Probing gas-phase radical reactions and modeling the detection of aerosol precursors using computational and experimental methods

Understanding the gas-phase chemistry of secondary organic aerosol (SOA) formation is critical for accurate estimation of the effect of these aerosol on Earth's radiative balance. Additionally, the direct detection of the precursor molecules involved in these chemical reactions at atmospheric pressure without pre-treatment is valuable. In this work, computational and experimental methods are employed to 1) elucidate the thermodynamics and the mechanisms of selected key radical-radical reactions in the atmosphere and 2) investigate the efficiency of some of the chemical ionization mass spectrometry methods in detecting the atmospherically relevant acids and precursor compounds involved in the formation of SOA. The main oxygen containing radical species in our atmosphere, and also the key focus of this study, are hydroxy (OH), hydroperoxy (HO2), alkoxy (RO) and peroxy (RO2) radicals. Our computational study on the favorability of the radical recycling product channels of RO2 + HO2 and RO2 + RO2 reactions (RO + OH + O2 and RO + RO + O2, respectively) for RO2s derived from the oxidation of a set of the highest globally emitted monoterpenes showed that the two reactions were thermodynamically favorable for all the studied systems, and that for some of them, especially the O3 oxidized systems, the rate-limiting transition state energies can be low enough to render the reactions competitive in atmospheric conditions. Peroxy radical reactions with the atmospheric oxidant OH and alkoxy radicals RO were found to first form a trioxide adduct (ROOOH and ROOOR, respectively). While the former rapidly decompose to RO + HO2 and R(O)OH + O2 products for the model β-oxo and acetyl RO2 systems, respectively, the ROOOR adducts from the latter can have lifetimes in the range of 10 - 100 s (for the homo and hetero alkyl and β-oxo systems). If the reacting RO2 and RO radicals are sufficiently large and oxidized, the product adducts can directly be involved in SOA formation. The modeling of iodide-based chemical ionization mass spectrometer (iodide-CIMS) using computational methods showed that relatively low-level computational theory can produce reasonable correlation between molecule•I- cluster binding enthalpies and iodide-CIMS instrumental sensitivities. While some outliers were observed (lower than expected binding enthalpies for clusters that were detected at the maximum possible sensitivity of the instrument, for example), the method outlined in our study can be a quick indicator of the detectibility of an analyte by an iodide-CIMS. Additionally, the direct detection of the HO2 radical experimentally using an iodide-CIMS was demonstrated. The comparison of iodide- and nitrate-CIMS spectra for a cyclohexene ozonolysis experiment showed that the iodide-CIMS method was capable of detecting the less oxidized (oxygen:carbon O/C ratio of 0.5 - 0.66) molecules more efficiently than nitrate-CIMS. Higher oxidized molecules (O/C ratio 1 - 1.5) were detected equally well by both methods. Finally, the use of a new chemical ionization inlet (Multi-scheme chemical IONization inlet, MION, Karsa Ltd, Helsinki, Finland), which is capable of switching between two different reagent ions, bromide and nitrate, in 1 s timescales was demonstrated and used to detect the ozonolysis products of cyclohexene and α-pinene. The successful demonstration of the MION inlet opens up the possibility to use multiple CIMS methods concurrently and detect a