Displays

Dimethyl sulfide (DMS), produced by marine organisms, represents the most abundant, biogenic sulfur emission into the Earth´s atmosphere. The gas-phase degradation of DMS is mainly initiated by the reaction with the OH radical forming first CH₃SCH₂O₂ radicals from the dominant H-abstraction channel. A fast CH₃SCH₂O₂ isomerization process was proposed as a result of quantum chemical calculations. In the present study, experimental investigations on the product formation from OH + DMS have been conducted in a free-jet flow system at 295 ± 2 K and 1 bar air. Very efficient detection of CH₃SCH₂O₂ isomerization products has been achieved by iodide-CI-APi-TOF measurements allowing to run the reaction for close to atmospheric conditions. It is experimentally shown that the CH₃SCH₂O₂ radicals undergo a two-step isomerization process finally forming a product consistent with the formula HOOCH₂SCHO. The isomerization process is accompanied by OH recycling. The rate-limiting first isomerization step, CH₃SCH₂O₂ → CH₂SCH₂OOH proceeds with k = (0.23 ± 0.12) s^-1 at 295 ± 2 K. Competing bimolecular CH₃SCH₂O₂ reactions with NO, HO₂ or RO₂ radicals are less important for trace-gas conditions over the oceans.  Results of atmospheric chemistry simulations demonstrate the predominance (≥95%) of CH₃SCH₂O₂ isomerization. The rapid peroxy radical isomerization, not yet considered in models, substantially changes the understanding of DMS´s degradation processes in the atmosphere.

How to cite: Berndt, T., Scholz, W., Mentler, B., Fischer, L., Hoffmann, E. H., Tilgner, A., Hyttinen, N., Prisle, N., Hansel, A., and Herrmann, H.: New pathways of the reaction of OH radicals with dimethyl sulfide based on CH3SCH2O2 isomerization, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10345, https://doi.org/10.5194/egusphere-egu2020-10345, 2020.

Autoxidation is a process whereby organic compounds become oxidized by molecular oxygen (O₂). It is ubiquitous in various reaction systems, contributing to the spoilage of food and wine, ignition in internal combustion engines, and formation of atmospheric secondary organic aerosol (SOA) from volatile emissions. Autoxidation thus greatly influences both engine operation and efficiency, and, via SOA, climate and air quality. Recent progress in atmospheric chemistry has identified double bonds and oxygen-containing moieties as structural facilitators for efficient autoxidation, and subsequent OA formation. Lacking either of these functionalities, alkanes, the primary molecular class in fuels for combustion engines and an important class of urban trace gases, have been expected to have low susceptibility to undergo autoxidation. In this work, we show that alkanes can indeed undergo efficient autoxidation both under combustion-relevant and atmospheric temperatures, consequently producing more highly oxygenated species than previously expected. By bridging methodologies and knowledge from both combustion and atmospheric chemistry, we mapped the autoxidation potential of a range of alkane structures under various conditions, from the combustion domain to the atmospheric domain. We identified the importance of isomerization steps driven by both peroxy and alkoxy radicals, and show that isomerization and production of low-volatile condensable vapors is efficient even under highly polluted ([NO]>10ppb) conditions. Our findings, currently under review, provide insights into the underlying chemical mechanisms causing highly variable SOA yields from alkanes, which were observed in previous atmospheric studies. The results of this inter-disciplinary effort provide crucial new information on oxidation processes in both combustion engines and the atmosphere, with direct implications for engine efficiency and urban air quality.

How to cite: Ehn, M., Wang, Z., Rissanen, M., Garmash, O., Quéléver, L., Monge-Palacios, M., Rantala, P., Donahue, N., Berndt, T., and Sarathy, M.: Alkane autoxidation and aerosol formation: new insights from combustion engines to the atmosphere, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20144, https://doi.org/10.5194/egusphere-egu2020-20144, 2020.

Intermediate volatility organic compounds (IVOCs) have been proposed to be great contributors to SOA formation. In this study, we performed comprehensive measurement of ambient IVOCs and calculated their SOA production at an urban site Peking University Urban Atmospheric Environment Monitoring Stations (PKUERS). Results showed that the campaign-average concentration IVOCs was 62.5 ± 45.2 μg·m^-3 (average ± standard deviation), which is comparable to the emission of VOCs. Only ~10% of the IVOCs could be speciated, with most of the IVOCs existing as unresolved complex mixture (UCM). Back trajectory analysis showed that clusters from near south has the highest IVOC concentration, suggesting the importance to control the IVOC emissions from the polluted regions of China. Using the OH exposure estimated by o-xylene to benzene and m/p-xylene to benzene, the estimated daily average SOA concentration was 5.8 ±3.4 μg·m^-3, in which IVOCs contributed 15 times that of single-ring aromatics. The estimated vehicular-SOA could be 1.04 ± 0.7 μg·m^-3. Considering its high SOA formation potential, this study highlights the importance to study the IVOC emissions in China.

How to cite: Tang, R., Lu, Q., Zhu, W., Song, K., Fan, H., Tan, R., Liu, K., Yu, Y., Shen, R., Wang, H., Chen, S., Robinson, A. L., and Guo, S.: Secondary Organic Aerosol Formation of Ambient Intermediate Volatility Organic Compounds: Implication from Gasoline Vehicles, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6360, https://doi.org/10.5194/egusphere-egu2020-6360, 2020.

The applicability of chamber-derived Secondary Organic Aerosol (SOA) yields to the atmosphere remains a key uncertainty in modeling SOA. The chemical and environmental conditions achieved in chambers are narrower than and often significantly biased from those experienced in the atmosphere. We present results from applying explicit chemical mechanisms in a dynamic gas-particle partitioning model (FOAM-WAM) to simulate SOA formation and evolution from a range of chamber experiments involving isoprene and monoterpenes. We focus on how such comparisons can highlight the applicability, or the lack thereof, of derived SOA yields, extrapolate measured SOA yields to more complex chemical or environmental conditions, and identify key gaps in chemical or physical mechanisms and thus feedback on chamber experiment design and earth system model parameterizations. In particular, we show that current mechanisms of low-NOx isoprene and a-pinene oxidation that incorporate RO2 H-shift reactions can adequately explain corresponding fresh SOA without the need for substantial vapor-pressure lowering accretion chemistry, while substantial particle-phase photo-chemistry is required to explain the dynamic evolution of SOA characteristics (volatility, O/C ratios, etc)observed in chambers at longer aging times. We find that chemical conditions, such as absolute concentrations, are as important as vapor wall loss, or even more so, at perturbing SOA yields from realistic values. Consistent with recent field studies but in contrast to previous chamber studies, our modeling predicts that low-NOx isoprene oxidation is unlikely to produce significant SOA in warm boundary layers, except through isoprene epoxy-diol multi-phase chemistry. Current mechanisms are unable to reproduce the non-linear response of isoprene-derived photochemical SOA with NOx observed in multiple chambers, suggesting a potentially important missing mechanism of volatility reduction at intermediate NOx concentrations in that system. 

How to cite: Thornton, J. A., Shilling, J., Pye, H., D'Ambro, E., Zawadowicz, M., and Liu, J.: Using explicit mechanisms of Secondary Organic Aerosol (SOA) formation and evolution to extrapolate chamber studies to the atmosphere, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3415, https://doi.org/10.5194/egusphere-egu2020-3415, 2020.

Biomass burning emissions, domestic- and wild-fires, agricultural burning, and fuel use, emit a blend of gases and particles with adverse effects on humans-health, climate and air quality. Furans are heterocyclic organic compounds (OVOC) that have been recently identified as important biomass burning emission-products. It is estimated that furan (C₄H₄O), 2-methylfuran (C₅H₆O), 2-furaldehyde (C₅H₄O₂) and benzofuran (C₈H₆O) emission levels are 70 to 120 times higher compared to CO. Once furans are emitted in the atmosphere, they will undergo gas phase chemistry and, to an extent, they will be photolyzed at actinic wavelengths. OH and NO₃ radicals, Cl atoms and O₃ chemistry might result in tropospheric O₃ and in secondary organic aerosols (SOA) formation, which might be enhanced due to their potent low volatility. Therefore, it is essential to investigate the kinetics and the mechanism of all the photochemically induced degradation pathways and identify and quantify SOA precursors, so as to evaluate their impact on Air-Quality and Climate.

