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Showing papers by "James B. Burkholder published in 1997"


Journal ArticleDOI
TL;DR: In this paper, the photodissociation quantum yields for acetone in the 290-320 nm wavelength region for pressures and temperatures characteristic of the upper troposphere were compared with those obtained during the NASA and NOAA sponsored Stratospheric Tracers of Atmospheric Transport (STRAT) field campaign.
Abstract: This paper summarizes measured photodissociation quantum yields for acetone in the 290-320 nm wavelength region for pressures and temperatures characteristic of the upper troposphere. Calculations combine this laboratory data with trace gas concentrations obtained during the NASA and NOAA sponsored Stratospheric Tracers of Atmospheric Transport (STRAT) field campaign, in which measurements of OH, HO_(2), odd-nitrogen, and other compounds were collected over Hawaii, and west of California during fall and winter of 1995/1996. OH and HO_(2) concentrations within 2 to 5 km layers just below the tropopause are ∼50% larger than expected from O_(3), CH_(4), and H_(2)O chemistry alone. Although not measured during STRAT, acetone is inferred from CO measurements and acetone-CO correlations from a previous field study. These inferred acetone levels are a significant source of odd-hydrogen radicals that can explain a large part of the discrepancy in the upper troposphere. For lower altitudes, the inferred acetone makes a negligible contribution to HO_(x) (HO+HO_(2)), but influences NO_(y) partitioning. A major fraction of HO_(x) production by acetone is through CH_(2)O formation, and the HO_(x) discrepancy can also be explained by CH_(2)O levels in the 20 to 50 pptv range, regardless of the source.

201 citations


Journal ArticleDOI
TL;DR: The ultraviolet absorption spectra of a number of alkyl iodides have been measured over the wavelength range 200-380 nm and at temperatures between 298 and 210 K as mentioned in this paper.
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°.

104 citations


Journal ArticleDOI
TL;DR: In this paper, the photodissociation quantum yield of methyl, ethyl and isopropyl nitrates was measured using a diode array spectrometer in the temperature range of 240 to 360 K.
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.

97 citations


Journal ArticleDOI
TL;DR: In this paper, the rate coefficients for the reaction of OD radical with between 220 and 410 K were measured and the results showed that OD reactions were slightly (ca.10-15%) higher than those for OH reactions.
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.

64 citations


Journal ArticleDOI
TL;DR: In this article, the rate coefficient for the IO + BrO → products (1) reaction was measured using pulsed laser photolysis with a discharge flow tube for radical production and pulsed Laser-induced fluorescence and UV absorption for detection of IO and BrO radicals, respectively.
Abstract: The rate coefficient for the IO + BrO → products (1) reaction was measured using pulsed laser photolysis with a discharge flow tube for radical production and pulsed laser-induced fluorescence and UV absorption for detection of IO and BrO radicals, respectively. Reaction 1 was studied under pseudo-first-order conditions in IO with an excess of BrO between 204 and 388 K at total pressures of 6−15 Torr. The Arrhenius expression obtained for non-iodine atom producing channels is k1a(T) = (2.5 ± 1.0) × 10-11 exp[(260 ± 100)/T] cm3 molecule-1 s-1 independent of pressure. The rate coefficient for the reaction BrO + BrO → products (2) and the UV absorption cross sections of BrO as a function of temperature were also determined as part of this study. The implications of these results to the loss rate of stratospheric ozone are discussed.

57 citations


Journal ArticleDOI
TL;DR: The quantum yields for the production of Cl, O, and ClO in the photolysis of ClONO2, measured by detecting Cl and O atoms by atomic resonance fluorescence, are reported in this paper.
Abstract: The quantum yields for the production of Cl, O, and ClO in the photolysis of ClONO2, measured by detecting Cl and O atoms by atomic resonance fluorescence, are reported The resonance fluorescence signals were calibrated by generating known concentrations of Cl and O atoms from the photolysis of Cl2, HCl, O3, or N2O The quantum yields for Cl in the 1932, 2220, 24825, and 30815 nm photolysis of ClONO2 were 053 ± 010, 046 ± 010, 041 ± 013, and 064 ± 020, respectively The yields for O atoms at these wavelengths were, respectively, 037 ± 008, 017 ± 005, <010, and <005 The quoted uncertainties are 2σ precision of the measurements ClO radical was converted to Cl via addition of NO and its signal compared to the Cl atom signal produced by photolysis of ClONO2 The obtained quantum yields for ClO were 029 ± 020, 064 ± 020, 039 ± 019, and 037 ± 019, at 1932, 2220, 24825, and 30815 nm, respectively It appears that Cl + NO3 and ClO + NO2 are the major products in the photodissociat

19 citations