The nitrate radical: Physics, chemistry, and the atmosphere

Some simple theories are described for calculating rate constants for neutral chemical reactions in the gas phase which are dominated by long-range forces and have no barriers in the potential energy surface. The methods involve combining an adiabatic capture approach with the quantum mechanical sudden approximation of non-reactive energy transfer. A version of the technique is shown to work well in calculations on the O + OH reaction. The methods are applied to the diatom-diatom reactions SO + OH, NO + BrO, NO + ClO, CN + O2 and CH + O2. The results provide new information on the origin of the negative temperature dependence of the rate constants for reactions such as these.

Rates of chemical reactions dominated by long-range intermolecular forces

Halogen oxides: Radicals, sources and reservoirs in the laboratory and in the atmosphere

We report rate coefficients for the reaction of OH with HNO3, k1, between 10 and 500 Torr of He, SF6, N2, and O2 and at 10 different temperatures between 200 and 375 K We generated OH via pulsed photolysis of HNO3 and monitored the [OH] temporal profile via pulsed laser induced fluorescence Below 300 K the value of k1 increases rapidly with decreasing temperature and depends on pressure The pressure dependence of k1 at low temperature is significantly larger than that obtained by extrapolation of the currently available data The pressure and temperature dependence is most likely due to a competition between direct abstraction and reactive complex formation A rate constant expression derived from such a mechanism gives a global fit for k1 that is applicable to atmospheric conditions The new rate constant alters the calculated NO2 to HNO3 ratio in the lower stratosphere

Reconsideration of the rate constant for the reaction of hydroxyl radicals with nitric acid

The I-atom sensitised decomposition of ozone in air at 1 atm pressure and ambient temperature has been investigated. Iodine atoms were produced by photolysis of I2 using visible light or of CH3I using ultraviolet light. In both cases, the quantum yield for O3 decomposition was 1.25 (±0.11) per I atom. An important role is proposed for the self-reaction of IO radicals leading to higher oxides of iodine, IO+IO(+M)→I2O2(+M)→higher oxides, which predominated over the bimolecular reaction leading to regeneration of I atoms, IO+IO→2I+O2, with k
2a/k
2b≥4. Simple computer modelling calculations indicate that reaction (2a) may be important in determining the fate of photolabile iodine species in the atmosphere. The consequences for the behaviour of radioiodine releases are also discussed.

Photochemical aspects of tropospheric iodine behaviour

The kinetics of the reactions of Cl atoms with HNO3, H2O2, and HO2 have been studied by the discharge flow–mass spectrometric technique. The following rate constants were obtained (in cm3 molecule−1⋅s−1): Cl+HNO3→HCl+NO3, k=1.5 ×10−11 exp(−8700/RT) in the range 298–633 K; Cl+H2O2 →HCl+HO2, k= (4.0±0.4) ×10−13 at 298 K; Cl+HO2→HCl +O2, k= (6.8±2.5) ×10−11 at 298 K. The stratospheric implication of these data is discussed, and it is estimated that the Cl+HO2 reaction could have a comparable effect to the Cl+CH4 reaction as a sink of chlorine atoms in the stratosphere.

Kinetic study of the reactions of Cl atoms with HNO3, H2O2, and HO2

The discharge flow‐EPR technique has been used to measure the rate constant and the product distribution for the reaction of OH with HNO3, which is important for stratospheric modeling. The rate constant is found to be (7.3±2)×10−15 exp[876±85)/T] cm3 molecule−1 s−1 in the range 251–403 K. Besides, the in situ titration of NO3 by NO has shown that the reaction of OH with HNO3 only produces NO3 and H2O at room temperature.

Determination of the rate parameters and products for the reaction of hydroxyl radicals with nitric acid

The rate constants for the reactions Br+HCO→HBr+CO (2) and Br+HO2→HBr+O2 (4) have been measured at 298 K by the discharge flow technique coupled to laser induced fluorescence and mass spectrometry for detection of reactive species. HO2 radicals were generated by reacting atomic bromine with formaldehyde using an excess of O2, and were monitored after conversion into OH and NO2 with added excess NO. k2 and k4 were found to be (2.8±1.2)×10−10 and (7.6±0.9)×10−13cm3 molecule −1 s−1, respectively. The experimental kinetic curves were also compared to the predicted profiles obtained from computer simulation of the chemical system used. Although k4 is found to be 3–4 times higher than an earlier direct measurement, this result confirms that HBr is not an efficient reservoir for stratospheric bromine.

G. Le Bras

Papers

Kinetic study of the reactions of Cl atoms with HNO3, H2O2, and HO2

Determination of the rate parameters and products for the reaction of hydroxyl radicals with nitric acid

Kinetics of the reactions of atomic bromine with HO2 and HCO at 298 K

Determination of the rate parameters and products for the reaction of hydroxyl radicals with nitric acid