Theo Kurtén

University of Helsinki, Helsinki, Uusimaa, Finland

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Publications (109)489.14 Total impact

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    ABSTRACT: The acid-mediated reaction of ketones with hydroperoxides generates radicals, a process with reaction conditions similar to those of the Baeyer-Villiger oxidation but with an outcome resembling the formation of hydroxyl radicals via ozonolysis in the atmosphere. The Baeyer-Villiger oxidation forms esters from ketones, with the preferred use of peracids. In contrast, alkyl hydroperoxides and hydrogen peroxide react with ketones by condensation to form alkenyl peroxides, which rapidly undergo homolytic OO bond cleavage to form radicals. Both reactions are believed to proceed via Criegee adducts, but the electronic nature of the peroxide residue determines the subsequent reaction pathways. DFT calculations and experimental results support the idea that, unlike previously assumed, the Baeyer-Villiger reaction is not intrinsically difficult with alkyl hydroperoxides and hydrogen peroxide but rather that the alternative radical formation is increasingly favored. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    Angewandte Chemie International Edition 08/2015; DOI:10.1002/anie.201505648 · 11.26 Impact Factor
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    ABSTRACT: Die säurevermittelte Reaktion von Ketonen mit Hydroperoxiden führt zur Bildung von Radikalen. Dieser Prozess verläuft unter ähnlichen Reaktionsbedingungen wie die Baeyer-Villiger-Oxidation, aber das Ergebnis ähnelt der Bildung von Hydroxylradikalen durch Ozonolyse von Olefinen in der Atmosphäre. Während die Baeyer-Villiger-Oxidation bevorzugt Persäuren verwendet, um aus Ketonen Ester zu bilden, kondensieren Alkylhydroperoxide und H2O2 mit Ketonen zu Alkenylperoxiden, die rasch homolytisch zerfallen. Beide Reaktionen verlaufen wahrscheinlich über Criegee-Addukte, wobei die elektronischen Eigenschaften der Peroxidreste den weiteren Reaktionspfad bestimmen. DFT-Rechnungen und experimentelle Befunde stützen die These, dass, anders als bisher angenommen, die Baeyer-Villiger-Reaktion mit Hydroperoxiden und Wasserstoffperoxid nicht grundsätzlich schwierig ist, die alternative Radikalbildung jedoch stärker bevorzugt wird.
    Angewandte Chemie 08/2015; DOI:10.1002/ange.201505648
  • Jonas Elm · Nanna Myllys · Noora Hyttinen · Theo Kurtén
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    ABSTRACT: We investigate the molecular interactions between sulfuric acid and a recently reported C6H8O7 ketodiperoxy acid formed through autoxidation from cyclohexene and ozone. Structurally similar but larger ELVOC (extremely low volatility organic compound) products formed from autoxidation of monoterpenes are believed to play a major role in the formation and early growth of atmospheric aerosol particles. Utilizing density functional theory geometries, with a DLPNO-CCSD(T)/def2-QZVPP single point energy correction, the stepwise Gibbs free energies of formation have been calculated, and the stabilities of the molecular clusters have been evaluated. C6H8O7 interacts weakly with both itself and sulfuric acid, with standard free energies of formation (Delta G at 298 K and 1 atm) around or above 0 kcal/mol. This is due to the presence of strong intramolecular hydrogen bonds in the peroxyacid groups of C6H8O7. These stabilize the isolated molecule with respect to its clusters, and lead to unfavourable interaction energies. The addition of sulfuric acid to clusters containing C6H8O7 is somewhat more favourable, but the formed clusters are still far more likely to evaporate than to grow further in atmospheric conditions. These findings indicates that the O/C-ratio cannot exclusively be used as a proxy for volatility in atmospheric new particle formation involving organic compounds. The specific molecular structure, and especially the number of strong hydrogen binding moieties, are equally important. The interaction between the C6H8O7 compound and aqueous phase sulfate ions indicates that ELVOC-type compounds can contribute to aerosol mass by effectively partitioning into the condensed phase.
