[1] Laboratory measurements of the quenching of CO 2 (n 2 ) by O atoms are presented over the 142-490 K temperature range relevant to the 75-120 km altitude region of the terrestrial atmosphere. The primary cooling mechanism in this region occurs when CO 2 is efficiently excited through collisions with ambient O atoms, populating the bending vibrational (n 2 ) modes. A significant fraction of the vibrationally excited CO 2 relaxes through spontaneous 15-mm emission that escapes into space, thereby removing kinetic energy from this region of the atmosphere and generating a local cooling effect. The rate coefficient for the vibrational relaxation of CO 2 (n 2 ) by O atoms, k O (n 2 ), is measured using transient diode laser absorption spectroscopy. A slight negative temperature dependence is observed for k O (n 2 ), with values ranging from 2.7 (AE0.4) Â 10 À12 cm 3 s À1 at 142 K to 1.3 (AE0.2) Â 10 À12 cm 3 s À1 at 490 K.