From AMS Glossary
(Abbreviated PAN; also called peroxyacetic nitric anhydride.) Organic compound formed in the atmosphere from the addition of nitrogen dioxide, NO2, to the peroxyacyl radical formed in the oxidation of acetaldehyde.
PAN and its larger homologs are irritants to the eyes and breathing system. They are formed in dangerous levels in photochemical smogs. The PANs are thermally quite stable "reservoir" species and can transport active nitrogen from polluted to pristine regions.