    The Journal of Physical Chemistry A 07/2015; DOI:10.1021/acs.jpca.5b04040 · 2.78 Impact Factor
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    ABSTRACT: Several Extremely Low Volatility Organic Compounds (ELVOCs) formed in the ozonolysis of endocyclic alkenes have recently been detected in laboratory and field studies. These experiments have been carried out with Chemical Ionization Atmospheric Pressure interface Time-of-Flight mass spectrometers (CI-APi-TOF) with nitrate ions as reagent ions. The nitrate ion binds to the detected species through hydrogen bonds, but it also binds very strongly to one or two neutral nitric acid molecules. This makes the measurement highly selective when there is an excess amount of neutral nitric acid in the instrument. In this work we used quantum chemical methods to calculate the binding energies between a nitrate ion and several highly oxidized ozonolysis products of cyclohexene. These were then compared to the binding energies of nitrate ion - nitric acid clusters. Systematic configurational sampling of the molecules and clusters was carried out at the B3LYP/6-31+G* and ωB97xD/aug-cc-pVTZ levels, and the final single point energies were calculated with DLPNO-CCSD(T)/def2-QZVPP. The binding energies were used in a kinetic simulation of the measurement system to determine the relative ratios of the detected signals. Our results indicate that at least two hydrogen bond donor functional groups (in this case, hydroperoxide, OOH) are needed for an ELVOC molecule to be detected in a nitrate ion CI-APi-TOF. Also, a double bond in the carbon backbone makes the nitrate cluster formation less favorable.
    The Journal of Physical Chemistry A 05/2015; 119(24). DOI:10.1021/acs.jpca.5b01818 · 2.78 Impact Factor
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    ABSTRACT: It has been postulated that secondary organic particulate matter plays a pivotal role in the early growth of newly formed particles in forest areas. The recently detected class of extremely low volatile organic compounds (ELVOC) provides the missing organic vapours and possibly contributes a~significant fraction to atmospheric SOA. ELVOC are highly oxidized multifunctional molecules (HOM), formed by sequential rearrangement of peroxy radicals and subsequent O2 addition. Key for efficiency in early particle growth is that formation of HOM is induced by one attack of the oxidant (here O3) and followed by an autoxidation process involving molecular oxygen. Similar mechanisms were recently observed and predicted by quantum mechanical calculations e.g. for isoprene. To assess the atmospheric importance and therewith the potential generality, it is crucial to understand the formation pathway of HOM. To elucidate the formation path of HOM as well as necessary and sufficient structural prerequisites of their formation we studied homologues series of cycloalkenes in comparison to two monoterpenes. We were able to directly observe highly oxidized multifunctional peroxy radicals with 8 or 10 O-atoms by an Atmospheric Pressure interface High Resolution Time of Flight Mass Spectrometer equipped with a NO3−-Chemical Ionization (CI) source. In case of O3 acting as oxidant the starting peroxy radical is formed on the so called vinylhydroperoxide path. HOM peroxy radicals and their termination reactions with other peroxy radicals, including dimerization, allowed for analysing the observed mass spectra and narrow down the likely formation path. As consequence we propose that HOM are multifunctional percarboxylic acids; with carbonyl-, hydroperoxy-, or hydroxy-groups arising from the termination steps. We figured that aldehyde groups facilitate the initial rearrangement steps. In simple molecules like cyloalkenes autoxidation was limited to both terminal C-atoms and two further C-atoms in the respective α-positions. In more complex molecules containing tertiary H-atoms or small constraint rings even higher oxidation degree were possible, either by simple H-shift of the tertiary H-atom or by initialisation of complex ring-opening reactions.
