Showing papers by "A. R. Ravishankara published in 1992"

Measurement of hydroxyl and hydroperoxy radical uptake coefficients on water and sulfuric acid surfaces

Abstract: A wetted wall flow tube reactor was used to measure the uptake coefficients, γ, of OH and HO 2 on pure water at 275 K and 28% w/w sulfuric acid at 249 K. The uptake coefficients are lower limits to the mass accommodation coefficients, α, and the γ were determined to be 0.0035 for OH and >0.01 for HO 2 on liquid water and >0.08 for OH and >0.05 for HO 2 on the sulfuric and solution. In addition, the binary diffusion coefficients for HO 2 and OH in water vapor were estimated to be 79±8 and 116±20 Torr cm 2 s -1 , respectively, at 275 K

Observation of methylthiyl radical addition to oxygen in the gas phase

Abstract: This report observes for the first time the reversible addition of: CH{sub 3}S + O{sub 2} {r_equilibrium} CH{sub 3}SOO, in the gas phase (T = 216-258 K) by detecting CH{sub 3}S radicals. Also briefly described is the thermochemical and kinetic information of this system and its importance to the atmospheric oxidation of reduced sulfur compounds.

Photodissociation of HNO3 at 193, 222, and 248 nm: Products and quantum yields

Abstract: The quantum yields for OH, O(3P), O(1D), and H(2S) from the photolysis of HNO3 have been determined at 248, 222, and 193 nm at 298 K. The quantum yield for OH was observed to be near unity at 248 [Φ(OH)=0.95±0.09] and 222 nm [Φ(OH)=0.90±0.11]. However, at 193 nm the quantum yield for OH was found to be 0.33±0.06. The quantum yield for O atoms [O(3P)+O(1D)], Φ(O), was observed to be 0.031±0.010, 0.20±0.03, 0.81±0.13, at 248, 222, and 193 nm, respectively. Both O(3P) and O(1D) were observed at 222 and 193 nm, but only O(3P) was detected at 248 nm. It was observed that 40% of the O atoms formed at both 193 and 222 nm were O(1D) atoms. The upper limits for H(2S) quantum yields: Φ(H)≤0.002 at 248 nm, ≤0.01 at 222 nm, and ≤0.012 at 193 nm were also determined. This present work quantifies the photolysis channel producing HONO+O and show that it accounts for a large fraction at HNO3 photolysis at 193 nm.

Photodissociation of H2O2 at 193 and 222 nm: Products and quantum yields

Abstract: The primary quantum yields of OH(X 2Π),H(2S), and oxygen atoms [O(1D)+O(3P)] produced in the photodissociation of H2O2 at 193 and 222 nm have been measured at 298 K. At 193 nm, the primary quantum yields were observed to be 1.51±0.18, 0.16±0.04, and <0.02, for Φ(OH), Φ(H), and the sum of Φ(O) and Φ(O 1S), respectively. At 222 nm, the OH yield was Φ(OH)=2.02±0.35, the H atom yield was Φ(H)=0.024±0.012, and Φ(O) was <0.002. The errors quoted above are 2σ, precision plus estimated systematic errors. The OH product was directly monitored by pulsed laser‐induced fluorescence, and the atomic species were detected via cw resonance fluorescence. The OH quantum yields reported here were measured relative to known product quantum yields in the dissocation of H2O2 at 248 nm. H(2S) yields were measured relative to those in photolysis of HBr and HCl, (at 193 nm) or CH3SH (at 222 nm), whereas O atoms yields were measured relative to O3 photolysis at both wavelengths. The present results indicate unit dissociation of H2...

Kinetics and mechanisms of the reactions of methylthio, methylsulfinyl, and methyldithio radicals with ozone at 300 K and low pressures

Abstract: The reactions of CH{sub 3}S + O{sub 3} {r_arrow} products (1), CH{sub 3}SO + O{sub 3} {r_arrow} products (2), and CH{sub 3}SS + O{sub 3} {r_arrow} (3) were investigated at 300 K in a discharge flow tube reactor coupled to a photoionization mass spectrometer. The measured value of {kappa}{sub 1} is (5.7 {plus_minus} 1.4) x 10{sup {minus}12} cm{sup 3} molecule {sup {minus}1} s{sup {minus}1} Torr He. The authors observed that OH was produced in this reaction or in subsequent steps and that complex branched chain reactions, which generate CH{sub 3}S from its precursor molecule, took place in the flow tube reactor. The authors found that CH{sub 3}SO was a product of reaction 1 and the branching ratio for this channel was 15 {plus_minus} 4%, between 0.7 and 2.2 Torr He, independent of pressure. A preliminary value of {kappa}{sub 2} = (6 {plus_minus} 3) x 10{sup {minus}13} cm{sup {minus}3} molecule{sup {minus}1} s{sup {minus}1} was measured. CH{sub 3}S was not a major product of reaction 2. These results suggest that the reaction with O{sub 3} is a major CH{sub 3}S removal process in the atmosphere. The rate coefficient for the reaction of CH{sub 3}SS with O{sub 3}(3) was measured to be {kappa}{submore » 3} = (4.6 {plus_minus} 1.1) x 10{sup {minus}13} cm{sup {minus}3} molecule{sup {minus}1}s{sup {minus}1}. 26 refs., 6 figs., 1 tab.« less