University of California, Davis



Camille Mouchel-Vallon NCAR cmv@ucar.eduGoAMAZON: Exploring the Impacts of a Metropolis on Amazonian Air with an Explicit Organic Chemistry Scheme In 2014 and 2015, the GoAMAZON field campaign took place in the vicinity of Manaus (Brazil). This 2 millions inhabitants metropolis is the most populated urban area in the Amazon forest, isolated in the middle of a vast expanse of rainforest. This makes it the ideal location to study the interplay between biogenic air masses and anthropogenic emissions.In this work, we build a detailed organic emissions inventory for both Manaus and the surrounding rainforest. Based on the MEGAN model and speciated monoterpene measurements, biogenic emissions include isoprene and 11 monoterpenes. We also produce an inventory of Manaus emissions including 47 organic compounds. The progressive oxidation of biogenic and anthropogenic precursors leads to the formation of highly oxygenated, semi-volatile compounds that compose Secondary Organic Aerosol.The Generator of Explicit Chemistry and Kinetics of Organics in the Atmosphere (GECKO-A) is therefore used to generate an hyper-explicit chemical scheme for our 59 organic precursors following a systematic protocol using experimental data when available, and Structure Activity Relationship when needed. The resulting mechanism of approx. 15 million reactions is integrated in a 0D boxmodel. We investigate this model?€?s ability to reproduce gas phase and particulate matter measurements. The molecular view of allows us to look in details into the urban-biogenic interactions.Julia Lee-Taylor (NCAR), Alma Hodzic (NCAR), Sasha Madronich (NCAR)Heather Simon US EPA simon.heather@Ongoing EPA efforts to evaluate modeled NOy budgets Recent studies have found that modeled concentration and deposition fields of NOy in the U.S. are biased high and have suggested that these over-predictions are the result of NOx emissions overestimates. We present a diagnostic model evaluation which uses measurements from field studies and monitoring networks. We look at model performance from recent versions of EPA?€?s modeling inventory implemented in both CMAQ and CAMx and assess the role of key processes that may contribute to model NOy biases including atmospheric mixing, chemistry, deposition, emissions magnitudes and emissions temporalization. For this assessment we use data from ambient monitoring networks to evaluate seasonal, diurnal, and spatial trends in NOx performance. In addition, we leverage the extensive measurement dataset from the DISCOVER-AQ Baltimore field campaign in July 2011 to further evaluate model performance for NOy species.Barron Henderson1, Deborah Luecken2, Kristen Foley2, Claudia Toro3, Brian Timin1, Alison Eyth1, Kirk Baker1, Norm Possiel11. US EPA, Office of Air Quality Planning and Standards, RTP, NC2. US EPA, National Exposure Research Laboratory, RTP, NC3. US EPA, Office of Transportation and Air Quality, Ann Arbor, MJennifer Kaiser Georgia Institute of Technology jennifer.kaiser@ce.gatech.eduCharacterization of Chemical Mechanisms used in Top-Down VOC Emission Estimates Uncertainties in bottom-up emission inventories can be one of the largest source of total uncertainty in air quality models. Satellite observations enable a top-down perspective of volatile organic compound (VOC) emissions, if the chemical link between the observable species and the emitted compound is well characterized. In this talk, we will discuss the use of formaldehyde observations to constrain isoprene emissions in the South East US. We show that the chemical dependency on NOx and is largely captured by most isoprene oxidation mechanisms, giving new confidence in top-down estimates. Finally, we discuss the range of VOC emission estimates that can be derived from satellite-observable oxidation products..Keding Lu Peking University k.lu@pku.Winter Haze in Beijing driven by fast Photochemical Smog Reactions Heavy haze conditions were frequently presented in the air-sheds of Beijing and surrounding areas, especially during winter time. To explore the trace gas oxidation and the subsequent formation of aerosols, comprehensive field campaigns were performed at both urban and regional sites in winter Beijing. Serious haze pollution processes were often observed with the fast increase of inorganic salt (especially nitrate) and these pollutions were always associated with enhanced humidity and the concentrations of PAN which is normally a marker of gas phase oxidations from NOx and VOCs. Moreover, based on the direct measurements of OH, NO2, VOCs, N2O5, particle concentrations/distributions/chemical compositions, and meteorological parameters, the gas phase oxidation rates that leads to the formation of nitrate and secondary organic aerosols were estimated. These determined formation rates were clearly enhanced by several folds during pollution episodes compared to that of the clean air masses. Analysis result showed that the gas phase formation potential of nitrate and secondary organic aerosols were capable to explain the observed concentrations of nitrate and SOA and associated with fast ozone production rates. Controlling factors of the gas phase formation potential of secondary aerosols and ozone are discussed in the framework of the empirical kinetic modeling approach.Hendrik Fuchs2, Andreas Hofzumahaus2, Zhaofeng Tan1, Haichao Wang1, Sebastian H. Schmitt2, Franz Rohrer2, Birger Bohn2, Sebastian Broch2, Huabin Dong1, Georgios I. Gkatzelis2, Thorsten Hohaus2, Frank Holland2, Xin Li1, Ying Liu1, Yuhan Liu1, Xuefei Ma1, Anna Novelli2, Patrick Schlag2, Min Shao1, Yusheng Wu1?€?, Zhijun Wu1, Limin Zeng1, Min Hu1, Astrid Kiendler-Scharr2, Andreas Wahner2, & Yuanhang Zhang11 State Key Joint Laboratory of Environmental Simulation and Pollution Control, College of Environmental Sciences and Engineering, Peking University, Beijing, China.2 IEK-8: Troposphere, Forschungszentrum J??lich, J??lich, Germany.Louisa Emmons National Center for Atmospheric Research emmons@ucar.eduImpact of anthropogenic and natural emissions on air quality in Korea Measurements over Korea during May-June 2016 as part of the KORUS-AQ (Korea-U.S. Air Quality) campaign showed an increase in ozone mixing ratios from May into June, coincident with increasing biogenic emissions of isoprene and other compounds as temperatures increased. Korea is heavily forested outside the Seoul megacity, resulting in a unique atmospheric environment with very high anthropogenic emissions next to seasonally high biogenic emissions. To accurately simulate air quality here, models must include appropriate oxidation schemes for high amounts of NOx, anthropogenic VOCs and biogenic VOCs. We have run a box model and CESM???/CAM-chem with chemical mechanisms with varying complexity. The standard MOZART-T1 chemistry of CESM2 has been compared to mechanisms with more detailed oxidation of alkanes (specifically representing butanes and pentanes). The MCM mechanism is compared to the MOZART mechanisms in box model simulations initialized by the KORUS-AQ observations. The simulations are evaluated with the full suite of observed ozone precursors, including NOx, organic nitrates, hydrocarbons and oxygenated volatile organic compounds (OVOCs). Evaluations of the online biogenic emissions model MEGAN in CAM-chem will be presented through comparisons with observations of isoprene and terpenes, as well as OVOCs, such as methanol and acetone.Rebecca Schwantes (NCAR), John Orlando (NCAR)William R. Stockwell University pf Texas at El Paso William.R.Stockwell@Review of the SAPRC-16 Chemical Mechanism and Comparison with the Regional Atmospheric Chemistry Mechanism, Version-2 Gas-phase chemical mechanisms are essential elements of air quality models where they are used to calculate concentrations of chemical species. Revision of the SAPRC series of chemical mechanism has become more and more automated. However, evaluation of new versions by expert examination of its core reactions and comparison with other standard air quality mechanisms is required to ensure its acceptance by the regulatory community. A reaction-by-reaction examination of the SAPRC-16 chemical mechanism has been performed and it has been compared with the Regional Atmospheric Chemistry Mechanism, version 2 (RACM2) and recent versions of the Carbon Bond Mechanism. The origin of the RACM series of chemical mechanisms is in regional air quality modeling, in contrast to SAPRC?€?s origins in highly polluted urban scale modeling. This difference in original design goals adds value to the comparison of SAPRC-16 to RACM2. Furthermore, RACM2 retains a more mechanism developer dependent approach to produce updated versions. Box-model simulations were made for typical real-world conditions and the results of the comparisons between both mechanisms will be presented. Finally, suggestions will be made for the future development of SAPRC and similar gas-phase atmospheric chemical mechanisms for air quality.Emily Saunders, NASA Goddard Space Flight Center; Rosa Fitzgerald, University of Texas El PasoAjith Kaduwela California Air Resources Board / Air Quality Research Center, UC Davis ajith.kaduwela@arb.Development of Future Atmospheric Chemical Mechanisms for Photochemical Modeling An atmospheric chemical mechanism is at the heart of each air quality model used in research and regulatory applications to predict and analyze the complex air pollutants: ozone, air toxics, and PM2.5. It is, therefore, essential that mechanisms continue to utilize the best, most up-to-date atmospheric chemistry information available so that policy development is based on air quality model predictions that are robust, transparent, and free from scientific challenge. During this presentation, we will discuss four discernible stages required for developing chemical mechanisms that can appropriately address the needs of future atmospheres: Information gathering, mechanism compilation, development of protocols to evaluate mechanisms, and systematic condensation of mechanisms. Peer review of an atmospheric chemical mechanism is also very important. We will describe the peer-review process of the California Air Resources Board and the utility of the UC Davis international conference series (Atmospheric Chemical Mechanisms (ACM) and International Aerosol Modeling Algorithms (IAMA)) in developing new chemical mechanisms.Chenxia Cai, Melissa A. Venecek, and John DaMassa, Air Quality Planning and Science Division, California Air Resources Board, Sacramento;Jeremy C. Avise, Air Quality Planning and Science Division, California Air Resources Board, Sacramento and Department of Civil and Environmental Engineering, Washington State University, Pullman;William P.L. Carter, Center for Environmental Research and Technology, University of California, Riverside;Michael J. Kleeman, Department of Civil and Environmental