Within this framework, a thorough laboratory study, using two complementary techniques has been carried out. First, major atmospheric oxidants reaction rate coefficients with furans were determined. Secondly, the degradation mechanisms were investigated from both kinetic and conversion-yields perspectives. A Teflon atmospheric simulation chamber, named THALAMOS (THermALly regulated AtMOSpheric simulation chamber), was used to study the reactions at atmospheric pressure. State-of-the-art in-line instrumentation, e.g., FTIR spectroscopy and Chemical ionization mass spectrometry, were used for the real-time monitoring of reactants and products. To further our understanding, the reactions rate coefficients were also measured at 2 mTorr, between 253 and 363 K, with the continuous flow technique of the Very Low Pressure Reactor, in which an effusive molecular beam is analyzed with Quadrupole Mass Spectrometry (VLPR/QMS). Intercomparing the results from the two techniques reactions mechanistic-scheme was mapped-out and their impact was evaluated.

OH and NO₃ radicals and Cl atoms reactions with all the furans were measured to be in the order of 10^-11, 10^-10 and 10^-12 cm³ molecule^-1 s^-1, respectively, leading to atmospheric-lifetimes between 2 and 10 hours. Temperature and pressure dependent kinetic measurements revealed association as the dominant reaction channel. However, experiments at very-low-pressure regime showed that HCl elimination cannot be excluded, especially when the furan-ring aromaticity is not breaking.

Finally, it is evident that furans degradation will occur at low altitudes and SOA precursors, i.e., end-oxidation products will be formed nearby their emission locations. Further, kinetics studies were used to study the structure-reactivity trend of furans and to estimate their Photochemical Ozone Creation Potential (POCP). Results from this study are expected to significantly improve our insight on furans tropospheric photochemistry and via identifying and quantifying end-products and SOA formation, to assess their indirect and direct impact, on Climate, Air-Quality and humans-health.

How to cite: Angelaki, M., Papadimitriou, V., and Romanias, M.: Atmospheric Degradation and Climate and Air-Quality Impact of Furan-based Biomass Burning Emission Products: A Kinetic and Mechanistic study, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1074, https://doi.org/10.5194/egusphere-egu2020-1074, 2020.

Biogenic volatile organic compounds (VOC) are important secondary organic aerosol (SOA) precursors. Whilst isoprene dominates VOC plant emissions globally, its yield of SOA mass is found to be modest in comparison to that of monoterpenes (MT). Tracers from isoprene oxidation have been observed in particles showing that they condense from the gas phase and yet new particle formation is suppressed by the presence of isoprene in mixtures of plant emissions containing MT.

Experiments were performed in the JPAC chamber in Jülich. We showed that isoprene can suppress both the instantaneous mass formation and overall yield of monoterpenes in mixtures by two effects: oxidant and product scavenging. Isoprene scavenged OH radicals from reacting with MT (oxidant scavenging). Subsequently, the resulting isoprene peroxy radicals reacted with highly oxygenated peroxy radicals from MT oxidation (product scavenging). These effects from isoprene, also demonstrated using CO or CH₄, reduced the yield of low-volatility, highly oxygenated molecules (HOM) from MT that would otherwise form SOA.

Our results show that in mixtures changes in particle mass and number are not additive, and yields from single precursor experiments cannot simply be linearly combined. Reactive, modest SOA yield compounds are not necessarily net SOA producers and isoprene oxidation can suppress both SOA number and mass. Global model calculations support that OH scavenging and product scavenging can also operate in the real atmosphere. Our results highlight a need for more realistic consideration of SOA formation in the atmosphere analogous to the treatment of ozone formation, where interactions between the mechanistic pathways involving peroxy radicals are recognised to be essential.

How to cite: Mentel, T., McFiggans, G., Wildt, J., and Kiendler-Scharr, A. and the JPAC-Team 2015: Secondary Organic Aerosol Reduced by Mixture of Atmospheric Vapours, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8607, https://doi.org/10.5194/egusphere-egu2020-8607, 2020.

Biogenic volatile organic compounds (BVOC) dominate the overall VOC emissions. Isoprene is the most common BVOC emitted from vegetation, accounting up to 50% of the total BVOC emissions. Despite being emitted in daytime it can accumulate in the stratified nocturnal layer. Thus, the oxidation of isoprene by nitrate radicals (NO₃) may be of high importance. A series of experiments were conducted in the atmospheric simulation chamber SAPHIR in Jülich, Germany, in order to investigate the gas and particle phase products of the oxidation of isoprene by NO₃, under a variety of conditions (e.g. high RO₂, high HO₂, nighttime to daytime transition, with and without seed aerosol) using a wide range of instrumentation. However, herein the focus is on the results of gas-phase product characterisation using high resolution time of flight chemical ionization mass spectrometers (HR-ToF-CIMS) using iodide or bromide as the primary reagent ion. The use of two HR-ToF-CIMS with different primary reagents provides possibilities to scrutinise the time profiles of isomers of selected products.

We will discuss qualitatively and quantitatively how the distribution of oxidation products change under different conditions, with a focus on the nighttime daytime transition of the major products and the role of subsequent OH oxidation on the products initially formed by NO₃ oxidation. Generally, the dominant gas phase products include compounds like nitrooxy hydroperoxide (INP) & dihydroxy nitrate (IDHN) (C₅H₉NO₅), carbonyl nitrate (ICN) (C₅H₇NO₅), hydroxy nitrate (IHN) (C₅H₉NO₄), hydroxy hydroperoxy nitrate (IHPN) (C₅H₉NO₆), as well as a C₄ compound (C₄H₇NO₅) among others.

How to cite: Tsiligiannis, E., Wu, R., Kang, S., Hantschke, L., Thornton, J., Fuchs, H., Mentel, T., and Hallquist, M.: Nighttime to daytime transition of the oxidation products of isoprene by NO3 radicals, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-961, https://doi.org/10.5194/egusphere-egu2020-961, 2020.

Field and laboratory studies have indirectly but conclusively established that reactions involving peroxy radicals (RO₂) play a key role in the gas-phase formation of accretion products, also commonly referred to as “dimers”, as they typically contain roughly twice the number of carbon atoms compared to their hydrocarbon precursors. Using computational tools, we have recently presented two different potential mechanisms for this process.

First, direct and rapid recombination of peroxy and alkoxy (RO) radicals, analogous to the recently characterized RO₂ + OH reaction, leads to the formation of metastable RO₃R’ trioxides, which may have lifetimes on the order of a hundred seconds. [1] However, due to both the limited lifetime of the trioxides, and the low concentration of alkoxy radicals, the RO₂ + R’O pathway is likely to be a minor, though not necessarily negligible, pathway for atmospheric dimer formation.

Second, we have shown that recombination of two peroxy radicals – phenomenologically known to be responsible for the formation of ROOR’ – type dimers – very likely occurs through a multi-step mechanism involving an intersystem crossing (ISC). [2]  In contrast to earlier predictions, we find that the rate-limiting step for the overall RO₂  + R’O₂ reaction is the initial formation of a short-lived RO₄R’ tetroxide intermediate. For tertiary RO₂, the barrier for the tetroxide formation can be substantial. However, for all studied species the tetroxide decomposition is rapid, forming ground-state triplet O₂, and a weakly bound triplet complex of two alkoxy radicals. The branching ratios of the different RO₂ + R’O₂ reaction channels are then determined by a three-way competition of this complex. For simple systems, the possible channels are dissociation (leading to RO + R’O), H-abstraction on the triplet surface (leading to RC=O + R’OH), and ISC and subsequent recombination on the singlet surface (leading to ROOR’). All of these can potentially be competive with each other, with rates very roughly on the order of 10⁹ s^-1. For more complex RO₂ parents, rapid unimolecular reactions of the daughter RO (such as alkoxy scissions) open up even more potential reaction channels, for example direct alkoxy – alkyl recombination to form (either singlet or triplet) ether-type (ROR’) dimers.

