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Oxygenated organic molecules (OOMs) are dominated by the N-containing species in polluted urban environments. As N-containing OOMs, especially those with more than one nitrogen atom, prevail in the high m/z (mass-to-charge) range (m/z> 350 Th), unambiguous identification of N-containing OOMs is highly desirable for understanding of their formation processes, precursors and influencing factors. To achieve this, we applied an ultra-high-resolution chemical-ionization Orbitrap (CI-Orbitrap) in a field campaign and found that OOMs contain one (1N-OOMs), two (2N-OOMs) and three (3N-OOMs) nitrogen atoms comprised 50 %, 26 % and 4 %, respectively, of total OOMs. More interestingly, the fraction of 2N-OOMs increased with the increase in carbon number (nC) and was dominated by the ones derived from aliphatic precursors (2N-OOMAli, 64.2 %), indicating the importance of multistep oxidation. Plausible precursors of 2N-OOMs were aliphatics (2N-OOMAli, 64.2 %), aromatics (2N-OOMAro, 16 %) and monoterpenes (2N-OOMMT, 15.4 %). The absolute concentrations of 2N-OOMs were greatly affected by the pollution level for most cases. The 2N-OOMAli was the most abundant 2N-OOM, and its fraction even increased on the polluted day with an enhanced proportion of the ones with nC >10. While 2N-OOMAli and 2N-OOMAro were dominated by daytime photochemical production, nighttime NO3-initiated oxidation played a comparable role to the daytime photochemistry in the formation of 2N-OOMMT. The 2N-OOMAro species were of the highest oxygenation level, followed by 2N-OOMMT and 2N-OOMAli, which were affected by photochemistry and NOx concentrations. These results highlight the significant formation of 2N-OOMs and the influencing factors on their formation in polluted urban environments, where various volatile organic compound (VOC) precursors and atmospheric oxidants are present.

Cyclic volatile methyl Siloxanes (cVMS) are a group of silicon-based organic compounds of anthropogenic origin that are found ubiquitously in the ambient air. They find wide application in many industrial, automotive products, consumer, and personal care products such as paints, solvents, adhesives, cosmetics, polymers, etc. About 90% of the environmentally released cyclic siloxanes diffuse directly into the atmosphere. They are high–production volume chemicals with some having an annual production rate of 45 to 227 thousand tons worldwide. Siloxanes have been observed in both urban and rural areas. Among the different siloxanes emitted into the atmosphere hexamethylcyclotrisiloxane (D3), octamethylcyclotetrasiloxane (D4), and decamethylcyclopentasiloxane (D5) are of great importance due to their subsequent concentrations. It should be mentioned that siloxanes are not readily biodegradable which means they can persist in the environment for a long duration. The D4 species has already been identified as a potential persistent organic pollutant (PoPs). D4 and D5 are very persistent and bioaccumulative and have evinced or potential hazards to both humans and the environment.Taking advantage of the Vocus proton-transfer-reaction time-of-flight mass spectrometer (VOCUS PTR–TOF–MS) the D3, D4, and D5 species were measured in real-time downtown Paris in the June–July 2022 period. The measurements were performed during the ACROSS (Atmospheric ChemistRy Of the Suburban foreSt) field campaign at the urban site located at Université Paris Cité (48.83°N, 2.38°E) sampling at about 25 m above the ground level. The data collected during the summer 2022 showed that the mean concentrations were 0.099±0.009, 0.045±0.009, and 0.131±0.089 ppb for D3, D4, and D5, respectively. A strong diurnal cycle was observed for the different siloxanes. Concentrations for the D5 siloxane peaked in the morning with an averaged maximum of 0.298± 0.119 ppb while the 25th and the 75th quantile are 0.221 and 0.377 ppb, respectively. In the afternoon concentrations show a minimum value, followed by a slight increase during the night. The D5 is the most abundant siloxane species and shows the strongest diurnal cycle compared to the other two identified siloxanes. Observations during the campaign and the potential reaction products and sources of siloxanes will be discussed in this work.KEYWORDS: Siloxanes; Cyclic Siloxanes; D3,D4 and D5; Ambient Air; PoPs; ACROSS; HR- Mass Spectrometry; VOCUS PTR-Tof

The complex refractive index (CRI) is one of the key parameter driving aerosol spectral optical properties and direct radiative effects (DRE). Its value and spectral variation under different conditions, such as anthropogenic− and biogenic−dominated environments and anthropogenic−biogenic mixing situations, remains not fully understood. As a consequence, oversimplified representations of aerosol optical properties are generally used in climate models. Therefore, measurements of aerosol CRI in different environments and their inclusion in models are needed. The field observations from the ACROSS campaign, performed in June-July 2022 in the Ile de France region, are used in this study to deepen the knowledge of aerosol optical properties, aiming to improve the aerosol representation in the CHIMERE model and provide the best constraint for DRE simulations. Measurements obtained both at the Paris city center and the Rambouilllet rural forest sites during ACROSS are considered, in order to explore the CRI variability from anthropogenic−dominated to biogenic−dominated environments, including anthropogenic−biogenic mixing situations. The CRI retrievals at seven different wavelengths, performed by combining the Mie theory with optical and size distribution measurements, are representative of different atmospheric conditions, aerosol loadings as well as type and chemical compositions. In fact, the June-July 2022 period was characterized by highly diversified weather conditions: 1) two strong heatwaves, promoting SOA build-up and favoring the export of the Paris pollution plume towards the forest site; 2) Saharan dust events transported from the upper atmosphere to the ground; 3) biomass burning episode; 4) periods with reduced anthropogenic influence. The CRI retrievals under these different conditions and their link to particulate chemical composition is investigated. Hence, the CRI dataset presented here constitutes a unique dataset from which models can benefit to validate and constrain simulations and DRE estimations, under both urban and biogenic emissions influence. These data, in conjunction with those from the aircraft observations during ACROSS, are used to initialize and perform sensitivity studies on the aerosol DRE, using the CHIMERE−WRF coupled model, the OPTSIM model for the aerosol optical properties and the Rapid Radiative Transfer Model for GCMs (RRTMG).Keywords: Complex refractive index, direct radiative effect, aerosol mixing, urban, forest

