Search Results - (Author, Cooperation:A. R. Ravishankara)

2011-03-12

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

Biology

Chemistry and Pharmacology

Computer Science

Medicine

Natural Sciences in General

Physics

2012-08-21

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

Biology

Chemistry and Pharmacology

Computer Science

Medicine

Natural Sciences in General

Physics

Atmosphere/*chemistry ; *Convection ; Ozone/*chemistry ; *Seasons ; *Steam ; *Ultraviolet Rays

2018-02-07

Wiley-Blackwell

0148-0227

Geosciences

Physics

2012-03-01

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

Biology

Chemistry and Pharmacology

Computer Science

Medicine

Natural Sciences in General

Physics

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

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.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

The primary quantum yields, ΦBr, for the formation of Br atom in the photodissociation of CF2Br2 and CH3Br at 248, 222, and 193 nm, and of CF3Br at 222 and 193 nm were measured at 298 K. The bromine atoms were directly detected via resonance fluorescence following pulsed laser photolysis of the molecules of interest. The Br atom quantum yields in CF2Br2 photolysis increased with decreasing wavelengths: 1.01±0.15, 1.63±0.19, and 1.96±0.27 at 248, 222, and 193 nm, respectively. The ΦBr values in CH3Br and CF3Br were close to unity at all the wavelengths: 0.92±0.15 and 1.12±0.16 at 222 and 193 nm, respectively, for CF3Br; 1.01±0.16, 1.10±0.20, and 1.05±0.11 at 248, 222, and 193 nm, respectively, for CH3Br. Quantum yield of H atom formation in the photolysis of CH3Br at 193 nm was measured to be 0.002±0.001. H atom could not be detected in the photolysis at 248 and 222 nm. In all cases the ΦBr values were found to be independent of buffer gas pressure or the photolysis laser fluence. Our results suggest that the quantum yields for dissociation of all the molecules considered here are unity; therefore, atmospheric lifetime calculations carried out by assuming a unit dissociation quantum yield are correct. The nature of the photodissociation process is discussed in the light of previous and present results.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

The quantum yields of the products, OH(X 2Π), O(3P) [plus O(1D)] and H(2S), in the photolysis of H2O2 and CH3OOH at 248 nm and 298 K have been measured. OH was directly observed by laser-induced fluorescence while the atomic species were detected by cw-resonance fluorescence. All quantum yield measurements were made using relative methods. The quantum yields of OH, O, and H in H2O2 photolysis were measured relative to the well known quantum yields of O(1D) and O(3P) in the photodissociation of O3, and H(2S) in CH3SH. The values we obtain are, 2.09±0.36, 〈0.002 and 〈0.0002 for OH, O, and H, respectively. For CH3OOH photolysis, the quantum yield of OH was measured relative to our value for OH quantum yield in H2O2 photolysis, and the quantum yields of O and H relative to those in O3 and CH3SH photodissociation, respectively. The values we obtain are, 1.00±0.18, 〈0.007 and 0.038±0.007 for OH, O, and H, respectively. In both H2O2 and CH3OOH photolysis, the observed O and H quantum yields showed an apparent dependence on the fluence of the photolysis light, the possible origin of which is discussed. The large quantum yield of OH we measure is consistent with the known continuous and unstructured absorption spectra of these molecules in this wavelength region, where the most important process is the dissociative (A˜ 1A←X˜ 1A) transition to give OH(X 2Π, v‘=0) fragment.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

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 H2O2 at both 222 and 193 nm with only two major products OH (∼80% at 193 nm, 98% at 222 nm) and H(2S) (∼20% at 193 nm, 2% at 222 nm). Up to 15% of the OH produced in the 193 nm photolysis may be vibrationally excited; however, no evidence for vibrationally excited OH was observed at 222 nm.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

The quantum yields for the formation of O(1D) and O(3P) from the photolysis of ozone have been measured at 222 and 193 nm. The quantum yield for O(3P) was observed to be Φ(3P)=0.13±0.02 at 222 nm, and the primary quantum yield for O(1D) was found to be Φ(1D)=0.87±0.04. This measurement is consistent with other studies of O3 photolysis within the Hartley band which indicate that a significant portion (5–15%) of the products are formed in the ground state. At 193 nm the quantum yield for the production of excited state O(1D) atoms is Φ(1D)=0.46±0.29, which is significantly less than what is observed during photolysis within the lower energy Hartley band. The quantum yield for O(3P) atoms at 193 nm was found to be 0.57±0.14. We have also observed that the quantum yield for O atoms [O(1D)+O(3P)] is greater than unity [Φ(O)=1.20±0.15] at 193 nm indicating the presence of a channel which produces three O(3P) atoms. O2(b 1Σ) has also been detected and quantified, Φ(b 1Σ)=0.50±0.38, from O3 photolysis at 193 nm. These measurements suggest that O(1D) and O2(b1Σ) are co-produced.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