Engineering, University of California, Davis;William Hutzell and Deborah Luecken, U.S. Environmental Protection Agency, Research Triangle Park, North Carolina.Michael Benjamin California Air Resources Board michael.benjamin@arb.Science-Based Policy Formation at the California Air Resources Board Air quality regulations that are designed to protect public health must be based on the most up-do-date scientific information and should be robust, transparent, and free from scientific challenges. During this presentation, we will outline the scientific underpinnings of the policy formation at the California Air Resources Board (CARB). This will include the institutional structure that favors science, use of state-of-the-science chemistry information in photochemical air quality modeling, sponsorship of several scientific conferences to obtain most up-to-date scientific information, and collaborations with world-renowned academics in scientific endeavors. Also highlighted would be the benefits of CARB’s sponsorship of this Atmospheric Chemical Mechanisms Conference.Michael Kleeman UC Davis mjkleeman@ucdavis.eduSource apportionment of O3 formation in California using SAPRC11 Ozone source apportionment tools are used to design emissions control programs to reduce ambient ozone concentrations. Widely available ozone source apportionment tools have been applied to the Carbon Bond mechanism available in CMAQ and CAMx, but State Implementation Plans (SIPs) in California are usually developed with the SAPRC chemical mechanisms. Only a single previous study has demonstrated ozone source apportionment with SAPRC99 by resolving one source at a time. In the current study we demonstrate ozone source apportionment calculations for SAPRC11 for nine simultaneous sources. NOx and VOC contributions to ozone formation are separately resolved. Calculations are performed for peak ozone events in the year 2010. The findings demonstrate the features of full ozone source apportionment methods in the SAPRC mechanism and provide useful insights into the optimal control strategies for ozone in California.Yusheng Zhao (UC Davis), Chao Wan (UC Davis), Ajith Kaduwela (CARB)Man Nin Chan The Chinese University of Hong Kong mnchan@cuhk.edu.hkFormation of Inorganic Sulfate in Heterogeneous OH Oxidation of Isoprene Epoxydiol-Derived Organosulfates Multiphase chemistry of epoxydiols formed from isoprene oxidation yields the most abundant organosulfates (i.e., methyltetrol sulfates) detected in atmospheric fine aerosols. However, chemical stability of these organosulfates remains unclear. We investigate the heterogeneous oxidation of aerosols consisting of potassium 3-methyltetrol sulfate ester (C5H11SO7K) by gas-phase hydroxyl radical (OH) through studying the oxidation kinetics and reaction products at a relative humidity (RH) of 70.8%. Real-time molecular composition of the aerosols is obtained by using a Direct Analysis in Real Time (DART) ionization source coupled to a high-resolution mass spectrometer. Aerosol mass spectra only show increases in the intensity of bisulfate ion (HSO4??’) after oxidation, suggesting the absence of functionalization processes that is likely attributable to the steric effect of substituted functional groups (e.g. methyl, alcohol and sulfate groups) on peroxy?€“peroxy radical reactions. Potassium 3-methyltetrol sulfate ester likely decomposes to form volatile fragmentation products and aerosol-phase sulfate radial anion (SO4?€???’). SO4?€???’ subsequently undergoes intermolecular hydrogen abstraction to form HSO4??’. These processes appear to explain the compositional evolution of the 3-methyltetrol sulfate ester during heterogeneous OH oxidation.Ho Ki Lam1, Kai Chung Kwong1, Hon Yin Poon1, James F. Davies2, Zhenfa Zhang3, Avram Gold3, Jason D. Surratt3, Man Nin Chan1, 4,*1Earth System Science Programme, Faculty of Science, The Chinese University of Hong Kong, Hong Kong, CHINA 2Department of Chemistry, UC Riverside, Riverside, USA 3Department of Environmental Sciences and Engineering, Gillings School of Global Public Health, The University of North Carolina at Chapel Hill, Chapel Hill, USA4The Institute of Environment, Energy and Sustainability, The Chinese University of Hong Kong, Hong Kong, CHINAAnke Mutzel Leibniz Institute for Tropospheric Research mutzel@tropos.deInterconnection of day- and night time chemistry for VOC degradation and SOA formation Compounds that are emitted during the daytime are continuously oxidized with OH radicals or ozone. In the evening VOCs and OVOCs that remain in the gas phase undergo nighttime oxidation by NO3 radicals. In the present study, we have conducted experiments with day time (OH radicals) and subsequent night time (NO3 radicals) processing (and vice versa) in one single chamber run with a-pinene, limonene and m-cresol as precursor. The data set demonstrates that night time oxidation with subsequent day time processing of m-cresol leads to a massive formation of organic mass of 30 ??g m-3 that is solely formed by secondary processes. APi-TOF and PTR-TOFMS measurements revealed the occurrence of two groups of compounds that are related to methyl benzoquinone structure and to nitro-cresol structure, respectively. The secondary formation is most likely attributed to the partitioning of the latter class of compounds because very strong correlation to the increase of organic mass were found. As this was not observed from day-night experiments, the present results highlight the importance of night time oxidation of anthropogenic VOCs emitted in the evening or remaining from previous day processing. The further oxidation of such compounds during night leads to gas-phase oxidation products that act as reservoir compounds which then very efficiently trigger SOA formation during the early morning hours of the next day.Hartmut Herrmann (Leibniz Institute for Tropospheric Research), Olaf B??ge (Leibniz Insitute for Tropospheric Research)Danielle Draper University of California, Irvine dcdraper@uci.eduFormation of Highly Oxidized Molecules from NO? Radical Oxidation of Δ-3-Carene: A Computational Mechanism NO??? radical oxidation of monoterpenes is estimated to be a significant source of monoterpene-derived secondary organic aerosol (SOA). However, structurally similar monoterpenes have been shown experimentally to have vastly different aerosol yields, requiring more research into the detailed oxidation mechanism leading to condensable products. We have computed reaction rates for the most likely intramolecular reactions (including RO??? and RO H-shifts) arising from NO??? oxidation of ?”-3-carene, which has moderate to high experimental SOA yields, to develop a mechanism leading to some of the highly oxidized products most likely to contribute to SOA formation. The computationally-derived mechanism is compared to Chemical Ionization Mass Spectrometry data of the same system using NO?????? as the reagent ion, which is selective for the highly oxidized molecules that are produced. Results indicate that both bimolecular and intramolecular reactions contribute to the final product distribution, with intramolecular reactions becoming increasingly competitive during later-generation oxidation steps.Nanna Myllys (University of California, Irvine), Noora Hyttinen (University of Helsinki), Kristian H. M??ller (University of Copenhagen), Henrik G. Kjaergaard (University of Copenhagen), Juliane L. Fry (Reed College), James N. Smith (University of California, Irvine), Theo Kurt??n (University of Helsinki)Fernanda Bononi Department of Chemistry, UC Davis fcbononi@ucdavis.eduModeling the Absorption Spectra of Phenol and Guaiacol at the Ice-Air Interface Snow packs and tropospheric clouds are important sites for environmentally significant reactions, notably the photolysis of organic molecules as they convert pollutants into more volatile molecules that can be released into the atmosphere. On the ice surface, such molecules are solvated in the quasi-liquid layer (QLL) at the air-ice interface or in liquid-like regions (LLR) within the ice matrix. Previous work shows that reaction rates for photolysis reactions could be enhanced at the air-ice interface when compared to solution, possibly due to a bathochromic shift of the UV-vis absorption spectrum. Hence it is essential to characterize the solvation shell of such molecules in water and at the air-ice interface at the molecular scale.Starting from a previously characterized ice model, we used classical molecular dynamics (MD) to characterize the solvation shell of phenol and guaiacol at the air-interface and in supercooled water. The absorption spectra of the molecules are then calculated using time-dependent density functional theory (TDDFT) on smaller models extracted from first-principles MD simulations, accounting for solvation shell variations within the QLL at finite temperatures.The results obtained from calculations are compared to experiments in order to optimize and validate model approaches that will be used to provide further insight into the nature of photochemical reactions rate enhancement in snow and ice when experimental measurements are not attainable. Ted Hullar - Department of Land, Air and Water Resources, UC DavisCort Anastasio - Department of Land, Air and Water ResourcesDavide Donadio - Department of Chemistry UC DavisFrank Winiberg Jet Propulsion Lab/Caltech fred.a.winiberg@jpl.Does water complexation affect the reaction of the ??-hydroxyethylperoxy radical with NO? Formation of organic nitrates from the oxidation hydrocarbons in the troposphere is a significant loss pathway for nitrogen oxides (NOx) and HOx radicals (OH and HO2). Unsaturated hydrocarbons are emitted in large quantities into the atmosphere. They are primarily oxidized by OH, producing ??-hydroxyperoxy radicals. One major loss process for these radicals is reaction with NO, producing organic nitrates or NO2. Organic nitrates are a sink for NOx through deposition and formation of secondary organic aerosols, whereas NO2 can photolyse to produce tropospheric ozone. Formed from the oxidation of ethene, ??-hydroxethylperoxy (??-HEP) serves as a model molecule for studying more complex systems, such as isoprene. Both experimental and theoretical works in the literature support the formation of a ??-HEP H2O complex, and a 6 fold increase in the ??-HEP self-reaction rate coefficient has been observed as a function of relative humidity (0 ?€“ 50 %) and temperature (270 ?€“ 298 K). In the upper troposphere up to 14% of RO2 radicals could be complexed with H2O and there are currently no chemical mechanisms for these reactions global models. Here we present results from an experimental study of the reaction of ??