Highly oxygenated organic molecules (HOMs) from α-pinene ozonolysis have been shown to be significant contributors to secondary organic aerosol (SOA), yet our mechanistic understanding of how the peroxy-radical-driven autoxidation leads to their formation in this system is still limited. The involved isomerisation reactions such as H-atom abstractions followed by O2 additions can take place on sub-second timescales in short-lived intermediates, making the process challenging to study. Similarly, while the end-products and sometimes radical intermediates can be observed using mass spectrometry, their structures remain elusive. Therefore, we propose a method utilising selective deuterations for unveiling the mechanisms of autoxidation, where the HOM products can be used to infer which C atoms have taken part in the isomerisation reactions. This relies on the fact that if a C−D bond is broken due to an abstraction by a peroxy group forming a −OOD hydroperoxide, the D atom will become labile and able to be exchanged with a hydrogen atom in water vapour (H2O), effectively leading to loss of the D atom from the molecule. In this study, we test the applicability of this method using three differently deuterated versions of α-pinene with the newly developed chemical ionisation Orbitrap (CI-Orbitrap) mass spectrometer to inspect the oxidation products. The high mass-resolving power of the Orbitrap is critical, as it allows the unambiguous separation of molecules with a D atom (mD=2.0141) from those with two H atoms (mH2=2.0157). We found that the method worked well, and we could deduce that two of the three tested compounds had lost D atoms during oxidation, suggesting that those deuterated positions were actively involved in the autoxidation process. Surprisingly, the deuterations were not observed to decrease HOM molar yields, as would have been expected due to kinetic isotope effects. This may be an indication that the relevant H (or D) abstractions were fast enough that no competing pathways were of relevance despite slower abstraction rates of the D atom. We show that selective deuteration can be a very useful method for studying autoxidation on a molecular level and likely is not limited to the system of α-pinene ozonolysis tested here.

Highly oxygenated organic molecules (HOMs) are a major source of new particles affecting Earth’s climate1,2. HOM production from the oxidation of volatile organic compounds (VOCs) occurs during both day and night, and can lead to new particle formation (NPF)3,4. However, NPF involving organic vapors has been reported much more often during daytime3-6 than during nighttime7,8. Here, we show that the nitrate radicals (NO3) - which arise predominantly at night – inhibit NPF during the oxidation of monoterpenes based on three lines of observational evidence: NPF experiments in the CLOUD chamber at CERN; radical chemistry experiments using an oxidation flow reactor; and field observations in a wetland that occasionally exhibits nocturnal NPF. Nitrooxy-peroxy radicals formed from NO3 chemistry suppress the production of ultra-low volatility organic compounds (ULVOCs) responsible for biogenic NPF, which are covalently bound RO2 dimer association products. The ULVOC yield of α-pinene in the presence of NO3 is one-fifth of that resulting from ozone chemistry alone. Even trace amounts of NO3 radicals, at sub parts per trillion level, suppress the NPF rate by a factor of 4. Ambient observations further confirm that when NO3 chemistry is involved, monoterpene NPF is completely turned off. Our results explain the frequent absence of nocturnal biogenic NPF in monoterpene-rich environments.

Secondary organic aerosol, formed through atmospheric oxidation processes, plays an important role in affecting climate and human health. In this study, we conducted a comprehensive campaign in the megacity of Shanghai during the 2019 International Import Expo (EXPO), with the first deployment of a chemical ionization─Orbitrap mass spectrometer for ambient measurements. With the ultrahigh mass resolving power of the Orbitrap mass analyzer (up to 140,000 Th/Th) and capability in dealing with massive spectral data sets by positive matrix factorization, we were able to identify the major gas-phase oxidation processes leading to the formation of oxygenated organic molecules (OOM) in Shanghai. Nine main factors from three independent sub-range analysis were identified. More than 90% of OOM are of anthropogenic origin and60% are nitrogen-containing molecules, mainly dominated by the RO

International audience

Oxygenated organic molecules (OOMs) are dominated by the N-containing species in polluted urban environment. As N-containing OOMs, especially those with more than one nitrogen atoms, prevailed in the high m/z range (m/z > 350Th), unambiguous identification of N-containing OOMs is highly desirable for understanding of their formation processes, precursors and influencing factors. To achieve this, we applied an ultra-high-resolution chemical ionization-orbitrap (CI-Orbitrap) in a field campaign and found that OOMs contain one (1N-OOMs), two (2N-OOMs) and three (3N-OOMs) nitrogen atoms respectively comprised 50 %, 26 % and 4 % of total OOMs. More interestingly, the fraction of 2N-OOMs increased with the increase of carbon number (nC) and were dominated by the ones derived from aliphatic precursors (2N-OOMAli, 64.2 %), indicating the importance of multistep oxidation. Plausible precursors of 2N-OOMs were aliphatics (2N-OOMAli, 64.2 %), aromatics (2N-OOMAro, 16 %), and monoterpenes (2N-OOMMT, 15.4 %). The 2N-OOMAli was the most abundant 2N-OOMs and its fraction even increased in the polluted day with enhanced proportion of the ones with nC>10. While 2N-OOMAli and 2N-OOMAro were dominated by daytime photochemical production, nighttime NO3-initiated oxidation dominated the formation of 2N-OOMMT. 2N-OOMAro were of highest oxygenation level, followed by 2N-OOMMT and 2N-OOMAli, which were affected by photochemistry and NOx concentrations. These results highlight the significant formation of 2N-OOMs and the influencing factors, on their formation in polluted urban environment, where various VOC precursors and atmospheric oxidants present.

Orbitrap Fourier transform mass spectrometry coupled with chemical ionization (CI) is a new-generation technique for online analysis in atmospheric chemistry. The advantage of the high resolving power of the CI-Orbitrap has been compromised by its relatively low sensitivity to trace compounds (e.g.,10

Atmospheric aerosols are an important part of Earth’s climate system. Further, they have great influence on air quality and human health. Secondary organic aerosols (SOA) generated from the oxidation of volatile biogenic precursors are an important contributor to the global aerosol budget. Therefore, understanding new particle formation and their subsequent growth is critical for our ability to predict the atmospheric aerosol composition and global climate change.The coupling of a chemical ionization source with an Orbitrap mass spectrometer provides soft ionization and a high mass resolution for the on-line measurements of laboratory-generated SOA. Through heating of the aerosol, it is possible to measure both the gas phase and the vaporized particle phase. We use this technique to compare the chemical composition of particles produced from the oxidation of -pinene and -carene. Although exhibiting a very similar chemical structure, they differ greatly in their resulting SOA, in terms of particle size and number concentration when oxidized with ozone in smog-chamber experiments. These differences lie in their abilities to form characteristic SOA precursors, which depending on their chemical structure, promote either new particle formation or the growth of existing ones. The extremely low volatile organic compounds (ELVOCs) required for these processes are generally believed to be formed by gas phase chemistry. However, newly formed particles provide a unique nanoscale chemical environment that affects chemical reactions in the condensed phase and heterogeneous reactions on their surface, making them a potential source of ELVOCs as well. The increasing pressure inside the particles with decreasing diameter (Laplace pressure) favors bond-forming reactions. The lower viscosity in nanometer-sized particles further promotes reactions within the particle and increases reactivity at the particle surface such as heterogeneous oxidation. We show the particle size-dependent heterogeneous oxidation in a model system and ongoing work on other size-dependent reactions.

