For comparison, sharks detect blood in water in the parts per million range (which itself is already super impressive.)

https://en.wikipedia.org/wiki/Geosmin

https://www.acsh.org/news/2018/07/28/geosmin-why-we-smell-ai...

1. The distinctive scent, caused by geosmin, which accompanies the first rain after a long, warm, dry spell. 2. The yellow organic oil that yields this scent.

From Wiktionary: "The term was coined by Australian scientist Richard Grenfell Thomas in 1964 for the article “Nature of Argillaceous Odour”, co-authored by Isabel Joy Bear and published in the journal Nature."

Some nuggets from the paper:

> There is some evidence that drought-stricken cattle respond in a restless manner to this 'smell of rain' which may drift with the wind for considerable distances.

> We have learned from the Indian Standard Association that the production and concentration of argillaceous odour from baked clay have been, for many years, the basis of a small perfumery industry centred near Kannauj, some 80 miles west of Lucknow, U.P., India. Baked clay disks, exposed there under the open sun during the hot summer months of May and June, are collected before the wet season and are steam-distilled and the vapours containing the odour and associated products are absorbed in sandalwood oil. The perfume so obtained is known as 'matti ka attar', which may be translated as 'earth perfume'.

> The diverse nature of the host materials has led us to propose the name 'petrichor' for this apparently unique odour which can be regarded as an 'ichor' or 'tenuous essence' derived from rock or stone. This name, unlike the general term 'argillaceous odour', avoids the unwarranted implication that the phenomenon is restricted to clays or argillaceous materials; it does not imply that petrichor is necessarily a fixed chemical entity but rather it denotes an integral odour, variable within a certain easily recognizable osmic latitude.

Otherwise known a one litre! Assuming that they mean US Customary gallon.