    Atmospheric Chemistry and Physics 01/2015; 15(2):2791-2851. DOI:10.5194/acpd-15-2791-2015 · 4.88 Impact Factor
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    ABSTRACT: Formation of highly oxidized, multifunctional products in the ozonolysis of three endocyclic alkenes, 1- methylcyclohexene, 4-methylcyclohexene, and α-pinene, was investigated using a chemical ionization atmospheric pressure interface time-of-flight (CI-APi-TOF) mass spectrometer with a nitrate ion (NO3(-)) based ionization scheme. The experiments were performed in borosilicate glass flow tube reactors at room temperature (T = 293 ± 3 K) and at ambient pressure. An ensemble of oxidized monomer and dimer products was detected, with elemental compositions obtained from the high-resolution mass spectra. The monomer product distributions have O/C ratios from 0.8 to 1.6 and can be explained with an autocatalytic oxidation mechanism (=autoxidation) where the oxygen-centered peroxy radical (RO2) intermediates internally rearrange by intramolecular hydrogen shift reactions, enabling more oxygen molecules to attach to the carbon backbone. Dimer distributions are proposed to form by homogeneous peroxy radical recombination and cross combination reactions. These conclusions were supported by experiments where H atoms were exchanged to D atoms by addition of D2O to the carrier gas flow. Methylcyclohexenes were observed to autoxidize in accordance with our previous work on cyclohexene, whereas in α-pinene ozonolysis different mechanistic steps are needed to explain the products observed.
    The Journal of Physical Chemistry A 01/2015; 119(19). DOI:10.1021/jp510966g · 2.78 Impact Factor
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    ABSTRACT: First generation product yields from the OH-initiated oxidation of methyl vinyl ketone (3-buten-2-one, MVK) under both low and high NO conditions are reported. In the low NO chemistry, three distinct reaction channels are identified leading to the formation of 1) OH, glycolaldehyde, and acetyl peroxy (R2a), 2) a hydroperoxide (R2b), and 3) an α-diketone (R2c). The α-diketone likely results from HOx-neutral chemistry previously only known to occur in reactions of HO2 with halogenated peroxy radicals. Quantum chemical calculations demonstrate that all channels are kinetically accessible at 298 K. In the high NO chemistry, glycolaldehyde is produced with a yield of 74 ± 6.0%. Two alkyl nitrates are formed with a combined yield of 4.0 ± 0.6%. We revise a 3-D chemical transport model to assess what impact these modifications in the MVK mechanism have on simulations of atmospheric oxidative chemistry. The calculated OH mixing ratio over the Amazon increases by 6%, suggesting that the low NO chemistry makes a non-negligible contribution toward sustaining the atmospheric radical pool.
    The Journal of Physical Chemistry A 12/2014; 119(19). DOI:10.1021/jp5107058 · 2.78 Impact Factor
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    ABSTRACT: We have used quantum chemical methods to investigate the molecular mechanism behind the recently reported ( Kampf , C. J. ; Environ. Sci. Technol . 2013 , 47 , 4236 - 4244 ) strong dependence of the Henry's law coefficient of glyoxal (C2O2H2) on the sulfate concentration of the aqueous phase. Although the glyoxal molecule interacts only weakly with sulfate, its hydrated forms (C2O3H4 and C2O4H6) form strong complexes with sulfate, displacing water molecules from the solvation shell and increasing the uptake of glyoxal into sulfate-containing aqueous solutions, including sulfate-containing aerosol particles. This promotes the participation of glyoxal in reactions leading to secondary organic aerosol formation, especially in regions with high sulfate concentrations. We used our computed equilibrium constants for the complexation reactions to assess the magnitude of the Henry's law coefficient enhancement and found it to be in reasonable agreement with experimental results. This indicates that the complexation of glyoxal hydrates with sulfate can explain the observed uptake enhancement.
    The Journal of Physical Chemistry A 11/2014; 119(19). DOI:10.1021/jp510304c · 2.78 Impact Factor
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    ABSTRACT: Sulphuric acid is generally considered one of the most important substances taking part in atmospheric particle formation. However, in typical atmospheric conditions in the lower troposphere sulphuric acid and water alone are unable to form particles. It has been suggested that strong bases may stabilize sulphuric acid clusters so that particle formation may occur. More to the point, amines – strong organic bases – have become the subject of interest as possible cause for such stabilisation. To probe whether amines play a role in atmospheric nucleation, we need to be able to measure accurately the gas-phase amine vapour concentration. Such measurements often include charging the neutral molecules and molecular clusters in the sample. Since amines are bases, the charging process should introduce a positive charge. This can be achieved for example using a positively charged reagent with a suitable proton affinity. In our study, we have used quantum chemical methods combined with a cluster dynamics code to study the use of acetone as a reagent in chemical ionization and compared the results with measurements performed with a chemical ionization atmospheric pressure interface time-of-flight mass spectrometer (CI-APi-TOF). The computational results indicate that protonated acetone is an effective reagent in chemical ionization. However, in the experiments the charger ions were not depleted at the predicted dimethylamine concentrations, indicating that either the modelling scheme or the experimental results – or both – contain unidentified sources of error.
