Browsing by Subject "OXIDIZED RO2 RADICALS"

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  • Kurten, Theo; Tiusanen, Kirsi; Roldin, Pontus; Rissanen, Matti; Luy, Jan-Niclas; Boy, Michael; Ehn, Mikael; Donahue, Neil (2016)
    COSMO-RS (conductor-like screening model for real solvents) and three different group-contribution methods were used to compute saturation (subcooled) liquid vapor pressures for 16 possible products of ozone-initiated alpha-pinene autoxidation, with elemental compositions C10H16O4-10 and C20H30O10-12. The saturation vapor pressures predicted by the different methods varied widely. COSMO-RS predicted relatively high saturation vapor pressures values in the range of 10(-6) to 10(-10) bar for the C10H16O4-10 "monomers", and 10(-11) to 10(-16) bar for the C20H30O10-12 "dimers". The group-contribution methods predicted significantly (up to 8 order of magnitude) lower saturation vapor pressures for most of the more highly oxidized monomers. For the differs, the COSMO-RS predictions were within the (wide) range spanned by the three group-contribution methods. The main reason for the discrepancies between the methods is likely that the group-contribution methods do not contain the necessary parameters to accurately treat autoxidation products containing multiple hydroperoxide, peroxy acid or peroxide functional groups, which form intramolecular hydrogen bonds with each other. While the COSMO-RS saturation vapor pressures for these systems may be overestimated, the results strongly indicate that despite their high O:C ratios, the volatilities of the autoxidation products of alpha-pinene (and possibly other atmospherically relevant alkenes) are not necessarily extremely low. In other words, while autoxidation products are able to, adsorb onto aerosol particles, their evaporation back into the gas phase cannot be assumed to be negligible, especially from the smallest nanometer-scale particles. Their observed effective contribution to aerosol particle growth may therefore involve rapid heterogeneous reactions (reactive uptake) rather than effectively irreversible physical absorption.
  • Mohr, Claudia; Lopez-Hilfiker, Felipe D.; Yli-Juuti, Taina; Heitto, Arto; Lutz, Anna; Hallquist, Mattias; D'Ambro, Emma L.; Rissanen, Matti P.; Hao, Liqing; Schobesberger, Siegfried; Kulmala, Markku; Mauldin III, Roy L.; Makkonen, Ulla; Sipilä, Mikko; Petäjä, Tuukka; Thornton, Joel A. (2017)
    We present ambient observations of dimeric monoterpene oxidation products (C16-20HyO6-9) in gas and particle phases in the boreal forest in Finland in spring 2013 and 2014, detected with a chemical ionization mass spectrometer with a filter inlet for gases and aerosols employing acetate and iodide as reagent ions. These are among the first online dual-phase observations of such dimers in the atmosphere. Estimated saturation concentrations of 10(-15) to 10(-6)mu gm(-3) (based on observed thermal desorptions and group-contribution methods) and measured gas-phase concentrations of 10(-3) to 10(-2)mu gm(-3) (similar to 10(6)-10(7)moleculescm(-3)) corroborate a gas-phase formation mechanism. Regular new particle formation (NPF) events allowed insights into the potential role dimers may play for atmospheric NPF and growth. The observationally constrained Model for Acid-Base chemistry in NAnoparticle Growth indicates a contribution of similar to 5% to early stage particle growth from the similar to 60 gaseous dimer compounds. Plain Language Summary Atmospheric aerosol particles influence climate and air quality. We present new insights into how emissions of volatile organic compounds from trees are transformed in the atmosphere to contribute to the formation and growth of aerosol particles. We detected for the first time over a forest, a group of organic molecules, known to grow particles, in the gas phase at levels far higher than expected. Previous measurements had only measured them in the particles. This finding provides guidance on how models of aerosol formation and growth should describe their appearance and fate in the atmosphere.
