Quantum Chemical Studies on Tropospheric Nucleation Mechanisms Involving Sulfuric Acid

Nucleation is the first step of the process by which gas molecules in the atmosphere condense to form liquid or solid particles. Despite the importance of atmospheric new-particle formation for both climate and health-related issues, little information exists on its precise molecular-level mechanisms. In this thesis, potential nucleation mechanisms involving sulfuric acid together with either water and ammonia or reactive biogenic molecules are studied using quantum chemical methods. Quantum chemistry calculations are based on the numerical solution of Schrödinger's equation for a system of atoms and electrons subject to various sets of approximations, the precise details of which give rise to a large number of model chemistries. A comparison of several different model chemistries indicates that the computational method must be chosen with care if accurate results for sulfuric acid - water - ammonia clusters are desired. Specifically, binding energies are incorrectly predicted by some popular density functionals, and vibrational anharmonicity must be accounted for if quantitatively reliable formation free energies are desired. The calculations reported in this thesis show that a combination of different high-level energy corrections and advanced thermochemical analysis can quantitatively replicate experimental results concerning the hydration of sulfuric acid. The role of ammonia in sulfuric acid - water nucleation was revealed by a series of calculations on molecular clusters of increasing size with respect to all three co-ordinates; sulfuric acid, water and ammonia. As indicated by experimental measurements, ammonia significantly assists the growth of clusters in the sulfuric acid - co-ordinate. The calculations presented in this thesis predict that in atmospheric conditions, this effect becomes important as the number of acid molecules increases from two to three. On the other hand, small molecular clusters are unlikely to contain more than one ammonia molecule per sulfuric acid. This implies that the average NH3:H2SO4 mole ratio of small molecular clusters in atmospheric conditions is likely to be between 1:3 and 1:1. Calculations on charged clusters confirm the experimental result that the HSO4- ion is much more strongly hydrated than neutral sulfuric acid. Preliminary calculations on HSO4- NH3 clusters indicate that ammonia is likely to play at most a minor role in ion-induced nucleation in the sulfuric acid - water system. Calculations of thermodynamic and kinetic parameters for the reaction of stabilized Criegee Intermediates with sulfuric acid demonstrate that quantum chemistry is a powerful tool for investigating chemically complicated nucleation mechanisms. The calculations indicate that if the biogenic Criegee Intermediates have sufficiently long lifetimes in atmospheric conditions, the studied reaction may be an important source of nucleation precursors.Nukleaatio on ensimmäinen askel prosessissa, jossa ilmakehän kaasumolekyylit tiivistyvät nestepisaroiksi tai kiinteiksi hiukkasiksi. Vaikka ilmakehän hiukkasmuodostus on ensiarvoisen tärkeä tutkimusaihe pienhiukkasten ilmasto- ja terveysvaikutusten takia, hiukkasmuodostuksen molekyylitason mekanismeista on olemassa hyvin vähän tietoa. Väitöskirjassa on tutkittu kvanttikemiallisten menetelmien avulla mahdollisia nukleaatiomekanismeja, jotka liittyvät rikkihappoon, sekä veteen ja ammoniakkiin, tai reaktiivisiin orgaanisiin molekyyleihin. Kvanttikemia tarkoittaa Schrödingerin aaltoyhtälön ratkaisemista systeemille, joka koostuu atomiytimistä ja näitä ympäröivästä elektroniverhosta. Käytännössä yhtälön ratkaiseminen edellyttää lukuisia likimääräisoletuksia. Eri likimääräisoletusten yhdistelmät muodostavat laajan kirjon erilaisia laskentamenetelmiä. Laskentamenetelmien vertailu osoittaa, että mikäli rikkihappo - vesi - ammoniakkiklustereille halutaan laskea tarkkoja tuloksia, tulee menetelmä valita huolella. Väitöskirjan tulokset osoittavat, että rikkihapon hydraatiota eli veteen sitoutumista koskevat kokeelliset tulokset voidaan kvantitatiivisesti toisintaa, mutta vain jos käytetään riittävän korkeatasoisia kvanttikemiallisia menetelmiä. Ammoniakin rooli rikkihappo-vesinukleaatiossa selvitettiin laskemalla muodostumisenergioita klustereille, joiden kokoa kasvatettiin joko rikkihappoa, vettä tai