Atmospheric fate of several alkyl nitrates Part 2UV absorption cross-sections and photodissociation quantum yields

Abstract

The UV absorption cross-sections of methyl, ethyl and isopropyl nitrate between 233 and 340 nm have been measured using a diode array spectrometer in the temperature range 240–360 K. The absorption cross-sections of these alkyl nitrates decrease with increasing wavelength and decrease with decreasing temperature for λ>280 nm. The photodissociation quantum yield for CH ₃ ONO ₂ to produce NO ₂ and CH ₃ O was found to be essentially unity at 248 nm using transient UV absorption methods. Production of O and H atoms in the photodissociation of methyl nitrate at 248 and 308 nm were found to be negligible using resonance fluorescence detection of the atoms. High quantum yields for O atoms were measured following 193 nm photolysis of methyl nitrate. The OH radical was measured to be a photoproduct with a very small quantum yield. Using the OH rate coefficients reported in the accompanying paper and the UV absorption cross-sections and the photodissociation quantum yields measured here, the first-order rate constants for atmospheric loss of methyl, ethyl and isopropyl nitrate were calculated. Photolysis was found to be the dominant atmospheric loss process for the three alkyl nitrates.