A. R. Ravishankara

Atmospheric Lifetimes of Long-Lived Halogenated Species

The atmospheric lifetimes of the fluorinated gases CF4, C2F6, c-C4F8, (CF3)2c-C4F6, C5F12, C6F14, C2F5Cl, C2F4C12, CF3Cl, and SF6 are of concern because of the effects that these long-lived compounds acting as greenhouse gases can have on global climate. The possible atmospheric loss processes of these gases were assessed by determining the rate coefficients for the reactions of these gases with O(1D), H, and OH and the absorption cross sections at 121.6 nanometers in the laboratory and using these data as input to a two-dimensional atmospheric model. The lifetimes of all the studied perfluoro compounds are >2000 years, and those of CF3Cl, CF3CF2Cl, and CF2ClCF2Cl are >300 years. If released into the atmosphere, these molecules will accumulate and their effects will persist for centuries or millennia.

On the role of iodine in ozone depletion

Ozone depletions in the lower stratosphere outside of polar regions are difficult to explain using only local chlorine and bromine chemistry. We speculate that iodine chemistry in combination with trends in anthropogenic chlorine and bromine may also be a factor in determining the widespread current depletion of lower stratospheric ozone. We also speculate on a related role for iodine in the sudden springtime surface ozone loss observed in the Arctic.

Do Hydrofluorocarbons Destroy Stratospheric Ozone

Hydrofluorocarbons, many of which contain a CF3 group, are one of the major substitutes for the chlorofluorocarbons and halons that are being phased out because they contribute to stratospheric ozone depletion. It is critical to ensure that CF3 groups do not cause significant ozone depletion. The rate coefficients for the key reactions that determine the efficiency of the CF3 radical as a catalyst for ozone loss in the stratosphere have been measured and used in a model to calculate the possible depletion of ozone. From these results, it is concluded that the ozone depletion potentials related to the presence of the CF3 group in hydrofluorocarbons are negligibly small.

The reaction probabilities of ClONO2 and N2O5 on polar stratospheric cloud materials

The reaction probabilities, γ, of ClONO2 and N2O5 on ice and nitric acid trihydrate (NAT) surfaces were determined, using reactant concentrations that are typical of the lower stratosphere, by measuring the first-order reactant loss rate coefficients over the substrate located on the walls of a fast-flow reactor. Reactants were detected using chemical ionization mass spectrometry, a sensitive technique which allows the use of low reactant concentrations. The reaction probabilities obtained for ClONO2 are: 0.3 (+0.7−0.1) on pure ice, 0.006 (±30%) over a NAT surface, and 0.3 (+0.7−0.1) on an HCl-doped NAT surface. Those for N2O5 are: 0.024 (±30%) on pure ice, 0.0006 (±50%) on a NAT surface, and 0.0032 (±30%) on an HCl-doped NAT surface. We observed that an ice surface will be converted into a less reactive HNO3-doped ice surface in a relatively short time, and we present arguments that this surface consisted of a NAT layer. The large differences between our results and previous measurements for some of these γ can be attributed to the relatively large reactant concentrations used in the previous work. The major implications of this work for polar stratospheric chlorine activation are: an efficient loss of ClONO2 on pure ice surfaces, a very rapid rate for the reaction ClONO2 + HCl on NAT surfaces, and that pure ice surfaces will become “passivated” when coated with one monolayer of NAT crystal.

Rate constants for the reaction OH+NO2+M → HNO3+M under atmospheric conditions

Abstract We present rate constants for the title reaction, k 1 , measured using a pulsed photolysis–laser induced fluorescence technique between 220 and 250 K in 20–250 Torr of N 2 and 20–50 Torr of O 2 . Our measured k 1 agree with literature values at low temperatures but show that the current recommendations for atmospheric modeling overestimate k 1 by 10–30% in the falloff region below 250 K. The revised values of k 1 help to better define the role of NO 2 in the stratosphere.

Atmospheric lifetimes and ozone depletion potentials of methyl bromide (CH3Br) and dibromomethane (CH2Br2)

The rate coefficients for the reactions of OH radical with CH3Br and CH2Br2 were measured as functions of temperature using the laser photolysis - laser induced fluorescence method. This data was incorporated into a semi-empirical model [Solomon et al., 1992] and a 2-D model to calculate the steady - state Ozone Depletion Potentials (ODP) and atmospheric lifetimes, τ, with greatly improved accuracy as compared to earlier studies. The calculated ODPs and τ are 0.65 and 1.7 years and 0.17 and 0.41 years for CH3Br and CH2Br2, respectively, using the semi-empirical model. These lifetimes agree well with those calculated using a 2-D model. This study better quantifies the ODPs and τ of these species which are needed inputs for discussion of possible regulation of human emissions currently under international considerations.

Photodissociation of H2O2 and CH3OOH at 248 nm and 298 K: Quantum yields for OH, O(3P) and H(2S)

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 de...

Absorption cross section of BrO between 312 and 385 nm AT 298 and 223 K

Abstract The absolute UV cross section of BrO at 338.1 ± 0.1 nm, the peak of the (7,0) band of the A( 2 Π)←X( 2 Π) transition, was measured at 298±2 and 223±4 K to be (1.71±0.14) × 10 −17 and (2.21±0.16) × 10 −17 cm 2 , respectively, using the technique of flash photolysis-ultraviolet absorption. The spectral resolution for these measurements was 0.18 nm. The absorption spectra of BrO in the wavelength range 312–385 nm were measured at 298±2 and 223±4 K using a flow tube reactor coupled to a diode array spectrometer. Using the (7,0) band cross sections, the absorption cross sections in the above wavelength range were calculated.

Quantum yields for production of O(1D) in the ultraviolet photolysis of ozone: Recommendation based on evaluation of laboratory data

[1] The quantum yield for O( 1 D) production in the photolysis of ozone in the ultraviolet region as a function of wavelength and temperature is a key input for modeling calculations in the atmospheric chemistry. To provide the modeling community with the best possible information, the available data are critically evaluated, and the best possible recommendations for the quantum yields are presented. Since the authors of this paper are the principal investigators of the groups which have provided most of the recent experimental data for the O( 1 D) quantum yields, the basic assumptions made by each group, the input parameters used in obtaining the quantum yields, and possible sources of systematic errors are well examined. The fitting expression of the O( 1 D) yield as a function of photolysis wavelength λ and temperature Tis presented in the ranges of 306 nm < X < 328 nm and 200 K < T < 300 K. The recommendation values of the O( 1 D) quantum yield for 290 nm < X < 306 nm and 328 nm < λ <350 nm are also presented. The formation mechanisms of O( 1 D) in the photolysis of ozone which result in the wavelength and temperature dependence of the O( 1 D) yields are interpreted.

LIF detection of IO and the rate coefficients for I + O3 and IO + NO reactions

Abstract Laser induced fluorescence (LIF) from the (0, 0), (2, 0), (3, 0) and (2, 1) bands of the A 2 Π 3 2 → X Π 3 2 system of IO was detected. Using LIF detection of IO, the rate coefficients for I + O3 ← IO + O2 (k1) and IO + NO → I + NO2 (k2) reactions were measured between 240 and 370 K to be k1(T) = (2.3 ± 0.7 × 10−11 exp[−(860 ± 100)/T] and k2(T) = (1.02 ± 0.31) × 10−11 exp[(185) ± 70)/T] cm3 molecule−1 s−1.