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

Abstract

Absorption cross-sections and quantum yields for NO2 production (ΦNO2) are reported for gaseous methyl, ethyl, n-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 ΦNO2 are 1.03 ± 0.05 (methyl nitrate), 0.98 ± 0.09 (ethyl nitrate), 1.01 ± 0.04 (n-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 (Z)-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 ΦNO2 ≤ 0.25 is obtained from the experimental measurements.