International audience; Northern China is regularly subjected to intense wintertime "haze events", with high levels of fine particles that threaten millions of inhabitants. While sulfate is a known major component of these fine haze particles, its formation mechanism remains unclear especially under highly polluted conditions, with state-of-the-art air quality models unable to reproduce or predict field observations. These haze conditions are generally characterized by simultaneous high emissions of SO2 and photosensitizing materials. In this study, we find that the excited triplet states of photosensitizers could induce a direct photosensitized oxidation of hydrated SO2 and bisulfite into sulfate S(VI) through energy transfer, electron transfer, or hydrogen atom abstraction. This photosensitized pathway appears to be a new and ubiquitous chemical route for atmospheric sulfate production. Compared to other aqueous-phase sulfate formation pathways with ozone, hydrogen peroxide, nitrogen dioxide, or transition-metal ions, the results also show that this photosensitized oxidation of S(IV) could make an important contribution to aerosol sulfate formation in Asian countries, particularly in China.

The formation of alkyl nitrates in various oxidation processes taking place throughout the diel cycle can represent an important sink of reactive nitrogen and mechanism for chain termination in atmospheric photo-oxidation cycles. The low-volatility alkyl nitrates (ANs) formed from biogenic volatile organic compounds (BVOCs), especially terpenoids, enhance rates of production and growth of secondary organic aerosol. Measurements of the NO3 reactivity and the mixing ratio of total alkyl nitrates (ΣANs) in the Finnish boreal forest enabled assessment of the relative importance of NO3-, O3- and OH-initiated formation of alkyl nitrates from BVOCs in this environment. The high reactivity of the forest air towards NO3 resulted in reactions of the nitrate radical, with terpenes contributing substantially to formation of ANs not only during the night but also during daytime. Overall, night-time reactions of NO3 accounted for 49 % of the local production rate of ANs, with contributions of 21 %, 18 % and 12 % for NO3, OH and O3 during the day. The lifetimes of the gas-phase ANs formed in this environment were on the order of 2 h due to efficient uptake to aerosol (and dry deposition), resulting in the transfer of reactive nitrogen from anthropogenic sources to the forest ecosystem.

Secondary organic aerosol (SOA) is known to impact both climate and air quality, yet molecular-level composition measurements remain challenging, hampering our understanding of SOA formation and evolution. Here, we reveal the importance of underestimated reaction pathways for the (trans)formation of SOA from monoterpenes, one of the largest SOA precursors globally. Utilizing mass spectrometric techniques to achieve a comprehensive characterization of molecular-level changes in the SOA, we were able to link the appearance of high-molecular weight (HMW) organic molecules to the concentration and level of neutralization of particulate sulfate. Interestingly, this oligomerization coincided with a decrease of highly oxygenated molecules (HOMs). Our findings highlight the role of particle-phase processing, and the underestimated importance of sulfate aerosol for monoterpene-SOA formation. The observations of these processes directly in the atmosphere reveal the need to account for the formation of HMW oligomers to fully understand the physicochemical properties of organic aerosol. Sulfate aerosols produced by industrial activities provide a key substrate for the formation of secondary organic aerosols (SOA) hundreds of km away. An international research group led by Matthieu Riva, now at the Universite de Lyon, applied detailed field observations and atmospheric transport modeling to show that SOA are produced when sulfate aerosols interact with monoterpenes — a volatile organic compound emitted by forests. Specifically, along the atmospheric transport pathway from the Russian pollution source to the research station in Finland, oxidized monoterpenes condensed onto sulfate particles, forming SOA. The team used laboratory experiments to confirm a powerful influence of sulfate levels on SOA formation. The research extends prior work showing similar pathways for isoprene-related SOA formation, and cements the importance of SOA formation through multiphase chemistry across of range of biogenically-derived atmospheric compounds.

International audience; Sulfate aerosol is responsible for a net cooling of the Earth's atmosphere due to its ability to backscatter light. Through atmospheric multiphase chemistry, it reacts with isoprene epoxydiols leading to the formation of aerosol and organic compounds, including organosulfates and high-molecular weight compounds. In this study, we evaluate how sulfate aerosol light backscattering is modified in the presence of such organic compounds. Our laboratory experiments show that reactive uptake of isoprene epoxydiols on sulfate aerosol is responsible for a decrease in light backscattering compared to pure inorganic sulfate particles of up to – 12% at 355 nm wavelength and – 21% at 532 nm wavelength. Moreover, while such chemistry is known to yield a core–shell structure, the observed reduction in the backscattered light intensity is discussed with Mie core–shell light backscattering numerical simulations. We showed that the observed decrease can only be explained by considering effects from the complex optical refractive index. Since isoprene is the most abundant hydrocarbon emitted into the atmosphere, and isoprene epoxydiols are the most important isoprene secondary organic aerosol precursors, our laboratory findings can aid in quantifying the direct radiative forcing of sulfates in the presence of organic compounds, thus more clearly resolving the impact of such aerosol particles on the Earth's climate.

Organic peroxy radicals (RO2) play a pivotal role in the degradation of hydrocarbons. The autoxidation of atmospheric RO2 radicals produces highly oxygenated organic molecules (HOMs), including low-volatility ROOR dimers formed by bimolecular RO2 + RO2 reactions. HOMs can initiate and greatly contribute to the formation and growth of atmospheric particles. As a result, HOMs have far-reaching health and climate implications. Nevertheless, the structures and formation mechanism of RO2 radicals and HOMs remain elusive. Here, we present the in-situ characterization of RO2 and dimer structure in the gas-phase, using online tandem mass spectrometry analyses. In this study, we constrain the structures and formation pathway of several HOM-RO2 radicals and dimers produced from monoterpene ozonolysis, a prominent atmospheric oxidation process. In addition to providing insights into atmospheric HOM chemistry, this study debuts online tandem MS analyses as a unique approach for the chemical characterization of reactive compounds, e.g., organic radicals., Organic peroxy radicals play a pivotal role in producing highly oxygenated organic molecules but the formation mechanisms remain elusive. Here, the authors show in-situ characterization of peroxy radicals and dimer structures in the gas-phase, using online tandem mass spectrometry analyses.

Organosulfates derived from monoterpene oxidation (MT-OSs) are ubiquitously abundant in atmospheric aerosol particles. However, to date, potential sampling artifacts in MT-OS detection and quantifi...

