Posted by: Prospector PJ12 SEP 2012
An aroma you may have detected frequently this summer is the scent of oncoming rain. As the wind picks up before a shower the sweet, pungent zing in your nostrils is the smell of ozone, which emanates from fertilisers and pollutants as well as natural sources. Christian Friedrich Schönbein, who first proposed that ozone was a distinct chemical substance in 1840, named it after the Greek verb ozein, to smell, after the peculiar odour in lightning storms.
The electrical charge from lightning splits atmospheric nitrogen and oxygen molecules into separate atoms which recombine to form nitric oxide. That in turn reacts with other atmospheric chemicals to form ozone, or O3. The aroma of ozone heralds stormy weather because a thunderstorm’s downdrafts carry ozone from higher altitudes down to the ground.
As a recent article in Scientific American goes on to explain, once the rain arrives other odours come with it. Rain drops displace odoriferous molecules from surfaces, particularly dry ones. For those near vegetation these smells will come from plants and trees, but townsfolk will detect aromas rising up from concrete constructions and bitumen in the roads. This collection of post precipitation scents was named “petrichor” in 1964. (For more on petrichor see Footler, PJ, 22/29 August 2009, p224.)
After a storm passes you may detect a hint of geosmin. This musty earth smell is a metabolic by-product of bacteria or blue-green algae (cyanobacteria). Communities whose water supplies depend on surface water can experience episodes of unpleasant-tasting water when a sharp drop in the population of these bacteria releases geosmin into the water supply.
The human nose is extremely sensitive to geosmin and is able to detect it at concentrations as low as five parts per trillion. It is responsible for the earthy taste of beets and the muddy smell in bottom dwelling freshwater fish such as carp and catfish. And because it decomposes into odourless substances in acidic conditions, vinegar can reduce the muddy flavour in certain fish recipes.
These weather smells could serve a useful purpose. Some biologists suspect that petrichor running into waterways acts as a cue for freshwater fish to spawn.
And geosmin could act as a beacon to help camels find desert oases. In return, the bacteria that produce the geosmin use camels as carriers for their spores.