    11/2014; 7(11):11011-11044. DOI:10.5194/amtd-7-11011-2014
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    ABSTRACT: The prompt formation of highly oxidized organic compounds in the ozonolysis of cyclohexene (C6H10) was investigated by means of laboratory experiments together with quantum chemical calculations. The experiments were performed in borosilicate glass flow tube reactors coupled to a Chemical Ionization Atmospheric Pressure interface Time-of-Flight mass spectrometer (CI-APi-TOF) with a nitrate ion (NO3-) based ionization scheme. Quantum chemical calculations were performed at the CCSD(T)-F12a/VDZ-F12//ωB97XD/aug-cc-pVTZ level, with kinetic modeling using multiconformer transition state theory (MC-TST), including Eckart tunneling corrections. The complementary investigation methods gave a consistent picture of a formation mechanism advancing by peroxy radical (RO2) isomerization through intramolecular hydrogen shift reactions, followed by sequential O2 addition steps, i.e., RO2 autoxidation, on a timescale of seconds. Dimerization of the peroxy radicals by recombination and cross-combination reactions is in competition with the formation of highly oxidized monomer species and is observed to lead to peroxides, potentially diacyl peroxides. The molar yield of these highly oxidized products (having O/C > 1 in monomers and O/C > 0.55 in dimers) from cyclohexene ozonolysis was determined as (4.5 ± 3.8)%. Fully deuterated cyclohexene and cis-6-nonenal ozonolysis, as well as the influence of water addition to the system (either H2O or D2O), were also investigated in order to strengthen the arguments on the proposed mechanism. Deuterated cyclohexene ozonolysis resulted in a less oxidized product distribution with a lower yield of highly oxygenated products and cis-6-nonenal ozonolysis generated the same monomer product distribution, consistent with the proposed mechanism and in agreement with quantum chemical modeling.
    Journal of the American Chemical Society 10/2014; 136(44). DOI:10.1021/ja507146s · 11.44 Impact Factor
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    ABSTRACT: We comment on a study by Nadykto et al. recently published in this journal. Earlier work from our group has been misrepresented in this study, and we feel that the claims made need to be amended. Also the analysis of Nadykto et al. concerning the implications of their own density functional calculations is incomplete. We present cluster formation simulations allowing more conclusions to be drawn from their data, and also compare them to recent experimental results not cited in their work.
    Chemical Physics Letters 10/2014; 624. DOI:10.1016/j.cplett.2015.01.029 · 1.99 Impact Factor
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    ABSTRACT: Models of formation and growth of atmospheric aerosols are highly dependent on accurate cluster binding energies. These are most often calculated by ab initio electronic structure methods but remain associated with significant uncertainties. We present a computational benchmarking study of the Gibbs free binding energies in molecular complexes and clusters based on gas phase FTIR spectroscopy. The acetonitrile-HCl molecular complex is identified via its redshifted H-Cl stretching vibrational mode. We determine the Gibbs free binding energy, ΔG°295 K, to between 4.8 and 7.9 kJ mol(-1) and compare this range to predictions from several widely used electronic structure methods, including five density functionals, Møller-Plesset perturbation theory, and five coupled cluster methods up to CCSDT quality, considering also the D3 dispersion correctional scheme. With some exceptions, we find that most electronic structure methods overestimate ΔG°295 K. The effects of vibrational anharmonicity is approximated using scaling factors, reducing ΔG°295 K by ca. 1.8 kJ mol(-1), whereby ΔG°295 K predictions well within the experimental range can be obtained.