[1] Iyer, S., Rissanen, M. P. and Kurtén, T. Reaction Between Peroxy and Alkoxy Radicals can Form Stable Adducts. Journal of Physical Chemistry Letters, Vol. 10, 2051-2057, 2019.

[2] Valiev, R., Hasan, G., Salo, V.-T., Kubečka, J. and Kurtén, T. Intersystem Crossings Drive Atmospheric Gas-Phase Dimer Formation. Journal of Physical Chemistry A, Vol. 123, 6596-6604, 2019.

How to cite: Kurtén, T., Iyer, S., Salo, V.-T., Hasan, G., Rissanen, M., and Valiev, R.: Computational studies of gas-phase accretion product formation involving RO2, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2614, https://doi.org/10.5194/egusphere-egu2020-2614, 2020.

The importance of peroxy radical hydrogen shift reactions in the atmosphere has gained acceptance in recent years. Recent theoretical calculations have suggested that these can be stereoselective i.e. that different stereoisomers react with significantly different rate coefficients. Combining experiments (GC-CIMS) with high-level calculations (MC-TST), we show that two hydroxy peroxy radical diastereomers formed in the oxidation of crotonaldehyde have rate coefficients for their peroxy radical hydrogen shift reactions that differ by more than a factor of 10. The difference is large enough that under urban atmospheric conditions, one diastereomer would react primarily by the unimolecular H-shift, while the other would react mainly by bimolecular reactions leading to diastreomeric enhancement of the products.

For a large set of peroxy radical hydrogen shift reactions in the oxidation of isoprene, the stereospecific rate coefficients are calculated to assess the global importance of this phenomenon in the atmosphere.  These calculated rate coefficients are implemented into the global chemistry-transport model GEOS-Chem to model the effect. Results show that more than 30 % of all isoprene molecules emitted undergo a minimum of one peroxy radical hydrogen shift reaction during its oxidation. Furthermore, the results show that the different diastereomers may react with rate coefficients differing by up to almost a factor of 1000, highlighting how important it is to account for this phenomenon.

How to cite: Møller, K. H., Praske, E., Xu, L., Crounse, J. D., Bates, K. H., Wennberg, P. O., and Kjaergaard, H. G.: Stereoselectivity in Atmospheric Autoxidation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6719, https://doi.org/10.5194/egusphere-egu2020-6719, 2020.

Production of ozone and secondary organic aerosols (SOA) in the troposphere is driven by the photo-oxidation of volatile organic compounds (VOCs). Crucial intermediates in these oxidation steps are peroxy radicals, which enable ozone generation when reacting with NO. Recent pioneering studies have shown peroxy radical chemistry to have much broader impacts on the atmosphere, with many of these species undergoing autoxidation and forming highly oxidised organic molecules (HOMs), including accretion products, which can form new particles, contribute to SOA growth and influence global climate. However, explicitly simulating the full complexity of this chemistry is impractical due to the many thousands of VOC species in the atmosphere; techniques for reducing complexity are therefore necessary. The Master Chemical Mechanism (MCM) is a near-explicit scheme, with ~6,000 species and ~19,000 reactions, but is almost exclusively used in box-model applications due to its high cost. The Common Representative Intermediates (CRIv2-R5) mechanism is an effective compromise, preserving the ozone forming potential of the MCM from the emission and atmospheric degradation of isoprene, α/β-pinene, and 19 other primary VOC species whilst reducing the number of species and reactions to be feasible in a 3D model (approximately 240 species and 650 reactions, including 47 non-transported peroxy radical species and their associated reactions).

We have implemented CRIv2R5 into a global chemistry-climate model, the UK Earth System Model (UKESM1). We present results from a present-day emissions scenario for the Coupled Model Intercomparison Project (CMIP6) to enable a broad scope of model simulations with more basic chemistry and observations to evaluate the model changes against. We find significant differences to tropospheric ozone production and oxidative capacity of the atmosphere, with a strong sensitivity to magnitude and speciation of VOC emissions, highlighting the importance of accurately simulating VOC chemistry to understand trends in tropospheric ozone under changing emissions and climate.

Moving forward, having the comprehensive CRIv2R5 mechanism within UKESM1 provides the framework for investigating the impacts of recently discovered peroxy radical chemical processes on global climate. We present further work that has focused on expanding the CRI mechanism in box-model studies with a semi-explicit treatment key peroxy radical processes including (i) the autoxidation of peroxy radicals from the hydroxyl radical and ozone initiated reactions of α-pinene, (ii) the formation of multiple generations of peroxy radicals, (iii) formation of accretion products (dimers) and (iv) isoprene-driven suppression of accretion product formation as observed in experiments. This new CRI-HOM mechanism is now being implemented into the global UKESM1 model and coupled with its aerosol mechanism. This work will enable pioneering investigations linking best process-level understanding of gas-phase peroxy radical chemistry to SOA formation and thus improving our understanding of the relationship between biogenic VOC emissions and global climate.

How to cite: Archer-Nicholls, S., Weber, J. M., Abraham, N. L., Russo, M. R., Knote, C., Griffiths, P. T., Lowe, D., Utembe, S., O'Connor, F., Wild, O., Berndt, T., Jenkin, M. E., and Archibald, A. T.: Linking Peroxy Radical Chemistry to Global Climate: The Common Representatives Intermediates Chemical Mechanism in the UK Earth System Model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18456, https://doi.org/10.5194/egusphere-egu2020-18456, 2020.

Monoterpenes are a group of volatile organic compounds that are emitted to the atmosphere in large amounts by natural sources. Some monoterpenes such as limonene and Δ³-carene are also widely used as additives in detergents and perfumes, and thus have a potential impact on indoor air quality and human health.

The volatile organic compounds like monoterpenes may undergo a series of autoxidation processes in the atmosphere to form highly oxygenated compounds, which have been linked to the formation of secondary organic aerosols. For this process to occur, the unimolecular reactions of the peroxy radicals formed during oxidation must have rate coefficients comparable to or greater than those of the competing bimolecular reactions with HO₂, NO or other RO₂ radicals.

We studied the hydrogen shift (H-shift) and the cyclization reactions of all 45 hydroxy peroxy radicals formed by hydroxyl radical (OH) and O₂ addition to six monoterpenes (α-pinene, β-pinene, Δ³-carene, camphene, limonene and terpinolene). The reaction rate coefficients of the possible unimolecular reaction were initially studied at a lower level of theory. Those deemed likely to be atmospherically competitive were then calculated using the multi-conformer transition states theory approach developed by Møller et al. (J. Phys. Chem. A, 120, 51, 10072-10087, 2016). This approach has been shown to agree with the experimental values to within a factor of 4 for other systems.

It was found that double bonds are key to fast unimolecular reactions in the first-generation monoterpene hydroxy peroxy radicals. The H-shift reactions abstracting a hydrogen from a carbon adjacent to a double bond are found to typically be fast enough to compete with the bimolecular reactions, likely due to the resonance stability of the nascent allylic radical. The reactivity of the cyclization reaction between the carbon-carbon double bonds and the peroxy group, which forms an endoperoxide ring, is high as well. The H-shifts abstracting the hydrogen from the hydroxy group may be competitive in some cases but the reaction rate coefficients for these reactions are more uncertain. Generally, the cyclization reaction and the allylic H-shift reactions are the dominant reaction paths for the studied peroxyl radicals. Since the OH radical addition consumes one double bond, we suggest that the monoterpenes with more than one double bond in their structure are likely to have unimolecular reactions that can be important for the first-generation monoterpene peroxy radicals. On the other hand, the ones with only one double bond initially are not likely to have fast unimolecular reactions that can compete with the bimolecular reactions under the atmospheric condition, unless a double bond can be formed during their oxidation process as found for α-pinene and β-pinene. This result greatly limits the amount of potentially important unimolecular reaction paths in atmospheric monoterpene oxidation.

How to cite: Chen, J., Møller, K. H., Otkjær, R. V., and Kjaergaard, H. G.: Double Bonds are Key to Fast Unimolecular Reactivity in First Generation Monoterpene Hydroxy Peroxy Radicals, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2661, https://doi.org/10.5194/egusphere-egu2020-2661, 2020.

