Isoprene oxidation and its impacts on the atmospheric composition

Terrestrische Vegetation, vor allem tropischer Regenwald, emittiert grosse Mengen flüchtiger organischer Verbindungen (VOCs) in die rnAtmosphäre, die durch Oxidationsreaktionen und Deposition der Reaktionsprodukte wieder entfernt werden. Die Oxidation wird vor allem durch Hydroxyl-Radikale (OH) initiiert, die hauptsächlich durch Photodissoziation von Ozon gebildet werden. Zuvor ging man davon aus, dass biogene VOCs OH in unverschmutzter Luft abbauen und dadurch die atmosphärische Oxidationskapazität verringern. Umgekehrt, führt rndie Oxidation von VOCs in verschmutzter Luft durch die katalytische Wirkung von Stickstoffoxiden (NOx = NO + NO2) zu schädlicher Oxidationsmittelbildung. Flugzeugmessungen atmosphärischer Spurengase, die über dem unberührten Amazonas-Regenwald durchgeführt worden sind, haben jedoch unerwartet hohe OH-Konzentrationen aufgezeigt. Das VOC mit der höchsten Emission in dieser Region war Isopren, dessen Oxidation als stärkeste OH-Senke berechnet wurde. In dieser Arbeit wurde die Hypothese genauestens untersucht, dass die natürliche Isopren-Oxidation in niedrig-NOx Luft OH effizient erneuert. Es wurde ein sehr detaillierter Oxidationsmechanismus für Isopren entwickelt, in dem neueste experimentelle und theoretische Fortschritte umgesetzt worden sind. Die Haupt-OH-Rückgewinnungswege wurden angewendet wodurch gezeigt wurde, dass sie wesentlich zur Oxidation unter niedrig-NOx Bedingungen beitragen. Verstärkte OH-Konzentrationen blieben unter verminderten Lichtverhältnissen, wie sie unter dichten Vegetationsdächern typisch sind, dauerhaft erhalten. Im Vergleich zu Flugzeugmessungen, der neue Oxidationsmechanismus reproduziert die OH-Konzentrationen innerhalb des Unsicherheitsbereiches. Darüber hinaus zeigten Simulationen eine erhebliche Produktion eines Isopren-Dihydroxyepoxids, das ein potenziell wichtiger Vorläufer organischer Aerosole in der Atmosphäre sein könnte. Es wurde einen neuen vereinfachten Oxidationsmechanismus auf Basis des traditionellen Wissenstands entwickelt und seine Anwendung für globale atmosphärische Studien getestet. Die Eingliederung der neuen Oxidationswege in diesen Mechanismus ermöglicht es folgende Auswirkungen der verstärkten VOC-Oxidation zu studieren die Zusammensetzung der Atmosphäre, den Austausch zwischen Erdoberfläche und Atmosphäre, Aerosole und Klima.

Within-plant isoprene oxidation confirmed by direct emissions of oxidation products methyl vinyl ketone and methacrolein

Isoprene is emitted from many terrestrial plants at high rates, accounting for an estimated 1/3 of annual global volatile organic compound emissions from all anthropogenic and biogenic sources combined. Through rapid photooxidation reactions in the atmosphere, isoprene is converted to a variety of oxidized hydrocarbons, providing higher order reactants for the production of organic nitrates and tropospheric ozone, reducing the availability of oxidants for the breakdown of radiatively active trace gases such as methane, and potentially producing hygroscopic particles that act as effective cloud condensation nuclei. However, the functional basis for plant production of isoprene remains elusive. It has been hypothesized that in the cell isoprene mitigates oxidative damage during the stress-induced accumulation of reactive oxygen species (ROS), but the products of isoprene-ROS reactions in plants have not been detected. Using pyruvate-2-13C leaf and branch feeding and individual branch and whole mesocosm flux studies, we present evidence that isoprene (i) is oxidized to methyl vinyl ketone and methacrolein (iox) in leaves and that iox/i emission ratios increase with temperature, possibly due to an increase in ROS production under high temperature and light stress. In a primary rainforest in Amazonia, we inferred significant in plant isoprene oxidation (despite the strong masking effect of simultaneous atmospheric oxidation), from its influence on the vertical distribution of iox uptake fluxes, which were shifted to low isoprene emitting regions of the canopy. These observations suggest that carbon investment in isoprene production is larger than that inferred from emissions alone and that models of tropospheric chemistry and biotachemistryclimate interactions should incorporate isoprene oxidation within both the biosphere and the atmosphere with potential implications for better understanding both the oxidizing power of the troposphere and forest response to climate change.