ammoniakkia lisäämällä. Kuten kokeelliset tulokset osoittavat, ammoniakki edistää klustereiden kasvua nimenomaan rikkihappojen lisäyksen kautta. Tässä väitöskirjassa esitetyt laskut ennustavat, että ilmiön merkitys ilmakehässä tulee merkittäväksi kun rikkihappomolekyylien lukumäärä kasvaa kahdesta kolmeen. Toisaalta, pienet molekyyliklusterit tuskin sisältävät enemmän kuin yhden ammoniakkimolekyylin kutakin rikkihappoa kohden. Tulos on merkittävä, sillä isommissa klustereissa on mitattu ammoniakkimolekyylejä olevan jopa tuplaten rikkihappomolekyyleihin verrattuna. Laskut siis osoittavat että pienten ja isojen aerosolihiukkasten kemiallinen koostumus voi erota merkittävästi. Varatuilla klustereilla suoritetut laskut vahvistavat kokeellisen tuloksen, jonka mukaan HSO4- ioni on huomattavan paljon voimakkaammin sitoutunut veteen kuin neutraali rikkihappo. Alustavat laskut HSO4- NH3 klustereille näyttävät osoittavan että ammoniakilla on korkeintaan vähäinen rooli rikkihappo- vesi - systeemin ioni-indusoidussa nukleaatiossa. Rikkihapon ja stabilisoitujen Criegee - väliaineiden väliselle reaktiolle lasketut termodynaamiset ja kineettiset parametrit osoittavat, että kvanttikemia on voimakas työkalu kemiallisesti monimutkaisten nukleaatiomekanismien tutkimiseen. Mikäli k.o. väliaineiden elinikä ilmakehässä on tarpeeksi pitkä, tutkittu reaktio saattaa laskentatulosten perusteella tuottaa merkittäviä määriä nukleaatioon tehokkaasti osallistuvia tiivistymiskykyisiä yhdisteitä.Nukleation är det första steget i processen där gasmolekyler i atmosfären bildar nya aerosolpartiklar. Fast uppkomsten av aerosolpartiklar i atmosfären är viktigt både ur klimat- och hälsoperspektiv, finns det endast lite information om nukleationsmekanismer på molekylnivå. I denna doktorsavhandling har potentiella nukleationsmekanismer som inbegriper svavelsyra, tillsammans med antingen vatten och ammoniak eller reaktiva biogeniska molekyler, undersökts med hjälp av kvantkemiska metoder. Kvantkemi, även kallad beräkningskemi, baserar sig på det numeriska lösandet av Schrödingerekvationen för ett system av atomer och elektroner med hjälp av vissa approximationer. Detaljerna i dessa approximationer ger upphov till ett stort antal så kallade modellkemier. En jämförelse av olika kvantkemiska metoder visar att modellkemin bör väljas noggrant om pålitliga beräkningsresultat för svavelsyra - vatten - ammoniak - kluster eftersträvas. Resultaten som rapporteras i denna doktorsavhandling visar att experimentella resultat gällande svavelsyramolekylens benägenhet att binda till sig vatten kan kvantitativt reproduceras av beräkningskemin, men endast med hjälp av tillräckligt avancerade metoder. Ammoniakets roll i nukleation av svavelsyra - vattenkluster har klargjorts genom att beräkna formationsenergier för kluster vars storlek ökades genom att tillsätta antingen svavelsyra-, vatten- eller ammoniakmolekyler. I enlighet med experimentella mätningar visar beräkningarna att ammoniak främjar tillväxten av kluster främst genom att binda nya svavelsyramolekyler starkare. Beräkningarna i denna doktorsavhandling visar att ammoniakets roll i atmosfäriska förhållanden blir viktig när mändgen svavelsyramolekyler ökar från två till tre. Å andra sidan visar beräkningarna att de minsta partiklarna i atmosfären antagligen innehåller högst en ammoniakmolekyl per svavelsyra, och antagligen mindre. Detta resultat är synnerligen intressant, eftersom större aerosolpartiklar enligt mätningar ofta innehåller upp till två ammoniakmolekyler per svavelsyra. Beräkningsresultaten visar alltså att den kemiska konsistensen av små och stora aerosolpartiklar kan skilja sig avsevärt från varandra. Beräkningar med laddade kluster bestyrker det experimentella resultatet at HSO4- jonen är mycket starkare bundet till vattenmolekyler än neutral svavelsyra H2SO4. Preliminära beräkningar med HSO4-*NH3 kluster tyder på att ammoniaket endast spelar en mycket liten roll i jon-inducerad svavelsyra - vatten nukleation Termodynamiska och kinetiska parametrar beräknade för reaktionen mellan svavelsyra och stabiliserade Criegee-intermediärer visar att kvantkemin är ett effektivt redskap för att studera kemiskt komplicerade nukleationsmekanismer. Om dessa intermediärers livstid i atmosfären är tillräckligt lång, kan den studerade reaktionsmekanismen producera betydliga mändger organosulfater som effektivt deltar i nukleationsprocesser