11 pags., 4 figs., New particle formation in the Arctic atmosphere is an important source of aerosol particles. Understanding the processes of Arctic secondary aerosol formation is crucial due to their significant impact on cloud properties and therefore Arctic amplification. We observed the molecular formation of new particles from low-volatility vapors at two Arctic sites with differing surroundings. In Svalbard, sulfuric acid (SA) and methane sulfonic acid (MSA) contribute to the formation of secondary aerosol and to some extent to cloud condensation nuclei (CCN). This occurs via ion-induced nucleation of SA and NH and subsequent growth by mainly SA and MSA condensation during springtime and highly oxygenated organic molecules during summertime. By contrast, in an ice-covered region around Villum, we observed new particle formation driven by iodic acid but its concentration was insufficient to grow nucleated particles to CCN sizes. Our results provide new insight about sources and precursors of Arctic secondary aerosol particles., The authors acknowledge European Research Council (GASPARCON, Grant no. 714621 and COALA, Grant no. 638703), Academy of Finland (Project nos. 296628, 306853, 317380, 316114, and 320094), INTERACT, European Regional Development Fund (project MOBTT42), Austrian Science Fund (FWF, Project J3951-N36), the European Union's Horizon 2020 Research and Innovation Program (Grant no. 689443) via project iCUPE (Integrative and Comprehensive Understanding on Polar Environments) and Project ERC-2016- COG 726349 CLIMAHAL, National Research Foundation (NRF) of the Ministry of Science, ICT and Future Planning (NRF-2018R1A2A1A19019281), Knut-and-Alice-Wallenberg Foundation within the Arctic Climate Across Scales (Project No. \,2016.0024), the Swedish EPA's (Naturvårdsverket) Environmental monitoring program (Miljöövervakning), the Swedish Research Council FORMAS (Project "Interplay between water, clouds and Aerosols in the Arctic," \# 2016-01427), Climate Change Tower – Integrated Project of the National Research Council of Italy and the National Interest Project by the Italian Minister of Education, University and Research (PRIN2007 and PRIN2009). The authors thank Department of Earth System Sciences and Technologies for the Environment, Department of Earth Sciences and Technology of the Environment of CNR and the doctoral program in atmospheric sciences at the University of Helsinki for financial support. Aarhus University acknowledge financial support from Danish Ministry of Environment and food and Ministry of Climate, Energy and Utilities by means of DANCEA.

International audience; Nighttime oxidation of biogenic volatile organic compounds (BVOCs) by nitrate radicals (NO3·) represents one of the most important interactions between anthropogenic and natural emissions, leading to substantial secondary organic aerosol (SOA) formation. The direct climatic effect of such SOA cannot be quantified because its optical properties and atmospheric fate are poorly understood. In this study, we generated SOA from the NO3·oxidation of a series BVOCs including isoprene, mono-terpenes, and sesquiterpenes. The SOA were subjected to comprehensive online and offline chemical composition analysis using high-resolution mass spectrometry and optical properties measurements using a novel broadband (315−650 nm) cavity-enhanced spectrometer, which covers the wavelength range needed to understand the potential contribution of the SOA to direct radiative forcing. The SOA contained a significant fraction of oxygenated organic nitrates (ONs), consisting of monomers and oligomers that are responsible for the detected light absorption in the 315−400 nm range. The SOA created from β-pinene and α-humulene was further photochemically aged in an oxidation flow reactor. The SOA has an atmospheric photochemical bleaching lifetime of >6.2 h, indicating that some of the ONs in the SOA may serve as atmosphere-stable nitrogen oxide sinks or reservoirs and will absorb and scatter incoming solar radiation during the daytime

Reactive uptake of isoprene epoxydiols (IEPOX), which are isoprene oxidation products, onto acidic sulfate aerosols is recognized to be an important mechanism for the formation of isoprene-derived secondary organic aerosol (SOA). While a mechanistic understanding of IEPOX-SOA formation exists, several processes affecting their formation remain uncertain. Evaluating mechanistic IEPOX-SOA models with controlled laboratory experiments under longer atmospherically relevant time scales is critical. Here, we implement our latest understanding of IEPOX-SOA formation within a box model to simulate the measured reactive uptake of IEPOX on polydisperse ammonium bisulfate seed aerosols within an environmental Teflon chamber. The model is evaluated with single-particle measurements of size distribution, volume, density, and composition of aerosols due to IEPOX-SOA formation at time scales of hours. We find that the model can simulate the growth of particles due to IEPOX multiphase chemistry, as reflected in increases of the mean particle size and volume concentrations, and a shift of the number size distribution to larger sizes. The model also predicts the observed evolution of particle number mean diameter and total volume concentrations at the end of the experiment. We show that in addition to the self-limiting effects of IEPOX-SOA coatings, the mass accommodation coefficient of IEPOX and accounting for the molar balance between inorganic and organic sulfate are important parameters governing the modeling of the IEPOX-SOA formation. Thus, models which do not account for the molar sulfate balance and/or diffusion limitations within IEPOX-SOA coatings are likely to predict IEPOX-SOA formation too high.

International audience; Nitrate anion (NO3–) is ubiquitous in the environment, and its photochemistry produces nitrous acid (HONO), a major source of tropospheric hydroxyl radical (OH). Enhanced HONO(g) emissions have been observed from NO3–(aq) photolysis in field studies, although the underlying reasons for this enhancement are debated. Here, we show that the enhancement is in part caused by changes in secondary nitrate anion photochemistry due to dissolved aliphatic organic matter (DAOM). Increased yields of superoxide radical (O2–) and HONO were observed when NO3– solutions (pH 6) were photolyzed in the presence of DAOM surrogates of varying solubility. In an additional experiment, nitrate titrated with additional DAOM showed a further simultaneous increase in the levels of O2–(aq) and HONO(g) with decreased yields of gaseous nitric oxide (NO) and nitrogen dioxide (NO2). To the best of our knowledge, this is the first time that superoxide was directly observed as an intermediate in nitrate photolysis experiments, produced through DOAM oxidation by OH(aq). Herein, we suggest that enhanced HONO(g) emissions from NO3–(aq) photolysis result from the reaction of O2–(aq) with NO2(aq) and NO(aq) to form peroxynitrate (OONO2–) and peroxynitrite (OONO–), respectively, which are precursors to nitrite (NO2–). Overall, this points to an important role of O2–(aq) in aqueous aerosol chemistry, which is currently underappreciated.

International audience; Brown carbon (BrC) is involved in atmospheric light absorption and climate forcing and can cause adverse health effects. Understanding the formation mechanisms and molecular structure of BrC is of key importance in developing strategies to control its environment and health impact. Structure determination of BrC is challenging, due to the lack of experiments providing molecular fingerprints and the sheer number of molecular candidates with identical mass. Suggestions based on chemical intuition are prone to errors due to the inherent bias. We present an unbiased algorithm, using graph-based molecule generation and machine learning, which can identify all molecular structures of compounds involved in biomass burning and the composition of BrC. We apply this algorithm to C12H12O7, a light-absorbing “test case” molecule identified in chamber experiments on the aqueous photo-oxidation of syringol, a prevalent marker in wood smoke. Of the 260 million molecular graphs, the algorithm leaves only 36,518 (0.01%) as viable candidates matching the spectrum. Although no unique molecular structure is obtained from only a chemical formula and a UV/vis absorption spectrum, we discuss further reduction strategies and their efficacy. With additional data, the method can potentially more rapidly identify isomers extracted from lab and field aerosol particles without introducing human bias.

