NO2 quantum yields from ultraviolet photodissociation of methyl and isopropyl nitrate

Abstract

Quantum yields, Φ_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 N₂ 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.