Alpha-pinene is the largest globally emitted monoterpene. Its oxidation reaction with ozone leads to peroxy radicals (RO₂) that can subsequently form highly oxygenated organic molecules (HOMs) through the process of autoxidation. HOMs are considered to play a critical role in the growth of early particles as they can have sufficiently low saturation vapor pressures.

Pseudo-unimolecular autoxidation reaction is generally thought to compete with bimolecular reactions of RO₂in the atmosphere. While these bimolecular reactions could potentially lead to radical recycling, [1] they are generally thought to lead to the formation of non-reactive products. In order to compete with these bimolecular reactions, the unimolecular autoxidation reaction must be rapid, especially in high RO₂/NO conditions.

The initial ozonolysis reaction of a-pinene leads to the first-generation RO₂with the 6-member ring broken. Current knowledge dictates the perpetuation of the inner 4-member cylobutyl ring in the first-generation RO₂. This ring has proven to be a hurdle for rapid unimolecular autoxidation reactions as the steric hindrance the ring affords leads to high barriers (and therefore slow reaction rates) for hydrogen-shift (H-shift) reactions central to autoxidation. [2]

In this work, we show that the ozonolysis of a-pinene could directly lead to the formation of a hitherto unexplored completely ring-opened RO₂ product. This pathway is made feasible by considering the large amount of excess energy channeled into the rovibrational modes of the vinoxy product after ozonolysis. This leads to the opening of the cyclobutyl ring of a significant fraction of the “hot” vinoxy radicals under atmospheric conditions, as opposed to all of them adding an O₂molecule as was previously thought. The breaking of the ring potentially leads to the formation of products with up to 8 oxygen atoms after a single hydrogen shift reaction following the formation of the vinoxy.

[1] Iyer, S.; Reiman, H.; Møller, K. H.; Rissanen, M. P.; Kjaergaard, H. G.; Kurtén, T. Computational Investigation of RO₂+ HO₂and RO₂+ RO₂Reactions of Monoterpene Derived First-Generation Peroxy Radicals Leading to Radical Recycling. J. Phys. Chem. A2018, 49, 9542-9552.

[2] Kurtén, T.; Rissanen, M. P. Rissanen, Mackeprang, K.; Thornton, J. A.; Jørgensen, S.; Ehn, M.; Kjaergaard, H. G. Computational Study of Hydrogen Shifts and Ring-Opening Mechanisms in a-Pinene Ozonolysis Products. J. Phys. Chem. A2015, 119, 11366-11375.

How to cite: Iyer, S., Rissanen, M., Valiev, R., Thornton, J., and Kurtén, T.: Rapid formation of HOMs from gas-phase alpha-pinene ozonolysis, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6865, https://doi.org/10.5194/egusphere-egu2020-6865, 2020.

OXIDATION PRODUCTS OF ALPHA-PINENE AND THEIR ELECTRICAL MOBILITIES
A. SKYTTÄ 1 , L. AHONEN 1 , R. CAI 1 and J. KANGASLUOMA 1
1 Institute for Atmospheric and Earth System Research/Physics, Faculty of Science, University of
Helsinki, Helsinki, 00140, Finland

α-pinene C10H16 is a monoterpene emitted by vegetation and its low volatile oxidation products are important source for secondary organic aerosols (SOA) in the atmosphere (Ehn et al., 2014). Because of the significant amount of α-pinene in the atmosphere, we investigated the oxidation
products of α-pinene.

In our setup we used parallel plate DMA (SEADM; (de la Mora et al., 2006)) at mobility resolution of about 80 coupled with APITOF-MS (Tofwerk AG; (Junninen et al., 2010)) and a flow tube system. A DMA can be used to measure the electrical mobility of the molecule or cluster and mass
spectrometer to measure the mass of those clusters. Based on the mass the chemical composition of the cluster can be determined.

The electrospray solution is sprayed through a thin capillary into the chamber through which neutral
sample is passed through. As a solute we used NaNO3 , NaI, LiCl and CH3CO2K dissolved in
methanol all charged in positive and negative mode. Particles that are charged by reagent ions are
led into the DMA via narrow inlet slit.

α-pinene was evaporated into a carrier gas flow and then oxidized using ozone produced from synthetic air with UV-light. The oxidation products are detected by charging them with ions sprayed from the electrospray solution and then directed into the DMA chamber. α-pinene oxidation products of oxidation state C10H16O2−7 were detected with almost all charger ions. Also, other products with different amounts of carbon and hydrogen were detected. Measurements made in negative mode were much more clear and because of this concentrated to examine them.

Mobility provides information on the structure of the compound. One cluster can have multiple peaks in the mobility spectrum if it has multiple different structures. In the mobility spectrum of C10H16O3 charged with NO3− we observe two peaks clearly separate mobility peaks that likely
correspond to two different structural isomers of the compound. We will present analysis of the mobility-mass measurements of α-pinene oxidation products, from where structural information will be obtained when combined to chemical reaction pathways and modeling of the electrical mobilities from the calculated structures.

REFERENCES
Ehn, M. et al, (2014). A large source of low-volatility secondary organic aerosol. (Nature, 506(7489), 476-+.
doi:10.1038/nature13032).

Fernández de la Mora et al, (2006). The potential of differen-
tial mobility analysis coupled to MS for the study of very large singly and multiply chargedproteins and protein complexes in the gas phase.
doi:10.1002/biot.200600070). (Biotechnology Journal, 1(9), 988-997.

Junninen, H. et al, (2010). A high-resolution mass spectrometer
to measure atmospheric ion composition. (Atmospheric Measurement Techniques, 3(4), 1039-
1053. doi:10.5194/amt-3-1039-2010).

How to cite: Skyttä, A., Ahonen, L., Cai, R., and Kangasluoma, J.: Oxidation products of alpha-pinene and their electrical mobilities, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8525, https://doi.org/10.5194/egusphere-egu2020-8525, 2020.

Highly oxygenated organic molecules (HOM) are formed in the atmosphere by autoxidation, i.e. peroxy radicals can undergo 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.
As other RO₂, HOM-RO₂ are terminated by reactions with RO₂, HO₂ and NO_x and in addition form efficiently stable accretion products. In this study, three noticeable effects on HOM formation were found by introducing NO_x in the photochemical system of monoterpenes. One effect is formation of highly oxygenated organic nitrates (HOM-ON) with sufficiently low vapor pressures allowing significant contributios to SOA formation. The second one is dimer suppression, because of competing dimer pathway (HOM-RO₂·+ RO₂·) and organic nitrate pathway (HOM-RO₂·+ NO_x). Thirdly, the reaction between peroxy radicals and NO increases 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 peroxy radicals, that eventually are higher oxygenated.

Because of their low volatility, HOM play a crucial role in new particle formation and secondary organic aerosol (SOA) formation. 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.

How to cite: Kang, S., Mentel, T., Pullinen, I., Springer, M., Kleist, E., Schmitt, S., Wu, C., Proff, S., Vereecken, L., Wildt, J., and Kiendler-Scharr, A.: The effect of NOx on formation of Highly Oxidized Multifunctional Molecules and SOA formation in photochemical system of α-pinene and β-pinene, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8308, https://doi.org/10.5194/egusphere-egu2020-8308, 2020.

Acetylperoxy radicals (CH₃C(O)O₂) play an important role in the tropospheric chemistry. They are produced by the photooxidation of most emitted biogenic non-methane hydrocarbons. Recent studies show that the CH₃C(O)O₂ + HO₂ reaction, which is the most important tropospheric loss reaction of acetylperoxy radicals in regions that are dominated by biogenic emissions (low NO emissions), does not only lead to radical chain terminating products but can also regenerate OH. The competing secondary chemistry, e. g., the CH₃C(O)O₂ self-reaction, complicate kinetic measurements. The detection of acetylperoxy radicals in previous kinetic laboratory studies was mainly done in the UV region. However, the spectral overlap of different peroxy species in this region is prone to systematic errors in the quantitative detection. These complications can be avoided, if acetylperoxy radicals are detected by absorption in the near IR.

