What atmospheric process widens gas absorption lines under higher pressure?

Pressure broadening is the atmospheric process that widens gas absorption lines under higher pressure.

In a denser gas, molecules collide more often. Those collisions interrupt or perturb the radiating molecule’s energy state, shortening the effective lifetime of the transition. The uncertainty principle then permits a wider range of transition frequencies, so the absorption line spreads beyond its low-pressure center.

Pressure broadening is also called collision broadening. At sufficiently low pressures, the width is approximately proportional to pressure, and the idealized line profile is Lorentzian. In real atmospheric spectra, pressure broadening commonly combines with Doppler broadening, producing a Voigt profile.

A common mix-up is to blame every widened line on temperature. Doppler broadening is primarily caused by molecular motion and has a Gaussian profile; pressure broadening instead depends mainly on collisions and gas density. Because atmospheric pressure decreases with altitude, pressure-broadened greenhouse-gas lines are generally narrower higher in the atmosphere.