Re: How do you measure the speed of aroma (smells) through air?

Dear User,

Scents are carried through air by diffusion and by convection. Convection
is movement of the air itself, as a medium and is much faster than
diffusion, normally. I assume that you refer to travel of scent through
still air, which is governed by diffusion.

Diffusion rates depend on several variables, including the concentration of
the molecules at the starting point and the type (and size) of the molecule
itself. Air molecules whizz around very fast (around the speeds of rifle
bullets, on the average). When you have a higher-than-average concentration
of something in the air, the molecular motion will eventually spread the
substance around until it is uniformly mixed. Larger molecules move slowly,
smaller molecules move faster. The larger the gradient of concentration,
the sooner the concentration of the scent molecules will reach the
threshold of detection. This is another factor, but a more subjective one.
Different substances have different detection thresholds by human nose, so
some things may *seem* to travel faster whereas they simply are detected
earlier. In general it is a safe approximation that the smaller the
molecule, the faster it travels.

Aerosol air freshners usually do not directly counteract vile smells. They
simply overload your nose and it becomes relatively insensitive to
unpleasant smells. Some aerosols in addition have substances which
"capture" smelly molecules in the aerosol drops. 

If you would like to read more on diffusion, I suggest to start with
Einsteinian models which are described in any semi-decent advanced physics
textbook.

Cheers,

A.G.E.

P.S. Or try this link to Brownian diffusion for a little more information
http://www.foresight.org/Nanomedicine/Ch03_1.html

Bill Reisdorf, Biophysics moderator