Photodissociation of Peroxynitric Acid in the Near-IR

Abstract

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.

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