OH Consumption Research Articles

Investigation of the nighttime decay of isoprene

A rapid nighttime decay of isoprene (2‐methyl‐1,3‐butadiene) has been observed at several forest sites. Data from the Program for Research on Oxidants: PHotochemistry, Emissions, and Transport (PROPHET) have been carefully examined with respect to this phenomenon. Essentially every evening (at PROPHET), isoprene concentrations fall from several ppb to levels below 100 ppt, with an average lifetime of 2.7 hours. Since this decay rate exceeds that expected from established nighttime chemistry, other possible mechanisms are suggested and discussed. Reaction with ozone will not occur at a rate consistent with the observed decay. Calculations of nitrate radical concentrations reveal that this oxidant only becomes an important sink for isoprene after the majority of the isoprene decay has taken place. The isoprene flux data were not consistent with dry deposition playing a significant role in nighttime forest loss. On the basis of ambient measurements of OH radical concentrations at the PROPHET site, calculated isoprene decay rates were compared with observations. For some nights the observed decay can be fit strictly by OH consumption; however, the reported OH data overpredict the isoprene loss rate on most nights. We estimate that vertical mixing with isoprene‐depleted air probably contributes to the fast isoprene decay observed; however, the measurements needed to support this suggestion have yet to be made.

Laser-induced fluorescence, mass spectrometric, and modeling studies of neat and NH 3-doped H 2/N 2O/Ar flames

A combined experimental and modeling study of neat and NH 3-doped (Φ = 1), 30 Torr flames is reported. The major species concentrations are measured by molecular beam mass spectrometry (MB/MS), whereas the minor species OH, NH, and O-atom concentrations are measured by laser-induced fluorescence (LIF). The species NO is measured both by LIF and MB/MS, and O 2 by MB/MS. The flame temperatures are measured both by OH and NH LIF and by thin wire thermometry. The flames are modeled with PREMIX using the temperature profiles and several detailed chemical mechanisms as input. The mechanisms include the GRI 2.11, SSLA, and their derivatives. The SSLA mechanism was developed previously in our laboratory from a critical literature review. Calculations using all the mechanisms predict fairly well the profiles of the major species for both neat and doped flames. However, both the SSLA and GRI 2.11 calculations fail to predict the postflame O 2 concentration in the neat flame, the drop in the O 2 concentration with the addition of NH 3, and the NH 3 decay in the doped flame. Sensitivity analyses suggest refinements to the SSLA and GRI 2.11 mechanisms. The experimental results are predicted rather well using a modified SSLA mechanism in which the NH + NO = N 2O + H reaction rate is decreased and the N 2O + M = N 2 + O + M reaction rate and/or H 2O third body efficiency is increased to the limit of their uncertainty. Rate analyses performed on the modeled calculations reveal the reactions important to NO, O 2, NH, OH, and O-atom production and consumption and NH 3 consumption. These reactions are presented and discussed.

Influence of montmorillonite on Fe(II) oxidation products

AbstractGoethite and maghemite are the stable species of Fe oxyhydroxides-oxides formed an acidic and alkaline environments from the oxidation of Fe(II) perchlorate solutions. The influence of montmorillonite on the oxidation products of 0.02 M ferrous perchlorate at pHs of 6.0 and 8.0 was studied by X-ray diffraction, infrared and transmission electron microscopic analyses. Increased rate of OH consumption during the oxidation at constant pH indicated that the presence of montmorillonite accelerated the rate of Fe(II) oxidation. The presence of montmorillonite, with high surface reactivity, at an initial montmorillonite/Fe (w/w) ratios of 1.4 and 3.4 retarded the formation of goethite, lepidocrocite and maghemite, and maghemite and goethite, and promoted the formation of ferrihydrite and lepidocrocite at pHs of 6.0 and 8.0, respectively. Kaolinite, on the other hand, with relatively low surface reactivity had no influence on the nature of the Fe(II) oxidation products formed at either pH.