Reaction between Peroxy and Alkoxy Radicals Can Form Stable Adducts

Peroxy (RO2) and alkoxy (RO) radicals are prototypical intermediates in any hydrocarbon oxidation. In this work, we use computational methods to (1) study the mechanism and kinetics of the RO2 + OH reaction for previously unexplored “R” structures (R = CH(O)CH2 and R = CH3C(O)) and (2) investigate a hitherto unaccounted channel of molecular growth, R′O2 + RO. On the singlet surface, these reactions rapidly form ROOOH and R′OOOR adducts, respectively. The former decomposes to RO + HO2 and R(O)OH + O2 products, while the main decomposition channel for the latter is back to the reactant radicals. Decomposition rates of R′OOOR adducts varied between 103 and 0.015 s–1 at 298 K and 1 atm. The most long-lived R′OOOR adducts likely account for some fraction of the elemental compositions detected in the atmosphere that are commonly assigned to stable covalently bound dimers.Peer reviewe

Temporal and Spatial Variation in Scots Pine Resin Pressure and Composition

Resin is a first-line defense in pine trees, but important questions regarding its temporal and spatial variation remain unsolved. Resin pressure varies according to water potential in dry conditions, but in moist conditions, it follows temperature dynamics for a yet unknown reason. Relations between resin composition, resin pressure, and shoot monoterpene emissions are also unquantified. To gain mechanistic understanding on the resin dynamics in a boreal forest, we measured temperature and water potential dependency of Scots pine resin pressure. We attempted to quantify the temperature dependency of resin pressure in terms of three contributions: 1) saturation vapor pressure, 2) thermal expansion, and 3) N2, O2, and CO2 solubility. We also analyzed monoterpene composition in the resin and the shoot emissions of 16 pines with gas chromatography mass spectrometry to study their interrelations. We show that in moist conditions, resin pressure is driven by temperature at a diurnal scale, but also affected by soil water potential at a day-to-day scale. Diurnal temperature dependency was explained by thermal expansion of resin and changes in bubble volume due to changes in gas solubility in resin with temperature. Resin pressures correlated also with total monoterpene and α-pinene content in resin and with total monoterpene and ∆3-carene and terpinolene emissions from shoots.Resin is a first-line defense in pine trees, but important questions regarding its temporal and spatial variation remain unsolved. Resin pressure varies according to water potential in dry conditions, but in moist conditions, it follows temperature dynamics for a yet unknown reason. Relations between resin composition, resin pressure, and shoot monoterpene emissions are also unquantified. To gain mechanistic understanding on the resin dynamics in a boreal forest, we measured temperature and water potential dependency of Scots pine resin pressure. We attempted to quantify the temperature dependency of resin pressure in terms of three contributions: (1) saturation vapor pressure, (2) thermal expansion, and (3) N2, O2, and CO2 solubility. We also analyzed monoterpene composition in the resin and the shoot emissions of 16 pines with gas chromatography mass spectrometry to study their interrelations. We show that in moist conditions, resin pressure is driven by temperature at a diurnal scale, but also affected by soil water potential at a day-to-day scale. Diurnal temperature dependency was explained by thermal expansion of resin and changes in bubble volume due to changes in gas solubility in resin with temperature. Resin pressures correlated also with total monoterpene and α-pinene content in resin and with total monoterpene and Δ3-carene and terpinolene emissions from shoots.Peer reviewe

