The concentrations of greenhouse gases in the atmosphere are very small compared to those of the main constituents, Nitrogen (78%) and Oxygen (21%), but nevertheless exert a significant effect on the temperature of the Earth's surface and its atmosphere

Greenhouse gas Concentration   
Water vapour 7600a ppmv
CO2 401 ppmv
N2O (nitrous oxide) 320 ppbv
CH4 (methane) 1780 ppbv
O3 (ozone) 28b ppbv

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 9-15 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 401 ppmv concentration of CO2 is 401 x 44/28.97 = 609 ppm. The relative molar mass of CO2 is 44 and the average value for the dry atmosphere is 28.97. Only chemists know about this! Chemists know the ppmv concentration as the mole fraction, i.e., the fraction of molecules of CO2 (say) in the gas sample, expressed as it is in ppmv. Mole fractions of gases are proportional to their partial pressures in the system.

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.