You’ve smelt it so many times before and it seems so familiar, but what is it? In this Everyday Olfactics we delve into those familiar smells to unearth their sometimes less familiar origins
You wake up one morning, open your window and the smell of fresh rain hitting the tarmac tells you it’s a bad day to hang the washing out. The next morning you open your window, there’s no rain but the smell of manure tells you it’s probably another bad day for hanging the washing out. What aromatic molecules are to blame for our ever growing pile of dirty laundry? Let’s take a look at the common smells that fill the air around us- the good, the bad and the gross!
RAIN, PETRICHOR AND PLANT MUNCHING BACTERIA
Some of us complain when it rains, some of us welcome the excuse for a rainy day, but we all treasure the smell of fresh rain on a summer’s day. The earthy aroma of warm rain water can be nostalgic and comforting and can evoke emotions of a fresh start and growth. This smell is known as Petrichor, a term coined in 1964, derived from the Greek ‘Petra’- stone and ‘ichor’-referring to the blood of the Gods.
The environment needs to be just right to get this smell filling the air (usually no God blood required). Firstly, a few warm days of the sun drying out the ground sets the stage, as the dryer the surface the more porous the surface. When rain hits these porous surfaces, it traps small air bubbles that then rise and ‘fizz’ over ground like champagne. The prolonged period prior, absent of rain, allows oils and bacteria in the ground to flourish and it is many of these compounds that release their aroma on that first rain. So in the depths of the forest, rain will smell much more earthy thanks to all those fungal spores, whilst a summer garden will smell floral as Geraniol and an abundance of Terpenes are hurled up into the air.
Along side all of these spores and terpenes is Geosmin (trans-1, 10-dimethyl-trans-9-decalol), the leading role in the Petrichor play. Geosmin is created by plant munching bacteria and detectable to the human nose at 5 parts per trillion 1. This molecule is what gives beetroot its earthy flavor and can make drinking water taste muddy when contaminated even at minuscule amounts. Geosmin is released just as the oils are, providing the overall earthy rain odour, topped off by the clean chlorine-like smell of ozone should lightning strike.
GAS, NUTS AND BRAINS
Gas, you’ve smelt it either from the Bunsen burner in a year 8 Chemistry class, the camping stove on that 2015 camping trip or perhaps that gas leak you heroically detected on the street outside somebody’s house. None of these times were you actually smelling gas however, because natural gas itself is odourless and undetectable to the human nose. Methanethiol is what we can smell, a molecule added by the gas industry to make any leaks easily detectable. Its odour is notoriously rancid, not too dissimilar to rotting cabbage but the molecule is actually prevalent in many other less harmful places, such as nuts, cheese… and our brains!