EVOLUTION OF SELECTED ISOPRENE OXIDATION PRODUCTS IN DARK AQUEOUS AMMONIUM SULFATE

The aqueous phase processing of glyoxylic acid, pyruvic acid, oxalic acid and methylglyoxal was studied simulating dark and radical free atmospheric aqueous aerosol. A novel observation of the cleavage of a carbon-carbon bond in pyruvic acid and glyoxylic acid leading to their decarboxylation was made in the presence of ammonium salts but no decarboxylation was observed from oxalic acid. The empirical rate constants for decarboxylation were determined. The structure of the acid, ionic environment of solution and concentration of species found to affect the decarboxylation process. A tentative set of reaction mechanisms was proposed involving nucleophilic attack by ammonia on the carbonyl carbon leading to fragmentation of the carbon-carbon bond between the carbonyl and carboxyl carbons. Whereas, the formation of high molecular weight organic species was observed in the case of methylglyoxal. The elemental compositions of the species were determined. It was concluded that, additional pathways that are not currently known likely contribute to aqueous phase processing leading to high molecular weight organic species. Under similar conditions in atmospheric aerosol, the aqueous phase processing will markedly impact the physicochemical properties of aerosol.

Total OH reactivity measurements at the biosphere-atmosphere interface

The biosphere emits copiously volatile organic compounds (VOCs) into the atmosphere, which are removed again depending on the oxidative capacity of the atmosphere and physical processes such as mixing, transport and deposition. Biogenic VOCs react with the primary oxidant of the atmosphere, the hydroxyl radical (OH), and potentially lead to the formation tropospheric ozone and aerosol, which impact regional climate and air quality. The rate of OH decay in the atmosphere, the total OH reactivity is a function of the atmospheric, reactive compound's concentration and reaction velocity with OH. One way to measure the total OH reactivity, the total OH sink, is with the Comparative Reactivity Method - CRM. Basically, the reaction of OH with a reagent (here pyrrole) in clean air and in the presence of atmospheric, reactive molecules is compared. This thesis presents measurements of the total OH reactivity at the biosphere-atmosphere interface to analyze various influences and driving forces. For measurements in natural environment the instrument was automated and a direct, undisturbed sampling method developed. Additionally, an alternative detection system was tested and compared to the originally used detector (Proton Transfer Reaction-Mass Spectrometer, PTR-MS). The GC-PID (Gas Chromatographic Photo-Ionization Detector) was found as a smaller, less expensive, and robust alternative for total OH reactivity measurements. The HUMPPA-COPEC 2010 measurement campaign in the Finish forest was impacted by normal boreal forest emissions as well as prolonged heat and biomass burning emissions. The measurement of total OH reactivity was compared with a comprehensive set of monitored individual species ambient concentration levels. A significant discrepancy between those individually measured OH sinks and the total OH reactivity was observed, which was characterized in detail by the comparison of within and above the forest canopy detected OH reactivity. Direct impact of biogenic emissions on total OH reactivity was examined on Kleiner Feldberg, Germany, 2011. Trans-seasonal measurements of an enclosed Norway spruce branch were conducted via PTR-MS, for individual compound's emission rates, and CRM, for total OH reactivity emission fluxes. Especially during summertime, the individually monitored OH sink terms could not account for the measured total OH reactivity. A controlled oxidation experiment in a low NOx environment was conducted in the EUPHORE reaction chamber (CHEERS, Spain 2011). The concentration levels of the reactant isoprene and its major products were monitored and compared to total OH reactivity measurements as well as to the results of two models. The individually measured compounds could account for the total OH reactivity during this experiment as well as the traditional model-degradation scheme for isoprene (MCM 3.2). Due to previous observations of high OH levels in the isoprene-rich environment of the tropics, a novel isoprene mechanism was recently suggested. In this mechanism (MIME v4) additional OH is generated during isoprene oxidation, which could not be verified in the conditions of the CHEERS experiment.

Photochemical Oxidation of Thioketones: Steric and Electronic Aspects

Oxidation of diaryl, aryl alkyl, and dialkyl thioketones by singlet oxygen generated via self-sensitization and other independent methods yielded the corresponding ketone and sulfine in varying amounts. A zwitterionic/ diradical intermediate arising out of the primary interaction of singlet oxygen with the thiocarbonyl chromophore is believed to be the common intermediate for the ketone and sulfine. While closure of the zwitterion/diradical to give 1,2,3-dioxathietane would lead to the ketone, competing oxygen elimination is believed to lead to the sulfine. This partitioning is governed by steric and electronic factors operating on the zwitterionic/diradical intermediate.

Photochemical oxidation of thio ketones: steric and electronic aspects

Oxidation of diaryl, aryl alkyl, and dialkyl thioketones by singlet oxygen generated via self-sensitization and other independent methods yielded the corresponding ketone and sulfine in varying amounts. A zwitterionic/ diradical intermediate arising out of the primary interaction of singlet oxygen with the thiocarbonyl chromophore is believed to be the common intermediate for the ketone and sulfine. While closure of the zwitterion/diradical to give 1,2,3-dioxathietane would lead to the ketone, competing oxygen elimination is believed to lead to the sulfine. This partitioning is governed by steric and electronic factors operating on the zwitterionic/diradical intermediate.

