J. M. Van Doren

Bio: J. M. Van Doren is an academic researcher from Hanscom Air Force Base. The author has contributed to research in topics: Reaction rate & Order of reaction. The author has an hindex of 4, co-authored 8 publications receiving 184 citations.

Studies of the reaction of O+2 with deuterated methanes

Abstract: In the gas phase O+2 reacts with methane at 300 K to produce a hydrogen atom and the CH3O+2 ion. The structure of this ion has recently been determined to be H2COOH+, methylene hydroperoxide ion. The reaction rate coefficients and product distributions have now been measured at 300 K for the CHnD4−n isotopes. The reaction shows both inter‐ and intramolecular isotope effects, e.g., CH2D2 reacts more slowly than methane and more rapidly than CD4, but loses hydrogen or deuterium with equal probability. The ion readily transfers HO+ to alkenes, CS2, and many other neutral molecules. The reaction with CS2 has been used to investigate the isotopic distribution within mixed isotope product ions. In addition, the reaction rate coefficients for both CH4 and CD4 have been measured as functions of temperature between 20 and 500 K; in both cases a clear minimum is observed in the reaction rate coefficient near room temperature. A mechanism for the reaction is proposed which allows us to model the temperature dependen...

Gas-phase reactions of oxide and superoxide radical anions with C6F6

Abstract: The reactions of O− and O2− with C6F6 and C6F6− with O2 were studied using the selected ion flow tube (SIFT) technique. The reaction rate constants and branching fractions were measured. The reactions of O− and O2− with C6F6 occur with near unit efficiency while that of C6F6− with O2 is about 2% efficient. The reaction of O− produces three ionic products, F−, C5F4−, and C6F5O−. The reaction of O2− with C6F6 produces five ionic decomposition products, C4F4−, F−, C4F2−, C5F4O−, C5F5O−, and the electron transfer product, C6F6−. The reaction of C6F6− with O2 produces products similar to those observed for the reaction O2− with C6F6.

Role of mineral aerosol as a reactive surface in the global troposphere

Abstract: A global three-dimensional model of the troposphere is used to simulate the sources, abundances, and sinks of mineral aerosol and the species involved in the photochemical oxidant, nitrogen, and sulfur cycles. Although the calculated heterogeneous removal rates on mineral aerosol are highly uncertain, mainly due to poorly known heterogeneous reaction rates, the reaction of SO2 on calcium-rich mineral aerosol is likely to play an important role downwind of arid source regions. This is especially important for regions in Asia, which are important and increasing emitters of sulfur compounds. Our results indicate that the assumption that sulfate aerosol follows an accumulation mode size distribution, is particularly in Asia likely to overestimate the sulfate aerosol climate-cooling effect. An even larger fraction of gas phase nitric acid may be associated with and neutralized by mineral aerosol. Interactions of N2O5, O3, and HO2-radicals with dust are calculated to affect the photochemical oxidant cycle, causing ozone decreases up to 10% in and nearby the dust source areas. Comparison of these results with limited available measurements indicates that the proposed reactions can indeed take place, although due to a lack of measurements a rigorous evaluation is not possible at this time.

Reaction of N2O5 on tropospheric aerosols: Impact on the global distributions of NO x , O3, and OH

