From AMS Glossary
Dinitrogen pentoxide, N2O5, is formed from the reaction of the nitrate radical with nitrogen dioxide.
It is, however, thermally unstable and is dissociated readily near the earth's surface to form its precursors. Higher in the atmosphere, it is an effective reservoir for active nitrogen. In solution it hydrolyzes to two nitric acid molecules. Removal of N2O5 on aqueous particles occurs very easily, causing an increase in acidity and a loss of active nitrogen (NO + NO2).