Photodissociation of Peroxynitric Acid in the Near-IR

Temperature-dependent near-IR photodissociation spectra were obtained for several vibrational overtone transitions of peroxynitric acid (HNO 4 ) with a tunable OPO photolysis/OH laser-induced-fluorescence system. Band-integrated photodissociation cross-sections ( ∫σ diss ), determined relative to that for the 3ν 1 OH stretching overtone, were measured for three dissociative bands. Assuming unit quantum efficiency for photodissociation of 3ν 1 we find 2ν 1 + ν 3 (8242 cm -1 ) = (1.21 × 10 -20 ) (independent of temperature), 2ν 1 (6900 cm -1 ) = 4.09 × 10 -18 * e (-826.5/ T ) (295 K > T > 224 K), and ν 1 + 2ν 3 (6252 cm -1 ) = 1.87 × 10 -19 * e (-1410.7/ T ) (278 K > T > 240 K) cm 2 molecule -1 cm -1 . The photodissociation cross-sections are independent of pressure over the range 2 to 40 Torr. Temperature-dependent quantum yields (φ) for these transitions were obtained using integrated absorption cross-sections (∫σ abs ) of HNO 4 overtone vibrations measured with a FTIR spectrometer. In the atmosphere, photodissociation in the infrared is dominated by excitation of the first overtone of the OH stretching vibration (2ν 1 ). Inclusion of all dissociative HNO 4 overtone and combination transitions yields a daytime IR photolysis rate of approximately 1 × 10 -5 s -1 . This process significantly shortens the estimated lifetime of HNO 4 in the upper troposphere and lower stratosphere.