# What does it mean that a substance can be smelled from far away?

Ok, Thioacetone takes the price for the World's smelliest chemical, I can accept it (why not?), but what about

You can smell one drop of this substance, almost instantly, from half a kilometer away

?

I'm an aerospace engineer, so I know pretty much about transport phenomena, but maybe I miss something about the kinetic theory of gases...

As far as I know, how far can I smell a substance depends on:

• how much substance there is,
• how good my nose is at detecting that kind of molecules (the substance),
• how favorable a lot of factors are for that molecules to actually reach my nose!

In the third point, by "a lot of factors" I mean temperature, density, ..., and, most importantly, convection! If the winds takes that molecules away from me, or if it just doesn't take them towards my nose, it doesn't matter how much substance there is and how good my nose is, does it?

The only thing I can think of that supports the quoted sentece is that despite fluid particles are not macroscopically driven to my nose (because driven away by wind or not driven at all by steady air), some molecules could still reach it thanks to their individual velocity.

Will they? I mean, in a drop there are many many molecules (order of 10 to the... let's say 15, shouldn't be far from reality), but they will still simply hit sourrounding air molecules (which should be not far from the same in number per cube centiemter), just like any other gas.

So what am I missing?

You are not missing anything. Rather I think you are placing too much emphasis on the scientific accuracy of something said for effect in a very chatty presentation.

The spoken words almost instantly are not repeated in the headlines, whereas the rest of the quote is. And the presenter does not develop this idea of being able to smell thioacetone almost instantly, so I doubt that he is making a scientific claim that it diffuses extraordinarily fast. What is being emphasized is the distance - which equates to being able to detect the chemical at very low concentrations.

The wikipedia article on thioacetone cites the experience of Esso researchers in 1967, who detected the chemical "downwind within seconds" from about $400$m away. It is not stated how many seconds, nor the wind speed. Since the average speed of air molecules is around 500 m/s, the transport of thioacetone by $400$m in say 5s is not physically impossible.

Diffusion in still air over distances of 400m usually takes a few hours rather than seconds, so I think that advection (the bulk movement of air) is likely to have been the dominant factor in the reported cases (as you suggested). Windspeeds at 10m above ground are typically 5m/s, decreasing downwards, so a transport time on the order of 100s is reasonable. Strong winds would reduce this time, but the descriptions "within seconds" and "almost instantly" seem somewhat exaggerated.

• Yes, maybe the chatty presentation needn't be very accurate, but the concept of "can be smelt from long distances" is widely used in a lot of scientific didactic presentation (even less chatty). Anyway, as you say in the last paragraph of your answer, that distance depends much more on the nose than on the substance. – Enrico Maria De Angelis Jul 2 '17 at 20:04
• Since the average speed of air molecules [...] is not impossible holds in general. But molecules don't travel freely; the mean free path is not so long, after all, is it? – Enrico Maria De Angelis Jul 3 '17 at 8:06
• Yes that is the point I am making. It takes the average air molecule 1s to travel 500m in a straight line, so reaching the same endpoint along a zig-zag path will take longer. If the smell were detected 500m away in air within much less than 1s - eg 1ms - then this would appear to defy the laws of physics. But diffusing this distance "within seconds" is reasonable. – sammy gerbil Jul 3 '17 at 8:28
• You point that convection is likely the dominant factor, but you offer no explanation to your conclusion or how the convection acts in this case. Doesn't convection play a role if the substance is lighter than air on average and would then tend to transport the substance upwards? How would that upward transport help detection in a horizontal distance? – Communisty Jul 3 '17 at 10:56
• @Communisty I was using the term "convection" in the loose sense of transportation by bulk movement of ambient fluid. I am alluding to dispersal on air currents. "Advection" is more precise so I shall edit my answer. – sammy gerbil Jul 3 '17 at 11:03

Obviously environmental conditions could prevent any substance being detectable at distance (wind in the wrong direction etc.). So, assuming the conditions are not against a substance being detectable, what makes Thioacetone so detectable (compared to other substances)?

From the Wikipedia page the key factors seem to me to be:

• The trimer of thioacetone [...] with a melting point of 24 °C, near room temperature
It will dissipate into the atmosphere very easily. It was also found "that dilution seemed to make the smell worse". Note as the side-box on that page states, the melting point of thioacetone itself is 40 °C.

• Like many low molecular weight organosulfur compounds [...]
Low weight will allow it to be carried for longer distances without sinking to the ground.

• [...] the smell is potent. And like such compounds it can be detected at high dilution.
Only very small quantities need to make it to your nose. The "sphere of detectability" will be much larger than for other compounds.

In other words, what makes one particular substance more detectable ("smelly") than an other is how easily its molecules can enter the air, how widely they can be dispersed without settling and how detectable they are if they reach your nose.

• Actually, Wikipedia says melting point 40°C in the info box and 24°C in the text. That article seems to be ... smelly. – Hagen von Eitzen Jul 3 '17 at 8:51
• @HagenvonEitzen Good spot, though having re-read the page, the 24°C is referring to the trimer of thioacetone whereas the box is presumably referring to the full polymer (edited to add details). I don't know enough chemistry to know all the ins and outs, but I assume the main argument still holds that the more volatile a substance is, the more easily it gets into the atmosphere and the more easily it can be detected. – TripeHound Jul 3 '17 at 9:02