    The Journal of Physical Chemistry A 07/2014; 118(28). DOI:10.1021/jp5037537 · 2.78 Impact Factor
  • Jonas Elm · Theo Kurtén · Merete Bilde · Kurt V Mikkelsen
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    ABSTRACT: We investigate the molecular interactions between the semi-volatile α -pinene oxidation product pinic acid and sulfuric acid using computational methods. The stepwise Gibb's free energies of formation have been calculated utilizing the M06-2X functional and the stability of the clusters are evaluated from the corresponding ∆G-values. The first two additions of sulfuric acid to pinic acid is found to be favourable with ∆G-values of -9.06 and -10.41 kcal/mol, respectively. Addition of a third sulfuric acid molecule is less favourable and leads to a structural rearrangement forming a bridged sulfuric acid - pinic acid cluster. The involvement of more than one pinic acid molecule in a single cluster is observed to lead to the formation of favourable (pinic acid)2 (H2 SO4 ) and (pinic acid)2 (H2 SO4 )2 clusters. The identified most favourable growth paths starting from a single pinic acid molecule lead to closed structures without the further possibility for attachment of neither sulfuric acid nor pinic acid. This suggest that pinic acid cannot be a key species in the first steps in nucleation, but the favourable interactions between sulfuric acid and pinic acid imply that pinic acid can contribute to the subsequent growth of an existing nucleus by condensation.
    The Journal of Physical Chemistry A 07/2014; 118(36). DOI:10.1021/jp503736s · 2.78 Impact Factor
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    ABSTRACT: We used quantum chemical formation free energies of hydrated sulfuric acid-containing molecular clusters and a dynamic model to simulate a flow tube measurement, and determined the effective diffusion coefficient of sulfuric acid as a function of relative humidity. This type of measurement was performed by Hanson and Eisele, who presented and applied a fitting method to obtain equilibrium constants K 1 and K 2 for the formation of sulfuric acid mono- and dihydrates, respectively, from the experimentally determined diffusion coefficients. The fit is derived assuming that only H2SO4 molecules hydrated by up to two water molecules are present. To study the sensitivity of the results to this assumption, we implemented the same fit to the modeled diffusion coefficient data, computed including also larger H2SO4 hydrates with more than two waters. We show that according to quantum chemical equilibrium constants, the larger hydrates are likely to be present in nonnegligible amounts, which affects the effective diffusion coefficient. This results in the fitted value obtained for K 1 being lower and for K 2 being higher than the actual values. The results are further altered if contaminant base molecules, such as amines, capable of binding to H2SO4 molecules, are able to enter the system, for example, with the water vapor. The magnitude and direction of the effect of the contaminants depends not only on the contaminant concentration, but also on the H2SO4 concentration and on the hygroscopicity of the H2SO4–base clusters.Copyright 2014 American Association for Aerosol Research
    Aerosol Science and Technology 05/2014; 48(6). DOI:10.1080/02786826.2014.903556 · 3.16 Impact Factor
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    ABSTRACT: A High-Resolution Time-of-Flight Chemical-Ionization Mass Spectrometer (HR-ToF-CIMS) using Iodide-adducts has been characterized and deployed in several laboratory and field studies to measure a suite of organic and inorganic atmospheric species. The large negative mass defect of Iodide, combined with soft ionization and the high mass-accuracy (< 20 ppm) and mass-resolving power (R > 5500) of the time-of-flight mass spectrometer, provides an additional degree of separation and allows for the determination of elemental compositions for the vast majority of detected ions. Laboratory characterization reveals Iodide-adduct ionization generally exhibits increasing sensitivity towards more polar or acidic volatile organic compounds. Simultaneous retrieval of a wide range of mass-to-charge ratios (m/Q from 25 to 625 Th) at a high frequency (> 1 Hz) provides a comprehensive view of atmospheric oxidative chemistry, particularly when sampling rapidly-evolving plumes from fast moving platforms like an aircraft. We present the sampling protocol, detection limits and observations from the first aircraft deployment for an instrument of this type, which took place aboard the NOAA WP-3D aircraft during the Southeast Nexus (SENEX) 2013 field campaign.