In our work, the near infrared CH₃C(O)O₂ spectrum was measured in the spectral ranges from 6094 cm^-1 to 6180 cm^-1 and 6420 cm^-1 to 6600 cm^-1. CH₃C(O)O₂ radicals were generated by pulsed photolysis of a acetaldehyde/Cl₂/O₂ mixture at 351 nm and were subsequently detected by time-resolved continuous-wave cavity ring-down spectroscopy (cw-CRDS). Experiments were done at 67 hPa in synthetic air and helium. The absorption cross sections of eight discrete absorption lines were determined relative to the absorption cross section of HO₂, which has previously been reported.

How to cite: Rolletter, M., Assaf, E., Assali, M., Fuchs, H., and Fittschen, C.: The Absorption Spectrum and Absolute Absorption Cross Sections of Acetylperoxy Radicals, CH3C(O)O2 in the near IR, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5733, https://doi.org/10.5194/egusphere-egu2020-5733, 2020.

β-Caryophyllene is the most common and abundant of the sesquiterpenes emitted into the atmosphere (Duhl et al., 2008). Although sesquiterpene emission rates were estimated to be only 9–16% of the total terpene emissions (Duhl et al., 2008), they are more reactive and larger in size than monoterpenes. Consequently, their aerosol mass yields are large and result in a significant contribution to the SOA budget in the atmosphere (Tasoglou and Pandis, 2015). Therefore, we studied the composition of both gas and particle phases as well as phase partitioning of SOA from ozonolysis of β-caryophyllene in presence and absence of NOx at five temperatures (213 K, 243 K, 273 K, 298 and 313 K) in the AIDA aerosol simulation chamber. This work focusses on the characterization of the SOA by mass spectrometry employing a FIGAERO-HR-TOF-CIMS operated with iodide ions and a HR-TOF-AMS (both Aerodyne Inc.). Particle phase analysis shows three groups of compound masses with m/z 240-400, (C_5-16),  (m/z 400-560, (C_20-34), and m/z 560-680, (C_35-40) classified as monomers, dimers, and trimers, respectively.  Trimeric compounds were observed preferentially in SOA formed at higher temperatures (273 K, 298 K, 313 K), while only monomeric and dimeric compounds were detected at lower temperatures (243 K and 213 K). Interestingly, dimeric compounds, including C_xH_yO_z and C_xH_yO_zN₁, contribute more to SOA mass for the lower temperatures. Comparing volatility distributions for the five different temperatures using the Volatility Basis Set (VBS) and thermal desorption information from FIGAERO-CIMS (298-473 K) we find more compounds with lower volatility for lower SOA formation temperatures. This contribution will discuss the volatility distributions obtained with and without NOx as well as the abundance of specific reaction products.

How to cite: Gao, L., Vallon, M., Song, J., Huang, W., Leisner, T., and Saathoof, H.: Chemical composition and volatility distribution of SOA formed by ozonolysis of β-caryophyllene between 213-313 K, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4415, https://doi.org/10.5194/egusphere-egu2020-4415, 2020.

Nitrate radical (NO₃) oxidation of biogenic volatile organic compounds (BVOCs) represents one of the most important interactions between anthropogenic emissions related to combustion and natural emissions from the biosphere. The functionalization process during this oxidation process leads to the formation of multifunctional compounds such as organic nitrates (ON). ON account for a significant fraction of total organic aerosols (OA) in ambient air, which influence atmospheric chemistry process, air quality, and climate through regional and global budgets for reactive nitrogen (particularly ON), ozone, and OA formation. Despite the significance of this process in atmospheric chemistry, the climatic effect of SOA from this process is undefined, largely due to a lack of knowledge about their optical properties with respect to their chemical composition. In this study, we generated SOA from NO₃ radical oxidation of a series BVOCs including isoprene, monoterpenes, and sesquiterpenes followed by photo-chemically aging in oxidation flow reactor (OFR/PAM). The chemical composition of the SOA was characterized online by high-resolution time-of-flight mass spectrometer (HR-Tof-AMS) and off-line by ultra-high-performance liquid chromatography (HPLC) coupled with photodiode array (PDA) detector coupled to a high-resolution Orbitrap mass spectrometer with a standard electrospray ionization (ESI) source (HPLC-PDA-HRMS). The UV-visible wavelength-resolved refractive index of the SOA, which is essential to understand their radiative forcing, was retrieved by measuring the light extinction using a novel broadband cavity-enhanced spectrometer (BBCES, 315-700 nm). We found that the SOA contain a large fraction of highly oxygenated ON, consisting of monomers and oligomers with single and multiple nitrate groups, which formed through bimolecular and unimolecular reactions. Strong absorption was detected in the UVA range which was attributed to the ON. The influence of the initial BVOCs/NO₃ ratio and the transition from nighttime oxidation to daytime aging on the SOA optical properties will be discussed. We will highlight the link between the SOA optical properties evolution and the chemical composition transformation with respect to the highly oxygenated ON formation and its atmospheric fate upon daytime photochemical aging.

How to cite: Rudich, Y., He, Q., Laskin, A., and Brown, S.: Optical Properties of Secondary Organic Aerosol from Nitrate Radical Oxidation of Biogenic Volatile Organic Compounds: The Role of Highly Oxygenated Organic Nitrates , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4136, https://doi.org/10.5194/egusphere-egu2020-4136, 2020.

Experiments at a set of atmospherically relevant conditions were performed in the atmospheric simulation chamber SAPHIR, investigating the oxidation of isoprene by the nitrate radical (NO3). A comprehensive set of instruments detected trace gases, radicals, aerosol properties and hydroxyl (OH) and NO3 radical reactivity. The chemical conditions in the chamber were varied to change the fate of the peroxy radicals (RO2) formed after the reaction between NO3 and isoprene, and seed aerosol of varying composition was added to initiate gas/aerosol partitioning. This presentation discusses observed gas/aerosol partitioning of the major organic nitrate products and summarizes the observations of secondary organic aerosol yield.

How to cite: Fry, J. L., Brownwood, B., Hohaus, T., Turdziladze, A., Carlsson, P., Tsiligiannis, E., Hallquist, M., Novelli, A., and Fuchs, H. and the NO3Isop Campaign at SAPHIR chamber, August 2018: New insights into secondary organic aerosol from nitrate oxidation of isoprene in the atmospheric simulation chamber SAPHIR, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11986, https://doi.org/10.5194/egusphere-egu2020-11986, 2020.

Experiments at a set of atmospherically relevant conditions were performed in the simulation chamber SAPHIR, investigating the oxidation of isoprene by the nitrate radical (NO₃)_. An extremely comprehensive set of instruments detected trace gases, radicals, aerosol properties and hydroxyl (OH) and NO₃ radical reactivity. The chemical conditions in the chamber were varied to change the fate of the peroxy radicals (RO₂) formed after the reaction between NO₃ and isoprene from either mainly recombining with other RO₂ or mainly reacting with hydroperoxyl radicals (HO₂). These major atmospheric pathways for RO₂ radicals lead to the formation of organic nitrate compounds which then have different atmospheric fates. The experimental concentration profiles are compared to box model calculations using both the current Master Chemical Mechanism (MCM) as well as recently available literature data alongside new quantum chemical calculations. The discussion here focusses on the resulting RO₂ distribution and deviations in the predictions of early products and total alkyl nitrate yields for the different chemical conditions. Preliminary results for instance show too high night time losses of alkyl nitrates due to ozonolysis in the current MCM. 

How to cite: Carlsson, P., Dewald, P., Shenolikar, J., Friedrich, N., Crowley, J., Brown, S., Bernard, F., Zhou, L., Fry, J., Brownwood, B., Hallquist, M., Tsiligiannis, E., Kangmin, X., Holzinger, R., Fuchs, H., Vereecken, L., Novelli, A., Bohn, B., Rohrer, F., and Mentel, T. and the NO3-Isoprene Campaign at Saphir: New insights into the gas-phase oxidation of isoprene by the nitrate radical from experiments in the atmospheric simulation chamber SAPHIR, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11196, https://doi.org/10.5194/egusphere-egu2020-11196, 2020.