Cyclohexene (C6H10) is commonly used as a proxy for biogenic monoterpenes, when studying their oxidation mechanisms and secondary organic aerosol (SOA) formation. The ozonolysis of cyclohexene has been shown to be effective at producing highly oxygenated organic molecules (HOM), a group of molecules known to be important in the formation of SOA. Here, we provide an in depth look at how, on a molecular level, the HOM formation and fate changed with perturbations from NOx and seed particles. HOM were produced in a chamber from cyclohexene ozonolysis, and measured with a chemical ionisation mass spectrometer (CIMS) using nitrate (NO3−) as reagent ion. As high-resolution CIMS instruments provide mass spectra with numerous ion signals and a wealth of information that can be hard to manage, we employed a primarily statistical approach for the data analysis. To utilise as many individual HOM signals as possible, each compound was assigned a parameter describing the quality of the observed signal. These parameters were then used as weights or to determine the inclusion of a given signal in further analyses. Under unperturbed ozonolysis conditions, known HOM peaks were observed in the chamber, including C6H8O9 as the largest HOM signal, and C12H20O9 as the largest dimer product. With the addition of nitric oxide (NO) into the chamber, the spectrum changed considerably, as expected. Dimer product signals decreased overall, but an increase in dimers with nitrate functionalities was seen, as a result of NO3 radical oxidation. The response of monomer signals to NO addition varied, and while nitrate-containing monomers increased, non-nitrate signals either increased or decreased, depending on the individual molecules. The addition of seed aerosol increased the condensation sink, which markedly decreased the signals of all low-volatility compounds. Larger molecules were seen to have a higher affinity for condensation, but a more detailed analysis showed that the uptake was controlled mainly by the number of oxygen atoms in each molecule. All non-nitrate compounds with at least 7 oxygen atoms were observed to condense onto the seed aerosol at close to equal rates. Nitrates required higher mass and higher oxygen content to condense at similar rates as the non-nitrate HOM. A comparison to experiments with alpha-pinene reported earlier, showed quite a similar relationship between elemental composition and volatility, although products from alpha-pinene ozonolysis appeared to require slightly higher oxygen numbers for the same decrease in volatility. In addition, two models developed for predicting volatilities of volatile organic compound (VOC) oxidation products were tested on the ozonolysis products of cyclohexene.

SSCI-VIDE+CARE+XWG:MRV:CGO; International audience;

Atmospheric organic vapors play essential roles in the formation of secondary organic aerosol. Source identification of these vapors is thus fundamental to understand their emission sources and chemical evolution in the atmosphere and their further impact on air quality and climate change. In this study, a Vocus proton-transfer-reaction time-of-flight mass spectrometer (PTR-TOF) was deployed in two forested environments, the Landes forest in southern France and the boreal forest in southern Finland, to measure atmospheric organic vapors, including both volatile organic compounds (VOCs) and their oxidation products. For the first time, we performed binned positive matrix factorization (binPMF) analysis on the complex mass spectra acquired with the Vocus PTR-TOF and identified various emission sources as well as oxidation processes in the atmosphere. Based on separate analysis of low- and high-mass ranges, fifteen PMF factors in the Landes forest and nine PMF factors in the Finnish boreal forest were resolved, showing a high similarity between the two sites. Factors representing monoterpenes dominate the biogenic VOCs in both forests, with less contributions from the isoprene factors and sesquiterpene factors. Particularly, various terpene reaction products were separated into individual PMF factors with varying oxidation degrees, such as lightly oxidized compounds from both monoterpene and sesquiterpene oxidations, monoterpene-derived organic nitrates, and monoterpene more oxidized compounds. These factors display similar mass profiles and diurnal variations between the two sites, revealing similar terpene reaction pathways in these forests. With the distinct characteristics of VOCs and oxygenated VOCs measured by the Vocus PTR-TOF, this study identified various primary emission sources and secondary oxidation processes of atmospheric organic vapors in the European pine forests, providing a more comprehensive understanding of gas-phase atmospheric chemistry.

Pyruvic acid (CH3C(O)C(O)OH, 2-oxopropanoic acid) is an organic acid of biogenic origin that plays a crucial role in plant metabolism, is present in tropospheric air in both gas-phase and aerosol-phase, and is implicated in the formation of secondary organic aerosols (SOAs). Up to now, only a few field studies have reported mixing ratios of gas-phase pyruvic acid, and its tropospheric sources and sinks are poorly constrained. We present the first measurements of gas-phase pyruvic acid in the boreal forest as part of the IBAIRN (Influence of Biosphere–Atmosphere Interactions on the Reactive Nitrogen budget) field campaign in Hyytiälä, Finland, in September 2016. The mean pyruvic acid mixing ratio during IBAIRN was 96 pptv, with a maximum value of 327 pptv. From our measurements we estimated the overall pyruvic acid source strength and quantified the contributions of isoprene oxidation and direct emissions from vegetation in this monoterpene-dominated forested environment. Further, we discuss the relevance of gas-phase pyruvic acid for atmospheric chemistry by investigating the impact of its photolysis on acetaldehyde and peroxy radical production rates. Our results show that, based on our present understanding of its photochemistry, pyruvic acid is an important source of acetaldehyde in the boreal environment, exceeding ethane and propane oxidation by factors of ∼10 and ∼20. Pyruvic acid (CH3C(O)C(O)OH, 2-oxopropanoic acid) is an organic acid of biogenic origin that plays a crucial role in plant metabolism, is present in tropospheric air in both gas-phase and aerosol-phase, and is implicated in the formation of secondary organic aerosols (SOAs). Up to now, only a few field studies have reported mixing ratios of gas-phase pyruvic acid, and its tropospheric sources and sinks are poorly constrained. We present the first measurements of gas-phase pyruvic acid in the boreal forest as part of the IBAIRN (Influence of Biosphere-Atmosphere Interactions on the Reactive Nitrogen budget) field campaign in Hyytiala, Finland, in September 2016. The mean pyruvic acid mixing ratio during IBAIRN was 96 pptv, with a maximum value of 327 pptv. From our measurements we estimated the overall pyruvic acid source strength and quantified the contributions of isoprene oxidation and direct emissions from vegetation in this monoterpene-dominated forested environment. Further, we discuss the relevance of gas-phase pyruvic acid for atmospheric chemistry by investigating the impact of its photolysis on acetaldehyde and peroxy radical production rates. Our results show that, based on our present understanding of its photochemistry, pyruvic acid is an important source of acetaldehyde in the boreal environment, exceeding ethane and propane oxidation by factors of similar to 10 and similar to 20.