widest possible range of volatile organic compound (VOC) oxidation products.Jotta voitaisiin arvioida sekundääristen orgaanisten aerosolihiukkasten (SOA) vaikutusta Maapallon säteilytaseeseen, meidän tulee ymmärtää niiden muodostukseen johtavaa kaasufaasin kemiaa. Lisäksi näiden kemiallisten reaktioiden lähtöainemolekyylien suora havaitseminen vallitsevassa ilmanpaineessa ilman esikäsittelyä on arvokasta. Tässä tutkimuksessa on käytetty laskennallisia- ja kokeellisia menetelmiä selvittämään 1) valittujen ilmakehässä merkittävien radikaali-radikaali reaktioiden termodynamiikkaa ja reaktiomekanismeja, ja 2) tiettyjen kemialliseen ionisaation perustuvien massaspektrometriamenetelmien (CIMS) tehokkuutta havaitsemaan SOA:n muodostumisessa oleellisia happoja ja lähtöaineyhdisteitä. Pääasialliset happea sisältävät ilmakehän radikaalit, jotka ovat myös tämän tutkimuksen painopisteet, ovat hydroksi- (OH), hydroperoksi- (HO2), alkoksi- (RO) ja peroksiradikaalit (RO2). Laskennallisessa tutkimuksessamme selvisi, että RO2 + HO2 ja RO2 + RO2 reaktioiden radikaaleja kierrättävät tuotekanavat (RO + O2 + OH ja RO + O2 + RO2) ovat termodynaamisesti suotuisia kaikille tutkituille peroksiradikaaleille, jotka ovat peräisin jostakin ilmakehässä eniten esiintyvien monoterpeenien hapetusreaktioista. Lisäksi useilla näistä tutkituista systeemeistä, etenkin otsonihapetuksessa muodostuvilla peroksiradikaaleilla, reaktion nopeuden määrittävä reaktiovaiheen siirtymätilan energia voi olla niin alhainen, että reaktio on tärkeä myös ilmakehän olosuhteissa. Peroksiradikaalien huomattiin reagoivan OH-radikaalin (ilmakehän tärkeimmän hapettimen) sekä myös alkoksiradikaalien kanssa, muodostaen trioksidiadduktin (ROOOH ja ROOOR). Ensiksimainittu hajoaa nopeasti RO + HO2 ja R(O)OH + O2 tuotteiksi mallinnetuilla β-okso- ja asetyyli-RO2 systeemeillä, mutta jälkimmäisessä tapauksessa muodostuvien ROOOR adduktien elinajat voivat olla jopa 10-100 sekuntia (homo- ja heteroalkyyli- ja β-oksosysteemeillä). Jodidi-ionisaatio massaspektrometrin (jodidi-CIMS) laskennallisessa mallintamisessa huomattiin, että melko alhaisellakin laskennan teoriatasolla voidaan saavuttaa järkevä korrelaatio molekyyli•I- -klusterin sidosentalpian ja jodidi-CIMS:n mittausherkkyyden välille. Muutamista poikkeavuuksista huolimatta (esimerkiksi odotettua alhaisemmat sidosentalpiat klustereille, jotka havaittiin mittalaitteen maksimiherkkyydellä) tutkimuksessamme esitettyä menetelmää voidaan käyttää nopeana kohteen havaittavuuden osoittimena jodidi-CIMS menetelmällä. Lisäksi tutkimuksessa näytettiin HO2-radikaalin suora havainnointi käyttämällä jodidi-CIMS menetelmää. Jodidi- ja nitraatti-CIMS –menetelmällä mitatut syklohekseenin otsonolyysikokeiden tulokset osoittivat, että jodidi-CIMS –menetelmällä voitiin havaita vähemmän hapettuneita molekyylejä (happi:hiili O/C suhdeluku 0.5 – 0.66) nitraatti-CIMS –menetelmää tehokkaammin. Enemmän hapettuneita molekyylejä (O/C suhdeluku 1 - 1.5) voitiin mitata kummallakin menetelmällä yhtä hyvin. Lopuksi, uuden kemiallisen ionisaation inletti (Multi-scheme chemical IONization inlet, MION, Karsa Ltd, Helsinki, Finland) käyttöönotto mahdollisti nopean vaihdon kahden eri reagenssi-ionin, bromidin ja nitraatin välillä sekunnin aikaskaalassa, mitä hyödynnettiin sykloheksaanin ja α-pineenin otsonolyysituotteiden havainnoinnissa. MION-läpiviennin hyödyllisyyden onnistunut havainnollistus avaa mahdollisuuden havaita laajimman mahdollisimman kirjon haihtuvien orgaanisten yhdisteiden (volatile organic compounds, VOC) hapettumistuotteita käyttämällä useita CIMS-menetelmiä rinnakkain