-HEP + NO over a range of water concentrations and temperatures, using Multiplexed-Photoionization Mass Spectrometry (MPIMS). Results will be discussed along with the mechanistic developments and atmospheric implications. Copyright 2018, California Institute of TechnologyAileen Hui,[1,2] Kristen Zuraski,[1] Matthew D. Smarte,[2] Rebecca L. Caravan,[3] Greg Jones,[2] Joseph Messinger,[2] Mitchio Okumura,[2] David Osborn,[3] Carl. J. Percival,[1] Craig Taatjes,[3] Stanley P. Sander.[1][1] Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109 USA[2] Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA 91125 USA[3] Combustion Research Facility, Mailstop 9055, Sandia National Laboratories, Livermore, California, 94551 USAJoseph Messinger California Institute of Technology jmessing@caltech.eduLaboratory Exploration of the Reactions between Aromatics and OH using Cavity Ringdown Spectroscopy Aromatic hydrocarbons are well-known atmospheric pollutants in urban environments, as they are produced from vehicle exhaust and lead to the formation of secondary organic aerosols. However, little is known about the intermediate steps in which these compounds are oxidized to form a variety of highly oxidized products, including epoxides, bicyclic compounds and glycolaldehyde. Here, we study the radicals produced in the initial reactions between toluene and benzene with the OH radical, as the two simplest and most abundant aromatic species in the atmosphere. These reactions serves as the main sink of these hydrocarbons and their first step towards oxidation. The resulting radicals, the hydroxy-methyl- and hydroxy-cyclohexadienyl radicals, are then detected using pulsed mid-infrared and visible cavity ringdown spectroscopy (CRDS). In conjunction with quantum chemical calculations and kinetic modeling, the vibrational and electronic spectra of these radicals are assigned for the first time. Implications for the oxidative mechanisms of toluene and benzene will be discussed.James S. Vinson (California Institute of Technology), Qinghui Meng (Tsinghua University), Mitchio Okumura (California Institute of TechnologyKelley Barsanti University of California Riverside kbarsanti@engr.ucr.eduDeveloping Reactivity- and Source-Based Monoterpene Parameterizations for Secondary Organic Aerosol Modeling Monoterpenes are a diverse category of volatile organic compounds (VOCs) that are emitted in significant quantities from both biogenic and biomass burning sources. The diversity of monoterpenes is expressed in their sources, structures, atmospheric reaction rates and products, and propensities to form secondary organic aerosol (SOA). Despite this known complexity and observed contribution to SOA formation (in both laboratory and field studies), the model representation of monoterpene-derived SOA is relatively underdeveloped. More specifically, the gas-phase oxidation of monoterpenes and SOA formation are typically represented using a single lumped surrogate. In this work we used the GECKO-A (Generator of Explicit Chemistry and Kinetics in the Atmosphere) model to simulate SOA formation from ten atmospherically relevant monoterpenes under a range of conditions. We then derived volatility basis set (VBS) parameters to represent monoterpene-derived SOA formation from three to six lumped surrogates, considering monoterpene reactivity and source emissions profiles. We also used time-dependent GECKO-A model output to evaluate the potential for accretion product formation in these different monoterpene systems. Results of these current research efforts will be presented, along with challenges and opportunities for further model development efforts.Isaac Afreh, Jia Jiang, Lindsay Hatch (UC Riverside); Christine Wiedinmyer (Cooperative Institute for Research in Environmental Sciences-CIRES); Annmarie Carlton (UC Irvine)Lea Hildebrandt Ruiz The University of Texas at Austin lhr@che.utexas.eduChlorine-Initiated Oxidation of Hydrocarbons: Mechanistic Insights from Measurements of Gas- and Particle-Phase Composition Chlorine atoms (Cl) are much more reactive than hydroxyl (OH) and can oxidize functional groups or whole molecules that are resistant to OH. Cl can also initiate radical propagation pathways which generate OH as secondary radical. Thus, Cl can rapidly initiate the oxidation cascade that results in the formation of highly oxidized molecules (HOMs) and particulate matter (PM), and drive the formation of different products than those derived from OH. I present results from laboratory chamber experiments on chlorine-initiated oxidation of different VOCs including isoprene, ?±-pinene, toluene, butyl carbitol and alkanes. Using measurements from a high resolution chemical ionization mass spectrometer (CIMS), we tracked several generations of oxidation chemistry. We identify known and previously unknown products of Cl-initiated oxidation, including highly oxidized molecules (HOMs). Using a filter inlet for gases and aerosols (FIGAERO) we analyzed the molecular composition of organic PM and found that Cl-initiated oxidation can form larger and lower-vapor pressure compounds than OH-initiated oxidation, which likely contributes to higher aerosol mass yields often observed from Cl-initiated chemistry. Organochlorides formed from all precursors investigated, even when the initial oxidation occurs via hydrogen-abstraction. Cl-initiated formation of organic PM is currently not represented in air-quality models used to support the development of environmental policies.Dongyu S. Wang, Catherine Masoud, Surya Dhulipala, Sahil BhandariMads Sulbaek Andersen California State University, Northridge sulbaek.andersen@csun.eduThe Atmospheric Chemistry of Nitriles The atmospheric chemistry of organic nitriles is diverse. Acetonitrile (CH3CN) is found in Earth?€?s atmosphere due to emissions from anthropogenic sources incl. vehicle exhaust, biomass burning and solvent use. Longer chain-length aliphatic nitriles have been detected in interstellar space and in the atmosphere of Saturn?€?s moon, Titan. Furthermore, (CF3)2CFCN has recently been developed as a replacement for the industrially important and potent greenhouse gas, SF6. Still, the literature on the atmospheric chemistry of nitriles is limited.We have employed experimental and computational techniques to investigate the atmospheric chemistry of the alkyl nitriles CH3(CH2)xCN (x=0-4), and (CF3)2CFCN. The oxidation kinetics and mechanisms were determined with respect to reactions with Cl atoms, OH radicals and O3. Trends in the kinetics for the aliphatic nitriles follow the patterns of other substituted alkanes. Atmospheric processing yields multiple different oxygenated products. The main atmospheric sink for (CF3)2CFCN is reaction with OH radicals and the sole atmospheric degradation products appear to be NO, COF2, and CF3C(O)F. The atmospheric lifetime of (CF3)2CFCN is approx. 22 years resulting in an estimated 100-year global warming potential of 1490, a factor of 15 less than that of SF6.This presentation provides an overview of the atmospheric chemistry of nitriles. Results are discussed in the context of impacts on air quality and the radiative forcing of climate change.Simone Thirstrup Andersen (University of Copenhagen), Jonathan Wilson Lengkong (California State University) and Ole John Nielsen (University of Copenhagen)Michael Rolletter Institute of Energy and Climate Research, IEK-8: Troposphere, Forschungszentrum J??lich GmbH, J??lich, Germany m.rolletter@fz-juelich.deInvestigation of the alpha-pinene & beta-pinene photooxidation by OH in the atmospheric simulation chamber SAPHIR This study investigates the photooxidation of alpha- and beta-pinene by OH in the atmospheric simulation chamber SAPHIR with a focus on the analysis of the budget of HOx (OH + HO2) radicals.Experiments were performed at low NO conditions (< 400 ppt) and atmospheric monoterpene concentrations (< 4.7 ppb). Measurements of OH and HO2 radicals, alpha-/beta-pinene, OH precursors and organic products (pinonaldehyde, nopinone, acetone and formaldehyde) allow testing our current understanding of the OH-oxidation mechanisms by comparing the measurements with model calculations using the Master Chemical mechanism (MCM 3.3.1).This study shows that the MCM model underestimates OH and HO2 for both alpha- and beta-pinene experiments. In addition, the MCM produces in the case of alpha-pinene significantly more pinonaldehyde compared to observations. Implementation of recent suggestions by Vereecken et al. (2007) improves the model-measurement agreement including radicals, but pinonaldehyde is still overestimated by the model in this case. Similarly, the MCM overpredicts the production of nopinone in the beta-pinene photodegradation. Utilizing an updated MCM mechanism by Vereecken and Peeters (2012) was able to describe the nopinone formation, but still needs an additional HO2 source to explain observed radical concentrations.Martin Kaminski (1,a), Ismail-Hakki Acir (1,b), Birger Bohn (1), Theo Bauers (1,?€ ), Hans-Peter Dorn (1), Rolf H?¤seler (1,?€ ), Frank Holland (1), Xin Li (1,c), Anna Lutz (2), Sascha Nehr (1,d), Franz Rohrer (1), Ralf Tillmann (1), Luc Vereecken (1), Robert Wegener (1), Andreas Hofzumahaus (1), Astrid Kiendler-Scharr (1), Andreas Wahner (1),Hendrik Fuchs (1) (1) Institute of Energy and Climate Research, IEK-8: Troposphere, Forschungszentrum J??lich GmbH, J??lich, Germany(2) Department of Chemistry and Molecular Biology, University of Gothenburg, Gothenburg, Sweden(a) now at: Federal Office of Consumer Protection and Food Safety, Department 5: Method Standardisation, Reference Laboratories, Resistance to Antibiotics, Berlin, Germany(b) now at: Institute of Nutrition and Food Sciences, Food Chemistry, University of Bonn, Bonn, Germany(c) now at: State Key Joint Laboratory of Environmental Simulation and Pollution Control, College of Environmental Sciences and Engineering, Peking University, Beijing, China(d) now at: INBUREX Consulting GmbH, Process Safety, Hamm, Germany(?