David M. Golden

Reaction of Chlorine Nitrate with Hydrogen Chloride and Water at Antarctic Stratospheric Temperatures

Laboratory studies of heterogeneous reactions important for ozone depletion over Antarctica are reported. The reaction of chlorine nitrate (ClONO2) with H20 and hydrogen chloride (HCl) on surfaces that simulate polar stratospheric clouds [ice and nitric acid (HNO3)—ice and sulfuric acid] are studied at temperatures relevant to the Antarctic stratosphere. The reaction of ClONO2 on ice and certain mixtures of HNO3 and ice proceeded readily. The sticking coefficient of ClONO2 on ice of 0.009 � 0.002 was observed. A reaction produced gas-phase hypochlorous acid (HOCl) and condensed-phase HNO3; HOC1 underwent a secondary reaction on ice producing dichlorine monoxide (Cl2O). In addition to the reaction with H20, ClONO2 reacted with HCl on ice to form gas-phase chlorine (Cl2) and condensed-phase HNO3. Essentially all of the HCl in the bulk of the ice can react with ClONO2 on the ice surface. The gaseous products of the above reactions, HOCl, Cl20, and Cl2, could readily photolyze in the Antarctic spring to produce active chlorine for ozone depletion. Furthermore, the formation of condensed-phase HNO3 could serve as a sink for odd nitrogen species that would otherwise scavenge the active chlorine.

Reactive uptake and hydration experiments on amorphous carbon treated with NO2, SO2, O3, HNO3, and H2SO4

The reactivity and hydration properties of amorphous carbon were studied in a low-pressure Knudsen cell reactor at room temperature (298 K). Reactions of NO2 (γ=0.11±0.04) and HNO3 (γ=0.038±0.008) were observed and may be important for nitrogen partitioning in the atmosphere. Water uptake was measured before and after exposure to various gases. Treating the amorphous carbon with NO2 and O3 does not alter the H2O uptake, while treatment with SO2, HNO3, and H2SO4 significantly increases the H2O uptake. The experimental results support current assumptions in jet aircraft plume models that sulfuric acid condensation is involved in the activation of soot particles as condensation nuclei.

A shock tube study of the enthalpy of formation of OH

The standard enthalpy, of formation of the hydroxyl radical (OH) at 298 K, Δ f H 0 298 (OH), has been determined from shock tube measurements spanning the tmeperature range 1964–2718 K and at pressures of 1–2.4 atm. Low-concentration, lean and stoichiometric mixtures of H 2 and O 2 in Ar produce well-controlled leels of OH in a “partial equilibrium” state, with little or no sensitivity to the reaction kinetics. the partial equilibrium OH concentrations are dependent only on the thermochemical parameters of the reacting species, with the heat of formation of OH being the most significant and uncertain parameter. Narrow line width UV laser absorption at 306.7 nm is used to measure OH concentrations with sufficient accuracy (2%–4%) to clearly determine the value of the enthalpy of formation. Over the whole range of experimental conditions, the average determination is Δ f H 0 298 (OH)=8.92±0.16 kcal/mol (37.3±0.67 kJ/mol) with a standard deviation of σ=0.04 kcal/mol (0.17 kJ/mol). This value is 0.40–0.48 kcal/mol (1.7–2.0 kJ/mol) below the previously accepted values, and it agrees with recent theoretical calculations, experimental studies using the positive-ion cycle, and calculations using thermochemical cycles.

Antarctic Ozone Depletion Chemistry: Reactions of N2O5 with H2O and HCl on Ice Surfaces

The reactions of dinitrogen pentoxide (N2O5) with H2O and hydrochloric acid (HCl) were studied on ice surfaces in a Knudsen cell flow reactor. The N2O5 reacted on ice at 185 K to form condensed-phase nitric acid (HNO3). This reaction may provide a sink for odd nitrogen (NOx) during the polar winter, a requirement in nearly all models of Antarctic ozone depletion. A lower limit to the sticking coefficient, γ, for N2O5 on ice is 1 x 10-3. Moreover, N2O5 reacted on HCl-ice surfaces at 185 K, with γ greater than 3 x 10-3. This reaction, which produced gaseous nitryl chloride (ClNO2) and condensed-phase HNO3, proceeded until all of the HCl within the ice was depleted. The ClNO2, which did not react or condense on ice at 185 K, can be readily photolyzed in the Antarctic spring to form atomic chlorine for catalytic ozone destruction cycles. The other photolysis product, gaseous nitrogen dioxide (NO2), may be important in the partitioning of NOx between gaseous and condensed phases in the Antarctic winter.