  • Riva, M.; Ehn, M.; Li, D.; Tomaz, S.; Bourgain, F.; Perrier, S.; George, C. (2019)
    While acknowledged as key components in the formation of new particles in the atmosphere, the accurate characterization of gaseous (highly) oxygenated organic compounds remains challenging and requires analytical developments. Earlier studies have successfully used the nitrate ion (NO3) based chemical ionization (CI) coupled to atmospheric pressure interface time-of-flight mass spectrometry (CI-APi-TOF) for monitoring these compounds. Despite many breakthroughs in recent years, the CI-APi-TOF has many limitations, preventing for instance the unambiguous ion identification of overlapping peaks. To tackle this analytical challenge, we developed a CI interface coupled to an ultrahigh-resolution Orbitrap mass spectrometer (CI-Orbitrap). We show that the CI-Orbitrap has similar sensitivity and selectivity as the CI-APi-TOF, but with over an order of magnitude higher mass resolving power (up to 140 000). Equally importantly, the CI-Orbitrap allows tandem mass spectrometry, providing the possibility for structural elucidation of the highly oxygenated organic molecules (HOM). As a proof of concept, we characterized HOM formed during the ozonolysis of two biogenic compounds (alpha-pinene and limonene), under different environmental conditions in a flow reactor. The CI-Orbitrap exhibited high sensitivity to both HOM and radical species, while easily separating ions of different elemental composition in cases where the more common TOF applications would not have been able to distinguish all ions. Our tandem mass spectrometry analyses revealed distinct fingerprint spectra for all the studied HOM. Overall, the CI-Orbitrap is an extremely promising instrument, and it provides a much-needed extension to ongoing research on HOM, with potential to impact also many other fields within atmospheric chemistry.
  • Quéléver, Lauriane L. J.; Kristensen, Kasper; Jensen, Louise Normann; Rosati, Bernadette; Teiwes, Ricky; Dällenbach, Kaspar; Peräkylä, Otso; Roldin, Pontus; Bossi, Rossana; Pedersen, Henrik B.; Glasius, Marianne; Bilde, Merete; Ehn, Mikael (2019)
    Highly oxygenated organic molecules (HOMs) are important contributors to secondary organic aerosol (SOA) and new-particle formation (NPF) in the boreal atmosphere. This newly discovered class of molecules is efficiently formed from atmospheric oxidation of biogenic volatile organic compounds (VOCs), such as monoterpenes, through a process called autoxidation. This process, in which peroxy-radical intermediates isomerize to allow addition of molecular oxygen, is expected to be highly temperature-dependent. Here, we studied the dynamics of HOM formation during a -pinene ozonolysis experiments performed at three different temperatures, 20, 0 and - 15 degrees C, in the Aarhus University Research on Aerosol (AURA) chamber. We found that the HOM formation, under our experimental conditions (50 ppb alpha-pinene and 100 ppb ozone), decreased considerably at lower temperature, with molar yields dropping by around a factor of 50 when experiments were performed at 0 degrees C, compared to 20 degrees C. At -15 degrees C, the HOM signals were already close to the detection limit of the nitrate-based chemical ionization atmospheric pressure interface time-of-flight (CI-APi-TOF) mass spectrometer used for measuring gas-phase HOMs. Surprisingly, comparing spectra measured at 0 and 20 degrees C, ratios between HOMs of different oxidation levels, e.g., the typical HOM products C10H14O7, C10H14O9, and C10H14O11, changed considerably less than the total HOM yields. More oxidized species have undergone more isomerization steps; yet, at lower temperature, they did not decrease more than the less oxidized species. One possible explanation is that the primary rate-limiting steps forming these HOMs occur before the products become oxygenated enough to be detected by our CI-APi-TOF (i.e., typically seven or more oxygen atoms). The strong temperature dependence of HOM formation was observed under temperatures highly relevant to the boreal forest, but the exact magnitude of this effect in the atmosphere will be much more complex: the fate of peroxy radicals is a competition between autoxidation (influenced by temperature and VOC type) and bimolecular termination pathways (influenced mainly by concentration of reaction partners). While the temperature influence is likely smaller in the boreal atmosphere than in our chamber, both the magnitude and complexity of this effect clearly deserve more consideration in future studies in order to estimate the ultimate role of HOMs on SOA and NPF under different atmospheric conditions.