Sulfur and nitrogen containing organic compounds, such as organosulfates (OSs) and nitrooxy organosulfates (NOSs), are recognized to be ubiquitously present in secondary organic aerosol (SOA). However, little is known about the chemical mechanisms or the required conditions for the formation of these compounds in the ambient atmosphere. Earlier studies have commonly suggested that OSs are predominantly formed through the reaction of organic gaseous epoxides with acidic sulfate particles. However, this epoxide pathway often fails to explain the formation of (N)OSs from monoterpenes. Moreover, recent studies highlight the potential role of gas-phase SO2 and organic peroxides for the formation of OSs, which might serve as predominant precursors for OSs and NOSs from atmospheric monoterpene oxidation.Here, we conducted a series of chamber experiments to elucidate the formation mechanisms of (N)OSs from α-pinene oxidation during daytime and nighttime conditions. In particular, we focused on the role of organic peroxides and S(IV) (i.e., gas-phase SO2 and particulate SO32–) in contrast to organic epoxides and isotope-labelled particulate sulfate (i.e., S(VI)). SOA particles were analyzed online by extractive electrospray ionization coupled with high-resolution Orbitrap mass spectrometry (EESI-Orbitrap MS) allowing an unambiguous identification of OS and NOS species with a high time resolution. Additionally, filter samples were collected and analyzed by liquid chromatography (LC) coupled with Orbitrap MS to determine the presence of isomeric compounds.Consistently, online and offline Orbitrap MS analysis showed that particulate sulfate played a minor role in the formation of OSs and NOSs. In contrast, (N)OSs were rapidly formed upon addition of either gaseous SO2 or particulate SO32–, suggesting S(IV) to react with organic peroxides that were formed through monoterpene oxidation. Based on these experiments, we identified specific NOS species that are formed only through either daytime or nighttime chemistry, and thus, might serve as marker molecules. Moreover, we present complete formation pathways for these species. Our study indicates that in contrast to previous work, the formation of OSs and NOSs does not require acidic sulfate particles, but rather involves the reaction of organic peroxides with S(IV) in the gas phase or the particle phase.

Secondary organic aerosol (SOA) forms a major part of the tropospheric submicron aerosol. Still, the exact formation mechanisms of SOA have remained elusive. Recently, a newly discovered group of oxidation products of volatile organic compounds (VOCs), highly oxygenated organic molecules (HOMs), have been proposed to be responsible for a large fraction of SOA formation. To assess the potential of HOMs to form SOA and to even take part in new particle formation, knowledge of their exact volatilities is essential. However, due to their exotic, and partially unknown, structures, estimating their volatility is challenging. In this study, we performed a set of continuous flow chamber experiments, supported by box modelling, to study the volatilities of HOMs, along with some less oxygenated compounds, formed in the ozonolysis of α-pinene, an abundant VOC emitted by boreal forests. Along with gaseous precursors, we periodically injected inorganic seed aerosol into the chamber to vary the condensation sink (CS) of low-volatility vapours. We monitored the decrease of oxidation products in the gas phase in response to increasing CS, and were able to relate the responses to the volatilities of the compounds. We found that HOM monomers are mainly of low volatility, with a small fraction being semi-volatile. HOM dimers were all at least low volatility, but probably extremely low volatility; however, our method is not directly able to distinguish between the two. We were able to model the volatility of the oxidation products in terms of their carbon, hydrogen, oxygen and nitrogen numbers. We found that increasing levels of oxygenation correspond to lower volatilities, as expected, but that the decrease is less steep than would be expected based on many existing models for volatility, such as SIMPOL. The hydrogen number of a compound also predicted its volatility, independently of the carbon number, with higher hydrogen numbers corresponding to lower volatilities. This can be explained in terms of the functional groups making up a molecule: high hydrogen numbers are associated with, e.g. hydroxy groups, which lower volatility more than, e.g. carbonyls, which are associated with a lower hydrogen number. The method presented should be applicable to systems other than α-pinene ozonolysis, and with different organic loadings, in order to study different volatility ranges. Secondary organic aerosol (SOA) forms a major part of the tropospheric submicron aerosol. Still, the exact formation mechanisms of SOA have remained elusive. Recently, a newly discovered group of oxidation products of volatile organic compounds (VOCs), highly oxygenated organic molecules (HOMs), have been proposed to be responsible for a large fraction of SOA formation. To assess the potential of HOMs to form SOA and to even take part in new particle formation, knowledge of their exact volatilities is essential. However, due to their exotic, and partially unknown, structures, estimating their volatility is challenging. In this study, we performed a set of continuous flow chamber experiments, supported by box modelling, to study the volatilities of HOMs, along with some less oxygenated compounds, formed in the ozonolysis of alpha-pinene, an abundant VOC emitted by boreal forests. Along with gaseous precursors, we periodically injected inorganic seed aerosol into the chamber to vary the condensation sink (CS) of low-volatility vapours. We monitored the decrease of oxidation products in the gas phase in response to increasing CS, and were able to relate the responses to the volatilities of the compounds. We found that HOM monomers are mainly of low volatility, with a small fraction being semi-volatile. HOM dimers were all at least low volatility, but probably extremely low volatility; however, our method is not directly able to distinguish between the two. We were able to model the volatility of the oxidation products in terms of their carbon, hydrogen, oxygen and nitrogen numbers. We found that increasing levels of oxygenation correspond to lower volatilities, as expected, but that the decrease is less steep than would be expected based on many existing models for volatility, such as SIM-POL. The hydrogen number of a compound also predicted its volatility, independently of the carbon number, with higher hydrogen numbers corresponding to lower volatilities. This can be explained in terms of the functional groups making up a molecule: high hydrogen numbers are associated with, e.g. hydroxy groups, which lower volatility more than, e.g. carbonyls, which are associated with a lower hydrogen number. The method presented should be applicable to systems other than alpha-pinene ozonolysis, and with different organic loadings, in order to study different volatility ranges.

Highly oxygenated organic molecules (HOMs) from atmospheric oxidation of alpha-pinene can irreversibly condense to particles and contribute to secondary organic aerosol (SOA) formation. Recently, the formation of nitryl chloride (ClNO2) from heterogeneous reactions, followed by its subsequent photolysis, is suggested to be an important source of chlorine atoms in many parts of the atmosphere. However, the oxidation of monoterpenes such as alpha-pinene by chlorine atoms has received very little attention, and the ability of this reaction to form HOMs is completely unstudied. Here, chamber experiments were conducted with alpha-pinene and chlorine under low- and high-nitrogen-oxide (NOx, NOx=NO+NO2) conditions. A nitrate-based CI-APi-ToF (chemical ionization–atmospheric pressure interface–time of flight) mass spectrometer was used to measure HOM products. Clear distributions of monomers with 9–10 carbon atoms and dimers with 18–20 carbon atoms were observed under low-NOx conditions. With increased concentration of NOx within the chamber, the formation of dimers was suppressed due to the reactions of peroxy radicals with NO. We estimated the HOM yields from chlorine-initiated oxidation of alpha-pinene under low-NOx conditions to be around 1.8 %, though with a substantial uncertainty range (0.8 %–4 %) due to lack of suitable calibration methods. Corresponding yields at high NOx could not be determined because of concurrent ozonolysis reactions. Our study demonstrates that also the oxidation of alpha-pinene by chlorine atoms and yield low-volatility organic compounds.