Isoprene is the major volatile organic compound that is released into the environment via biogenic emissions and its oxidation can result in formation of secondary organic aerosol (SOA). Although isoprene emission occurs mainly at daytime, it can accumulate at nighttime and be oxidized by the nitrate radical (NO₃) to form organic nitrates that can partition to the particle phase. A detailed understanding of the reaction between isoprene and NO₃ is thus required to predict its role in e.g. NO_X lifetimes and SOA formation.

The reaction between NO₃ and isoprene was investigated under varying experimental conditions (high or low RO₂/HO₂, temperature, humidity, seed aerosols) during the NO3ISOP campaign at the atmospheric simulation chamber SAPHIR of the research centre in Jülich (Germany). Direct measurement of the NO₃ reactivity was carried out with means of a flowtube coupled to a cavity-ring-down spectroscopy (FT-CRDS) setup which enabled the evolution of the NO₃ lifetime during the isoprene oxidation process to be monitored.

By comparing direct NO₃ reactivity measurements with those calculated from VOC mixing ratios and those calculated from a stationary-state analysis we identify the contributions of isoprene, secondary oxidation products and peroxy radicals to NO₃ losses.

How to cite: Dewald, P., Shenolikar, J., Friedrich, N., Rohrer, F., Tillmann, R., Reimer, D., Xu, K., Holzinger, R., Bernard, F., Zhou, L., Brown, S., Fuchs, H., and Crowley, J.: Chamber Studies of NO3 reactivity during the oxidation of isoprene, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4597, https://doi.org/10.5194/egusphere-egu2020-4597, 2020.

Theoretical, laboratory and chamber studies have shown fast regeneration of hydroxyl radical (OH) in the photochemistry of isoprene largely due to previously disregarded unimolecular reactions which were previously thought not to be important under atmospheric conditions. Based on early field measurements, nearly complete regeneration was hypothesized for a wide range of tropospheric conditions, including areas such as the rainforest where slow regeneration of OH radicals is expected due to low concentrations of nitric oxide (NO). In this work the OH regeneration in the isoprene oxidation is directly quantified for the first time through experiments covering a wide range of atmospheric conditions (i.e. NO between 0.15 and 2 ppbv and temperature between 25 and 41°C) in the atmospheric simulation chamber SAPHIR. These conditions cover remote areas partially influenced by anthropogenic NO emissions, giving a regeneration efficiency of OH close to one, and areas like the Amazonian rainforest with very low NO, resulting in a surprisingly high regeneration efficiency of 0.5, i.e. a factor of 2 to 3 higher than explainable in the absence of unimolecular reactions. The measured radical concentrations were compared to model calculations and the best agreement was observed when at least 50% of the total loss of isoprene peroxy radicals conformers (weighted by their abundance) occurs via isomerization reactions for NO lower than 0.2 parts per billion (ppbv). For these levels of NO, up to 50% of the OH radicals are regenerated from the products of the 1,6 α-hydroxy-hydrogen shift (1,6-H shift) of Z-δ-RO₂ radicals through photolysis of an unsaturated hydroperoxy aldehyde (HPALD) and/or through the fast aldehyde hydrogen shift (rate constant ~10 s^-1 at 300K) in di-hydroperoxy carbonyl peroxy radicals (di-HPCARP-RO₂), depending on their relative yield. The agreement between all measured and modelled trace gases (hydroxyl, hydroperoxy and organic peroxy radicals, carbon monoxide and the sum of methyl vinyl ketone, methacrolein and hydroxyl hydroperoxides) is nearly independent on the adopted yield of HPALD and di-HPCARP-RO₂ as both degrade relatively fast (< 1 h), forming OH radical and CO among other products. Taking into consideration this and earlier isoprene studies, considerable uncertainties remain on the oxygenated products distribution, which affect radical levels and organic aerosol downwind of unpolluted isoprene dominated regions.

How to cite: Novelli, A., Vereecken, L., Bohn, B., Dorn, H.-P., Gkatzelis, G., Hofzumahaus, A., Holland, F., Reimer, D., Rohrer, F., Rosanka, S., Taraborrelli, D., Tillmann, R., Wegener, R., Yu, Z., Kiendler-Scharr, A., Wahner, A., and Fuchs, H.: Importance of isomerization reactions for the OH radical regeneration from the photo-oxidation of isoprene investigated in the atmospheric simulation chamber SAPHIR, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5475, https://doi.org/10.5194/egusphere-egu2020-5475, 2020.

The oxidation of most organic matter emitted to the atmosphere proceeds by radical reaction steps, where peroxy radicals, ROO^•, are critical intermediates formed by addition of O₂ molecules to carbon-based radicals. The chemistry of these RO₂ radicals in high-NOx conditions is well-known, forming alkoxy radicals and NO₂. In low-NOx and pristine conditions, the RO₂ radicals react with HO₂ and other R'O₂ radicals, but can have a sufficiently long lifetime to also undergo unimolecular reactions. Hydrogen atom migration, forming a hydroperoxide (-OOH) and a new peroxy radical site after addition of an additional O₂ on the newly formed radical site, has been studied extensively in some compounds, such as isoprene where it was shown to be the a critical step in OH radical regeneration. RO₂ ring closure reactions have likewise been studied, where for β-pinene it has been shown to be a critical step governing the yield of the decomposition products such as acetone and nopinone.

Despite the interest in RO₂ unimolecular reactions, and the potential impact on atmospheric chemistry, no widely applicable structure-activity relationships (SARs) have been proposed to allow systematic incorporation of such unimolecular reactions in gas phase atmospheric kinetic models. In this work, we present a series of systematic theoretical predictions on the site-specific rate coefficients for such reactions for a wide range of molecular substitutions. Combined with extensive literature data this allows for the formulation of a SAR for RO₂ unimolecular reactions, covering aliphatic, branched, and unsaturated RO₂ with oxo, hydroxy, hydroperoxy, nitrate, carboxylic acid, and ether substitutions.

The predictions are compared to experimental and theoretical data, including multi-functionalized species. Though some molecular classes are well represented in the training set (e.g. aliphatic RO₂), other classes have little data available and additional work is needed to enhance and validate the reliability of the SAR. Direct experimental data is scarce for all RO₂ classes. The fastest H-migrations are found to be for unsaturated RO₂, with the double bond outside the H-migration TS ring. Ring closure of unsaturated RO₂ are likewise fast if the product radical carbon is exocyclic to the newly formed peroxide ring.

How to cite: Vereecken, L., Vu, G. H. T., and Nguyen, H. M. T.: Structure-activity relationships for unimolecular reactions of peroxy radicals, RO2, at atmospheric temperatures, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2922, https://doi.org/10.5194/egusphere-egu2020-2922, 2020.

Atmospheric aerosols impact climate and health. Most of the smallest atmospheric nanoparticles are formed by oxidation of volatile organic compounds (VOC) and subsequent condensation of resulting low-volatile vapors. Biogenic terpenes are the largest atmospheric secondary organic aerosol (SOA) source, and among these, a-pinene likely the single most important compound.

 Recently, autoxidation changed the paradigm of long processing time-scales in the formation of SOA [1, 2]. Previous experiments with cyclic unsaturated compounds have indicated the autoxidation to be very rapid, forming compounds with even 10 O-atoms infused to the carbon structure in a few seconds timeframe [3-6]. Berndt et al. noted that the whole process was apparently finished already at about 1.5 seconds reaction time in cyclohexene ozonolysis initiated autoxidation, indicated by the “frozen” peroxy radical product distribution beyond this reaction time [4].

Here we performed sub-second time-scale flow reactor experiments of a-pinene ozonolysis initiated autoxidation under ambient atmospheric conditions to constrain the timeframe needed to form the first highly-oxidized reaction products, and to inspect the peroxy radical dynamics at significantly shorter reaction times than have been previously possible. The shortest achievable reaction time was around 0.1 seconds and was enabled by the new Multi-scheme chemical IONization (MION) inlet setup [7]. Nitrate and bromide were used as reagent ions in this work.

References:

How to cite: Rissanen, M., Barua, S., Krechmer, J., Kurtén, T., and Iyer, S.: a-pinene autoxidation at sub-second time-scales, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9638, https://doi.org/10.5194/egusphere-egu2020-9638, 2020.

