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This touches on how smell works, but my intent is ask about the mechanism for why do ionised smell like they do? I would like to get one step past ozone if that is the primary mechanism.

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Regarding our sense of smell, from the Wikipedia:

The perception of odors, or sense of smell, is mediated by the olfactory nerve. The olfactory receptor (OR) cells are neurons present in the olfactory epithelium, which is a small patch of tissue at the back of the nasal cavity. There are millions of olfactory receptor neurons that act as sensory signaling cells. Each neuron has cilia in direct contact with the air. Odorous molecules bind to receptor proteins extending from cilia and act as a chemical stimulus, initiating electric signals that travel along the olfactory nerve's axons to the brain.2

Regarding the odor of ozone, from the Wikipedia:

Ozone (/ˈoʊzoʊn/), or trioxygen, is an inorganic molecule with the chemical formula O 3. It is a pale blue gas with a distinctively pungent smell.

From this site:

While oxygen is not detectable by the human nose, ozone has an extremely low odor threshold. Humans have the ability to detect ozone [concentrations of ~10 ppb] before many expensive measuring devices. Also, ozone in air at high levels (greater than 0.1 ppm) is hazardous to human health.

Ozone is a powerful oxidant (far more so than dioxygen) and has many industrial and consumer applications related to oxidation. This same high oxidizing potential, however, causes ozone to damage mucous and respiratory tissues in animals, and also tissues in plants, above concentrations of about 0.1 ppm. While this makes ozone a potent respiratory hazard and pollutant near ground level [...]

There are quite a few articles available online that discuss the "odor of ozone". It is described by some as "metallic" or as "similar to chlorine" (although note that "chlorine smell" is often "pool smell" caused by chloramines).

Given the reactivity of ozone (it oxidizes just about anything containing carbon to $\ce{CO2}$, which explains why it can cause inflammation and is generally regarded as deleterious to health), it is an interesting question whether there exists a distinct odor receptor that senses ozone or whether the response is nonspecific or mediated by pain receptors and registered as the "pungent smell" of ozone. Or perhaps it is other molecules altogether rather than the interaction of ozone with nasal receptors that causes the smell.

All of this is made even more complicated by the apparent loss of olfactory perception caused by exposure to ozone:

  • An acute exposure to 0.2 ppm ozone impairs odor detection threshold.
  • The cause is not clear, different modes of action might play at role.
  • Olfactory threshold seems sensitive to photochemical air pollutants’ effects.

The "The Curious Dr. Z" blog hypothesizes on the source of "ozone smell" (see also reference 1 cited in the blog):

It turns out that our olfactory receptors might be able to bind free radicals, and, since free radicals and ozone frequently occur together, that’s what we’re actually smelling.

In 1991, a group of patients undergoing irradiation for cancer all complained about a strong, pungent odor they later identified as ozone. This lead to a study where researchers discovered that humans can actually detect the irradiation of tissue by detecting free radicals trapped in mucus.

Another, older theory, is that ozone acts as the signal for irradiation. Ozone binds to and reacts with olfactory receptors the just as other odorants do, and effectively acts as a warning of more damaging chemicals nearby.

We don’t know too much beyond this, and as far as I can tell, ozone olfaction is not a very active area of research. Bummer.

Since we are so sensitive to the smell of ozone, and we don’t smell much else when we do smell ozone, I suspect that ozone/free radicals have many potential binding sites (a lot of those “hands” can grab ozone). When the ozone/free radicals does bind, it does so with high affinity, meaning it easily binds to receptors. This persistent and overwhelming smell experience is similar to other smells that indicate something noxious, including smoke and rotting tissue.

References

  1. Lloyd M. Beidler. Olfaction, Handbook of Sensory Physiology, Chemical Senses. Springer Science & Business Media, 2012, ISBN 3642651267, 9783642651267
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