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quote:Originally posted by jyskI understand that this "fresh and clean" smell is in fact, ground-level ozone. The irony is that is it's mildly toxic rather then cleansing.
Title: Genesis of petrichor Authors: Bear, I. J.; Thomas, R. G. Publication: Geochimica et Cosmochimica Acta, vol. 30, Issue 9, pp.869-879 (GeCoA Homepage) Publication Date: 09/1966 Origin: ELSEVIER Bibliographic Code: 1966GeCoA..30..869B Abstract"Petrichor", well known to mineralogists as argillaceous odour, is commonly observed as the pleasant and refreshing odour which frequently accompanies the first rains after a warm dry period. Several possible mechanisms have been considered in connection with the origin of this odour. These include the synthesis of odorous compounds on the clay or rock surface by spontaneous catalysis of atmospheric gases, the sorption of organic compounds from the atmosphere, catalytic transformation of sorbed compounds and microbial activity. Evidence is presented which suggests the atmosphere contains, as general contaminants, lipids, terpenes, carotenoids and other volatile decomposition products from animal and vegetable matter. The sorption of these compounds, or their oxygenated derivatives, by rocks and clays is controlled by the properties of the sorbent and the partial water vapour pressure of the atmosphere, low relative humidities favouring maximum uptake. Oxidation and transformation of sorbates take place on the rock surface and are accelerated by warm to hot climatic conditions. The odorous and volatile products of these processes are subsequently displaced from the pores of the rock by moisture when the relative humidity of the atmosphere approaches saturation. The possibility of a relationship between petrichor and petroleum formation is discussed.
What does ozone smell like?Ozone's characteristic odor can be smelled immediately after a lightening storm or near photocopiers and electric motors, all which generate ozone. Its odor has also been described as the smell of clean bed sheets or forest air.How is ozone formed by nature?Trees emit hydrocarbons, by-products of photosynthesis; together with sunlight, they produce ozone. The electrical discharge of lightning can also produce ozone. That is why the air always smells so fresh after a thunderstorm and in a forest.
The characteristic smell in the air that follows a recent shower is down to bacteria. These didn't come down in the last shower (!) but instead they're a family of ubiquitous filamentous soil-dwellers called the actinomycetes. This bacterial species make tiny "spores", which are metabolically inert or dormant forms of the bacteria and which remain resident in the soil when the ground dries out. The purpose of these spores is to enable the organism to survive dessication and other severe conditions for extended periods of time. When it rains the water hitting the ground ejects millions of them into the air where they drift up peoples' noses, contributing to the fresh and earthy smell of a recent shower. As a rule they don't pose a threat to humans - at least not to people with a healthy immune system - and making the air smell is not their primary goal. What will actually happen shortly after the shower is that the newly uncovered and dampened spores will settle back onto the ground and germinate. So you could look upon this process as part of the way that they disperse and spread themselves around the planet.Chris[This question has been answered by others above, including Paul (thanks). The purpose of this post is to draw strands together and give a thorough answer.]