Highly oxygenated molecules (HOMs) are important atmospheric oxidation products that may contribute to new particle formation and initial particle growth. Thousands of such compounds were quantified in both winter and summer of 2016 in Beijing by using online nitrate ion chemical ionization time-of-flight mass spectrometry. Positive-matrix factorization of the time series of the high-resolution mass spectra identified at least 10 major groups of gaseous HOMs in Beijing. We compared these PMF factors with the HOMs produced in a Potential Aerosol Mass (PAM) flow reactor in the laboratory from the oxidation of typical biogenic and aromatic precursors under various oxidation conditions. Our results show that four of the ten PMF factors perhaps correspond to biogenic precursors, and another four factors are likely related to aromatic precursors. The chemistry of these aromatic HOMs are discussed based on the results from the PAM experiments.

How to cite: Chen, Q., Cheng, X., Li, Y., Zheng, Y., Liao, K., Huang, G., Liu, Y., Zhu, T., and Canagaratna, M. R.: Highly oxygenated molecules and their chemistry in polluted urban environments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4329, https://doi.org/10.5194/egusphere-egu2020-4329, 2020.

Oxidation of volatile organic compounds (VOC) by ozone (O₃), hydroxyl radicals (OH) and nitrogen oxide radicals (NO₃, NOx) reduces their volatility and leads to the formation of secondary organic aerosols (SOA) through gas-particle partitioning. Recent studies have shown that monoterpene (C₁₀H₁₆) oxidation products can participate in all stages of aerosol formation, especially in forested boreal environments. However, deposition of these semi-volatile and (extremely) low-volatility organic compounds (SVOC, LVOC, ELVOC) to surfaces in the canopy directly competes with the gas-particle partitioning and has a substantial effect (~50%) on organic aerosol loading. Hence understanding the fate of these oxidation products is crucial in determining the organic aerosol budget and thereby constraining their contribution to climate-relevant processes such as new-particle formation and cloud formation.

Oxidation products of monoterpenes were measured at the station for measuring ecosystem atmosphere relations (SMEAR II), a boreal forest research station in Hyytiälä, Finland, in spring/summer 2019. The forest is dominated by Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies (L.) H. Karst) which are well known high monoterpene emitters. Eddy covariance (EC) flux measurements of oxygenated organic compounds in the gas phase were performed using an iodide-adduct high-resolution time-of-flight chemical ionization mass spectrometer (I-CIMS) with high frequency (5 Hz) co-located with a sonic anemometer (METEK USA-1) on a tower, 35 m above the forest floor. The ion-molecule reaction (IMR) chamber of I-CIMS was actively humidified to mitigate the dependence of the sensitivity of the measurements on the ambient relative humidity. The EC data were analysed following standard correction procedures like lag correction, coordinate rotation and uncertainty analysis. VOCs and oxygenated VOCs were also measured at ground level using a Vocus proton-transfer-reaction time-of-flight mass spectrometer (Vocus PTR-MS), which is sensitive also to the majority of compounds measured by I-CIMS.

We present the first continuous I-CIMS dataset at high time resolution (5 Hz) from a tall tower and calculate the Eddy covariance fluxes of a wide range of monoterpene oxidation products during the primary plant-growth season in a boreal forest. Bidirectional fluxes for formic acid (HCOOH) were observed at a higher temporal resolution than reported in earlier studies. We found an increasing trend in the deposition velocity for heavier monoterpene oxidation products which enables us to constrain the net flow of organics between the atmosphere and the canopy layer using the continuity/mass balance equation. When coupled to ground-based measurements using Vocus-PTR, our EC flux measurements will give further insight about the abundance of organics above the canopy vs near ground-level. We also plan to integrate our observations with a chemical transport model containing details of monoterpene oxidation chemistry (ADCHEM) to simulate the sources and sinks and to derive parameterizations for representing the dry deposition rates of monoterpene oxidation products in the boreal forested environments.

How to cite: Vettikkat, L., Ylisirniö, A., Pullinen, I., Feijó Barreira, L. M., Miettinen, P., and Schobesberger, S.: Eddy covariance (EC) fluxes of monoterpene oxidation products from a boreal forest canopy, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15188, https://doi.org/10.5194/egusphere-egu2020-15188, 2020.

Recent developments in mass spectrometry have brought huge advancements to the field of atmospheric science. For example, mass spectrometers are now able to detect ppq-level (10^-15) concentrations of both clusters and precursor vapours in atmospheric samples (Junninen et al., 2010; Jokinen et al., 2012), as well as directly explore the chemistry of new particle formation (NPF) in the atmosphere (Kulmala et al., 2014; Bianchi et al., 2016; Ehn et al., 2014). One of the most common mass spectrometers used to measure online cluster composition and concentration in the atmosphere is the Atmospheric Pressure interface Time Of Flight Mass Spectrometer (APi-TOF MS).
Identification of atmospheric molecular clusters and measurement of their concentrations by APi-TOF may be affected by systematic error due to possible decomposition of clusters inside the instrument. Indeed, the detection process in the APi-TOF involves energetic interactions between the carrier gas and the clusters, possibly leading to their decomposition, and thus altering the measurement results.
Here we use a theoretical model to study in detail the decomposition of clusters involving so-called Highly-Oxygenated organic Molecules (HOM), which have recently been identified as a key contributor to NPF (Bianchi et al., 2019). HOM are molecules formed in the atmosphere from Volatile Organic Compounds (VOC). Some VOC with suitable functional groups can undergo an autoxidation process involving peroxy radicals, generating polyfunctional low-volatility vapors (i.e. HOM) that subsequently condense onto pre-existing particles. 
Our study involves a specific kind of representative HOM (C₁₀H₁₆O₈) in the APi. This elemental composition corresponds to one of the most common mass peaks observed in experiments on ozone-initiated autoxidation of α-pinene, which also fulfills the “HOM” definition of Bianchi et al. (2019). The precise molecular structure was adopted from Kurtén et al. (2016), and corresponds to the lowest-volatility structural isomer of the three C₁₀H₁₆O₈ compounds investigated in that study.
The main scope of this work is to determine to what extent we are able to perform measurements of atmospheric cluster concentrations using APi-TOF mass spectrometers. More specifically, we want to determine whether decomposition can possibly be responsible for the lack of observations of some HOM-containing clusters in an APi-TOF. Here, we predict both an upper bound for decomposition energy necessary for decomposition in the APi-TOF, and a lower bound for new-particle formation in the atmosphere given realistic vapor concentrations.
Our results show that decomposition is highly unlikely for the considered clusters, provided their bonding energy is large enough to allow formation in the atmosphere in the first place.

How to cite: Zanca, T., Kubečka, J., Zapadinsky, E., Passananti, M., Kurtén, T., and Vehkamäki, H.: HOM cluster decomposition in APi-TOF mass spectrometers, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10708, https://doi.org/10.5194/egusphere-egu2020-10708, 2020.

Chemical characterization of oxygenated organic compounds in gas-phase and particle-phase in the Pearl River Delta using Iodide-CIMS with FIGAERO

Chenshuo Ye¹, Yi Lin², Zelong Wang², Tiange Li², Caihong Wu², Chaomin Wang², Weiwei Hu³, Shan Huang², Wei Song³, Xinming Wang³, Bin Yuan²*, Min Shao^2,1**

1 College of Environmental Sciences and Engineering, Peking University, Beijing

2 Institute for Environmental and Climate Research, Jinan University, Guangzhou

3 Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou

* e-mail: byuan@jnu.edu.cn

** e-mail: mshao@pku.edu.cn

Abstract

The Pearl River Delta region (PRD) is a highly industrialized and urbanized area in southeastern China, strongly influenced by both anthropogenic and biogenic emissions. The atmospheric processes in PRD involves the interactions between organics and inorganics, anthropogenic pollutants and natural emissions, leading to the formation of various  secondary products. The iodide chemical ionization time-of-flight mass spectrometer installed with FIGAERO inlet was applied at an urban site in PRD region during the autumn of 2018, to measure a number of oxygenated organic compounds in both gas phase and particle phase. Using the dataset, we find: (1) Oxygenated organic compounds and N-containing organics accounted for the majority of detected species. The most abundant organics were formic acid and multifunctional organics containing 3-6 oxygens. Nitrophenols, dinitrophenols and organic nitrates derived from isoprene and monoterpenes made a substantial contribution to N-containing organics. (2) Isoprene oxidation products peaked in the afternoon, while monoterpene oxidation products and oxidized aromatics had various diurnal patterns due to their different chemical pathways during their formation processes. (3) We detected many biomass burning tracers previously described in the literature, among which levoglucosan, along with other monosaccharide derivatives and serval guaiacol derivatives, were highly correlated with each other, with their concentrations peaked during the harvest season for local crops. (4) Photochemical processes generally create smaller products of higher oxidation states, whereas nighttime chemistry plays an important role in producing larger molecule but less oxidized products particularly nitrogen-containing oxygenated organics. The variations of dominant atmospheric processes as well as the emissions of precursors lead to the variations of the bulk properties of secondary products.