Abstract: Using a three-dimensional global model of the troposphere, we show that the heterogeneous reactions of NO 3 and N205 on aerosol particles have a substantial influence on the concentrations of NOx; 03, and OH. Due to these reactions, the modeled yearly average global NOx burden decreases by 50% (80% in winter and 20% in summer). The heterogeneous removal of NOx in the northern hemisphere (NH) is dominated by reactions on aerosols; in the tropics and southern hemisphere (SH), with substantial smaller aerosol concentrations, liquid water clouds can provide an additional sink for N205 and NO 3. During spring in the NH subtropics and at mid-latitudes, O3-concentrations are lowered by 25%. In winter and spring in the subtropics of the NH calculated OH concentrations decreased by up to 30%. Global tropospheric average 03 and OH burden (the latter weighted with the amount of methane reacting with OH) can drop by about 9% each. By including reactions on aerosols, we are better able to simulate observed nitrate wet deposition patterns in North America and Europe. 03 concentrations in springtime smog situations are shown to be affected by heterogeneous reactions, indicating the great importance of chemical interactions resulting from NOx and SO2 emissions. However, a preliminary analysis shows that under present conditions a change in aerosol concentrations due to limited SO2 emission control strategies (e.g., reductions by a factor of 2 in industrial areas) will have only a relatively minor influence on 03 concentrations. Much larger reductions in SO2 emissions may cause larger increases in surface 03 concentrations, up to a maximum of 15%, if they are not accompanied by a reductio!a in NOx or hydrocarbon emission.

Heterogeneous reactions in sulfuric acid aerosols: A framework for model calculations

Abstract: A framework for applying rates of heterogeneous chemical reactions measured in the laboratory to small sulfuric acid aerosols found in the stratosphere is presented. The procedure for calculating the applicable reactive uptake coefficients using laboratory-measured parameters is developed, the necessary laboratory-measured quantities are discussed, and a set of equations for use in models are presented. This approach is demonstrated to be essential for obtaining uptake coefficients for the HOCl + HCl and ClONO2 + HCl reactions applicable to the stratosphere. In these cases the laboratory-measured uptake coefficients have to be substantially corrected for the small size of the atmospheric aerosol droplets. The measured uptake coefficients for N2O5 + H2O and ClONO2 + H2O as well as those for other heterogeneous reactions are discussed in the context of this model. Finally, the derived uptake coefficients were incorporated in a two-dimensional dynamical and photochemical model thus for the first time the HCl reactions in sulfuric acid have been included. Substantial direct chlorine activation and consequent ozone destruction is shown to occur due to heterogeneous reactions involving HCl for volcanically perturbed aerosol conditions at high latitudes. Smaller but significant chlorine activation also is predicted for background aerosol loadings at extreme high latitudes, suggesting chlorine activation can occur on background sulfuric acid aerosol in these regions. The coupling between homogeneous and heterogeneous chemistry is shown to lead to important changes in the concentrations of various reactive species. The basic physical and chemical quantities needed to better constrain the model input parameters are identified.

Model study indicating halogen activation and ozone destruction in polluted air masses transported to the sea

Abstract: A chemical box model of the marine boundary layer has been developed. It treats reactions in the gas phase and in deliquesced sea-salt aerosol particles. A quasi-size-dependent approach was used to obtain mean values of those aerosol properties that depend on the droplet radii. A residence time of 2 days was assumed for the return of the aerosol particles to the sea surface. Emission and deposition fluxes simulate exchange processes with seawater and air masses surrounding the air parcel. Apart from the well-known reactions, the chemical reaction mechanism includes a large set of reactions of halogen compounds that are of potential importance for the ozone budget. Photochemical reactions are switched on during the day, assuming a semisinusoidal diurnal cycle of the photolysis rates. In our model runs, a heavily polluted urban air mass (in which O3 has been formed by photochemical smog reactions over land) is advected over the ocean. We found two processes that convert aqueous phase bromide into reactive bromine compounds. First, when concentrations of nitrogen oxides are still high, NO3 is scavenged during nighttime by the aerosol particles and oxidizes bromide: Br− + NO3 → Br + NO−3. The second process is a cycle in which the aqueous phase reaction H+ + HOBr + Br− → Br2 + H2O plays a central role. Br2 is only slightly soluble, volatilizes, and is dissociated into Br atoms. Subsequently, rapid destruction of ozone takes place via the gas phase reactions Br + O3 → BrO + O2, BrO + HO2 → HOBr + O2, and HOBr + hv → Br + OH. Sensitivity analyses have been performed to investigate how our results are influenced by input parameters that are not well known.