€ ) deceasedMike Newland University of York mike.newland@york.ac.ukStructural dependence of stabilised CH2OO yield in terminal alkene ozonolysis The reaction of alkenes with ozone produces Criegee intermediates (CI). In the atmosphere, these short lived reactive species are an important non-photolytic source of radicals, participate in aerosol formation, and can themselves act as atmospheric oxidants. CI are produced, together with a carbonyl co-product, from the decomposition of an energy-rich primary-ozonide (POZ) formed in the initial ozonolysis reaction. The CI population is formed with a broad energy distribution: a fraction is formed chemically activated (CI*) and undergoes prompt decomposition or collisional stabilisation, to yield stabilised CI (SCI); another fraction may be formed stabilised.One of the largest current uncertainties for modelling the impacts of alkene ozonolysis in the atmosphere, is the fraction of each CI that is stabilised from a particular alkene. Current models generally assume stabilisation of a given CI, based on the SCI yield from its symmetrical parent alkene.Here we report measurements from the ozonolysis of a range of terminal alkenes at the atmospheric simulation chamber, EUPHORE. We show that stabilisation of the smallest CI, CH2OO, increases rapidly with increasing size of carbonyl co-product. These experiments provide, for the first time, measurements of stabilisation of a given CI across a range of structurally different alkenes. These results suggest current models may significantly underestimate production of stabilised CH2OO from large alkenes.Beth Nelson (University of York, UK); Andrew Rickard (National Centre for Atmospheric Science (NCAS), University of York, UK); Amalia Munoz, Milagros Rodenas, Joan Tarrega Gallego, Teresa Vera, Esther Borras (all Fundaci??n CEAM, EUPHORE Laboratories, Spain)Mixtli Campos-Pineda University of California, Riverside mcamp007@ucr.eduDirect measurements of vinoxy radicals and formaldehyde from ozonolysis of trans- and cis-2-butenes: new insights into OH radical formation and secondary chemistry Ozonolysis of alkenes is one of the major OH radical production channels in the troposphere. Production of the OH radical involves isomerization of a Criegee intermediate (CI) into a peroxide and its subsequent decomposition. In the case of trans- and cis-2-butene, acetaldehyde oxide CH3CHOO (CI) isomerizes into vinylhydroperoxide and subsequent decomposition yields OH and vinoxy radicals. However, the fate of the vinoxy radical in the troposphere has not yet been completely elucidated and further study of its reactions and products is necessary. In this work, we use a flow reactor and cavity ring-down spectroscopy to directly observe and measure the concentration of vinoxy radicals and formaldehyde (HCHO) produced from ozonolysis of trans- and cis-2-butene under several reaction conditions. Reactions without significant presence of O2 show that the yield of HCHO from both cis- and trans-2-butene is similar at ~0.18. However, the yield of vinoxy radical from cis-2-butene ozonolysis is around 60% of that from trans-2-butene ozonolysis, indicating HCHO production from both syn- and anti-CH3CHOO. In addition, measurements of vinoxy radicals and formaldehyde under excess O2 and at short reaction times (~0.3 s) were performed. The use of 2-butene ozonolysis mechanisms to simulate the experimental measurements provided more information on the possible reaction pathways of the vinoxy radical and its contribution to the formation of additional OH radicals, formaldehyde, and glyoxal.Prof. Jingsong Zhang (University of California, Riverside)Rebecca Caravan Sandia National Laboratories rcarava@The role of Criegee Intermediate + ROOH reactions towards secondary organic aerosol formation ?€“ laboratory, modelling and field studies Criegee Intermediates (CIs) are formed in the troposphere principally from the ozonolysis of alkenes. However, CIs are short lived and have a low steady-state concentration following production via ozonolysis reactions, making laboratory studies of their reactivity challenging. The advent of photolytic production methods has enabled direct studies of the reactivity of CIs, which have shown some bimolecular reactions of CIs to be orders of magnitude faster than previously thought. The rapid reactions of CIs with organic acids lead to the formation of functionalized hydroperoxides (ROOH). In chamber studies of alkene ozonolysis, further reactions of CIs with these initial reaction products have been implicated in the formation of secondary organic aerosols (SOA); however, no direct studies of these reactions to elucidate their kinetics and mechanisms have been undertaken. We examine herein the role of CI reactions with ROOH on SOA formation with evidence from experimental work through to field measurements and global impact modelling studies. Direct measurements of reaction kinetics and mechanistic information for CI + ROOH reactions are obtained through multiplexed photoionization mass spectrometry and compared with alkene ozonolysis studies using a jet-stirred reactor, supported by ab initio calculations and computational kinetics. Evidence for the importance of CI + ROOH reactions in the troposphere will be presented from field measurement data and global modelling studies.Rebecca L. Caravan,1* Tom Bannan,2 Frank A. F. Winiberg,3 M. Anwar H. Khan,4 Aric Rousso,1,5 Ahren Jasper,6 Stephen Klippenstein,6 Stephen Worrall,2 Asan Bacak,2 Paulo Artaxo,7 Joel Brito,7 Stanley P. Sander,3 James Allan,2 Hugh Coe,2 David L. Osborn,1 Nils Hansen,1 Dudley E. Shallcross,4 Craig A. Taatjes,1 and Carl J. Percival.2,31Combustion Research Facility, Mailstop 9055, Sandia National Laboratories, Livermore, California, 94551 USA2The School of Earth, Atmospheric and Environmental Science, The University of Manchester, Simon Building, Brunswick Street, Manchester, M13 9PL, UK3Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109 USA4School of Chemistry, Cantock?€?s Close, University of Bristol, BS8 1TS, UK5Department of Mechanical and Aerospace Engineering, Princeton University, Engineering Quadrangle, Olden Street, Princeton, NJ 08544, USA6Chemical Sciences Division, Argonne National Laboratories, Lemont, IL, USA7Physics Institute, University of S??o Paulo ?€“ IFUSP/USP, S??o Paulo, BrazilDeborah Leucken luecken.deborah@Melissa Venecek AQPSD, California Air Resources Board and Department of Land, Air and Water Resources, University of California, Davis melissa.venecek@arb.Analysis of SAPRC16 Chemical Mechanism for Ambient Simulations SAPRC16 is an interim update to the SAPRC series of chemical mechanisms that includes updated rate constants, a revised representation of radical chemistry and a new speciation lumping scheme to better develop predictions of SOA precursors. In this study, the ability of the SAPRC16 chemical mechanism to simulate regional ozone episodes is tested using the UCD-CIT 3-D airshed model and Community Multiscale Air Quality Modeling system. Concentrations of ozone, OH radical and HO2 radical predicted from both models were compared to measured values in order to observe the trends independently of the air quality model used. SAPRC16 predicted slightly lower ozone concentrations than the former SAPRC11 chemical mechanism. Predictions from SAPRC11 are in better agreement with the measurements in the western U.S. however SAPRC16 outperforms in some eastern and southern U.S. cities. Differences in ozone concentrations predicted by SAPRC16 and SAPRC11 increased as emissions decreased suggesting that the two mechanisms will predict different outcomes from future emissions control programs. A box model analysis shows that the SAPRC16 mechanism quenches ozone production earlier than SAPRC11 as NOx concentrations increase. This could be caused by more detailed HO2+RO2 reactions and RO2 isomerization reactions in SAPRC16 that compete with the HO2+NO reaction. Further analysis of the updated SAPRC16 chemical mechanism should be carried out before widespread adoption of the new mechanism.Chenxia Cai - AQPSD, California Air Resources BoardAjith Kaduwela - AQPSD, California Air Resources Board and Air Quality Research Center, University of California at DavisJeremy Avise - AQPSD, California Air Resources Board and Department of Civil and Environmental Engineering, Washington State UniversityWilliam P. L. Carter - Center for Environmental Research and Technology, University of California at Riverside Michael J. Kleeman - Department of Civil and Environmental Engineering, University of California at DavisAnna Novelli Institute of Energy and Climate Research, IEK-8: Troposphere, Forschungszentrum J??lich GmbH, J??lich, Germany a.novelli@fz-juelich.deThe impact of the aldehyde-hydrogen shift on the OH radical budget in the isoprene oxidation mechanism in pristine environments Measurements of trace gases, OH radicals and OH reactivity were performed during the OH-initiated oxidation of isoprene for a range of NO concentrations in the SAPHIR chamber. A good agreement between measured and modelled OH radicals was observed only when the MCM 3.3.1 model, which contains a revised version of LIM1, was modified with an increased yield of di-HPCARP (0.75) and a faster unimolecular rate for the 1,6-hydrogen shift (~ 0.4 s-1) as observed in laboratory studies. Di-HPCARP for NO < 0.3 ppbv is expected to undergo unimolecular decomposition and theoretical calculations confirm that its major degradation path proceeds via an aldehyde-hydrogen shift with a proposed rate coefficient of > 0.7 s-1 and with formation of OH radicals. The aldehyde-hydrogen shift mechanism is a combination of 1,4-, 1,5- and 1,6-hydrogen shifts that takes place after the much faster hydrogen-scrambling across the isomeric peroxy radicals. With the modified MCM model there is a strong decrease in the formation of HPALD while the chemistry of di-HPCARP is enhanced becoming the largest source of regenerated OH radicals in the experiments presented. This alternative chemistry appears in better agreement with the measured OH radicals and OH reactivity. These reaction paths, combined with the updated yield in the 1,6-hydrogen shift, were included in a global chemistry model to assess the impact of the aldehyde-hydrogen shift on the OH concentration.Luc Vereecken, Institute of Energy and Climate Research, IEK-8: Troposphere, Forschungszentrum J??lich GmbH, J??lich,GermanyBirger Bohn, Institute of Energy and Climate Research, IEK-8: Troposphere, Forschungszentrum J??lich GmbH, J??lich,GermanyHans-Peter Dorn, Institute of Energy and Climate Research, IEK-8: Troposphere, Forschungszentrum J??lich GmbH, J??lich,GermanyAndreas Hofzumahaus, Institute of Energy and Climate Research, IEK-8: Troposphere, Forschungszentrum J??lich GmbH, J??lich,GermanyFrank Holland, Institute of Energy and Climate Research, IEK-8: Troposphere, Forschungszentrum J??lich GmbH, J??lich,GermanyXin Li, College of Environmental Sciences and Engineering, Peking University, Beijing, ChinaMartin Kaminsky, Bundesamt f??r Verbraucherschutz, Abteilung 5, Berlin, GermanyZhujun Yu, Institute of Energy and Climate Research, IEK-8: Troposphere, Forschungszentrum J??lich GmbH, J??lich,GermanySimon Rosanka, Institute of Energy and Climate Research, IEK-8: Troposphere, Forschungszentrum J??lich GmbH, J??lich,GermanyDavid Reimer, Institute of Energy and Climate Research, IEK-8: Troposphere, Forschungszentrum J??lich GmbH, J??lich,GermanyGeorgios I. Gkatzelis, Institute of Energy and Climate Research, IEK-8: Troposphere, Forschungszentrum J??lich GmbH, J??lich,GermanyDomenico Taraborrelli, Institute of Energy and Climate Research, IEK-8: Troposphere, Forschungszentrum J??lich GmbH, J??lich,GermanyFranz Rohrer, Institute of Energy and Climate Research, IEK-8: Troposphere, Forschungszentrum J??lich GmbH, J??lich,GermanyRalf Tillman, Institute of Energy and Climate Research, IEK-8: Troposphere, Forschungszentrum J??lich GmbH, J??lich,GermanyRobert Wegener, Institute