ATMOSPHERIC FATE OF SEVERAL ALKYL NITRATES .2. UV ABSORPTION CROSS-SECTIONS AND PHOTODISSOCIATION QUANTUM YIELDS

The UV absorption cross-sections of methyl, ethyl and isopropyl nitrat e between 233 and 340 nm have been measured using a diode array spectr ometer in the temperature range 240-360 K. The absorption cross-sectio ns of these alkyl nitrates decrease with increasing wavelength and dec rease with decreasing temperature for lambda > 280 nm. The photodissoc iation quantum yield for CH3ONO2 to produce NO2 and CH3O 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 mm were found to be negligible using resonance fluores cence detection of the atoms. High quantum yields for O atoms were mea sured following 193 nm photolysis of methyl nitrate. The OH radical wa s measured to be a photoproduct with a very small quantum yield. Using the OH rate coefficients reported in the accompanying paper and the U V absorption cross-sections and the photodissociation quantum yields m easured here, the first-order rate constants for atmospheric loss of m ethyl, ethyl and isopropyl nitrate were calculated Photolysis was foun d to be the dominant atmospheric loss process for the three alkyl nitr ates.