Computational studies on chemical ionization and reaction rates of atmospherically relevant oxidized multifunctional compounds

High pressure chemical ionization has recently been used with mass spectrometers to measure atmospheric molecules and molecule clusters. In anion chemical ionization, negatively charged reagent ions ionize the neutral sample molecules (or clusters) mainly by forming ion-molecule clusters. The detection of neutral molecules is highly dependent on how effective the chemical ionization process-es are, since the mass spectrometers can only detect charged molecules or clusters. This causes uncertainties in the measurements of most atmospheric trace gas molecules. In addition, mass spectrometers are able to detect only the molecular mass of the sample molecules, which means that other methods are needed to find the molecular structures of the detected compounds. In this work, we take a look at how quantum chemistry can be used to model different chemical ionization processes in a typical chemical ionization instrument, and to calculate reaction rates for unimolecular gas-phase reactions. With our computations, we were able to explain some of the experimental observations regarding the differences in the detection efficiencies of some reagent ions. The cause for the low detection efficiency of some sample molecules was found to be less stable ion-molecule clusters. Our calculations showed an increasing cluster stability for each of the studied reagent anions with the increase of the number of oxygen atoms in the sample molecule. This means that less oxygenated molecules generally have lower detection efficiencies than the more oxygenated ones. In addition, the computed reaction rate coefficients of two different unimolecular HO2 loss reaction mechanisms showed that, due to collisional stabilization, this reaction is too slow to compete with bimolecular reactions under atmospheric conditions, especially if the reactant is an oxygenated organic molecule.Maan ilmakehä sisältää typen, hapen ja hiilidioksidin lisäksi muun muassa suuren määrän erilaisia orgaanisia yhdisteitä. Aerosolihiukkasten muodostumisen kannalta erityisen mielenkiintoisia yhdisteitä ovat paljon happea sisältävät, alhaisen haihtuvuuden omaavat molekyylit. Nämä yhdisteet muodostuvat helposti haituvien molekyylien hapettumisreaktioiden tuotteina kaasufaasissa. Tuotteita voidaan mitata suoraan kaasufaasista käyttämällä massaspektrometriaa, mutta neutraalit molekyylit on varattava, ennen kuin niitä pystytään mittaamaan. Väitöskirjassa käydään läpi, kuinka laskennallisia menetelmiä on käytetty sekä hapettumisreaktioiden tutkimiseen että molekyylien varausmekanismien mallintamiseen

Particle formation and surface processes on atmospheric aerosols: a review of applied quantum chemical calculations

Aerosols significantly influence atmospheric processes such as cloud nucleation, het- erogeneous chemistry, and heavy-metal transport in the troposphere. The chemical and physical complexity of atmospheric aerosols results in large uncertainties in their climate and health effects. In this article, we review recent advances in scientific understanding of aerosol processes achieved by the application of quantum chemical calculations. In particular, we emphasize recent work in two areas: new particle for- mation and heterogeneous processes. Details in quantum chemical methods are pro- vided, elaborating on computational models for prenucleation, secondary organic aerosol formation, and aerosol interface phenomena. Modeling of relative humidity effects, aerosol surfaces, and chemical kinetics of reaction pathways is discussed. Because of their relevance, quantum chemical calculations and field and laboratory experiments are compared. In addition to describing the atmospheric relevance of the computational models, this article also presents future challenges in quantum chemical calculations applied to aerosols

