Mechansims operating a greenhouse is not the trapping of infrared radiation, but the restriction of convective losses when the air is warmed due to contact with the ground which is heated by solar radiation (heat sink).

This was demonstrated by R.W Wood whom built a model greenhouse and demonstrated that there is no difference in the temperature of the air inside by replacing the glass with rock salt (transparent to infrared radiation)

What matters is important is that solar radiation is not hindered by glass and the ground once heated warms up adjacent air, must be noted that turbulent movement of air does not remove heat rapidly.

THEREFORE greenhouses do NOT trap terrestrial radiation.

As we move up in the atmosphere the density decreases therefore adsorption of terrestrial radiation will take place near the surface (exception is ozone which occurs higher in contentrations in stratosphere and exerts a difference on radiative balance). Since water vapor is the most important absorber of terrestrial radiation we would expect the greatest part of absorption and emission to occur at the bottom of the atmosphere.

If a warming scenario were to occur where concentration of radiatively active gases increases and solar radiation absorption remains constant, lowest level of the atmosphere will warm up. The warming of the lower atmosphere will consequently emit more infrared radiation both upwards and downwards, achieving “balance” between incoming and outgoing radiation leading to surface and lower atmosphere warming and upper atmosphere cooling.

The consequence of having additional absorbing gases is for the surface to warm up the atmosphere expressed as a whole and effectively radiate from higher levels. This quantified as an average of troposphere and is expressed as radiative forcing and is the parameter which determines if the greenhouse effect is due to natural causes or human activity.

“So it is better not to regard these gases as putting a lid on the atmosphere, but rather of subtly altering how it redistrubes the heat absorbed from the Sun”.

Reference:

Burroughs, W. J. (2001). Climate Change A Multidisciplinary Approach, Cambride University Press.