Why is the atmosphere a selective absorber of electromagnetic radiation and why does the surface behave almost like a black body?

As explained below.

Explanation:

The atmosphere on earth is mostly a result of the interaction of solar winds and earth gravitational pull. The planet takes incoming shortwave insolation that heats the surface from above and charges the surface from below. At night it gives this absorbed energy as a black body out in the space when it completes a cycle of 12 hours and this in long waveforms of energy with IR radiation that is quite less as compared to that of suns. Like a black body, the earth radiated this heat bk as it cannot afford to keep much as is to balance the energy flow within and out of the earth system.