Combustion Inhibition by Methyl Chloride and the Effects of Steam Injection in a Two Stage Turbulent Flow Reactor

Abstract An experimental and modeling study of the combustion inhibition of CH3Cl and the effects of steam injection in a two stage, turbulent flow reactor is presented. Premixed C2H4, air, and nitrogen with or without CH3Cl were fed to the first stage of the reactor. Steam was injected into the second stage. The feed fuel equivalence ratio was kept constant at 0.6 or 1.3. The loading of CH3Cl was changed from a feed molar CH3Cl/C2H4 ratio of 0 to 0.4. Reactor temperatures and CO, CO2, and light hydrocarbon concentrations were measured at various locations along the reactor. Experiments showed that the presence of CH3Cl inhibited CO burnout and increased the yield of light hydrocarbons. Predicted species concentrations from detailed reaction modeling agree well with the experimental observations. Rate-of-production (ROP) calculations indicate that OH + HCl = Cl + H2O is a major channel for OH consumption. The resulting depletion of OH inhibits the CO burnout reaction OH + CO = CO2 +H2O. Experiments showed...

Soil Solutions of Organic Horizons in Some Acid Forest Soils

AbstractThe equilibrium soil solution (ESS) of thirteen O horizons from six Dystrochrepts and one Spodosol on granitic, phyllitic and basaltic solifluction material in the Fichtelgebirge mountain range was analyzed for organic carbon (DOC), optical density at 465 nm (OD465), base neutralization capacity (BNC), and various cations. The pH of the ESS was from 2.95 to 4.0 and from 4.1 to 4.2 in the phyllitic and granitic soils and in the basaltic soils, respectively. The concentration (mmol/L) of Ca and K ranged between 0.2 and 0.9 but was only < 0.18 for Mg, corresponding to the Mg deficiency in spruce identified in this area. Al (0.07–0.56 mmol/L) was partitioned into “labile” and “non‐labile” (complexed) fractions on the basis of polarographic measurements and a fast reaction with hydroxyquinoline (HQ). The polarograms indicated one hundred percent Al complexation in 9 out of 11 ESSs whereas an average of only 63% was nonreactive with HQ. Complexed Al was correlated with DOC (R2 = 0.47*), OD465 (0.63**) and the corrected BNC for pH 7 (0.82***). By adding small increments of Al to the 9 “non‐Al‐saturated” ESSs, an Al complexing capacity was polarographically determined. It ranged between 0.3 and 2.8 mmol/L and was saturated in the natural state to more than 90% in 7 out of the 9 ESSs. The average composition of the Al‐organic complex in M NH4Cl extracts (pH 2.7–3.8) as deduced from OH consumption was [Al Lc.66]2.34+ (L = monovalent ligand).Gleichgewichtsbodenlösungen von O‐Horizonten einiger saurer Waldböden.

Investigation of OH-H2 and OH-CO reactions using argon-sensitized pulse radiolysis

Gas-phase OH radical reactions with H2 and CO were studied using pulse radiolysis absorption spectroscopy in Ar-H2O-H2 and Ar-H2O-CO systems. Computer simulation was used to examine the contribution of various reactions involved in OH decay and to confirm the experimentally measured bimolecular rate constants of OH+H2 and OH+CO reactions. In the hydrogen-argon-water system, it was found that the OH+H reaction makes a substantial contribution to OH decay in competition with the process OH+H2. The value of the OH+H2 abstraction rate constant, (3.2±0.7)×10−15 cm3 molecules−1 s−1, obtained empirically from a plot of the pseudo-first-order rate constant for OH consumption versus H2 pressure was about 50% lower than most literature values, but proved to be acceptable in the dynamic simulations. Although some 30 reactions are needed to describe the Ar-H2O-CO system, the OH concentration decreases almost exponentially, mostly by the OH+CO addition reaction, with small contributions by other processes. The empirically determined OH+CO bimolecular rate constant of (1.55±0.13)×10−13 cm3 molecule−1 s−1 agreed well with literature values determined in the presence of Ar buffer gas and also led to consistent predictions in the computer simulation work.

The maturation of crystalline calcium phosphates in aqueous suspensions at physiologic pH.

