Formation of Highly Oxygenated Organic Molecules from alpha-Pinene Ozonolysis : Chemical Characteristics, Mechanism, and Kinetic Model Development

Terpenes are emitted by vegetation, and their oxidation in the atmosphere is an important source of secondary organic aerosol (SOA). A part of this oxidation can proceed through an autoxidation process, yielding highly oxygenated organic molecules (HOMs) with low saturation vapor pressure. They can therefore contribute, even in the absence of sulfuric acid, to new particle formation (NPF). The understanding of the autoxidation mechanism and its kinetics is still far from complete. Here, we present a mechanistic and kinetic analysis of mass spectrometry data from α-pinene (AP) ozonolysis experiments performed during the CLOUD 8 campaign at CERN. We grouped HOMs in classes according to their identified chemical composition and investigated the relative changes of these groups and their components as a function of the reagent concentration. We determined reaction rate constants for the different HOM peroxy radical reaction pathways. The accretion reaction between HOM peroxy radicals was found to be extremely fast. We developed a pseudo-mechanism for HOM formation and added it to the AP oxidation scheme of the Master Chemical Mechanism (MCM). With this extended model, the observed concentrations and trends in HOM formation were successfully simulated.Peer reviewe

Influence of temperature on the molecular composition of ions and charged clusters during pure biogenic nucleation

It was recently shown by the CERN CLOUD experiment that biogenic highly oxygenated molecules (HOMs) form particles under atmospheric conditions in the absence of sulfuric acid, where ions enhance the nucleation rate by 1-2 orders of magnitude. The biogenic HOMs were produced from ozonolysis of alpha-pinene at 5 degrees C. Here we extend this study to compare the molecular composition of positive and negative HOM clusters measured with atmospheric pressure interface time-of-flight mass spectrometers (APi-TOFs), at three different temperatures (25, 5 and -25 degrees C). Most negative HOM clusters include a nitrate (NO3-) ion, and the spectra are similar to those seen in the nighttime boreal forest. On the other hand, most positive HOM clusters include an ammonium (NH4+) 4) ion, and the spectra are characterized by mass bands that differ in their molecular weight by similar to 20 C atoms, corresponding to HOM dimers. At lower temperatures the average oxygen to carbon (O : C) ratio of the HOM clusters decreases for both polarities, reflecting an overall reduction of HOM formation with decreasing temperature. This indicates a decrease in the rate of autoxidation with temperature due to a rather high activation energy as has previously been determined by quantum chemical calculations. Furthermore, at the lowest temperature (-25 degrees C), the presence of C-30 clusters shows that HOM monomers start to contribute to the nucleation of positive clusters. These experimental findings are supported by quantum chemical calculations of the binding energies of representative neutral and charged clusters.Peer reviewe

Reduced anthropogenic aerosol radiative forcing caused by biogenic new particle formation

The magnitude of aerosol radiative forcing caused by anthropogenic emissions depends on the baseline state of the atmosphere under pristine preindustrial conditions. Measurements show that particle formation in atmospheric conditions can occur solely from biogenic vapors. Here, we evaluate the potential effect of this source of particles on preindustrial cloud condensation nuclei (CCN) concentrations and aerosol-cloud radiative forcing over the industrial period. Model simulations show that the pure biogenic particle formation mechanism has a much larger relative effect on CCN concentrations in the preindustrial atmosphere than in the present atmosphere because of the lower aerosol concentrations. Consequently, preindustrial cloud albedo is increased more than under present day conditions, and therefore the cooling forcing of anthropogenic aerosols is reduced. The mechanism increases CCN concentrations by 20-100% over a large fraction of the preindustrial lower atmosphere, and the magnitude of annual global mean radiative forcing caused by changes of cloud albedo since 1750 is reduced by 0.22 W m-2 (27%) to -0.60 W m-2. Model uncertainties, relatively slow formation rates, and limited available ambient measurements make it difficult to establish the significance of a mechanism that has its dominant effect under preindustrial conditions. Our simulations predict more particle formation in the Amazon than is observed. However, the first observation of pure organic nucleation has now been reported for the free troposphere. Given the potentially significant effect on anthropogenic forcing, effort should be made to better understand such naturally driven aerosol processes

