Observation of gas-phase peroxynitrous and peroxynitric acid during the photolysis of nitrate in acidified frozen solutions

► Photolysis of nitrate in frozen solutions produces volatile nitrogen oxides. ► Under neutral or basic conditions, the dominant species emitted is NO2. ► Under acidic conditions, the nitrogen oxides released are mainly NOz species. ► Peroxynitrous and peroxynitric acid were observed by chemical ionization mass spectrometry. ► HOONO may be an important but overlooked intermediate in the troposphere.