Novel Oxidation of Pyridoxal in Ternary Metal Complexes. An X-Ray Study of the Products

Attempts to prepare ternary metal complexes of pyridoxylidene-amino acid Schiff bases culminated in the oxidation of pyridoxal to pyridoxic acid or to its lactone and their complex formation, as evidenced from an X-ray study.

Electro-Oxidation of Borohydride on Rhodium, Iridium, and Rhodium-Iridium Bimetallic Nanoparticles with Implications to Direct Borohydride Fuel Cells

Electrochemical oxidation of borohydride is studied on nanosized rhodium, iridium, and bimetallic rhodium-iridium catalysts supported onto Vulcan XC72R carbon. The catalysts are characterized by X-ray diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy in conjunction with cyclic voltammetry and polarization studies. The studies reveal that a 20 wt % bimetallic Rh-Ir catalyst supported onto carbon (Rh-Ir/C) is quite effective for the oxidation of borohydride. Direct borohydride fuel cell with Rh-Ir/C as the anode catalyst and Pt/C as the cathode catalyst exhibits a peak power density of 270 mW/cm(2) at a load current density of 290 mA/cm(2) as against 200 mW/cm(2) at 225 mA/cm(2) for Rh/C and 140 mW/cm(2) at 165 mA/cm(2) for Ir/C while operating at 80 degrees C. The synergistic catalytic activity for the bimetallic Rh-Ir nanoparticles toward borohydride oxidation is corroborated by density-functional theory calculations using electron-localization function. (C) 2010 The Electrochemical Society. [DOI:10.1149/1.3442372] All rights reserved.

Role of hydroxyl group on the mesomorphism of alkyl glycosides:synthesis and thermal behavior of alkyl 6-deoxy-beta-D-glucopyranosides

A homologous series of alkyl 6-deoxy-beta-D-glucopyranoside amphiphiles was prepared,in an effort to identify the role of hydroxyl group in the mesomorphic behavior of alkyl glycosides. Synthesis was performed by a chlorination of the sugar moiety in alkyl-beta-D-glucopyranosides with methylsulfonyl chloride in DMF, followed by a metal mediated dehalogenation to secure alkyl 6-deoxy-beta-D-glucopyranosides, wherein the alkyl chain length varied from C-9 to C-16. The mesomorphic behavior of these 6-deoxy alkyl glycosides was assessed using polarizing optical microscopy, differential scanning calorimetry and X-ray diffraction method. Whereas the lower homologues exhibited a monotropic SmA phase till sub-ambient temperatures, the higher homologues formed a plastic phase. A partial interdigitized bilaye structure of SmA phase is inferred from experimental d-spacing and computationally derived lengths of the molecules. The results were compared with those of normal alkyl glucopyranosides, retained with hydroxyl groups at C-2-C-6 carbons, and alkyl 2-deoxy-glucopyranosides, devoid of a hydroxyl group at C-2 and the comparison showed important differences in the mesomorphic behavior.(C)2010 Elsevier Ireland Ltd. All rights reserved.

Oxidation of methanol on the surfaces of model Cu/ZnO catalysts containing Cu1+ and Cu-0 species

Interaction of CH3OH with Cu clusters deposited on ZnO films grown on a Zn foil as well as on a ZnO(0001)Zn crystal, has been examined by X-ray photoelectron spectroscopy. On clean Cu clusters, reversible molecular adsorption or formation of CH3O is observed. However if the Cu clusters are pretreated with oxygen, both CH3O and HCOO- species are produced. Model Cu/ZnO catalyst surfaces, containing both Cu1+ and Cu-0 species, show interesting oxidation properties. On a Cu-0-rich catalyst surface, only CH3O species is formed on interaction with CH3OH. On a Cu1+-rich surface, however, HCOO- ion is the predominant species.

Oxidation and decomposition of NH3 over combustion synthesized Al2O3 and CeO2 supported Pt, Pd and Ag catalysts

The catalytic oxidation and decomposition of NH3 have been carried out over combustion synthesized Al2O3 and CeO2 supported Pt, Pd and Ag catalysts using temperature programmed reaction (TPR) technique in a packed bed tubular reactor. Metals are ionically dispersed over CeO2 and fine metal particles are found on Al2O3. NH3 oxidation occurs over 1% Pt/Al2O3, 1% Pd/Al2O3 and 1% Ag/Al2O3 at 175, 270 and 350 C respectively producing N-2, NO, N2O and H2O, whereas 1% Pt/CeO2, 1% Pd/CeO2 and 1% Ag/CeO2 give N-2 along with NO, N2O and H2O at 200, 225 and 250degreesC respectively. N-2 predominates over other nitrogen-containing products during the reaction on all catalysts. At less O-2 concentration, N-2 and H2O are the only products obtained during NH3 Oxidation. NH3 decomposition over all the catalysts occurs above 450degreesC.