SSCI-VIDE+CARE+MRV:DLI:SPR:CGO; International audience; Chemical ionization Orbitrap mass spectrometry (CI-Orbitrap) represents a promising new technique for gas-phase analysis in analytical and atmospheric chemistry mainly due to its very high mass resolving power. In this work, we performed the first side-by-side comparison between a CI-Orbitrap and the widely used atmospheric pressure interface time-of-flight mass spectrometry (CI-APi-TOF) using two different chemical ionization methods, i.e., acetate-ion-based (CH3COO–) and aminium-ion-based (n-C3H7NH3+) schemes. The capability of the CI-Orbitrap at accurately measuring low concentrations of gaseous species formed from the oxidation of α-pinene was explored. Although this study reveals a lack of linearity of the CI-Orbitrap when measuring product ions at very low concentrations (

Cis-3-hexen-1-ol (cis-HXO) is a green leaf volatile emitted from plants under stress and belongs to an important class of biogenic volatile organic compounds. In this study, we have investigated the potential formation of organosulfates (OSs) from the hydroxyl radical (OH)-initiated oxidation and ozonolysis of cis-HXO using either non-acidified or acidified sulfate seed aerosols under different relative humidity (RH) conditions. For selected ozonolysis experiments, an OH scavenger was utilized. Ultra performance liquid chromatography interfaced to high-resolution quadrupole time-of-flight mass spectrometry with electrospray ionization (UPLC/ESI-HR-Q-TOFMS) was used to characterize cis-HXO-derived secondary organic aerosol (SOA) formation. Chemical characterization of cis-HXO-derived SOA products reveals that OSs were generated in significant quantity from multiphase chemistry of gas-phase oxidation products of cis-HXO. Ambient fine aerosol (PM2.5) samples collected from Rio de Janeiro, Brazil, were also analyzed. Seven cis-HXO-derived OSs identified in the lab study with molecular weights 154, 186, 170, 210, 212, 226 and 270 were also found in the PM2.5 samples collected in Brazil. This study provides direct evidence that the oxidation of cis-HXO by OH and O3 yields biogenic SOA through the formation of polar OSs.

Light-absorbing brown carbon (BrC) constituents of organic aerosol (OA) have been shown to significantly absorb ultraviolet (UV) and visible light and thus impact radiative forcing. However, molecular identification of the BrC constituents is still limited. In this study, we characterize BrC constituents at the molecular level in (i) aerosols emitted by combustion of peat, fern/leaf, and charcoal from Indonesia and (ii) ambient aerosols collected in Singapore during the 2015 haze episode. Aerosols were analyzed using ultra performance liquid chromatography instrument interfaced to a diode array detector and electrospray ionization high-resolution quadrupole time-of-flight mass spectrometer operated in the negative ion mode. In the laboratory-generated aerosols, we identified 41 compounds that can potentially absorb near-UV and visible wavelengths, such as oxygenated-conjugated compounds, nitroaromatics, and S-containing compounds. The sum of BrC constituents in peat, fern/leaf, and charcoal burning aerosols are 16%, 35%, and 28% of the OA mass, respectively, giving an average contribution of 24%. On average, the BrC constituents account for 0.4% of the ambient OA mass; however, large uncertainties in mass closure remain because of the lack of authentic standards. This study highlights the potential of light-absorbing BrC OA constituents from peat, fern/leaf, and charcoal burning and their importance in the atmosphere.

Ozonolysis of alkenes results in the formation of primary ozonides (POZs), which can subsequently decompose into carbonyl compounds and stabilized Criegee intermediates (sCIs). The sCIs generated from isoprene ozonolysis include the simplest congener, formaldehyde oxide (CH2OO), and isomers of C4-sCI. Although the bimolecular reaction with H2O is expected to be the main fate of sCIs, it was reported that sCIs can also react with carboxylic acids and/or organic hydroperoxides leading to gas-phase oligomeric compounds. While the impact of the gas-phase composition (H2O, sCI scavenger) on the formation of such products was recently studied, their fate remains unclear. In the present work, formation of oligomeric hydroperoxides from isoprene ozonolysis, proposed as reaction products composed of the sCI as a chain unit and formed from the insertion of sCI into a hydroperoxide or a carboxylic acid, was systematically examined in the presence of aerosol with varying compositions. The effect of hydroxyl (OH) radicals on the gas- and particle-phase compositions was investigated using diethyl ether as an OH radical scavenger. Thirty-four oligomeric compounds resulting from the insertion of sCIs into organic hydroperoxides or carboxylic acids were identified using iodide chemical ionization high-resolution mass spectrometry. Large reactive uptake onto acidified sulfate aerosol was observed for most of the characterized gaseous oligomeric species, whereas the presence of organic coatings and the lack of aerosol water significantly reduced or halted the reactive uptake of these species. These results indicate that highly oxidized molecules, such as hydroperoxides, could undergo multiphase reactions, which are significantly influenced by the chemical composition of seed aerosol. Furthermore, in addition to functionalization and accretion, decomposition and re-volatilization should be considered in SOA formation.

With the recent developments in mass spectrometry, combined with the strengths of factor analysis techniques, our understanding of atmospheric oxidation chemistry has improved significantly. The typical approach for using techniques like positive matrix factorization (PMF) is to input all measured data for the factorization in order to separate contributions from different sources and/or processes to the total measured signal. However, while this is a valid approach for assigning the total signal to factors, we have identified several cases where useful information can be lost if solely using this approach. For example, gaseous molecules emitted from the same source can show different temporal behaviors due to differing loss terms, like condensation at different rates due to different molecular masses. This conflicts with one of PMF's basic assumptions of constant factor profiles. In addition, some ranges of a mass spectrum may contain useful information, despite contributing only minimal fraction to the total signal, in which case they are unlikely to have a significant impact on the factorization result. Finally, certain mass ranges may contain molecules formed via pathways not available to molecules in other mass ranges, e.g. dimeric species versus monomeric species. In this study, we attempted to address these challenges by dividing mass spectra into sub-ranges and applying the newly developed binPMF method to these ranges separately. We utilized a dataset from a chemical ionization atmospheric pressure interface time-of-flight (CI-APi-TOF) mass spectrometer as an example. We compare the results from these three different ranges, each corresponding to molecules of different volatilities, with binPMF results from the combined range. Separate analysis showed clear benefits in dividing factors for molecules of different volatilities more accurately, in resolving different chemical processes from different ranges, and in giving a chance for high-molecular-weight molecules with low signal intensities to be used to distinguish dimeric species with different formation pathways. In addition, daytime dimer formation (diurnal peak around noon) was identified, which may contribute to NPF in Hyytiälä. Also, dimers from NO3 oxidation were separated by the sub-range binPMF, which would not be identified otherwise. We recommend PMF users to try running their analyses on selected sub-ranges in order to further explore their datasets.