Pressure and temperature dependence of reactions proceeding via a bound complex. 2. Application to 2CH3 → C2H5 + H

Abstract Elementary bimolecular processes that involve the formation of a chemically activated intermediate are common. In a previous paper we described the information required to ensure that a kinetic model will extrapolate the rate constants involved over wide ranges of temperature and pressure. This approach is illustrated here for the system centered around the ethane intermediate. It is necessary and sufficient to specify the temperature and pressure dependence of three rate constants k ( T , P ) and the temperature dependence of two equilibrium constants K ( T ), viz., Rate constants are cast in the form of an analytical expression and appropriate parameters are tabulated.

Heterogeneous atmospheric reactions - Sulfuric acid aerosols as tropospheric sinks

The collisional reaction probabilities of several atmospheric species on bulk sulfuric acid surfaces indicate that heterogeneous processes may be important in tropospheric chemistry.

Infrared multiphoton generation of radicals: A new technique for obtaining absolute rate constants. Application to reactions of CF3

The infrared multiphoton dissociation of CF3I is used as a convenient source of CF3 radicals under very low pressure conditions. Measurement of thermal absolute rate constants at 298 K for the reactions CF3+Br2→CF3Br+Br, CF3+ClNO→CF3Cl+NO, CF3+O3→CF3O+O2, and CF3+NO2→CF3O+NO yield (7.8±1.3) ×108, (3.5±0.5) ×108, (5.6±0.8) ×108, and (1.6±0.3) ×109 M−1 s−1, respectively. This new technique shows great promise for production of other free radicals of interest and measurement of thermal absolute rate constants over a wide temperature range.

Reactions of methyl radicals of importance in combustion systems

Abstract Methyl radicals have been shown not to undergo a rapid bimolecular reaction with oxygen or nitric oxide at temperatures up to 1220 K.

Experimental study and modeling of the reaction H + O2+ M → HO2+ M (M = Ar, N2, H2O) at elevated pressures and temperatures between 1050 and 1250 K

The H + O2 + M → HO2 + M reaction was investigated at temperatures between 1050 and 1250 K and pressures from 7 to 152 bar behind reflected shock waves in gas mixtures of H2, O2, NO, and bath gases of Ar, N2 and H2O. Narrow linewidth laser absorption of NO2 at 472.7 nm was used to measure quasi-steady NO2 concentration plateaus in experiments designed to be sensitive only to the H + O2 + M → HO2 + M and the relatively well-known H + NO2 → NO + OH and H + O2 → OH + O reaction rates. The pressure dependence of the reaction was studied by measuring the fall-off of the reaction for M = Ar over a 10–152 bar pressure range. A simple modified Hindered-Gorin model of the transition state is used in an RRKM analysis of the results to facilitate comparisons of this work with measurements from other researchers at lower pressures. The RRKM calculations can also be described, using the simple functional form suggested by Troe, with the following: k∞/cm3 molecule−1 s−1 = 4.7 × 10−11 (T/300)0.2; k0(Ar)/cm6 molecule−2 s−1 = 2.0 × 10−32 (T/300)−1.2; k0(N2)/cm6 molecule−2 s−1 = 4.4 × 10−32 (T/300)−1.3; k0(H2O)/cm6 molecule−2 s−1 = 3.4 × 10−31 (T/300)−1.0; Fc = 0.7 for Ar and N2 and 0.8 for H2O. Measured values of the reaction rate for M = Ar in the highest pressure experiments fall below both simple RRKM analysis and the more sophisticated treatment of Troe using an ab initio potential energy surface. Collision efficiencies of N2 and H2O relative to Ar at 1200 K are 3.3 and 20 respectively.

Infrared multiphoton dissociation yields via a versatile new technique: intensity, fluence, and wavelength dependence for CF3I

Abstract Intensity, fluence, and wavelength dependence of the collisionless infrared multiphoton dissociation yield for CF 3 I have been determined via “titration” of radical photoproducts and molecular-beam-sampling-mass spectrometry. The importance of nonlinear effects in the pumping of this molecule is confirmed.