Trace gas analysis of oxidation processes at the CLOUD experiment

Flüchtige organische Verbindungen (VOC) sind allgegenwärtig in unserem täglichen Leben. Ihre Quellen können sowohl anthropogener wie auch biogener Natur sein. Sie sind verantwortlich für den Großteil an Gerüchen, sowohl in Kosmetika oder Reinigungsmitteln, so wie ebenfalls für den Geruch eines Nadelwaldes oder den einer Blume. So vielfältig und allgegenwärtig ihre Quellen sind, so breit gefächert ist auch ihre Rolle in der Atmosphäre. Flüchtige organische Verbindungen spielen eine wichtige Rolle in der Kommunikation von Pflanzen untereinander, sowie der Kommunikation zwischen Flora und Fauna. Aber sie können auch eine Gefahr darstellen für die menschliche Gesundheit, die Umwelt schädigen und das Klima beeinflussen. Die chemische Umwandlung von biogenen VOCs in der Atmosphäre und ihr Einfluss auf das Klima der Erde insbesondere im Hinblick auf Partikelneubildung aus der Gasphase , ist Hauptfokus der vorliegenden Thesis. Mit einem jährlichen Fluss von mehr als 1150 Terragram Kohlenstoff tragen biogene flüchtige organische Verbindungen den größten Teil zur Gesamtmenge an atmosphärischen VOCs bei. Unter den biogenen VOCs, mit Ausnahme von Methan, ist Isopren (C5H8) die vorherrschend emittierte flüchtige organische Verbindung und ist seit Jahrzenten für die Atmosphärenforschung von großem Interesse. In der CLOUD Kammer am CERN wurde Isoprenoxidation unter einer Vielzahl atmosphärischer Bedingungen sowie Partikelneubildung ausgehend von Isopren untersucht, um ein besseres Verständnis für atmosphärische Prozesse zu erhalten. Die Gasphasenkomposition wurde mittels SRI-ToF-MS, einer Modifikation von Protonentausch Reaktion Flugzeit Massenspektrometrie (PTR-ToF-MS), sowie einer neu entwickelten hoch auflösenden PTR-ToF-MS Variante (PTR3-TOF) analysiert. So wichtig die Untersuchungen von Isopren auch sein mögen, werden sie jedoch von ihren vielleicht unerwarteten Herausforderungen begleitet. Der Vergleich von Isoprenoxidationsexperimenten bei niedrigen NOx Konzentrationen mit Modellsimulationen haben gezeigt, dass oberflächenkatalysierte Reaktionen auf Metalloberflächen der Reaktionskammer die Oxidationsprodukte des Isoprens von den Produkten aus niedrigen NOx Bedingungen hin zu den Produkten aus hohen NOx Bedingungen verschieben können. Statt der wenig volatilen Hydroxyhydroperoxide, ISOPOOH, die eine Rolle in der Bildung von sekundärem organischem Aerosol spielen, werden die hochflüchtigen Oxidationsprodukte Methylvinylketon, Methacrolein und Formaldehyd erzeugt. Es ist bekannt, dass die homolytische Spaltung der Peroxide auch intra-instrumentell in Gaschromatographen oder konventionellen PTR( ToF) MS Instrumenten auftreten kann. Das Verschieben der Oxidationsprodukte vom nieder-NOx Regime in das hoch-NOx Regime kann zu einer signifikanten Fehlinterpretation von atmosphärischen Prozessen und ihrer wissenschaftlichen Beurteilung in ihrer Bedeutung für die Erdatmosphäre führen. Studien zur Partikelneubildung ausgehend von Isoprenoxidation haben den Einfluss selbst geringer Konzentrationen von Kontaminationen aufgezeigt. Konzentrationen von gerade einmal 1 % an [4+2] Cycloadditionsprodukten von Isopren mit gleichem Oxidationsverhalten wie Monoterpene, haben einen tiefgreifenden Einfluss auf die Verteilung der Oxidationsprodukte, insbesondere hinsichtlich der sogenannten hochgradig oxygenierten organischen Moleküle (HOMs). Der Großteil der in dieser Studie beobachteten HOMs wurde durch Oxidation der Kontamination, und nicht des eigentlichen Vorläufers Isopren, erhalten. Zur eingehenden Studie der Partikelneubildung und Bildung von HOMs ausgehend von Isopren ist es daher notwendig, die Reinheit des verwendeten Vorläufers zu garantieren. Dies kann erfolgreich durch der Oxidation vorgeschaltetes Ausfrieren von möglichen weniger flüchtigen Kontaminationen erreicht werden. Die bereits erwähnten hoch oxygenierten organischen Moleküle, insbesondere der Monoterpene, sind von besonderem Interesse für die Atmosphärenforschung, da ihre Bedeutung für die Partikelneubildung und deren Wachstum in Laborstudien belegt werden konnte. Experimente an der CLOUD Kammer haben gezeigt, dass es entgegen dem bisherigen Verständnis nicht zwingend notwendig ist, dass Schwefelsäure vorhanden ist, damit Partikelneubildung stattfinden kann. Stattdessen kann Partikelneubildung auch direkt von HOMs ausgehend stattfinden. CLOUD Experimente konnten ebenso die Bedeutung von extrem