NO2 quantum yields from ultraviolet photodissociation of methyl and isopropyl nitrate

Quantum yields, Phi(NO2), are reported for the NO2 formation channel (RO + NO2) following photodissociation of methyl and isopropyl nitrate (CH3ONO2 and C3H7ONO2) at the tropospherically relevant wavelengths of 308-320 nm. Measurements were made using ultra-violet (UV) laser absorption and cavity ring down spectroscopy (CRDS) detection of the NO2 photoproducts. Analysis of CRDS data to obtain NO2 quantum yields required account to be taken of the spatially inhomogeneous NO2 distribution resulting from photolysis with a laser beam with a Gaussian intensity profile. Photodissociation quantum yields were measured to be unity (with <= 10% uncertainties (2SD)) for both methyl and isopropyl nitrate at wavelengths of 308, 315 and 320 nm in the presence of N-2 bath gas at pressures up to 700 Torr, and at a temperature of 294 K. An apparent decrease in the NO2 yields, observed at bath gas pressures below 500 Torr, may result from photochemical formation of internally excited NO2 which is not thermalized by collisions on the timescale of the measurement, or from diffusion of the photoproducts. Quantum yield values from the current work are in agreement with previously reported values at 308 and 248 nm photo-excitation wavelengths.