    Environmental Science & Technology 05/2014; 48(11). DOI:10.1021/es500362a · 5.48 Impact Factor
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    ABSTRACT: Formation of new particles through clustering of molecules from condensable vapors is a significant source for atmospheric aerosols. The smallest clusters formed in the very first steps of the condensation process are, however, not directly observable by experimental means. We present here a comprehensive series of electronic structure calculations on the hydrates of clusters formed by up to four molecules of sulfuric acid, and up to two molecules of ammonia or dimethylamine. Though clusters containing ammonia, and certainly dimethylamine, generally exhibit lower average hydration than the pure acid clusters, populations of individual hydrates vary widely. Furthermore, we explore the predictions obtained using a thermodynamic model for the description of these hydrates. The similar magnitude and trends of hydrate formation predicted by both methods illustrate the potential of combining them to obtain more comprehensive models. The stabilization of some clusters relative to others due to their hydration is highly likely to have significant effects on the overall processes that lead to formation of new particles in the atmosphere.
    The Journal of Physical Chemistry A 03/2014; 118(14). DOI:10.1021/jp500712y · 2.78 Impact Factor
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    ABSTRACT: Forests emit large quantities of volatile organic compounds (VOCs) to the atmosphere. Their condensable oxidation products can form secondary organic aerosol, a significant and ubiquitous component of atmospheric aerosol, which is known to affect the Earth's radiation balance by scattering solar radiation and by acting as cloud condensation nuclei. The quantitative assessment of such climate effects remains hampered by a number of factors, including an incomplete understanding of how biogenic VOCs contribute to the formation of atmospheric secondary organic aerosol. The growth of newly formed particles from sizes of less than three nanometres up to the sizes of cloud condensation nuclei (about one hundred nanometres) in many continental ecosystems requires abundant, essentially non-volatile organic vapours, but the sources and compositions of such vapours remain unknown. Here we investigate the oxidation of VOCs, in particular the terpene α-pinene, under atmospherically relevant conditions in chamber experiments. We find that a direct pathway leads from several biogenic VOCs, such as monoterpenes, to the formation of large amounts of extremely low-volatility vapours. These vapours form at significant mass yield in the gas phase and condense irreversibly onto aerosol surfaces to produce secondary organic aerosol, helping to explain the discrepancy between the observed atmospheric burden of secondary organic aerosol and that reported by many model studies. We further demonstrate how these low-volatility vapours can enhance, or even dominate, the formation and growth of aerosol particles over forested regions, providing a missing link between biogenic VOCs and their conversion to aerosol particles. Our findings could help to improve assessments of biosphere-aerosol-climate feedback mechanisms, and the air quality and climate effects of biogenic emissions generally.
    Nature 02/2014; 506(7489):476-9. DOI:10.1038/nature13032 · 42.35 Impact Factor
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    ABSTRACT: Oxidation processes in Earth's atmosphere are tightly connected to many environmental and human health issues and are essential drivers for biogeochemistry. Until the recent discovery of the atmospheric relevance of stabilized Criegee intermediates (sCI), atmospheric oxidation processes were thought to be dominated by few main oxidants: ozone, hydroxyl radicals (OH), nitrate radicals and, e.g. over oceans, halogen atoms such as chlorine. Here, we report results from laboratory experiments at 293 K and atmospheric pressure focusing on sCI formation from the ozonolysis of isoprene and the most abundant monoterpenes (α-pinene and limonene), and subsequent reactions of the resulting sCIs with SO2 producing sulphuric acid (H2SO4). The measured sCI yields were (0.15 ± 0.07), (0.27 ± 0.12) and (0.58 ± 0.26) for the ozonolysis of α-pinene, limonene and isoprene, respectively. The ratio between the rate coefficient for the sCI loss (including thermal decomposition and the reaction with water vapour) and the rate coefficient for the reaction of sCI with SO2, k(loss) / k(sCI + SO2), was determined at relative humidities of 10% and 50%. Observed values represent the average reactivity of all sCIs produced from the individual alkene used in the ozonolysis. For the monoterpene derived sCIs, the relative rate coefficients k(loss) / k(sCI + SO2) were in the range (2.0-2.4) × 1012 molecule cm-3 and nearly independent on the relative humidity. This fact points to a minor importance of the sCI + H2O reaction in the case of the sCI arising from α-pinene and limonene. For the isoprene sCIs, however, the ratio k(loss) / k(sCI + SO2) was strongly dependent on the relative humidity. To explore whether sCIs could have a more general role in atmospheric oxidation, we investigated as an example the reactivity of acetone oxide (sCI from the ozonolysis of 2,3-dimethyl-2-butene) toward small organic acids, i.e. formic and acetic acid. Acetone oxide was found to react faster with the organic acids than with SO2; k(sCI + acid) / k(sCI + SO2) = (2.8 ± 0.3) for formic acid and k(sCI + acid) / k(sCI + SO2) = (3.4 ± 0.2) for acetic acid. This finding suggests that sCIs can play a role in the formation and loss of several atmospheric constituents besides SO2.