schwerflüchtigen HOMs für das anfängliche Partikelwachstum sowie den Einfluss von etwas weniger schwerflüchtigen HOMs im späteren Partikelwachstum belegen.Volatile organic compounds (VOC) are omnipresent in our daily lives. They can be either of anthropogenic or biogenic nature and are responsible for most scents or odours, may it be in cosmetics or cleaning agents or the smell of a pine tree forest or flower. As numerous, varied and ubiquitous as their sources, as manifold are their roles in the atmospheric cycle. VOCs play an important role in communication between plants and between flora and fauna. But they may also be dangerous to human health, cause harm to the environment, and influence climate. The oxidative processing of biogenic VOCs in the atmosphere and their impact on earths climate especially new particle formation is the main focus of this thesis. With an estimated flux of more than 1150 teragram carbon per year, biogenically produced VOCs are the main contributors to the global VOC budget. Of all non-methane biogenic VOCs, isoprene (C5H8) is predominantly emitted and has been of great interest for atmospheric research over the past decades. Laboratory studies of isoprene oxidation under a variety of atmospheric conditions as well as studies of new particle formation from isoprene oxidation have been conducted at the CLOUD chamber at CERN to gain a better understanding of atmospheric processes. Gas-phase composition during these experiments has been analysed with Selective Reagent Ionisation Time-of-Flight Mass Spectrometry (SRI ToF MS), a modification of Proton Transfer Reaction Time-of-Flight Mass Spectrometry (PTR-ToF-MS), as well as a newly developed high resolution PTR ToF MS instrument (PTR3-TOF). As important as these isoprene studies are for our understanding, they are not without their, maybe unexpected, challenges. The comparison of low-NOx oxidation experiments with model simulations of isoprene oxidation has shown that surface catalysed decomposition reactions on the metal surface of the reaction chamber can shift the oxidation products of isoprene from the low NOx products to the high NOx products. Instead of the expected lower volatile hydroxy hydroperoxides, ISOPOOH, that have been shown to be of importance for SOA formation, the highly volatile oxidation products methyl vinyl ketone, methacrolein and formaldehyde are obtained as main oxidation products despite low NOx conditions. The underlying homolytic cleavage of the peroxides is also known to occur intra-instrumentally in gas chromatographs and conventional PTR-(ToF)-MS. The shift from low NOx products to high NOx products on contact with metal surfaces in the experimental setup may lead to a significant misrepresentation of atmospheric processes and their scientific conclusions for earths atmosphere. New particle formation studies from isoprene oxidation revealed the impact of minute trace concentrations of higher volatility contaminants. Even as little as 1 % of [4+2] cycloaddition products of isoprene with the same oxidative behaviour as monoterpenes, contained within the isoprene source, e.g. gas bottle, can have a profound impact on oxidation product distribution, especially on the highly oxygenated organic molecules (HOMs). The majority of HOMs observed in this study was formed from oxidation of the contaminant instead of the actual precursor isoprene. To comprehensively study new particle formation (NPF) and formation of HOMs from isoprene it is therefore necessary to assure the purity of the precursor which can successfully be achieved by cryogenic removal of possible low volatility contaminants before oxidation. Said highly oxygenated organic molecules, especially from monoterpene oxidation, are of particular interest in atmospheric sciences as their importance in new particle formation and growth has been shown in chamber studies. Experiments at the CLOUD facility revealed that contrary to current understanding, the presence of sulfuric acid is not imperatively required for new particle formation but that it can also proceed directly via HOMs. CLOUD studies have also highlighted the importance of extremely low-volatility HOMs in initial particle growth and the influence of slightly more volatile HOMs in subsequent growth of smallest particles.by Dipl. Chem. Anne-Kathrin BernhammerKumulative Dissertation aus vier ArtikelnZusammenfassung in deutscher SpracheUniversity of Innsbruck, Dissertation, 2018OeBB(VLID)277941

