07 How is the composition of air described? (Or, what is a 'mixing ratio'?)
Description
This article is from the Ozone Depletion FAQ, by Robert Parson rparson@spot.colorado.edu with numerous contributions by others.
07 How is the composition of air described? (Or, what is a 'mixing ratio'?)
The density of the air in the atmosphere depends upon altitude, and
in a complicated way because the temperature also varies with
altitude. It is therefore awkward to report concentrations of
atmospheric species in units like g/cc or molecules/cc. Instead,
it is convenient to report the "mole fraction", the relative
number of molecules of a given type in an air sample. Atmospheric
scientists usually call a mole fraction a "mixing ratio". Typical
units for mixing ratios are parts-per-million, billion, or
trillion by volume, designated as "ppmv", "ppbv", and "pptv"
respectively. (The expression "by volume" reflects Avogadro's Law -
for an ideal gas mixture, equal volumes contain equal numbers of
molecules - and serves to distinguish mixing ratios from "mass
fractions" which are given as parts-per-million by weight.) Thus
when someone says the mixing ratio of hydrogen chloride at 3 km
is 0.1 ppbv, he means that 1 out of every 10 billion molecules in
an air sample collected at that altitude will be an HCl molecule.
[Wayne] [Graedel and Crutzen]
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