Showing papers by "A. R. Ravishankara published in 1997"

Temperature dependence of UV absorption cross sections and atmospheric implications of several alkyl iodides

Abstract: The ultraviolet absorption spectra of a number of alkyl iodides which have been found in the troposphere, CH3I, C2H5I, CH3CH2CH2I, CH3CHICH3, CH2I2, and CH2ClI, have been measured over the wavelength range 200–380 nm and at temperatures between 298 and 210 K. The absorption spectra of the monoiodides CH3I, C2H5I, CH3CH2CH2I, and CH3CHICH3 are nearly identical in shape and magnitude and consist of single broad bands centered near 260 nm. The addition of a chlorine atom in CH2ClI shifts its spectrum to longer wavelengths (σmax at 270 nm). The spectrum of CH2I2 is further red-shifted, reaching a maximum of 3.85×10−18 cm2 molecule−1 at 288 nm and exhibiting strong absorption in the solar actinic region, λ>290 nm. Atmospheric photolysis rate constants, J values, have been calculated assuming quantum efficiencies of unity for different solar zenith angles as a function of altitude using the newly measured cross sections. Surface photolysis rate constants, calculated from the absorption cross sections measured at 298 K, range from 3×10−6 s−1 for CH3I to 5×10−3 s−1 for CH2I2 at a solar zenith angle of 40°.

Atmospheric fate of several alkyl nitrates Part2UV absorption cross-sectionsand 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.

Atmospheric fate of several alkyl nitrates Part 1Rate coefficients of the reactions of alkyl nitrates with isotopically labelled hydroxyl radicals

Abstract: The rate coefficients for the reaction of OH radical with CH 3 ONO 2 (k 1 ), CD 3 ONO 2 (k 6 ), C 2 H 5 ONO 2 (k 2 ) and 2-C 3 H 7 ONO 2 (k 3 ) between 220 and 410 K were measured to be: k 1 =(8.2±1.6)×10 -13 exp{-(1020±60)/T}, k 6 =(1.6±0.2)×10 -12 exp{-(1730±40)/T}, k 2 =3.68×10 -12 exp(-1077/T)+5.32×10 -14 exp(126/T) and k 3 =4.3×10 -12 exp(-1250/T)+2.5×10 -13 exp(-32/T) cm 3 molecule -1 s -1 . The quoted errors in the Arrhenius expressions are at the 95% confidence limit and include estimated systematic errors. The measured rate coefficients for the reactions of 18 OH were roughly the same as those for 16 OH reactions while the rate constants for OD reactions were slightly (ca. 10–15%) higher than those for OH reactions. The hydroxyl radicals were generated by pulsed laser photolysis and detected by pulsed laser induced fluorescence. The observed large primary kinetic isotope effect (k 1 /k 6 =4.5 at 298 K), the independence of the measured rate coefficients on the gas composition, and the trend in the reactivity with the alkyl group suggests that the above reactions proceed via abstraction of an H atom from the alkyl group. Our results are compared with those from previous studies.