Quantum Yields for Photochemical Production of NO₂ from Organic Nitrates at Tropospherically Relevant Wavelengths

Abstract

Absorption cross-sections and quantum yields for NO₂ production (Φ_NO₂) are reported for gaseous methyl, ethyl, <i>n</i>-propyl, and isopropyl nitrate at 294 K. Absorption cross-sections in the wavelength range of 240–320 nm agree well with prior determinations. NO₂ quantum yields at photoexcitation wavelengths of 290, 295, and 315 nm are unity within experimental uncertainties for all of the alkyl nitrates studied and are independent of bath gas (N₂) pressure for total sample pressures in the range of 250–700 Torr. When averaged over all wavelengths and sample pressures, values of Φ_NO₂ are 1.03 ± 0.05 (methyl nitrate), 0.98 ± 0.09 (ethyl nitrate), 1.01 ± 0.04 (<i>n</i>-propyl nitrate), and 1.00 ± 0.05 (isopropyl nitrate), with uncertainties corresponding to 1 standard deviation. Absorption cross-sections for ethyl nitrate, isopropyl nitrate, and two unsaturated dinitrate compounds, but-3-ene-1,2-diyl dinitrate and (<i>Z</i>)-but-2-ene-1,4-diyl dinitrate in acetonitrile solution, are compared to gas-phase values, and over the wavelength range of 260–315 nm, the gas-phase values are well-reproduced by dividing the liquid-phase cross-sections by 2.0, 1.6, 1.7, and 2.2, respectively. Reasonable estimates of the gas-phase absorption cross-sections for low-volatility organic nitrates can therefore be obtained by halving the values for acetonitrile solutions. The quantum yield for NO₂ formation from photoexcitation of but-3-ene-1,2-diyl dinitrate at 290 nm is significantly lower than those for the alkyl (mono) nitrates: a best estimate of Φ_NO₂ ≤ 0.25 is obtained from the experimental measurements