A pulsed laser photolysis technique has been employed to study the kinetics of the important stratospheric reaction O+ClO→k1Cl+O2 in N2 buffer gas over the temperature and pressure ranges 231–367 K and 25–500 Torr. 351 nm pulsed laser photolysis of Cl2/O3/N2 mixtures produced Cl atoms in excess over O3. After a delay sufficient for the reaction Cl+O3→ClO+O2 to go to completion, a small fraction of the ClO was photolyzed at 266 nm to produce O(3P). The decay of O(3P) in the presence of an excess, known concentration of ClO was then followed by time-resolved resonance fluorescence spectroscopy. We find that k1 is independent of pressure, but that k1(T) increases with decreasing temperature. Our results suggest that the Arrhenius expression k1(T)=(1.55±0.33)×10−11 exp{(263±60)/T} cm3 molecule−1 s−1 is appropriate for modeling stratospheric chemistry. Errors in the Arrhenius expression are 2σ and represent precison only. The absolute accuracy of k1 at any temperature within the range studied is estimated to be ±20%. Our results agree with other recent measurements of k1 at 298 K but give significantly faster rate coefficients at stratospheric temperatures. A few measurements of the rate coefficient for the reaction ClO+ClO→k7products were also carried out. These measurements were necessary to assess the time dependence of [ClO].

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

A tunable pulsed dye laser was used to pump OD to the (A 2∑+,v′=1) level. The fluorescence excitation spectrum of OD in the wavelength region 287.26–290.03 nm in the A→X system was recorded. The rate coefficients for quenching of OD (A 2∑+) from v′=0 and v′=1 levels by O2, N2, air, and SF6 were measured. The rate coefficient for vibrational relaxation of OD (A 2∑+) from v′=1 to v′=0 level was separated from the overall rate coefficient for quenching of OD (A 2∑+) from v′=1 level to the ground (X 2Π) state for the above four gases. The rate coefficients for quenching of OD (A 2∑+) from v′=0 level to the ground (X 2Π) state by 15 common gases were measured by using excess SF6 to rapidly quench the OD (A 2∑+) from v′=1 to v′=0 level. All measurements were carried out in excess helium and are for rotationally thermalized OD (A 2∑+) radicals.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

Rate coefficients for the reaction OH+HBr → Br+H2O have been measured at 298 K using two different experimental techniques: (1) 266 nm pulsed laser photolysis of O3/H2O/HBr/He mixtures followed by time resolved resonance fluorescence detection of OH (LFP-RF) and (2) 248 nm pulsed laser photolysis of H2O2/HBr/Ar mixtures followed by pulsed laser induced fluorescence detection of OH (LFP-LIF). In both sets of experiments, the HBr concentration was monitored in situ in the slow flow system by UV photometry. Measured rate coefficients in units of 10−12 cm3 molecule−1 s−1 are 11.4±0.3 (LFP-RF) and 10.7±0.3 (LFP-LIF) where the uncertainties are 2σ and represent precision only. The absolute accuracy of the results is estimated to be ±10%. The experimental k1 values are in good agreement with a previous LFP-RF study in our laboratory but significantly faster than values reported by several other groups.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

Time resolved resonance fluorescence detection of O(3P) and H(2S) has been employed in conjunction with pulsed laser photolysis of O3/HX/He mixtures to determine absolute rate coefficients and product yields for reactions of O(1D) with HCl(1) and HBr(2) at 297 K. Total rate coefficients for O(1D) removal are found to be, in units of 10−10 cm3 molecule−1 s−1, k1=1.50±0.18 and k2=1.48±0.16, where the quoted errors are 2σ and represent precision only. The absolute accuracy of these rate coefficients is estimated to be ±20%. Formation of O(3P)+HX is found to account for 9%±5% of the overall rate of reaction (1) and 20%±7% of the overall rate of reaction (2). Formation of H(2S)+XO is found to account for 24%±5% of the overall rate of reaction (1) but 〈4.5% of the overall rate of reaction (2). By difference, the OH+X(2P) channel accounts for 67%±10% of the overall rate of reaction (1) and 80%±12% of the overall rate of reaction (2).

Electronic Resource

1476-4687

Nature Archives 1869 - 2009

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

[Auszug] Of all the trace tropospheric species (that is, excluding H2O and CO2) methane contributes most to the infrared heating of the atmosphere1'2. Methane is also the most abundant hydrocar-bon in the troposphere where it modulates the concentration of the OH free radical and serves as a source of CO. ...

