The particles in the atmosphere are mostly nitrogen molecules (N2) and oxygen molecules (O2). These molecules reflect infrared radiation; they do not absorb it. Other particles in the atmosphere include water vapor, methane, and carbon dioxide (CO2). These molecules absorb infrared radiation.
But why does CO2 absorb infrared radiation when N2 and O2 don’t? This has to do with the structure of the molecules. N2 and O2 only vibrate one way. CO2 vibrates the same way, and also in another way.
The way CO2 vibrates changes the distribution of charges in the CO2. This means the particle is excited and will eventually “relax” and emit electromagnetic radiation.