The role of low-volatility organic compounds in initial particle growth in the atmosphere

About half of present-day cloud condensation nuclei originate from atmospheric nucleation, frequently appearing as a burst of new particles near midday1. Atmospheric observations show that the growth rate of new particles often accelerates when the diameter of the particles is between one and ten nanometres2,3. In this critical size range, new particles are most likely to be lost by coagulation with pre-existing particles4, thereby failing to form new cloud condensation nuclei that are typically 50 to 100 nanometres across. Sulfuric acid vapour is often involved in nucleation but is too scarce to explain most subsequent growth5,6, leaving organic vapours as the most plausible alternative, at least in the planetary boundary layer7,8,9,10. Although recent studies11,12,13 predict that low-volatility organic vapours contribute during initial growth, direct evidence has been lacking. The accelerating growth may result from increased photolytic production of condensable organic species in the afternoon2, and the presence of a possible Kelvin (curvature) effect, which inhibits organic vapour condensation on the smallest particles (the nano-Köhler theory)2,14, has so far remained ambiguous. Here we present experiments performed in a large chamber under atmospheric conditions that investigate the role of organic vapours in the initial growth of nucleated organic particles in the absence of inorganic acids and bases such as sulfuric acid or ammonia and amines, respectively. Using data from the same set of experiments, it has been shown15 that organic vapours alone can drive nucleation. We focus on the growth of nucleated particles and find that the organic vapours that drive initial growth have extremely low volatilities (saturation concentration less than 10−4.5 micrograms per cubic metre). As the particles increase in size and the Kelvin barrier falls, subsequent growth is primarily due to more abundant organic vapours of slightly higher volatility (saturation concentrations of 10−4.5 to 10−0.5 micrograms per cubic metre). We present a particle growth model that quantitatively reproduces our measurements. Furthermore, we implement a parameterization of the first steps of growth in a global aerosol model and find that concentrations of atmospheric cloud concentration nuclei can change substantially in response, that is, by up to 50 per cent in comparison with previously assumed growth rate parameterizations.ISSN:0028-0836ISSN:1476-468

Multicomponent new particle formation from sulfuric acid, ammonia, and biogenic vapors

A major fraction of atmospheric aerosol particles, which affect both air quality and climate, form from gaseous precursors in the atmosphere. Highly oxygenated organic molecules (HOMs), formed by oxidation of biogenic volatile organic compounds, are known to participate in particle formation and growth. However, it is not well understood how they interact with atmospheric pollutants, such as nitrogen oxides (NOx) and sulfur oxides (SOx) from fossil fuel combustion, as well as ammonia (NH3) from livestock and fertilizers. Here, we show how NOx suppresses particle formation, while HOMs, sulfuric acid, and NH3 have a synergistic enhancing effect on particle formation. We postulate a novel mechanism, involving HOMs, sulfuric acid, and ammonia, which is able to closely reproduce observations of particle formation and growth in daytime boreal forest and similar environments. The findings elucidate the complex interactions between biogenic and anthropogenic vapors in the atmospheric aerosol system.Peer reviewe