of Energy and Climate Research, IEK-8: Troposphere, Forschungszentrum J??lich GmbH, J??lich,GermanyAstrid Kiendler-Scharr, Institute of Energy and Climate Research, IEK-8: Troposphere, Forschungszentrum J??lich GmbH, J??lich,GermanyAndreas Wahner, Institute of Energy and Climate Research, IEK-8: Troposphere, Forschungszentrum J??lich GmbH, J??lich,GermanyHendrik Fuchs, Institute of Energy and Climate Research, IEK-8: Troposphere, Forschungszentrum J??lich GmbH, J??lich,GermanyBenjamin Brown-Steiner AER bbrownst@Development of furan oxidation mechanism from OH and NO3 oxidation within biomass-burning regimes via chamber experiments We present the preliminary development of a furan oxidation mechanism within the Aerosol Simulation Program (ASP) based on laboratory chamber experiments at the Georgia Tech Environmental Chamber (GTEC) facility. ASPv2.2 is a young biomass burning plume chemical mechanism that contains over 600 chemical species that merges portions of the MCMv3.2 with portions of RACM2. Furan species found within biomass burning plumes, especially furfural and methylfurans, are quickly oxidized by OH and NO3, but there remain many uncertainties as to their oxidation products, their reaction rates, their branching ratios, and their ultimate impact on O3 and Secondary Organic Aerosols (SOAs). The CTEC chamber experiments, which use a High Resolution Time-of-Flight Chemical Ionization Mass Spectrometer (HR-ToF-CIMS) coupled with a Filter Inlet for Gases and AEROsols (FIGAERO) for the real-time measurement of oxidation products, are designed to determine the furan oxidation products and the O3 and SOA production under a range of biomass burning chemical regimes by testing different NOx levels (to simulate different NOx:VOC ratios), relative humidity, and temperature conditions. We compare the existing ASPv2.2 furan oxidation scheme with a complex NOAA-derived furan oxidation mechanism and, constrained by the results of the chamber experiments, present proposed updates and constrains to the ASP furan oxidation mechanism, as well as potential simplified furan oxidation mechanisms.Matthew Alvarado, Nga Lee Ng, Taekyu JooBernard Aumont LISA bernard.aumont@lisa.u-pec.frStructure-activity relationships for the development of MCM/GECKOA mechanisms Gas‐phase atmospheric oxidation of organic compounds can be represented by a limited number of reaction types, repeated many times up to full oxidation of the given parent compound. The redundancy in these chemical sequences allows the design of generic protocols that can be automatically applied to develop chemical mechanism on a systematic basis. This approach is based on an extensive use of Structure-Activity Relationships (SARs) to estimate unknown kinetic or thermodynamic parameters. The GECKOA modelling tool was designed to codify SARs and generate comprehensive oxidation mechanism for various hydrocarbons emitted in the atmosphere. The recent advances in the protocol and related SARs implemented in this tool for the generation of MCM/GECKOA mechanism will be presented. 1. LISA, UMR CNRS 7583, Université Paris Est Créteil et Université Paris Diderot, France2. Atmospheric Chemistry Services, Okehampton, Devon, UK.3. National Centre for Atmospheric Science, Wolfson Atmospheric Chemistry Laboratories, Department of Chemistry, University of York, York, UK4. National Center for Atmospheric Research, Boulder,. Aumont1, M. Camredon1, R. Valorso1, M. Jenkin2, A. Rickard3, P. Brauer3, M. Newland3, M. Evans3, C. Mouchel-Vallon4, S. Madronich4Bernhard Reischl Institute for Atmospheric and Earth System Research / Physics, University of Helsinki bernhard.reischl@helsinki.fiMolecular Dynamics simulations of sulfuric acid cluster collisions The first steps in atmospheric particle formation involve small clusters often containing sulfuric acid and ammonia, or amines. The detection and quantification of these clusters has recently been made possible due to the development of the chemical ionization atmospheric pressure interface mass spectrometers (CI-APi-TOF). While this technique can detect clusters at environmentally low concentration, the clusters can undergo fragmentation inside the instrument due to chemical ionization, low pressure and energetic collisions with neutral molecules, leading to systematic errors. Therefore modeling of cluster collision and fragmentation is an essential prerequisite for understanding the processes occurring inside the APi during the measurement ?€“ as well as in the atmosphere.While ab initio molecular dynamics simulations could reveal the atomistic details of the cluster collisions, the large number of individual trajectories with different initial conditions and parameters (cluster composition, conformer, velocity, and impact parameter) required to obtain statistically significant results, is prohibitive. We present equilibrium configuration and energy benchmarks for sulfuric acid clusters, using empirical force fields, against ab initio calculations using PW91/aug-cc-pVQZ level of theory, as well as initial results on the molecular dynamics simulations of the cluster collision and fragmentation.Monica Passananti, Dina Alfaouri, Nanna Myllys, Roope Halonen, Evgeni Zapadinsky and Hanna Vehkam?¤ki, Institute for Atmospheric and Earth System Research / Physics, University of HelsinkiLuc Vereecken Forschungszentrum J??lich GmbH l.vereecken@fz-juelich.deDevelopment, Extension, and Validation of Theory-based Structure-Activity Relationships (SARs) for Atmospheric Modeling The need to better understand and control air quality and climate change places increasing demands on the predictive capabilities of atmospheric models, with more subtle, and more complex chemistry. The construction of mechanisms is hampered by lack of kinetic data, especially for multi-functionalized species that are poorly addressed in SARs.1 Based on a recent perspective article,1 an overview is given of the needed experimental and theoretical data, and methodological work, for further improvement of SARs. Theory-based calculations, verified against accurate experimental data, are a powerful tool to generate the systematic data series needed for SAR development. A summary of recent efforts to derive and extend theory-based SARs, e.g. for carbonyl oxide chemistry, and thoughts on how to validate SARs and determine their shortcomings, is given.[1] Vereecken, L.; Aumont, B.; Barnes, I.; Bozzelli, J. W.; Goldman, M. J.; Green, W. H.; Madronich, S.; Mcgillen, M. R.; Mellouki, A.; Orlando, J. J.; Picquet-Varrault, B.; Rickard, A. R.; Stockwell, W. R.; Wallington, T. J.; Carter, W. P. L.: Perspective on Mechanism Development and Structure-Activity Relationships for Gas-Phase Atmospheric Chemistry, Int. J. Chem. Kinet., 50(6), 435?€“469, 2018.N/ADaniel Ellis University of York daniel.ellis@york.ac.ukUnderstanding the Atmosphere: Graph clustering methods for mechanism reduction. Numerical models of atmospheric chemistry are essential to understand, predict and mitigate air quality and climate issues. The description of the chemistry within these models is known as a ?€?mechanism?€?, with different models using differing levels of chemical complexity depending on their individual foci.As our understanding improves, along with advances in technology, our appreciation of the scale and intricacy of chemistry occurring has increased. With tools to automatically generate mechanisms, it is now apparent that a ?€?full?€? description of atmospheric chemistry results in >10e7 species and reactions. The size and numerically stiff nature of these mechanisms pose too much of a computational burden to be used in chemical transport models. These mechanisms need to be ?€?reduced?€? such that the relationships between key compounds are maintained. Using graph-based mathematics (akin to Google and Netflix), a novel approach is attempted here. This allows the transmutation of a chemical mechanism into a graphical network. Presenting model output in this way allows a more holistic representation of the chemistry and can be used as an indicator for important species.Graph clustering methods can also be used to identify communities of well connected and/or like species within a network. These groupings may then presupposition the lumping stage of mechanism reduction and offer a new methodology to evaluate and construct reduced mechanisms.Dr Andrew R. Rickard (NCAS, University of York), Prof. Mathew J. Evans(NCAS, University of York)David Topping University of Manchester ping@manchester.ac.ukPredicting instrument response as a function of composition. Our ability to model the chemical and thermodynamic processes that lead to secondary organic aerosol (SOA) formation is thought to be hampered by the complexity of the system. While there are fundamental models now available that can simulate the tens of thousands of reactions thought to take place, validation against experiments is challenging. Integrative analytical methods such as the Aerosol Mass Spectrometer (AMS) are capable of quantifying mass, but because of their inability to isolate individual molecules, comparisons have been limited to simple data products such as total organic mass and the O:C ratio. More detailed comparisons could be made if more of the mass spectral information could be used, but because a discrete inversion of AMS data is not possible, this activity requires a system of predicting mass spectra based on molecular composition.Rather than build a method from first principles, in this talk, the ability to train supervised methods to predict electron impact ionisation (EI) mass spectra for the AMS is discussed. A discussion around the general use of machine learning techniques are also discussed given the increased popularity in atmospheric science.James Allan1,2 , M. Rami Alfarra1,2, Bernard Aumont3Eleanor Browne University of Colorado Boulder eleanor.browne@colorado.eduDetection of novel organic nitrogen compounds with protonated ethanol cluster chemical ionization mass spectrometry Organic nitrogen is a ubiquitous atmospheric component that affects biogeochemistry, air quality, and climate. Assessing the impact of organic nitrogen on these processes remains challenging because traditional measurement techniques have lacked the sensitivity and chemical resolution to characterize the speciation and chemistry of organic nitrogen. Here, we discuss our work with protonated ethanol cluster chemical ionization time-of-flight mass spectrometry as a selective and sensitive measurement technique for organic nitrogen compounds. Previously used for measurements of amines, this technique can also measure several novel classes of nitrogen compounds including imines, amides, and nitrogen-containing heterocycles. We illustrate these capabilities by investigating how the multi-phase reactions of amines and carbonyls lead to the formation of imines, enamines, and heterocycles. We present both gas-phase and particle-phase measurements and discuss how these reactions may contribute to aerosol growth.Mitchell Alton, Aroob Abdelhamid, Jennifer Berry, University of Colorado BoulderHanna Vehkam?