Identification of molecular cluster evaporation rates, cluster formation enthalpies and entropies by Monte Carlo method

We address the problem of identifying the evaporation rates for neutral molecular clusters from synthetic (computer-simulated) cluster concentrations. We applied Bayesian parameter estimation using a Markov chain Monte Carlo (MCMC)algorithm to determine cluster evaporation/fragmentation rates from known cluster distributions, assuming that the clustercollision rates are known. We used the Atmospheric Cluster Dynamic Code (ACDC) with evaporation rates based on quantumchemical calculations to generate cluster distributions for a set of electrically neutral sulphuric acid and ammonia clusters. We then treated these concentrations as synthetic experimental data, and tested two approaches for estimating the evaporation rates. First we have studied a scenario where at one single temperature time-dependent cluster distributions are measured before thesystem reaches a time-independent steady-state. In the second scenario only steady-state cluster distributions are measured, butat several temperatures. This allowed us to use multiple sets of concentrations at different temperatures. Additionally, in thelatter case the evaporation rates were represented in terms of cluster formation enthalpies and entropies which were considered to be free parameters. This reparametrization reduced the number of unknown parameters, since several evaporation ratesdepend on the same cluster formation enthalpy and entropy values. We show that in the second setting, even if only two temperatures were used, the temperature-dependent steady-state dataoutperforms the first setting for parameter identification. We can thus conclude that for experimentally determining evaporationrates, cluster distribution measurements at several temperatures are recommended over time-dependent measurements at one temperature.Peer reviewe

Criegee Intermediates React with Ozone

We have investigated the reaction of the one-carbon stabilized Criegee intermediate (H_2COO, formaldehyde oxide) with ozone, theoretically, using high level coupled cluster ab initio methods. Key to the reactivity of the Criegee intermediate with ozone is the strongly exothermic formation of an intermediate consisting of five oxygen and one carbon atoms (H_2CO_5) in a six-membered ring structure. This intermediate proceeds via a spin-allowed route over two transition states with low energy barriers to form molecular oxygen and formaldehyde. The reaction may contribute to the loss of these biradicals in the atmosphere

How well can we predict cluster fragmentation inside a mass spectrometer?

Fragmentation of molecular clusters inside mass spectrometers is a significant source of uncertainty in a wide range of chemical applications. We have measured the fragmentation of sulfuric acid clusters driving atmospheric new-particle formation, and developed a novel model, based on first principles calculations, capable of quantitatively predicting the extent of fragmentation.Peer reviewe

Comparing simulated and experimental molecular cluster distributions

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New Particle Formation from the Vapor Phase : From Barrier-Controlled Nucleation to the Collisional Limit

Studies of vapor phase nucleation have largely been restricted to one of two limiting cases—nucleation controlled by a substantial free energy barrier or the collisional limit where the barrier is negligible. For weakly bound systems, exploring the transition between these regimes has been an experimental challenge, and how nucleation evolves in this transition remains an open question. We overcome these limitations by combining complementary Laval expansion experiments, providing new particle formation data for carbon dioxide over a uniquely broad range of conditions. Our experimental data together with a kinetic model using rate constants from high-level quantum chemical calculations provide a comprehensive picture of new particle formation as nucleation transitions from a barrier-dominated process to the collisional limit.Peer reviewe