Multicomponent new particle formation from sulfuric acid, ammonia, and biogenic vapors

A major fraction of atmospheric aerosol particles, which affect both air quality and climate, form from gaseous precursors in the atmosphere. Highly oxygenated organic molecules (HOMs), formed by oxidation of biogenic volatile organic compounds, are known to participate in particle formation and growth. However, it is not well understood how they interact with atmospheric pollutants, such as nitrogen oxides (NOx) and sulfur oxides (SOx) from fossil fuel combustion, as well as ammonia (NH3) from livestock and fertilizers. Here, we show how NOx suppresses particle formation, while HOMs, sulfuric acid, and NH3 have a synergistic enhancing effect on particle formation. We postulate a novel mechanism, involving HOMs, sulfuric acid, and ammonia, which is able to closely reproduce observations of particle formation and growth in daytime boreal forest and similar environments. The findings elucidate the complex interactions between biogenic and anthropogenic vapors in the atmospheric aerosol system.publishedVersionPeer reviewe

Aqueous phase oxidation of sulphur dioxide by ozone in cloud droplets

The growth of aerosol due to the aqueous phase oxidation of sulfur dioxide by ozone was measured in laboratory-generated clouds created in the Cosmics Leaving OUtdoor Droplets (CLOUD) chamber at the European Organization for Nuclear Research (CERN). Experiments were performed at 10 and −10 °C, on acidic (sulfuric acid) and on partially to fully neutralised (ammonium sulfate) seed aerosol. Clouds were generated by performing an adiabatic expansion – pressurising the chamber to 220 hPa above atmospheric pressure, and then rapidly releasing the excess pressure, resulting in a cooling, condensation of water on the aerosol and a cloud lifetime of approximately 6 min. A model was developed to compare the observed aerosol growth with that predicted using oxidation rate constants previously measured in bulk solutions. The model captured the measured aerosol growth very well for experiments performed at 10 and −10 °C, indicating that, in contrast to some previous studies, the oxidation rates of SO2 in a dispersed aqueous system can be well represented by using accepted rate constants, based on bulk measurements. To the best of our knowledge, these are the first laboratory-based measurements of aqueous phase oxidation in a dispersed, super-cooled population of droplets. The measurements are therefore important in confirming that the extrapolation of currently accepted reaction rate constants to temperatures below 0 °C is correct

Aqueous phase oxidation of sulphur dioxide by ozone in cloud droplets

The growth of aerosol due to the aqueous phase oxidation of sulfur dioxide by ozone was measured in laboratory-generated clouds created in the Cosmics Leaving OUtdoor Droplets (CLOUD) chamber at the European Organization for Nuclear Research (CERN). Experiments were performed at 10 and −10 °C, on acidic (sulfuric acid) and on partially to fully neutralised (ammonium sulfate) seed aerosol. Clouds were generated by performing an adiabatic expansion – pressurising the chamber to 220 hPa above atmospheric pressure, and then rapidly releasing the excess pressure, resulting in a cooling, condensation of water on the aerosol and a cloud lifetime of approximately 6 min. A model was developed to compare the observed aerosol growth with that predicted using oxidation rate constants previously measured in bulk solutions. The model captured the measured aerosol growth very well for experiments performed at 10 and −10 °C, indicating that, in contrast to some previous studies, the oxidation rates of SO2 in a dispersed aqueous system can be well represented by using accepted rate constants, based on bulk measurements. To the best of our knowledge, these are the first laboratory-based measurements of aqueous phase oxidation in a dispersed, super-cooled population of droplets. The measurements are therefore important in confirming that the extrapolation of currently accepted reaction rate constants to temperatures below 0 °C is correct