Electronic Resource

0538-8066

Chemistry ; Physical Chemistry

Wiley InterScience Backfile Collection 1832-2000

Chemistry and Pharmacology

The reactions of hydroxyl radicals with eight substituted aromatic hydrocarbons and four olefins were studied utilizing the flash photolysis-resonance fluorescence technique. The rate constants were measured at 298°K using either Ar or He as the diluent gas. The values of the rate constants (k × 1012) in the units of cm3/molec. sec are (a) OH + o-xylene → products: (12.9±3.8), 20 torr He; (13.0±0.3), 20 torr Ar; (12.4±0.1), 200 torr He;(b) OH + m-xylene → products: (15.6±1.4), 3 torr Ar; (19.4±0.8), 20 torr Ar; (21.4±0.2), 20 torr He; (20.3±1.9), 200 torr Ar; (20.6±1.3), 200 Torr He;(c) OH + p-xylene → products: (8.8±1.2), 3 torr Ar; (10.1±1.0), 20 torr He; (10.5±0.6), 200 torr He;(d) OH + ethyl benzene → products: (7.50±0.38), 3 torr He; (7.06±0.26), 20 torr He; (7.95±0.28), 200 torr He;(e) OH + n-propylbenzene → products: (6.40±0.36), 20 torr He; (5.86±0.16), 200 torr He;(f) OH + isopropylbenzene → products: (7.79±0.40), 200 torr He;(g) OH + hexafluorobenzene → products: (0.221±0.020), 20 torr He; (0.219±0.016) 200 torr He;(h) OH + n-propyl pentafluorobenzene → products: (2.52±0.54), 3 torr He; (3.01±0.76), 20 torr He; (3.06±0.24), 200 torr He;(i) OH + propylene → products: (25.6±1.2), 20 torr He; (26.3±1.2), 200 torr He;(j) OH + 1-butene → products: (29.6±1.9), 3 torr He; (29.4±1.4), 20 torr He;(k) OH + cis-2-butene → products: (43.2±4.1), 3 torr He; (42.6±2.5), 20 torr He;(l) OH + tetramethylethylene → products: (56.9±1.3), 20 torr He.

15 Tab.

Electronic Resource

0538-8066

Chemistry ; Physical Chemistry

Wiley InterScience Backfile Collection 1832-2000

Chemistry and Pharmacology

The third order rate coefficients for the addition reaction of Cl with NO2, Cl + NO2 + M → ClNO2 (ClONO) + M; k1, were measured to be k1(He) = (7.5 ± 1.1) × 10-31 cm6 molecule-2 s-1 and k1(N2) = (16.6 ± 3.0) × 10-31 cm6 molecule-2 s-1 at 298 K using the flash photolysis-resonance fluorescence method. The pressure range of the study was 15 to 500 torr He and 19 to 200 torr N2. The temperature dependence of the third order rate coefficients were also measured between 240 and 350 K. The 298 K results are compared with those from previous low pressure studies.

1 Tab.

Electronic Resource

0538-8066

Chemistry ; Physical Chemistry

Wiley InterScience Backfile Collection 1832-2000

Chemistry and Pharmacology

Absolute rate coefficients for the reactions of the hydroxyl radical with ethane (k1, 297-300 K) and propane (k2, 297-690 K) were measured using the flash photolysis-resonance fluorescence technique. The rate coefficient data were fit by the following temperature-dependent expressions, in units of cm3/molecule·s: k1(T) = 1.43 × 10-14T1.05 exp (-911/T) and k2(T) = 1.59 × 10-15T1.40 exp (-428/T). Semiquantitative separation of OH-propane reactivity into primary and secondary H-atom abstraction channels was obtained.

2 Ill.

Electronic Resource

0538-8066

Chemistry ; Physical Chemistry

Wiley InterScience Backfile Collection 1832-2000

Chemistry and Pharmacology

Rate coefficients, k1, for the reaction OH + HONO → H2O + NO2, have been measured over the temperature range 298 to 373 K. The OH radicals were produced by 266 nm laser photolysis of O3 in the presence of a large excess of H2O vapor. The temporal profiles of OH were measured under pseudo-first-order conditions, in an excess of HONO, using time resolved laser induced fluorescence. The measured rate coefficient exhibits a slight negative temperature dependence, with k1 = (2.8 ± 1.3) × 10-12 exp((260 ± 140)/T) cm3 molecule-1 s-1. The measured values of k1 are compared with previous determinations and the atmospheric implications of our findings are discussed.

5 Ill.

Electronic Resource

0538-8066

Chemistry ; Physical Chemistry

Wiley InterScience Backfile Collection 1832-2000

Chemistry and Pharmacology

The flash photolysis resonance fluorescence technique has been utilized to determine the rate constants for three reactions involving the hydroxyl radical [OH(2≅)] and three halogenated C2 alkanes. The nominal temperature range covered was 250-375 K. The compounds studied and the resulting Arrhenius expressions in units of cm3/molec·sec are The error limits in these expressions are such that they include any possible systematic errors due to the presence of impurities in the halocarbon samples. Tropospheric lifetimes have been calculated for the above species by combining the above rate constant data with global seasonally and diurnally averaged hydroxyl radical concentrations.

1 Ill.

Electronic Resource

0538-8066

Chemistry ; Physical Chemistry

Wiley InterScience Backfile Collection 1832-2000

Chemistry and Pharmacology

These tables of evaluated rate constants for use in stratospheric modeling have been taken from the most recent report of the NASA Panel that has been periodically producing such reviews. They are reproduced here to make a broader community aware of their existence. This article should NOT be cited, nor should these rate constants be used without consulting the full report. All citations should be to that original report (JPL Publ. 85-37), which contains extensive documentation and discussion of the rationale of the evaluation. Copies may be obtained by requesting JPL Publ. 85-37 from Documentation Services, 111-116B, Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91109.

2 Tab.

Electronic Resource