    Atmospheric Chemistry and Physics 12/2013; 14(2). DOI:10.5194/acpd-14-3071-2014 · 4.88 Impact Factor
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    ABSTRACT: Sulfuric acid clusters stabilized by base molecules are likely to have a significant role in atmospheric new particle formation. Recent advances in mass spectrometry techniques have permitted the detection of electrically charged clusters. However, direct measurement of electrically neutral clusters is not possible. Mass spectrometry instruments can be combined with a charger, but the possible effect of charging on the composition of neutral clusters must be addressed before the measured data can be linked to properties of neutral clusters. In the present work we have used formation free energies from quantum chemical methods to calculate the evaporation rates of electrically charged (both positive and negative) sulfuric acid-ammonia/dimethylamine clusters. To understand how charging will affect the composition of these clusters, we have compared the evaporation rates of charged clusters with those of the corresponding neutral clusters. We found that the only cluster studied in this paper which will retain its composition is H2SO4 · NH3 when charged positively; all other clusters will be altered by both positive and negative charging. In the case of charging clusters negatively, base molecules will completely evaporate from clusters with 1 to 3 sulfuric acid molecules in the case of ammonia, and from clusters with 1 or 2 sulfuric acid molecules in the case of dimethylamine. Larger clusters will maintain some base molecules, but the H2SO4 : base ratio will increase. In the case of positive charging, some of the acid molecules will evaporate, decreasing the H2SO4 : base ratio.
    Atmospheric Chemistry and Physics 12/2013; 14(2). DOI:10.5194/acpd-14-1317-2014 · 5.51 Impact Factor
  • Oona Kupiainen-Määttä · Tinja Olenius · Theo Christian Kurtén · Hanna Vehkamaki
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    ABSTRACT: Quantum chemical calculations have been performed on negatively charged nitric acid - sulfuric acid - dimethylamine clusters. The cluster energies were combined with a kinetic model to study the chemical ionization of sulfuric acid molecules and sulfuric acid - dimethylamine clusters with nitrate ions. Both the sulfuric acid monomer and the H2SO4·(CH3)2NH cluster get ionized, but the cluster has a much higher dipole moment, and thus a higher collision rate with charger ions. Clustering of sulfuric acid with bases will therefore increase its detection probability in the CIMS, instead of decreasing it as has been suggested previously. However, our comparison of different quantum chemical methods shows some uncertainty on the extent of sulfuric acid-dimethylamine cluster formation in typical ambient conditions, and no experimental data is available for comparison. Apart from affecting CIMS measurements, the degree of clustering is directly linked to the formation rate of larger clusters, and needs to be quantified in order to understand atmospheric new-particle formation. Based on the different charging efficiencies of the monomer and the cluster, a method is proposed for determining experimentally the binding energies of H2SO4·base clusters by measuring the extent of cluster formation as a function of base concentration.
    The Journal of Physical Chemistry A 12/2013; 117(51). DOI:10.1021/jp4049764 · 2.78 Impact Factor