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The rate coefficients for the removal of O2(¹%@mt;sys@%Σ%@sx@%g%@be@%+%@sxx@%%@mx@% ) (from now on referred to as O2(¹Σ)) by H2 were measured between 209 and 373 K by generating O2(¹Σ) via pulsed laser excitation near 762 nm. The rate coefficient was determined to be k1 = (6.8 ± 1.2) × 10^-12 exp[−(590 ± 52)/T] cm³ molecule^-1 s^-1. Reaction of O2(¹Σ) with O3 yielded O(³P), which was detected by vacuum ultraviolet (VUV)-resonance fluorescence. O2(¹Σ) was also prepared by the interaction of O(¹D) with O2. In separate experiments, the yield for OH from the title reaction was measured to be <4 × 10^-5 at 298 and 209 K. This upper limit was used in a one-dimensional atmospheric model to show that the contribution of this reaction to the total OH production in the atmosphere is minimal under most conditions.

Rate coefficients for the reaction, OH + CCl3CHO (chloral) → products (k5), were measured over the temperature range 233−415 K using the pulsed laser photolysis-laser-induced fluorescence method, and were determined to be k5(T) = (1.79 ± 0.17) × 10^-12 exp[−(240 ± 60)/T] cm³ molecule^-1 s^-1. The quoted uncertainties are at the 95% confidence level with σA = AσlnA, and include estimated systematic errors. Our results are compared with those from previous work and the differences are discussed. UV absorption cross sections of CCl3CHO were measured between 200 and 345 nm and over the temperature range 240−360 K. These measurements are in good agreement with previously reported values. The quantum yields for O, H, and Cl atoms in CCl3CHO photolysis were measured via atomic resonance fluorescence detection following pulsed excimer photolysis. Upper limits for the O atom quantum yield in CCl3CHO photolysis at 308 and 248 nm were measured to be <0.01 and <0.02, respectively. The H atom quantum yields in CCl3CHO photolysis at 193, 248, and 308 nm were 0.04 ± 0.01, <0.01, and <0.002, respectively. The Cl atom quantum yield in the photolysis of CCl3CHO at 308 nm was 1.3 ± 0.3. The rate coefficient for the reaction Cl + CCl3CHO was determined to be k8(298 K) = (5.4 ± 0.7) × 10^-12 cm³ molecule^-1 s^-1. These results are used to evaluate the tropospheric lifetime of CCl3CHO and the significance of chlorine transport from methylchloroform degradation to the lower stratosphere.

The rate constant for the reaction of hydroxyl radicals with nitric acid has an unusual pressure and temperature dependence. To explore the mechanism for this reaction, we have measured rate constants for reactions of isotopically substituted species OD + DNO3, OH + DNO3, OD + HNO3, and ¹⁸OH + HNO3 and the yield of NO3 product. Deuterium substitution on nitric acid results in more than a 10-fold reduction in the rate constant, removes the pressure dependence (over the observed range of 20−200 Torr in He and SF6), and leads to a strongly curved Arrhenius temperature dependence. Deuterium substitution on hydroxyl increases the rate constant slightly but does not change the pressure dependence. There is no evidence for exchange reactions in the isotopically mixed reactions. Absorption measurements of the NO3 product yield show that the title reaction produces nitrate radical with unit efficiency over all temperatures and pressures studied. We discuss the implications of the measured rate constants, product yields, and lack of isotopic exchange in terms of a mechanism that involves formation of a hydroxyl radical−nitric acid complex and its subsequent reaction to give NO3 and H2O.

Rate coefficients for the reaction of OH radical with HCl (k1) between 200 and 400 K, were measured to be 3.28 × 10^-17 T^1.66 exp [184/T] cm³ molecule^-1 s^-1 by producing OH via pulsed laser photolysis and detecting it via laser induced fluorescence. The rate coefficients for the reactions of OH with DCl (k2), OD with HCl (k3), and OD with DCl (k4) were also measured using the same method to be k2 = 2.9 × 10^-12 exp [−728/T], k3 = 8.1 × 10^-18 T^1.85 exp [300/T], and k4 = 2.5 × 10^-12 exp [−660/T] cm³ molecule^-1 s^-1. k1 − k4 were computed for 200 < T < 400 K using the variational transition theory, including tunneling corrections, to be k1 = 1.49 × 10^-16 T^1.35 exp [262/T], k2 = 9.04 × 10^-20 T^2.34 exp[429/T], k3 = 1.01 × 10^-16 T^1.4 exp[309/T], and k4 = 1.64 × 10^-19 T^2.27 exp [385/T]cm³ molecule^-1 s^-1, in reasonable agreement with experiments. They were also used to analyze the origin of the isotope effects. A two-dimensional chemical transport model calculation shows that our measured higher values of k1 at low temperatures do not significantly alter the calculated ozone abundance, or its trend, in the stratosphere.

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.

The rate coefficients for the reaction of OH with CH3D (k1), CH2D2 (k2), CHD3 (k3), CD4 (k4), and CH4 (k5) as well as that of OD with CH4 (k6) have been measured using the pulsed photolytic production of OH followed by its detection via pulsed laser induced fluorescence. k1−k4 and k6 were measured between ~220 and ~415 K, while k5 was measured down to 195 K. The measured rate coefficients do not strictly obey the Arrhenius expression. However, below 298 K, they can be represented by the expressions (in cm³ molecule^-1 s^-1):  k1 = (3.11 ± 0.44) × 10^-12 exp[−(1910 ± 70)/T]; k2 = (2.3 ± 1.2) × 10^-12 exp[−(1930 ± 250)/T]; k3 = (1.46 ± 0.22) × 10^-12 exp[−(1970 ± 70)/T]; k4 = (1.00 ± 0.22) × 10^-12 exp[−(2100 ± 120)/T]; k5 = (1.88 ± 0.11) × 10^-12 exp[−(1695 ± 30)/T]; k6 = (1.68 ± 0.12) × 10^-12 exp[−(1640 ± 40)/T]. The obtained values of the rate coefficients and kinetic isotope effects are compared with values previously measured or calculated by other groups. The atmospheric implications of this data are briefly discussed.

A simple but elegant spectroscopic technique using two narrow-band dye lasers has been demonstrated for analyzing single-color resonant multi-photon-ionization spectra of atoms. This technique provides a direct identification of the starting level of the multi-photon-ionization pathway. This method can also be used to determine intermediate levels, which play an important role in the ionization process. Some typical results for uranium are presented.