  • Riva, Matthieu; Rantala, Pekka; Krechmer, Jordan E.; Peräkylä, Otso; Zhang, Yanjun; Heikkinen, Liine; Garmash, Olga; Yan, Chao; Kulmala, Markku; Worsnop, Douglas; Ehn, Mikael (2019)
    The impact of aerosols on climate and air quality remains poorly understood due to multiple factors. One of the current limitations is the incomplete understanding of the contribution of oxygenated products, generated from the gas-phase oxidation of volatile organic compounds (VOCs), to aerosol formation. Indeed, atmospheric gaseous chemical processes yield thousands of (highly) oxygenated species, spanning a wide range of chemical formulas, functional groups and, consequently, volatilities. While recent mass spectrometric developments have allowed extensive on-line detection of a myriad of oxygenated organic species, playing a central role in atmospheric chemistry, the detailed quantification and characterization of this diverse group of compounds remains extremely challenging. To address this challenge, we evaluated the capability of current state-of-the-art mass spectrometers equipped with different chemical ionization sources to detect the oxidation products formed from alpha-Pinene ozonolysis under various conditions. Five different mass spectrometers were deployed simultaneously for a chamber study. Two chemical ionization atmospheric pressure interface time-of-flight mass spectrometers (CI-APi-TOF) with nitrate and amine reagent ion chemistries and an iodide chemical ionization time-of-flight mass spectrometer (TOF-CIMS) were used. Additionally, a proton transfer reaction time-of-flight mass spectrometer (PTR-TOF 8000) and a new "vocus" PTR-TOF were also deployed. In the current study, we compared around 1000 different compounds between each of the five instruments, with the aim of determining which oxygenated VOCs (OVOCs) the different methods were sensitive to and identifying regions where two or more instruments were able to detect species with similar molecular formulae. We utilized a large variability in conditions (including different VOCs, ozone, NOx and OH scavenger concentrations) in our newly constructed atmospheric simulation chamber for a comprehensive correlation analysis between all instruments. This analysis, combined with estimated concentrations for identified molecules in each instrument, yielded both expected and surprising results. As anticipated based on earlier studies, the PTR instruments were the only ones able to measure the precursor VOC, the iodide TOF-CIMS efficiently detected many semi-volatile organic compounds (SVOCs) with three to five oxygen atoms, and the nitrate CI-APi-TOF was mainly sensitive to highly oxygenated organic (O > 5) molecules (HOMs). In addition, the vocus showed good agreement with the iodide TOF-CIMS for the SVOC, including a range of organonitrates. The amine CI-APi-TOF agreed well with the nitrate CI-APi-TOF for HOM dimers. However, the loadings in our experiments caused the amine reagent ion to be considerably depleted, causing nonlinear responses for monomers. This study explores and highlights both benefits and limitations of currently available chemical ionization mass spectrometry instrumentation for characterizing the wide variety of OVOCs in the atmosphere. While specifically shown for the case of alpha-Pinene ozonolysis, we expect our general findings to also be valid for a wide range of other VOC-oxidant systems. As discussed in this study, no single instrument configuration can be deemed better or worse than the others, as the optimal instrument for a particular study ultimately depends on the specific target of the study.
  • Elm, Jonas; Myllys, Nanna; Olenius, Tinja; Halonen, Roope; Kurten, Theo; Vehkamäki, Hanna (2017)
    Using computational methods, we investigate the formation of atmospheric clusters consisting of sulfuric acid (SA) and 3-methyl-1,2,3-butanetricarboxylic acid (MBTCA), identified from a-pinene oxidation. The molecular structure of the clusters is obtained using three different DFT functionals (PW91, M06-2X and oB97X-D) with the 6-31++ G(d, p) basis set and the binding energies are calculated using a high level DLPNO-CCSD(T)/ Def2-QZVPP method. The stability of the clusters is evaluated based on the calculated formation free energies. The interaction between MBTCA and sulfuric acid is found to be thermodynamically favourable and clusters consisting of 2-3 MBTCA and 2-3 SA molecules are found to be particularly stable. There is a large stabilization of the cluster when the amount of sulfuric acid-carboxylic acid hydrogen bonded interactions is maximized. The reaction free energies for forming the (MBTCA) 2-3(SA) 2-3 clusters are found to be similar in magnitude to those of the formation of the sulfuric acid-dimethylamine cluster. Using cluster kinetics calculations we identify that the growth of the clusters is essentially limited by a weak formation of the largest clusters studied, implying that other stabilizing vapours are required for stable cluster formation and growth.
