^a This is the average concentration at sea level and varies considerably at that level. It also falls off with increasing altitude since it is not a 'permanent' gas like the other greenhouse gases

^b This is the average concentration in the troposphere; the first ~15 km of the atmosphere. There is a higher concentration in the ozone layer in the stratosphere around 35 km altitude.

The units of concentration need some explanation. Those for water vapour and CO₂ are parts per million by volume; ppmv. The other greenhouse gases have lower concentrations that are given as parts per billion by volume; ppbv.

By comparison the concentration of nitrogen [N₂], 78% by volume is equivalent to 780,000 parts per million by volume.

Sometimes concentrations are given in terms of parts per million by mass (ppm) and are considerably different numbers from ppmv. This is because they take into account the relative molar masses of the gases. For example the 380 ppmv concentration of CO₂ is 380 x 44/28.9 = 579 ppm. The relative molar mass of CO₂ is 44 and the average value for the dry atmosphere is 28.9. Only chemists know about this!

The variation of CO₂ concentration in the atmosphere from pre-industrial times is very important in assessing the possible enhancement of the greenhouse effect.

There are small concentrations of other gases in the atmosphere including the inert gases [He, Ne, Ar, Kr, and Xe] and carbon monoxide and hydrogen. And at even lower concentrations are lots of smelly things.