Dinitrogen Pentoxide - A compound that contributes to ozone depletion during springtime in the Antarctic. This occurs because ozone converts some nitrogen dioxide radical to nitrogen trioxide radical, which combines with other dinitrogen radicals to produce dinitrogen pentoxide. The reaction is reversible except in the presents of stratospheric liquid droplets where nitric acid occurs. This reaction ties up the nitrogen dioxide radical allowing more chlorine atoms to be present in the active form. 
Atmospheric Chemistry Glossary