  • Molteni, Ugo; Simon, Mario; Heinritzi, Martin; Hoyle, Christopher R.; Bernhammer, Anne-Kathrin; Bianchi, Federico; Breitenlechner, Martin; Brilke, Sophia; Dias, António; Duplissy, Jonathan; Frege, Carla; Gordon, Hamish; Heyn, Claudia; Jokinen, Tuija; Kürten, Andreas; Lehtipalo, Katrianne; Makhmutov, Vladimir; Petäjä, Tuukka; Pieber, Simone M.; Praplan, Arnaud P.; Schobesberger, Siegfried; Steiner, Gerhard; Stozhkov, Yuri; Tomé, António; Tröstl, Jasmin; Wagner, Andrea C.; Wagner, Robert; Williamson, Christina; Yan, Chao; Baltensperger, Urs; Curtius, Joachim; Donahue, Neil M.; Hansel, Armin; Kirkby, Jasper; Kulmala, Markku; Worsnop, Douglas R.; Dommen, Josef (2019)
    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.
  • Molteni, Ugo; Bianchi, Federico; Klein, Felix; El Haddad, Imad; Frege, Carla; Rossi, Michel J.; Dommen, Josef; Baltensperger, Urs (2018)
    Anthropogenic volatile organic compounds (AV-OCs) often dominate the urban atmosphere and consist to a large degree of aromatic hydrocarbons (ArHCs), such as benzene, toluene, xylenes, and trimethylbenzenes, e.g., from the handling and combustion of fuels. These compounds are important precursors for the formation of secondary organic aerosol. Here we show that the oxidation of aromatics with OH leads to a subsequent autoxidation chain reaction forming highly oxygenated molecules (HOMs) with an O:C ratio of up to 1.09. This is exemplified for five single-ring ArHCs (benzene, toluene, o-/m-/p-xylene, mesitylene (1,3,5-trimethylbenzene) and ethylbenzene), as well as two conjugated polycyclic ArHCs (naphthalene and biphenyl). We report the elemental composition of the HOMs and show the differences in the oxidation patterns of these ArHCs. A potential pathway for the formation of these HOMs from aromatics is presented and discussed. We hypothesize that AV-OCs may contribute substantially to new particle formation events that have been detected in urban areas.
  • Ehn, Mikael; Berndt, Torsten; Wildt, Juergen; Mentel, Thomas (2017)
    Recent advances in chemical ionization mass spectrometry have allowed the detection of a new group of compounds termed highly oxygenated molecules (HOM). These are atmospheric oxidation products of volatile organic compounds (VOC) retaining most of their carbon backbone, and with O/C ratios around unity. Owing to their surprisingly high yields and low vapor pressures, the importance of HOM for aerosol formation has been easy to verify. However, the opposite can be said concerning the exact formation pathways of HOM from major aerosol precursor VOC. While the role of peroxy radical autoxidation, i.e., consecutive intramolecular H-shifts followed by O-2 addition, has been recognized, the detailed formation mechanisms remain highly uncertain. A primary reason is that the autoxidation process occurs on sub-second timescales and is extremely sensitive to environmental conditions like gas composition, temperature, and pressure. This, in turn, poses a great challenge for chemical kinetics studies to be able to mimic the relevant atmospheric reaction pathways, while simultaneously using conditions suitable for studying the short-lived radical intermediates. In this perspective, we define six specific challenges for this community to directly observe the initial steps of atmospherically relevant autoxidation reactions and thereby facilitate vital improvements in the understanding of VOC degradation and organic aerosol formation. (C) 2017 Wiley Periodicals, Inc.
  • Elm, Jonas; Myllys, Nanna; Kurten, Theo (2017)
    We investigate the molecular interactions between phosphoric acid and common atmospheric nucleation precursors using computational methods. The equilibrium geometries and vibrational frequencies are obtained using the three DFT functionals M06-2X, PW91 and B97X-D. The single-point energy is corrected using a high-level CCSD(T)-F12a/VDZ-F12 calculation. The molecular interaction between phosphoric acid and sulphuric acid is found to be strong with reaction free energy of similar magnitude as the interaction between dimethylamine and sulphuric acid. The strong hydrogen bonding of phosphoric acid to sulphuric acid indicates that concentrations of as low as 10(2)-10(4) molecules/cm(3) will offer equivalent or higher stability as the sulphuric acid dimer for the formation of atmospheric molecular clusters. We assess and utilise the DLPNO-CCSD(T) method for studying larger clusters involving phosphoric acid and sulphuric acid and find that having a phosphoric acid molecule present in the cluster enhances the further addition of sulphuric acid molecules. [GRAPHICS] .