¤ki University of Helsinki hanna.vehkamaki@helsinki.fiCharacterising cluster fragmentation in an Atmospheric Pressure interface Time of Flight (APi-ToF) mass spectrometer Mass spectrometric instruments, for example atmospheric Pressure interface Time of Flight (APi-ToF) and the Chemical Ionization APi-ToF (CI-APi-ToF) can be used to detect molecules and small clusters, which are involved in the first stages of new particle formation, even at the low concentration they are present in the atmosphere. Understanding collision induced cluster fragmentation is however of vital importance for retrieving the actual ambient cluster distribution in the experiments. We developed a model for the cluster fragmentation inside the APi. In this model, the charged clusters move through the APi under applied constant and uniform electrical field (defined by the tuning of the instrument). The fate of cluster is simulated as a random process involving collisions with carrier gas, energy transfer and fragmentation. To validate the model, we produced negatively charged sulphuric acid clusters by ElectroSpray Ionization and selected negatively charged three acid clusters using a Differential Mobility Analyser to inject into the APi-ToF. We studied fragmentation in the three vacuum chambers where an electric field is applied to guide the ions through the interface. We evaluated the effects of the voltages applied to the electrodes to the fragmentation rate. Both experiments and simulations indicate clusters are mainly fragmented at the interface between the first and second chamber, and the model captures the observed extent of fragmentation. M. Passananti 1, E. Zapadinsky 1, J. Kangasluoma 1, N. Myllys 1, T. Zanca 1, M. Attoui 2, H. Vehkam?¤ki 11 Department of Physics, University of Helsinki, Helsinki, FIN-00014, Finland 2 LISA, University Paris Est Creteil, Creteil, 94010, FranceJohn Crounse Caltech crounjd@caltech.eduQuantification of multifunctional molecules in chamber and ambient air using gas-chromatography chemical ionization mass spectrometry (GC-CIMS) The inability to quantify functionalized organic species has been a long-standing barrier limiting our understanding of the oxidative degradation mechanisms of volatile organic compounds in the atmosphere. Here we describe a mass spectrometry-based method which allows for isomeric separation and quantification of a number of multifunctional species formed in the atmospheric oxidation of VOC, having adequate sensitivity for making ambient observations. Examples from laboratory and field studies will be presented which demonstrate the utility of this method for improving our understanding of VOC oxidation mechanisms.Krystal Vasquez, CaltechLu Xu, CaltechEric Praske, CaltechPaul Wennberg, CaltechMark Goldman Massachusetts Institute of Technology goldmanm@mit.eduBulk vs. stochastic kinetics to describe the oxidation of organic aerosol components The kinetics of reactions within the atmospheric condensed phase (e.g., aerosol particles) are typically described in terms of bulk concentrations, analogous to chemistry occurring in the gas and solution phases. However, the assumptions underlying bulk concentrations can break down when only a few molecules exist within a single particle. Here we discuss how applying bulk kinetics assumptions impacts the calculated oxidation rate, by comparing it to the oxidation rate determined using stochastic distributions of molecules within particles. We explore how bulk and stochastic descriptions of the heterogeneous oxidation chemistry differ as a function of oxidant concentration, kinetic rates, and particle size We find that under some conditions ?€“ e.g., small aerosols and low oxidant concentrations ?€“ the use of bulk kinetics can underpredict oxidation rates by orders of magnitude. We then apply this framework to analyze the impact of solver type (bulk vs. stochastic) on the oxidation rate during a nighttime particle growth event, where we predict the largest error using bulk concentrations.William H. Green, Jesse H. Kroll (both also at Massachusetts Institute of Technology)Ugo Molteni PSI ugo.molteni@Free troposphere wintertime gas-phase composition using CI-APi-TOF Aerosols influence radiative directly and as cloud condensation nuclei, of which up to 45% are formed via new particle formation (NPF) and to large extents in the free troposphere (FT). Measurements at high altitude are however scarce and FT NPF instead modelled on sulphuric acid concentration, relative humidity and temperature. The CLOUD experiment showed how ammonia, amines and oxidized organics interact with sulfuric acid in ternary nucleation processes.1-3 Further, NPF driven by highly oxygenated molecules (HOMs) in absence of sulfuric acid was observed, and the findings considered important in defining climate sensitivity.4-5 High-altitude and low temperature measurements of NPFs and associated organic precursors and HOMs instead are yet scarce.6 We present two intensive winter campaigns, NU-CLACE2013 & 2014, with a nitrate chemical ioniza-tion mass spectrometer (CI-APi-TOF) deployed at Jungfraujoch (3580 m.a.s.l) in the Swiss Alps. We describe chemistry and oxidation processes linked to NPF episodes, and focus on sulphur containing species, halogens and HOMs. Positive Matrix Factorization reduces the complex dataset into few significant profiles, indicating a rich scenario where HOMs from bio- and anthropogenic organic precursors are identified to be produced via different ambient oxidation paths. 1 Almeida2013, Nature2 Kirkby2011, Nature3 Riccobono2014, Science4 Kirkby2016, Nature5 Gordon2016, PNAS6 Bianchi2016, Science7 Paatero1994, EnvironmetricsU. Molteni1, Y. Sosedova1, J. Dommen1, & the NUCLACE collaboration1,21 Laboratory of Atmospheric Chemistry, Paul Scherrer Institute, CH-5232 Villigen, Switzerland2 Department of Physics, University of Helsinki, FI-00014 Helsinki, Finland3 Goethe University Frankfurt, Institute for Atmospheric and Environmental Sciences, 60438 Frankfurt am Main, Germany4 University of Eastern Finland, FI-70211 Kuopio, Finland5 Aerodyne Research Inc., Billerica, Massachusetts 01821, USAPaul Ziemann University of Colorado Boulder paul.ziemann@colorado.eduGas- and Particle-Phase Products and their Mechanisms of Formation from the Reactions of Monoterpenes with NO3 Radicals: Comprehensive Measurements and Modeling Most laboratory studies of VOC oxidation and SOA formation have focused on reactions initiated by OH radicals, and to a lesser extent by O3. However, large amounts of gaseous and particulate organic nitrates have been reported in field studies worldwide, with a significant fraction apparently being formed by nighttime reactions of monoterpenes with NO3 radicals. Because this chemistry is not understood, we conducted a comprehensive laboratory/modeling study of the reaction of NO3 radicals with ??-pinene, a major monoterpene emission, with a focus on gas- and particle-phase products and their mechanisms of formation. Experiments were conducted in an environmental chamber and products were analyzed using online and offline methods. Molar yields of 4 gas-phase products and 8 particle-phase products were ~40% and ~60%, thus achieving mole balance for product analysis. The SOA consisted almost entirely of acetal oligomers apparently formed by acid-catalyzed reactions in phase-separated particles. A kinetic model was developed to extract rate constants for RO2 + RO2 reactions, gas-phase reaction branching ratios, and a rate constant for particle-phase oligomer formation from the measurements. Functional group analyses of SOA formed from reactions of NO3 radicals with other monoterpenes indicate the occurrence of similar chemistry. The results demonstrate that methods are available for developing detailed quantitative models of VOC oxidation and SOA formation for atmospheric systems.NoneLucy Carpenter University of York lucy.carpenter@york.ac.ukOceanic physicochemical processes affecting tropospheric O3 Globally, the budget and spatio-temporal variations of tropospheric ozone are reasonably well understood. Models can broadly capture the seasonal cycle of ozone and changes over interannual to decadal periods, although longer-term trends are not well reproduced. The recent Tropospheric Ozone Assessment Report concluded that of the ozone budget terms, models differ by a factor of ~2 in chemical production and loss, and by a factor of ~ 3 in deposition, with the magnitude of the oceanic deposition flux being the dominant driver of differences in this term. Oceanic deposition not only directly removes O3 from the atmosphere, but also results in emissions of trace gases including volatile iodine compounds which lead to further O3 destruction, representing a negative feedback mechanism. This talk examines such physicochemical processes affecting surface O3 which are not well defined, but which appear to be significant in understanding the ozone budget and potentially in understanding long-term O3 trends.Lucy. J. Carpenter, Rosie Chance, Liselotte Tinel, Mat. J. Evans, Ryan Pound,, Adam Saint, Tomas Sherwen Wolfson Atmospheric Chemistry Laboratories (WACL), Department of Chemistry, University of YorkMatthew Smarte California Institute of Technology msmarte@caltech.eduWater vapor dependence of the ??-hydroxyethylperoxy self reaction Organic peroxy radicals are ubiquitous intermediates in the atmosphere whose diverse fates can terminate or propagate free radical chemistry in an oxidation mechanism. Despite the importance of these species, the impact of humidity on their chemistry remains unclear. ??-hydroxyethylperoxy (??-HEP) is the simplest peroxy radical formed from the hydroxyl-initiated oxidation of alkenes and its chemistry provides general insight into the mechanism of alkene oxidation. It was recently reported that the rate of the ??-HEP self reaction is enhanced by the presence of water vapor, suggesting that complexation of a hydroxyl-substituted organic peroxy radical with water may significantly alter its reactivity. While faster radical termination from the HO2 self reaction through an HO2?€“H2O complex is well established, there is less experimental understanding of the fates of organic RO2?€“H2O complexes, where both radical propagating and terminating channels of the self reaction exist. Here, we present an experimental investigation of ??-HEP self reaction kinetics measured as a function of water vapor concentration and temperature using multiplexed synchrotron-photoionization mass spectrometry. The flexibility of this technique allows for the simultaneous, time-resolved observation of nearly every radical intermediate and stable product in the oxidation mechanism. We will discuss the impact of humidity on the rate constant and product branching ratio of the ??