Toward atomic-based understanding of some reactive and non-reactive surfaces

The thesis is composed of two broad themed sections with the underlying aim of understanding on a precise atomic basis, the electronic and structural factors governing the reactive and non-reactive surfaces of two metal oxides belonging to the same group in the periodic table; boron (B) and aluminium (Al). Using accurate density functional theory (DFT) computations, we first elucidate the initial reaction steps of the surface oxidation of elemental boron into its respective oxide; boron trioxide (B₂O₃). The highly exoergic reaction obtained for the dissociative adsorption of molecular oxygen over the boron surface coincides with the widely used boron oxidation reaction as secondary energy source in rockets. The relatively large activation energy for the O-O dissociation step marks the non-spontaneity of elemental boron oxidation at room temperature. Having established routes for the formation of B₂O₃-like precursors, we then investigate the relative stability of four low-index surfaces of the low-pressure B₂O₃ phase; namely the B₂O₃-I configuration. We demonstrate that none of the investigated low-index surfaces have dangling bonds, which reasonably relates to the experimentally observed low reactivity of this compound. The most stable surface terminations of B₂O₃ orientations entail tetrahedral BO₄ units. Such termination incurs a lower surface energy than orientations that consist of only triangular BO₃ units. Electronic and structural factors provide atomic-base elucidation of the observed inertness of B₂O₃. Combined experimental techniques (i.e. diffuse reflectance infrared spectroscopy) and DFT simulation are used to answer some of the most intriguing questions pertinent to factors underpinning the well-documented catalytic inhibition by B₂O₃ and its hygroscopic behaviour. We investigate the adsorption and dissociation mechanisms of two hydrogen chalcogenides, namely water (H₂O) and hydrogen sulfide (H₂S) molecules over B₂O₃-I (101) surfaces. We show that the diboron trioxide surface exhibits high physiochemical reactivity towards water molecules. The Lewis acid properties of B₂O₃-I lead to the formation of a molecular adsorption state (rather than dissociative adsorption) of the H₂S molecule via the acceptance of an electron pair into the low-energy orbital of the boron valence shell. While acting as water scavenger to generate dissociated radicals, B₂O₃ exhibits an inhibitor characteristic towards the dissociation of H₂S molecules, representing an ideal reactor wall coating in such systems. Alumina have been widely utilised as independent catalysts or as support materials for other catalysts. From an environmental perspective, alumina nanoclusters dispersed on surfaces of particulate matter PM₁₂ generate from various combustion processes play a critical role in the synthesis of environmental persistent free radicals (EPFR). Of particular importance are phenoxy-type EPFR that often acts as building blocks for the formation of notorious pollutants. Herein, we provide a comprehensive thermo-mechanistic account of alumina-surface mediated formation of phenoxy-type EPFR on different structural alumina models encompassing the following surfaces: dehydrated alumina surface, fully hydrate alumina surface, surfaces with different hydration coverage, and silicon-alumina doped surface. We show that fission of the phenol’s hydroxyl bond over dehydrated alumina systematically incurs lower energy barriers in reference to the hydrate surfaces. The catalytic activity of the alumina surface in producing the phenoxy/phenolate species reversibly correlates with the degree of hydroxyl coverage. Furthermore, we clarify the effect doping on the catalytic activity of alumina. The activation energy barrier required to form phenoxy moiety on Si-substituted Al₂O₃(0001) surface is ~40% lower than that of analogous barriers encountered over undoped dehydrate alumina surface. Overall, all considered models of alumina configurations are shown to produce adsorbed phenolate; however, desorption of the latter into the gas phase requires a rather sizable energy. Thus, the fate of absorbed phenolate is most likely to be dictated by decomposition affording carboneous layer of self-decomposition into other stable molecules

Theoretical investigations of surface chemistry in space

In this Thesis, computational models for carbonaceous dust grains were examined and compared to known experimental data. Different formation routes of molecules, important to the astrochemical evolution of the universe, have been investigated and their relative energies were analysed with respect to the harsh conditions in interstellar dark clouds of extremely low pressure (10‐17 bar) and temperature (10 – 20 K). Dust grains are present in the universe, and evidence shows they are siliceous or carbonaceous, possible with an icy mantle surrounding the core. In this research, only carbonaceous surfaces were examined. Two models were used to represent polycyclic, aromatic carbonaceous surfaces: coronene, C24H12, representing a relatively small hydrocarbon, and graphene – a single graphite sheet – which represents an extended carbonaceous surface. The main aims of this Thesis were to examine the validity of computationally modelled astrochemical reactions and to investigate the catalytic effect of dust grain surfaces on these reactions. Several formation reactions were examined, including water, methanol and carbonyl sulfide formation. The abundance of these molecules in dark molecular clouds cannot be explained by solely considering gas phase type reactions, and the influence that the carbonaceous surfaces have on these reactions was investigated in order to examine any catalytic effect that they may have

A Kinetic Study of Novel Gas-Phase Reaction Pathways for Pyruvic Acid and Hydrogen Halides with Hox Radicals.

Hydroperoxy (HO2) and hydroxyl (OH) radicals are known to play important roles in both combustion and atmospheric chemistry. Although less reactive than hydroxyl radicals, HO2&nbsp;has been found to be important to the ignition process in combustion systems and has been found in greater concentrations than that of OH in the troposphere. In this work, gas-phase reactions between HO2&nbsp;and hydrogen halides (HX, X=F, Cl, Br, I) are examined due to the catalytic effect halides can have in the ozone destruction process, as well as their ability to inhibit flame speeds under combustion settings. During the study of the abstraction reaction, a novel double exchange process between HO2&nbsp;and hydrogen halides was discovered. These new exchange reactions were found to have barriers significantly lower than the corresponding abstraction process due to a highly structured transition state which is characterized as a planar ring. The kinetics of the abstraction and the exchange process were also compared. It was found that due to a competition between entropic and energetic effects the abstraction process dominates at high temperatures and the exchange reaction is more significant at low temperatures. The kinetics of gas-phase reactions between HO2&nbsp;and OH with pyruvic acid were also examined. Pyruvic acid is an important keto-acid intermediate formed from the oxidation pathway of isoprene. This molecule is unique due to slow oxidation rates by OH and the ability to absorb in the UV-vis region. These characteristics lead to photolysis being the primary channel for degradation in the troposphere. In this work, accurate rate coefficients are determined for use in atmospheric models and compared to experimental estimates. Due to the number of possible hydrogen transfer reactions in pyruvic acid, reactions with hydroperoxy radicals were also examined to see if barrier lowering effects could play a role under atmospheric conditions. It was found that although HO2&nbsp;mediation produced a catalytic effect on the reactions barriers, the high energy products of the reactions made this effect negligible on the rate constants.</p