How to cite: Ye, C.: Chemical characterization of oxygenated organic compounds in gas-phase and particle-phase in the Pearl River Delta using iodide-CIMS with FIGAERO, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12127, https://doi.org/10.5194/egusphere-egu2020-12127, 2020.

 Abstract

Atmospheric particles are known to cause adverse health effects and premature deaths in European cities. To improve air quality, a detailed understanding of particle sources is thus essential in order to reduce their emissions. Secondary organic aerosols (SOA) produced from the oxidation of volatile organic compounds emitted by anthropogenic sources such as road vehicles and solid fuel combustion is an important air pollution source in urban areas. It is demonstrated that SOA contribute significantly to the atmospheric particle loading, and could even be the major contributor at specific locations. Yet, state of the art models are still not able to reproduce SOA formation despite recent advances. Clearly, further work is needed to improve our understanding of the processes related to SOA formation.

In this context, a field campaign has been conducted at a monitoring station in Cork City, Ireland during winter 2019 (26^th January to 8^th February). The chemical composition of organic compounds in both gas and particle phases was investigated online using a Time-of-Flight Chemical Ionisation Mass Spectrometer (ToF-CIMS) coupled with a Filter Inlet for Gases and Aerosols (FIGAERO). PM_2.5 concentration, ozone and nitrogen oxides (NO_x) were also monitored during the campaign, as well as meteorological parameters. Finally, air mass backward trajectories were computed using the HYSPLIT model.

A strong night-time air pollution event was observed during the field campaign, characterized by PM_2.5 concentrations up to 180 µg m^-3. Using iodide as reagent, the FIGAERO-ToF-CIMS detected hundreds of ions simultaneously in gas and particulate phases. Among the identified compounds were a range of well-known atmospheric tracers of solid fuel burning, including phenolic compounds such as guaiacol and catechol, and numerous oxygenated polycyclic aromatic hydrocarbons (OPAHs). A number of nitrated aromatic compounds were also detected. In this work, the gas/particle partitioning of some of these key compounds has been investigated to provide information on phase transfer of solid fuel emissions over time. The thermograms produced by the FIGAERO analysis are also used to determine the volatility of the species detected. Finally, the FIGAERO-ToF-CIMS data is used to explore the extent to which oxidation of the gaseous emissions by the nitrate radical (NO₃) leads to the formation of nitrated compounds in the particulate phase. This work thus provides unique insights into the night-time oxidation processes that can lead to SOA formation from anthropogenic sources.

Acknowledgments

This work was supported by the Irish Research Council (GOIPG/2017/1364) and by the European Union’s Horizon 2020 research and innovation programme (EUROCHAMP-2020, grant no. 730997; Marie Skłodowska-Curie grant agreement No. 751527).

How to cite: Kammer, J., O’Sullivan, N., Gomez Alvarez, E., Hellebust, S., and Wenger, J.: Characterization of gaseous and particulate phase organic compounds during a winter-time air pollution event, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18854, https://doi.org/10.5194/egusphere-egu2020-18854, 2020.

Organic aerosol (OA) constitutes a significant fraction of the atmospheric fine particulate matter that influences both air quality and climate. Secondary organic aerosol (SOA), which is formed through photo-oxidation of organic vapors in the atmosphere, is a major component of OA. There are some studies indicating the major role of vehicles emissions in SOA formation in urban cities of China. However, SOA formation is complex and uncertain.

Historically, the China fleet has been dominated by vehicles equipped with port-fuel injected (PFI), but the market share of vehicles equipped with gasoline direct injection engines (GDI) has increased dramatically. And 10% of renewable energy ethanol (E10) may be added to the gasoline of China market in the future. Go-PAM is one kind of potential aerosol mass for simulating SOA formation, which is designed and made by the University of Gothenburg.

In this study, we focus on the influence of ethanol content (0% or 10%), engine types (GDI or PFI) and different engine loads (idling or constant velocity) to the SOA formation potential from gasoline motor cars emissions. We exposed the diluted emissions to a range of oxidation (O₃ and OH) concentrations in the Go-PAM, resulting different OH exposures. We observed variations of different cases in SOA formation.

Firstly, compared to PFI engine, GDI engine at idling loading has larger SOA mass concentrations. The peak SOA production of PFI engine at idling load occurred at equivalent photochemical age (EPA) of 3.8 days, which peak point occurred at larger EPA (4.8 days) for GDI engines. Secondly, there is no large difference between E10 and gasoline. Thirdly, OA enhancement is more obvious at idling (about 30-180 times) than at constant velocity (about 3-4 times) whatever engine is used. Generally, densities of particles at size of 70nm,140nm and 200nm keep growing from about 1.25 up to 1.45 g/cm³.

The results of this study highlight the utility of Go-PAM for studying SOA formation potential from vehicle exhaust, and provide indications of the influence of ethanol content and different engines to SOA formation in China.

How to cite: Wang, H., Tang, R., Shen, R., Yu, Y., Liu, K., Tan, R., Zhang, W., Zhang, Z., Shuai, S., and Guo, S.: Secondary Organic Aerosol Formation from On-road Gasoline Vehicles in China, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8093, https://doi.org/10.5194/egusphere-egu2020-8093, 2020.

Unexpectedly high daytime concentrations of nitrous acid (HONO) measured by field observations cannot be explained by theoretical calculations, implying that there may be a missing source of HONO in the daytime (P_missing). The value of P_missing near the ground (P_Gmissing) is different from that measured higher in the atmosphere (P_Hmissing) according to previous field studies, but the contribution of the vertical P_missing profile in the atmospheric boundary layer (ABL) to air quality remains unknown. We derived a new formula P_Gmissing = 0.180 × J(NO₂) [ppb s^-1] based on field measurements near the ground, where J(NO₂) is the photolysis frequency of NO₂, and used the value of P_Hmissing inferred from Zeppelin measurements in the troposphere to parameterize P_missing in the ABL. This parameterization was incorporated into the Weather Research and Forecasting model with Chemistry (WRF-Chem) to quantify the vertical effects of P_missing on the concentrations of HONO, O₃ and secondary organic aerosols (SOAs) in eastern China. Our results showed that P_Gmissing and P_Hmissing together further narrowed the gap between the simulations and observations, leading to a daytime increase in HONO concentrations of about 160 ppt near the ground compared with P_Gmissing only, an increase in the daytime concentrations of O₃ of 8–37 ppb within the ABL in almost all of the studied domain in summer (1–19 ppb in winter and 4–21 ppb in autumn) and the largest hourly increase in the concentration of SOAs of 22.5 (18.6) μg m^-3 in winter (summer). The results indicated that HONO sources near the ground have a limited effect on the HONO concentrations in the upper ABL even in summer in the presence of strong convective activities, while the HONO increase in the upper ABL can affect the concentration of HONO near the ground. When P_Gmissing was inserted into each model layer in the ABL, the concentrations of HONO higher in the atmosphere were substantially overestimated, suggesting that observations of the vertical distribution of HONO in the ABL are required in polluted areas.

How to cite: Guo, Y., An, J., Zhang, J., and Qu, Y.: Effect of vertical parameterization of a missing daytime source of HONO on concentrations of HONO, O3 and secondary organic aerosols in eastern China, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6301, https://doi.org/10.5194/egusphere-egu2020-6301, 2020.