-HEP self reaction.Matthew D. Smarte [1], Frank A. F. Winiberg [2], Aileen Hui [1,2], Gregory Jones [1], Joseph Messinger [1], Rebecca L. Caravan [3], Kristen Zuraski [2], David L. Osborn [3], Craig A. Taatjes [3], Carl J. Percival [2], Stanley P. Sander [2], Mitchio Okumura [1][1] Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, United States[2] NASA Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109, United States[3] Combustion Research Facility, Sandia National Laboratories, Livermore, California 94551, United StatesGabriel da Silva University of Melbourne gdasilva@unimelb.edu.auIsomerization and Decomposition of Isoprene?€?s Delta-(Z)-Hydroxyperoxyl Radicals Isoprene is one of the most emitted compounds to Earth?€?s atmosphere, and largely controls chemical composition in remote, forested environments. Despite the importance of isoprene in atmospheric science, the chemistry of many of its key chemical degradation pathways are not yet fully understood. In the troposphere isoprene reacts rapidly with OH and then O2 to produce a suite of hydroxyperoxyl radicals (ISOPO2), incorporating beta-OH and E/Z delta-OH functionality. It has been shown that the delta-(Z)-ISOPO2 radicals can undergo a facile intramolecular hydrogen shift to produce a 1-hydroxyalkyl radical intermediate which can react with O2 to yield hydroperoxyl-aldehydes. This study uses computational chemistry calculations to demonstrate that delta-(E)-OH isoprene peroxyl radicals can undergo further rearrangements with lower barriers, leading to enol epoxide products with direct OH radical reformation. Master equation simulations predict appreciable OH radical reformation yields via direct chemical activation, which should be incorporated into chemical kinetic models for isoprene atmospheric chemistry.NoneGeoff Tyndall NCAR/ACOM tyndall@ucar.eduDependence of alkyl nitrate yields on structure for mid-sized alkanes Oxidation of alkanes by OH leads to formation of oxygenated VOCs (OVOCs), and organic nitrates, RONO2. With increasing activities involving the extraction of oil and natural gas in the United States, alkane chemistry is taking on a renewed importance. It has been known since the 1980s that the yields of organic nitrates increase with (1) the size of the alkane, (2) increasing pressure, and (3) decreasing temperature. Yet, these nitrate formation yields remain uncertain, even from fairly simple alkanes. In particular, it has been accepted that the yields of nitrates from secondary RO2 are roughly twice those of primary and tertiary alkanes, although a few recent studies have cast doubt on this. We are undertaking a systematic study of nitrate yields from mid-size alkanes (4-8 carbon atoms, linear and branched). The experiments are conducted in a 47-L stainless steel reactor with in-situ detection by FTIR, and external sampling by GC-FID. We have measured nitrate yields from a number of linear and branched alkanes at room temperature, using authentic standards to characterize the GC retention times and sensitivity where possible. In contrast to the older experiments, the yields of primary and tertiary nitrates (corrected for site of attack) are found to be similar to the secondary nitrates from the same molecule. A full analysis of the products also gives insight as to the relative sites of attack in a given molecule.John Orlando NCAR/ACOMFrank Flocke NCAR/ACOMJames Smith Smith Univ. of Calif., Irvine jimsmith@uci.eduEffect of relative humidity on the mechanism of new particle formation from monoterpene oxidation It has been widely observed that the frequency and intensity of new particle formation (NPF) events are reduced during periods of high relative humidity (RH). The current study explores how RH affects the formation of highly oxidized molecules (HOMs), which are key components of NPF and initial growth caused by oxidized organics. The ozonolysis of selected monoterpenes, with and without OH-scavenger, were studied under low NOx conditions and 3 - 90% RH in a temperature-controlled flow tube. A novel transverse-ionization chemical ionization mass spectrometer detected HOMs and size distributions were determined using SMPS. A major finding from this work is that neither the detected HOMs nor their abundance changed significantly with RH, which indicates that the detected HOMs must be formed from water-independent pathways. In fact, observed OH- and O3-derived peroxy radicals (RO2), HOM monomers, and HOM dimers could mostly be explained by the autoxidation of RO2 followed by bimolecular reactions with other RO2 or hydroperoxy radicals (HO2), rather than from a water-influenced pathway such as the formation of a stabilized Criegee intermediate (sCI). However, as RH changed from 3 to 90% the particle number concentrations decreased by a factor of 2-3 while particle mass concentrations increased or decreased only slightly. Thus while high RH appears to inhibit NPF, this reduction is not caused by a decrease in RO2-derived HOMs formation. Possible explanations are discussed.Xiaoxiao Li, Jiming Hao, and Jingkun Jiang (Tsinghua Univ.)Sabrina Chee (Univ. of Calif., Irvine)Jonathan Abbatt (Univ. of Toronto)Joel Thornton University of Washington thornton@atmos.uw.eduPeroxy radical autoxidation and dimer formation in alpha-pinene oxidation: constraints from flow tubes, chambers, and the field We use a suite of flow reactor studies, chamber experiments, and field observations with high-resolution time-of-flight chemical ionization mass spectrometry (HRToF-CIMS) to evaluate the importance and mechanisms of peroxy radical autoxidation and cross-reactions in the oxidation of a-pinene by the hydroxyl radical an ozone. Using radical scavengers and competitive radical cross reactions with nitric oxide in controlled flow reactors, we provide constraints on the rate constants and branching ratios for peroxy radical autoxidation pathways and cross-reactions to form gas-phase dimers. Based on these experiments, we have developed a working detailed chemical mechanism involving the first and second generation products of alpha-pinene oxidation that couples to the Master Chemical Mechanism via the F0AM model. We evaluate the atmospheric importance of these pathways by comparing to steady-state chamber experiments and ambient conditions. We find that autoxidation and dimer formation can explain a large majority of the secondary organic aerosol that forms both during low and high NOx oxidation of alpha-pinene with significant implications for the sources of organic aerosol in forested regions.Emma D'Ambro (University of Washington), Havla Pye (U.S. EPA), Ben H. Lee (University of Washington), Yue Zhao (Shanghai Jiao Tong University), Masayuki Takeuchi (GeorgiaTech), Sally Ng (GeorgiaTech)Kristian H. M??ller University of Copenhagen khm@chem.ku.dkUnimolecular Peroxy Radical Hydrogen Shift Reactions in Isoprene Oxidation Recently, the gas-phase chemistry of isoprene (2-methyl-1,3-butadiene, C??…H???), the most highly emitted non-methane hydrocarbon, and its major oxidation products, was reviewed (Wennberg et al., Chem. Rev., 2018). While much of its chemistry is well constrained by either experiment or theory, the rates of many of the unimolecular peroxy radical hydrogen shift (H-shift) reactions remain speculative. Using a high-level multi-conformer transition state theory (MC-TST) approach, we determine recommended temperature dependent reaction rate coefficients for a number of the H-shift reactions in the isoprene oxidation, to better model the atmospheric competition between uni- and bimolecular reactions. Based on the available data, we find that most of the (1,4; 1,5 and 1,6) aldehydic and (1,5 and 1,6) ?±-hydroxy H-shifts have rate coefficients in the range 10????? s????? - 1 s????? and that these reaction classes thus need to be considered in atmospheric oxidation. Furthermore, we find that the chirality of the reactant is crucial to consider as diastereomers can have rate coefficients that differ by up to almost three orders of magnitude. Implementation of these newly calculated reaction rate coefficients into the most recent GEOS-Chem model for isoprene oxidation shows that at least 30 % of all isoprene molecules emitted to the atmosphere undergo a minimum of one peroxy radical hydrogen shift reaction, highlighting the importance of this reaction class in atmospheric oxidation.Kelvin H. Bates, Harvard University and Henrik G. Kjaergaard, University of CopenhagenKrystal Vasquez California Institute of Technology kvasquez@caltech.eduObservational constraints on the fate of the hydroxy nitrates produced in the reaction of isoprene peroxy radicals with NO The formation of hydroxy nitrates (IHN) from the daytime oxidation of isoprene affects the concentration and distribution of nitrogen oxides (NOx) in the troposphere. It has been suggested that IHN may act as an important sink of NOx, though the relative importance of its individual isomers is unknown due to a lack of observational data. In this work, we deployed instrumentation capable of observing in situ isomer distributions of isoprene oxidation products to two locations to assess the formation and fate of the IHN isomers at varying peroxy radical lifetimes. Under low NOx / low OH conditions, the average daytime ratio of the most abundant IHN isomers (1,2:4,3-IHN = 2.6) differed significantly from the corresponding ISOPOOH isomer ratio (7.6), despite the similar formation pathways. In a high NOx / high OH environment, an average daytime ratio of ~1.6 was observed?€”roughly one third of what is expected when accounting for known IHN sinks?