The Station for Measuring Ecosystem Atmosphere Relations (SMEAR) II is well known among atmospheric scientists due to the immense amount of observational data it provides of the earth–atmosphere interface. Moreover, SMEAR II plays an important role in large European research infrastructures, enabling the large scientific community to tackle climate and air pollution related questions, utilising the high-quality long-term data sets recorded at the site. So far, the well-documented site was missing the description of the seasonal variation of aerosol chemical composition that is crucial for understanding the complex biogeochemical and -physical processes governing the forest ecosystem. Here, we report the sub-micron aerosol chemical composition and its variability utilising data measured between 2012 and 2018 using an Aerosol Chemical Speciation Monitor (ACSM). We observed a bimodal seasonal trend in the sub–micron aerosol concentration culminating in February (2.7, 1.6, 5.1 µg m−3 for median, 25th, 75th percentiles, respectively) and July (4.2, 2.2, and 5.7 µg m−3 for median, 25th, 75th percentiles, respectively). The wintertime maximum was linked to an enhanced presence of inorganic aerosol species (ca. 50 %) whereas the summertime maximum (ca. 80 % organics) to biogenic secondary organic aerosol (SOA) formation. During the exceptionally hot Julys of 2014 and 2018, the organic aerosol concentrations were up to 70 % higher than the 7–year July mean. The projected increase of heat wave frequency over Finland will most likely influence the loading and chemical composition of aerosol particles in the future. Our findings suggest strong influence of meteorological conditions such as radiation, ambient temperature, wind speed and direction on aerosol chemical composition. To our understanding, this is the longest time series reported describing the aerosol chemical composition measured online in the boreal region, but the continuous monitoring will be maintained also in the future.

Highly oxygenated organic molecules (HOMs) from atmospheric oxidation of alpha-pinene can irreversibly condense to particles and contribute to secondary organic aerosol (SOA) formation. Recently, the formation of nitryl chloride (ClNO2) from heterogeneous reactions, followed by its subsequent photolysis is suggested to be an important source of chlorine atoms in many parts of the atmosphere. However, the oxidation of monoterpenes such as alpha-pinene by chlorine atoms has received very little attention, and the ability of this reaction to form HOM is completely unstudied. Here, chamber experiments were conducted with alpha-pinene and chlorine under low and high nitrogen oxide (NOx) conditions. A NO3-based CI-APi-TOF was used to measure HOM products. Clear distributions of monomers with 9–10 carbon atoms and dimers with 18–20 carbon atoms were observed under low NOx conditions. With increased concentration of NOx within the chamber, the formation of dimers was suppressed due to the reactions of peroxy radicals with NO. We estimated the HOM yields from chlorine-initiated oxidation of alpha-pinene under low-NOx conditions to be around 1.8 %, though with a substantial uncertainty range (0.8–4 %) due to lack of suitable calibration methods. Corresponding yields at high NOx could not be determined because of concurrent ozonolysis reactions. Our study demonstrates that chlorine atoms also initiated oxidation of alpha-pinene and yields low volatility organic compounds.

Pyruvic acid, CH3C(O)C(O)OH, is an organic acid of biogenic origin that plays a crucial role in plant metabolism, is present in tropospheric air in both gas-phase and aerosol-phase and is implicated in the formation of secondary organic aerosols (SOA). Up to now, only a few field studies have reported mixing ratios of gas-phase pyruvic acid and its tropospheric sources and sinks are poorly constrained. We present the first gas-phase measurements of pyruvic acid in the boreal forest as part of the IBAIRN (Influence of Biosphere–Atmosphere Interactions on the Reactive Nitrogen budget) field campaign in Hyytiälä, Finland, in September 2016. The mean pyruvic acid mixing ratio during IBAIRN was 96 pptv, with a maximum value of 327 pptv. From our measurements we derived the overall pyruvic acid source strength and quantified the contributions of isoprene oxidation and direct emissions from vegetation in this monoterpene-dominated, forested environment. Further, we discuss the relevance of gas-phase pyruvic acid for atmospheric chemistry by investigating the impact of its photolysis on acetaldehyde and peroxy radical production rates. Our results show that, based on our present understanding of its photo-chemistry, pyruvic acid is an important source of acetaldehyde in the boreal environment, exceeding ethane/propane oxidation by factors of ~ 10 and ~ 20.

Secondary organic aerosols (SOA) produced in the atmosphere from sequential oxidation of biogenic or anthropogenic volatile organic compounds (VOC) represent a significant part of the atmospheric organic aerosol. Aerosol particles can act as cloud condensation nuclei or can be scavenged by cloud droplets, where organic carbon undergo chemical and biological transformations. Due to its high complexity, a nontargeted analysis by FT-ICR MS (Fourier-transform ion cyclotron resonance mass spectrometry) has been used to characterize the dissolved organic matter at a molecular level. SOA compounds have been detected in six aqueous samples collected in 2017 at the Puy de Dome station (PUY, France). SOA tracers, produced by VOC oxidation in environmental chambers or in ambient air and characterized in previous studies by mass spectrometry, were searched by FT-ICR MS in cloudwaters. Results clearly indicate the presence of oxidation products of isoprene, α- and β-pinene, and other monoterpenes and sesquiterpenes. I...

The formation of alkyl nitrates in various oxidation processes taking place throughout the diel cycle can represent an important sink of reactive nitrogen and mechanism for chain-termination in atmospheric photo-oxidation cycles. The low volatility alkyl nitrates formed from biogenic volatile organic compounds (BVOCs), especially terpenoids, enhance rates of production and growth of secondary organic aerosol. Measurements of the NO3-reactivity and the mixing ratio of total alkyl nitrates in the Finnish boreal forest enabled assessment of the relative importance of NO3−, O3− and OH-initiated formation of alkyl-nitrates from BVOCs in this environment. The high reactivity of the forest air towards NO3 resulted in reactions of the nitrate radical with terpenes contributing substantially to formation of ANs not only during the night but also during daytime. Overall, night-time reactions of NO3 accounted for 49 % of the local production rate of ANs, with contributions of 21 %, 18 % and 12 % for NO3, OH and O3, during the day. The lifetimes of the gas-phase ANs formed in this environment were of the order of 2 hours implying that the lifetime of NOx is strongly controlled by biogenic emissions from the forest. As the organic nitrates are lost to the particle phase and via dry-deposition to foliar surfaces, the overall result is transfer of reactive nitrogen from anthropogenic sources to the forest ecosystem.