is experienced by all humans and animals when it rains after a long dry period, and, as it is always accompanied by rain, I had always assumed that it is the smell of the rain itself. I discovered recently that this is not true; it is actually caused by chemicals in plants, bacteria in the soil and ozone in the atmosphere. 

Plants produce two oil compounds during periods of dry weather called Stearic Acid and Palmitic Acid, which can build up between rocks and in the soil around the plants. Stearic Acids and Palmitic Acids are fatty acids (long chains of hydrogen, carbon and oxygen molecules), which you may have heard about in chemistry or biology. These oils are then broken down into smaller molecules that remain in the soil until it rains, when we can smell them as petrichor. 

Streptomyces bacteria in the soil produce the chemical geosmin, which are, similarly to the fatty acid molecules, odourless until they are in contact with water, as they remain in the soil. Geosmin molecules are combined with the smaller molecules from plants in the soil and form what will produce petrichor when it is wet. Bacteria thrive in damp soil and will secrete more geosmin when they are in dry, unpleasant soil because it attracts small invertebrates, so they carry the Streptomyces spores elsewhere. 

The reason the fatty acids and geosmin molecules don’t produce the smell of petrichor until they are wet is because they are not released into the atmosphere yet, but are stuck in the soil, accumulating over the long period of dry weather. When it rains lightly and the water droplets reach the soil, they trap the air bubbles in the dry soil underneath them. These air bubbles rise from under the rain drops, through them, then out of the soil as a fine mist which enters the atmosphere (similarly to the bubbles in a fizzy drink). 

The air now contains the geosmin and molecules from the plant so the mist carries the smell of petrichor into the air around us, not just in the soil. Furthermore, if there is rain and lightning after a period of dry weather, the ozone molecules produced by lightning enhance the smell of petrichor. In the atmosphere, there are diatomic oxygen molecules and diatomic nitrogen molecules (two oxygens or two nitrogens bonded together). 

When lightning strikes, the bonds holding the two oxygens or two carbons together are broken and the molecules are rearranged. This produces nitric oxide and ozone. The ozone molecules dissolve in the rain droplets, which fall to the earth and contribute to the smell of petrichor. 

An important thing to note is that more heavy rainfall doesn’t result in a stronger smell of petrichor. This is because heavy, continuous rain droplets actually reduce the smell of petrichor as they push down the air bubbles in the soil that contain the plant oils and geosmin, preventing them from being released to the atmosphere.