€”and observed changes in this ratio appeared to be closely linked to changes in relative humidity. In combination, these observations strongly suggest a rapid non-photochemical loss of the 1,2-IHN isomer, likely heterogeneous hydrolysis. A steady-state box model constrained by ambient measurements suggests that a hydrolysis lifetime of 3-6 hours can explain observations at both field sites. If this hypothesis is correct, hydrolysis of 1,2-IHN would represent a significant source of HNO3 globally, especially over isoprene-influenced regions.John D. Crounse, California Institute of TechnologyLu Xu, California Institute of TechnologyHannah M. Allen, California Institute of TechnologyEric Praske, California Institute of TechnologyKelvin H. Bates, Harvard University Paul O. Wennberg, California Institute of TechnologyLavinia Onel University of Leeds L.Onel@leeds.ac.ukAn inter-comparison of methods for HO2 and CH3O2 detection and kinetic study of the HO2 + CH3O2 cross-reaction in the Highly Instrumented Reactor for Atmospheric Chemistry (HIRAC) The hydroperoxy radical, HO2, and methylperoxy radicals, CH3O2, participate in a rapid chemical cycling at the heart of tropospheric oxidation. Laser-induced fluorescence (LIF) spectroscopy at low-pressure, known as the Fluorescence Assay by Gas Expansion (FAGE) technique, is most commonly used for the measurements of HO2 in the atmosphere by conversion of HO2 to OH by reaction with added NO followed by OH on-resonance LIF at 308 nm. A new method has been developed for the sensitive and selective detection of CH3O2. The method is similar to the FAGE method for HO2 detection and consists in the titration of CH3O2 to CH3O by reaction with NO followed by the detection of the resultant CH3O by off-resonant LIF. Recently, the first near infrared CRDS measurements of HO2 and CH3O2 in an atmospheric simulation chamber (HIRAC) were inter-compared against FAGE. The good agreement between HO2 and CH3O2, respectively concentrations measured using the two techniques provides a validation for the FAGE method for both HO2 and CH3O2 detection. The HO2 + CH3O2 cross-reaction is important under clean, low NOx levels, yet there are large uncertainties associated with its kinetics. The FAGE technique has been used to measure kinetic decays of HO2 and CH3O2 radicals by the cross-reaction in the temperature range from 268 K to 343 K and at 1000 mbar of air in the HIRAC chamber to obtain results in good agreement with the IUPAC preferred values.Alexander Brennan (University of Leeds), Freja F. ??sterstr??m (University of Leeds), Michele Gianella (University of Oxford), Gus Hancock (University of Oxford), Lisa Whalley (University of Leeds), Paul Seakins (University of Leeds), Grant Ritchie (University of Oxford) and Dwayne Heard (University of Leeds)Mani Sarathy KAUST mani.sarathy@kaust.edu.saFormation of highly oxidized multifunctional compounds in alkane autoxidation ?€“ relevance to atmospheric and combustion chemistry Highly oxidized multifunctional compounds (HOM) produced from autooxidation of volatile organic compounds (VOC) are important in secondary organic aerosol (SOA) formation and fuel ignition chemistry. Autoxidation leads to low volatility compounds under atmospheric conditions or highly reactive radical chain branching intermediates under combustion conditions. Peroxy (RO2) radical intermediates rapidly isomerize and further react with molecular oxygen to form larger molecules. Studies have reported HOM formation via ozonolysis of endocyclic alkenes, but few studies report autooxidation of alkanes under atmospheric conditions. Meanwhile, alkane autooxidation under combustion conditions has been widely investigated, due to its relevance to ignition processes. This presentation covers a series of recent theoretical and experimental studies performed to investigate OH-initiated alkane autooxidation under atmospheric and combustion conditions. Autooxidation experiments were performed with various alkanes in quartz flow tube reactors and jet stirred reactors. HOMs were measured using nitrate chemical ionization mass spectrometry (CIMS) and synchrotron vacuum ultraviolet photoionization MS (SVUV-PIMS). Chemical kinetic models were developed and coupled with reactor simulations to predict the formation of HOMs under various conditions. New insights into the role of temperature, reactor inlet conditions, and alkane molecular structure on HOM formation will be presented.Zhandong Wang, Manuel Monge-Palacios - KAUST. Matti Rissanen, Mikael Ehn - University of HelsinkiRasmus V. Otkj?r Department of Chemistry, University of Copenhagen otkjaer@chem.ku.dkTrends in Peroxy Radical Hydrogen Shift Rate Constants Unimolecular hydrogen shift reactions in peroxy radicals have been shown to be important in the atmospheric oxidation of isoprene and ?±-pinene. [P. Wennberg et al., Chem. Rev. 118, 3337-3390 (2017); T. Berndt et al. Nature Comm., 7, 13677 (2016)] These studies also report the efficient generation of highly oxidized organic molecules known to contribute to particle formation and growth. Unimolecular hydrogen shifts are also known to be important in the combustion of organic materials. The role of these reactions in the oxidation of organics in the atmosphere has received less attention due, in part, to the lack of kinetic data at relevant temperatures.Here we use an experimentally verified theoretical approach based on Multi-Conformer Transition State Theory (MC-TST) to calculate rate constants for a systematic set of H-shifts. [Submitted to J. Phys. Chem. A] Our results show that substitution at the abstraction site, with OH, OOH, OCH???, C=C or C=O leads to increases in the rate constant by factors of up to ~1000. In addition, reactions leading to secondary carbon radicals (alkyl substituent) are 100 times faster than those leading to primary carbon radicals, and those leading to tertiary carbon radicals a factor of 30 faster again. When the ring size in the TS is 6, 7 or 8 atoms (1,5; 1,6 or 1,7 H-shift), the H-shift reactions rate constants can reach 1 s?????. Thus H-shift reactions are likely much more prevalent in the atmosphere than previously considered.Henrik G. Kjaergaard, Department of Chemistry, University of CopenhagenSungah Kang Forschungszentrum Juelich IEK-8 s.kang@fz-juelich.deThe effect of NOx on formation of Highly Oxidized Multifunctional Molecules and SOA formation in photochemical system of ?±-pinene and ??-pinene Highly oxygenated organic molecules (HOM) are formed in the atmosphere by autoxidation. It is consist of peroxy radicals go through H-shift followed by O2 addition. A sequence of these very fast steps leads to highly oxygenated peroxy radicals (HOM-RO2) and finally to stable termination products with O/C>1. Because of their low to extreme low volatility, HOM play a crucial role in new particle formation and secondary organic aerosol (SOA) formation. As other RO2, HOM-RO2 are terminated by reactions with RO2, HO2 and NOx. In this study, three noticeable effects on HOM formation were found by introducing NOx in the photochemical system of monoterpenes. One effect is formation of highly oxygenated organic nitrates (HOM-ON), with sufficient to low vapor pressures that significantly contribute to SOA formation. The second one is dimer suppression. This is caused by competition between dimer reaction pathway (HOM-RO2??+ RO2??) and organic nitrate formation pathway (HOM-RO2??+NOx). Thirdly, the reaction between peroxy radicals and NO brings more alkoxy radicals in the system. The fragmentation of alkoxy radicals produces volatile compounds that should result in decrease of SOA yield. However, the effect of fragmentation is offset: alkoxy radicals also undergo H-shifts that produce alkyl radicals and after O2 addition new peroxy radicals. Suppression of dimers and increased degree of oxidation of the HOM monomer play together with the result of only a small reduction of the SOA yields.Thomas Mentel1, Iida Pullinen1,3, Monika Springer1, Sebastian Schmitt1, Einhard Kleist2, J??rgen Wildt1,2, Astrid Kiendler-Scharr11Institute for Energy and Climate Research (IEK-8), Forschungszentrum J??lich GmbH2Institute of Bio- and Geosciences (IBG-2), Forschungszentrum J??lich GmbH3 Department of Applied Physics, University of Eastern Finland, Kuopio, FinlandTheo Kurt??n University of Helsinki theo.kurten@helsinki.fiEvaluating mechanisms for dimer formation from RO2 + RO2 reactions Self- and cross-reactions of complex RO2 have been observed to produce dimers with the elemental composition ROOR (Berndt et al., Angew. Chem. 57, 3820, 2018). It is widely accepted that RO2 + RO2 reactions proceed through a RO4R ?€?tetroxide?€? intermediate. However, subsequent mechanisms forming O2 and either ROOR, RO + RO or RC=O + ROH remain open questions. Lee et al. (PCCP 18, 23673, 2016) suggest that RO4R decomposes on a singlet surface to a RO?€?O2?€?RO complex, in which both O2 and the RO?€?RO pair have triplet multiplicities. The two RO can then either dissociate (leading to RO + RO), react on the triplet surface (leading to RC=O + ROH), or undergo an intersystem crossing (ISC) and subsequent recombination on a singlet surface (leading to ROOR). Unfortunately, the rate-limiting barrier for RO4R decomposition reported by Lee is much too high compared to experimental overall RO2 + RO2 rates. CASPT2 and NEVPT2 calculations yield RO4R decomposition barriers in rough agreement with overall RO2 + RO2 rates. Furthermore, we estimate the ISC rate of triplet CH3O?€?CH3O to the singlet surface to be 4E8 1/s. The rate of dissociation is significantly higher, explaining why CH3OOCH3 is not formed. Larger RO?€?RO triplet complexes, corresponding to systems studied by Berndt et al., are somewhat more strongly bound than CH3O?€?CH3O, potentially making ISC competitive. ISC and recombination of RO?€?RO triplet complexes could thus be a feasible source of ROOR dimers from RO2 + RO2 reactions.Rashid Valiev (University of Helsinki), Siddharth Iyer (University of Helsinki)Torsten Berndt Leibniz Institute for Tropospheric Research (TROPOS), 04318 Leipzig, Germany berndt@tropos.deAccretion product formation from self- and cross-reactions of RO2 radicals in the atmosphere The global emission rate of non-methane hydrocarbons from vegetation and human activities into the atmosphere is estimated to be about 1.3 ?— 10(9) metric tons of carbon per year. Their gas-phase degradation process is mainly initiated by the reaction with hydroxyl (OH) or nitrate (NO3) radicals, chlorine atoms or ozone (O3). After initial attack of the oxidant, RO2 radicals are almost exclusively formed as intermediates, which rapidly react further with NO, HO2 or other RO2 radicals or via RO2 radical self-reaction. Here we show that self- and cross-reactions of two RO2 radicals produce accretion products composed of the carbon backbone of both reactants.(1)RO2 + R??O2??’ROOR?? + O2(R1)RO2 radicals bearing functional groups show fast accretion product formation rates competing with those of the corresponding reactions with NO and HO2. This pathway, not considered yet in the modelling of atmospheric processes, can be important for the fate of RO2 radicals in all areas of the atmosphere. Moreover, the formed accretion products can be featured by remarkably low vapour pressure characterizing them as effective source for secondary organic aerosol. (1) Berndt et al., Angew. Chem. Int. Ed. 57, 3820-3824 (2018)W. Scholz (Institute for Ion Physics and Applied Physics, University of Innsbruck, 6020 Innsbruck, Austria)B. Mentler (Institute for Ion Physics and Applied Physics, University of Innsbruck, 6020 Innsbruck, Austria)L. Fischer (Institute for Ion Physics and Applied Physics, University of Innsbruck, 6020 Innsbruck, Austria)H. Herrmann (Leibniz Institute for Tropospheric Research (TROPOS), 04318 Leipzig, Germany)M. Kulmala (Department of Physics, University of Helsinki, Helsinki 00014, Finland)A. Hansel (Institute for Ion Physics and Applied Physics, University of Innsbruck, 6020 Innsbruck, Austria) ................
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