The maturation of calcium phosphate crystals formed by the conversion of spontaneously precipitated amorphous calcium phosphate (ACP) was studied in aqueous media at temperatures ranging from 20° to 37°. Reaction pH was kept at 7.4 with either Hepes buffer or by the pH-stat addition of base. Reaction kinetics were followed by monitoring solution calcium and total phosphate, and, in the pH-stat controlled reaction, by recording the amount of KOH needed to maintain pH. Reaction products were examined chemically and by X-ray diffraction and transmission electron microcopy. The first crystals to form deviated markedly from apatite in morphology, composition, structure, and solubility. They were extremely thin and flaky in appearance, had a low Ca/P molar ratio (1.4), contained an appreciable amount of acid phosphate (16%), and had an exceptionally largea-axis (10.5 A vs. 9.4 A for apatite). With maturation, the crystals became thicker but smaller in lateral dimensions, more apatitelike in structure and composition, and less soluble. However, this ripening of the crystals was accompanied by unusual inflections in the solution Ca and total PO4 curves, and, in the case of the pH-stat experiments, in the OH consumption profiles as well. These anomalous post-ACP solution changes suggest that a phase change had taken place during crystal maturation. Although the observed structural and compositional changes are not inconsistent with the perfection of an initially defective apatite, the changes in crystal morphology and the anomalous behavior of the reaction solution may more accurately reflect a conversion of the ACP first into an OCP-like crystalline phase which subsequently hydrolyzes into apatite.

Isocyanate Reactions with Difunctional Polyisobutylenes

Abstract The reactions of isocyanates with carboxy terminated polyisobutylenes, CTPIB, and with hydroxy terminated polyisobutylenes, HTPIB, have been studied in detail. In the case of HTPIB specific emphasis has been given to an hydroxy-ester functionality prepared by the base catalyzed reaction of CTPIB with propylene oxide. Isocyanate reactions with polymeric carboxyl groups were studied to observe if conditions could be established to remove quickly the undesirable carbon dioxide by-product. A potential advantage of this reaction would be the formation of a more stable amide link compared with that of a urethane linkage. In capping reactions with CTPIB and diisocyanates (where NCO group concentrations are in excess), the course of the reaction essentially follows second order kinetics with respect to carboxyl utilization. Bulk reactions, run under vacuum, facilitated the removal of CO2 and markedly increased the rate of reaction. Even so, the reaction required relatively high concentrations of tertiary amine catalysts suggesting a dual role for the base. Aromatic diisocyanates with chlorine substitution were several fold more reactive with CTPIB than was toluene diisocyanate, and gave indications of a better selectivity. Sulfonyl isocyanates possess still greater reactivity. The selectivity of the isocyanate reaction with polymeric COOH is poor when using common diisocyanates such as TDI. The predominant extension of prepolymers is far less probably than in the case of hydroxyl based systems. However, tough, dense, and flexible networks can be formed from initial products of 2000 number average molecular weight. The reactivity of the secondary hydroxyl ester terminal functionality of polyisobutylene, 2° HTPIB, with diisocyanates was comparable to that of commercial polyether or polyester diols which are largely primary hydroxyl. This comparable activity is explained by the fact that in bulk reactions the hydrocarbon backbone of 2° HTPIB provides a reaction medium with a lower dielectric constant and thus a more advantageous environment. In capping reactions followed by IR monitoring of OH consumption, reaction rates also followed second order kinetics with respect to OH consumption when the NCO concentration was in excess. In contrast to isocyanate-polymeric COOH systems, the reaction with HTPIB required no catalysts for extensive consumption of OH groups at moderate temperatures. The HTPIB-toluene diisocyanate reaction was far more selective, and this resulted in a greater potential for extension with the prepolymer. The physical properties of extended and crosslinked networks reflected this selectivity. For a given molecular weight level, networks with HTPIB-diisocyanate prepolymers were more extensible and had higher strengths than did CTPIB based counter parts. Fractionation of original starting materials into narrower molecular weight ranges with slightly improved degrees of functionality improved tensile strengths and extensibilities of subsequent HTPIB based networks. Interesting blocked polymer networks were formed with HTPIB and polyether diols (for example polytetramethyleneglycol). These two liquids which were immiscible, in the molecular weight range of Mn−2000, formed transparent elastic networks of high strength after mutual capping with TDI and subsequent extension and crosslinking by a combination of aromatic diamines and low molecular weight aliphatic diols.