An analysis of the sources and sinks for Criegee intermediates: An extended computational study

This thesis is centred around a common theme of using computational chemistry to investigate reactions that either produce or deplete atmospheric Criegee intermediates (CIs). The computational work investigates many novel reaction mechanisms and generates kinetic and product branching data for these reactions which is then reviewed in the context of their impact on local tropospheric environments. Firstly, the ozonolysis of a large array of alkenes is examined, as known sources of CIs. The results of this study indicate that factors including the number of α-H atoms in the alkene substituents and others besides all have a significant influence on the ozonolysis rate constants and the fractional distribution of different CI yields. Assessments of bimolecular CI sinks are also examined, particularly via reaction with gaseous alcohols. The high reactivity of many CI + alcohol reactions shows that in geographical areas such as Sao Paulo, where biofuel use is prevalent, that alcohols are likely a significant sink of CIs. The bimolecular chemistry of a series of CIs derived from a new range of synthetic hydrofluoroolefin refrigerants are also examined, because their refrigerant precursors are being emitted in ever larger quantities, and their fluorinated substituents make their CI chemistry distinctive. Both the CIs and their alkene precursors are classified into several sets of taxonomic groups on the basis of common structural features and similar bimolecular chemistries. By linking computational ozonolysis chemistry to the structural alkene features, this classification allows the author to generate a new theoretical “FESP” model designed to predict the ozonolysis chemistry of lengthy, conformationally flexible alkenes. This model is used to determine the reaction rate and branching fractions of the O3 + Z-2-hexene reaction, but could be applied to many other alkenes, and perhaps even adapted to explore the bimolecular reactivity of lengthy CIs

Atomistic modeling of the charge process and optimization of catalysts positioning in porous cathodes of lithium/air batteries

The reversibility and capacity of current lithium/air cells are severely limited by the high overpotential between the charge and discharge process and the occlusion of the pores of the active cathode surface due to non-uniform deposition of Li2O2 as the discharge product. In this thesis we present a study of these capacity-limiting issues on the lithium/air battery in two parts. First we present a combined classical and density functional theory based molecular dynamics study of the mechanisms underlying the oxygen evolution reaction during the charging of lithium/air batteries. As models for the Li2O2 material at the cathode we employ small amorphous clusters with a 2:2 Li:O stoichiometry, whose energetically most stable atomic configurations comprise both O atoms and O-O pairs with mixed peroxide/superoxide character, as revealed by their bond lengths, charges, spin moments, and densities of states. The oxidation of Li8O8 clusters is studied in unbiased density functional theory based molecular dynamics simulations upon removal of either one or two electrons, either in vacuo or immersed in dimethyl sulfoxide solvent molecules with a structure previously optimized by means of classical molecular dynamics. Whereas removal of one electron leads only to an enhancement of the superoxide character of O-O bonds, removal of two electrons leads to the spontaneous dissolution of either an O2 or a LiO2 molecule. These results are interpreted in terms of a two-stage process in which a peroxide-to-superoxide transition can take place in amorphous Li2O2 phases at low oxidation potentials, later followed by the dissolution of dioxygen molecules and Li ions at higher potentials. In the second part we solve numerically a reaction-diffusion equation to determine the Li2O2 deposition profiles in a model porous cathode in the absence and presence of discrete catalytic sites, considering four commonly used electrolytes. We implement a Greedy optimization algorithm to maximize the cathode capacity before pore clogging by optimal positioning of the discrete catalysts along the pore. The results indicate that a maximal capacity is limited by the oxygen solubility and diffusivity in each electrolyte in the absence of catalysts and vary widely in the four cases considered. However, optimal catalyst distributions can effectively compensate for these differences, suggesting a rational way of designing cathode structures with high performances according to the required operation conditions