Does indoor odor smell travel up or down?

Question

Does odor smell, let's say it's from caulk off-gasing, travel upward or downward in the air?

Are all odor smell lighter than air?

Answer 1

For reasons of simplicity, suppose you are talking about a building that is left alone, and hence, there is no forced air movement (e.g. from people walking around, dehumidifiers, draught, thermal buoyancy due to heating, etc.).

Whether odor smells will travel upward or downward then is not determined by the mass of their constituting particles (related to their vapor densities) but rather by their initial concentrations (or rather their gradients). If the initial odor concentrations are already at their thermodynamic equilibrium values, nothing will "travel" at all.

But if you have initially high concentrations of, say, acetic acid fumes from fresh silicone caulk in the basement of your building, while the top floor is still uncaulked and free from acetic acid vapor, the acid smell will definitely travel upward. Conversely, if you have first caulked the top floor before the basement, the odor will travel downwards.

In both cases, the process just follows the concentration gradients according to Fick's laws, and it will continue until thermodynamic equilibrium is reached, or rather say approximated (at least if you do not disturb equilibrium by ventilating the building or caulking for the rest of your life). For all practical matters, "equilibrium" simply means "equal concentration everywhere" (see point 1 below for an explanation).

What is determined by the particle masses are

1. the equilibrium pressures and concentrations (very slightly less equilibrium odor density at the top floor compared to the basement); since mass differs by gas, the different gases in a mixture change differently with height; see this answer for the calculation of the relative particle concentrations (e.g. for the mixture of air and some defined odor) with height; the differences are minute: only 0.4% for a whopping 100u particle mass difference and a height difference of 10 meters; probably not even a detection dog, walking from basement to top floor, can notice a 0.4% relative concentration difference before the background concentration (dogs can, of course, detect very small absolute concentrations before zero); therefore, we can practically assume that equilibrium concentration is homogeneous for every component gas and is determined by Dalton's law applied at ground level
2. the speed of travel; at the same pressure, regions with a higher proportion (see again Dalton's law) of heavier particles tend to be denser and will cause diffusive buoyancy of the less dense regions, which can be considered a fast bypass to the otherwise slow diffusion processes; this does in fact constitute an up/down preference, theoretically; however, as already noted by Philip Wood, the typical concentrations of odor molecules are very very small (for a typical building I would say, a few grams of acetic acid in a few hundred kilograms of air, so more like 1:100000 to 1:1000000 and less), i.e. even big relative changes in the concentrations of heavy odor molecules will change the density of the air/odor mixture only negligibly; therefore, diffusive buoyancy is irrelevant for smell distribution speeds
3. the diffusivity of the particles, but this is not a gravitational effect but rather an inertial (collisional) effect, i.e. some substances travel faster than others, but it is not that diffusion prefers up or down (other than due to concentration gradients)

If, contrary to my above assumption, the building is not left alone, there is an extremely wide range of possible convection scenarios, some of which might change the speed of odor travel dramatically (by many orders of magnitude). But then again, this does not have any much significance with respect to gravitational mass/density of the odor in question.

As to the question if all odor molecules are lighter than air: most odor molecules are organic and often contain a benzene ring or a chain of several carbon atoms (e.g. esters in scents), making them clearly heavier than all air molecule types. Nitrogen has a molar mass of 28 grams, oxygen has 32 grams. By comparison, benzene has a molar mass of 78 grams, whereas my silicone caulk example, acetic acid has 60 grams. The lightest olfactorily active substance is methane (no! see p.s. at the bottom of this answer) with a molar mass of 16 grams, followed by ammonia with 17 grams and hydrogen fluoride with 20 grams. Ethylene is already at 28 grams and ethane at 30 grams. So, if we exclude hydrogen fluoride (which you will probably only smell right before you die), only methane and ammonia are stinks that are significantly lighter than air. All others are at least about as heavy as air or even (much) heavier.

Also note that the transport speed or concentration of an odor is often much less relevant than the olfactoric activity itself. Some substances can be detected by the nose in extremely low concentrations, which can be achieved very rapidly, surprisingly far away from the source, even if nominal diffusivity seems low. If, after first detection, concentrations rise, the nose gets numb to these smells, and we think nothing has changed ("the smell isn't moving anymore") although concentration changes more than ever. An extreme example for that numbing effect is hydrogen sulfide (fowl eggs' smell), which can be smelled very sensibly at first, but after a few minutes of exposure can't be smelled at all! Since hydrogen sulfide is toxic in higher concentrations, this effect can be very dangerous.

Sorry, small correction necessary: methane is characterized as odorless. People (including myself) often think that it stinks because it mostly occurs (like in cow dung) together with other stinky substances, e.g. sulfuric compounds. For methane (as well as propane and butane) fuel the stinky compounds are even deliberately added for safety reasons in order to make the (toxic and flammable) methane easily detectable by humans.

Answer 2

Most smells consist of relatively few (probably fewer than 1 in 1000) molecules mixed in with air molecules, and the spread of the smell is largely governed by the draughts and convection currents which the air would experience even without the odour molecules.

In still air the odour molecules will spread by diffusion, that is by the speeds (usually several hundred metres per second) that they naturally possess at a given temperature; but they travel only very short distances before encountering collisions with air molecules, so diffusion is at far lower speeds.

For gases that are denser than air, that is those with molecules of more mass than those of oxygen or nitrogen, the pull of the Earth's gravity will favour downward diffusion, but upward diffusion will be favoured for lighter molecules. So to know which will be favoured you must find out whether the molecular mass of the odour molecule is larger or smaller than about 30 u (that is 30 Da). I believe that most odour molecules (particularly those of scents) have a significantly greater mass than 30 u, but it would be unsafe to assume that this is the case for all odour molecules. Note also that although downward diffusion is favoured for heavier molecules, they will still diffuse upward and sideways as well.

Answer 3

I follow here the concept that laconic, not elaborated questions, if not already closed, do not ask for long, elaborated, detailed and exhaustive answers.

If the smelling stuff is locally a major air component, affecting air density, then it depends on the overall mean effective molar mass. As gas density, if taken as an ideal gas, is $\rho=\frac{pM}{RT}$. A dense gas would flow down and vice versa. Near all smelling stuff would be denser than air, with few exceptions like ammonia or hydrogen fluoride or methane (smelling if scented as NG).

If it is just a trace addition to air, changed air density does not play any role. Therefore, the direction of smell spreading depends on already existing air convection, either thermal or forced.

If both effects are in place then the result is subject of vector addition of both effects.

Aside of both ways of macroscopic convection, there is omnidirectional gas diffusion according to the concentration gradient.