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Air ions are the key components for a series of atmospheric physicochemical interactions, such as ion-catalyzed reactions, ion-molecule reactions, and ion-induced new particle formation (NPF). They also control atmospheric electrical properties with effects on global climate. We performed molecular-level measurements of cluster ions at the high-altitude research station Chacaltaya (CHC; 5240 m a.s.l.), located in the Bolivian Andes, from January to May 2018 using an atmospheric-pressure-interface time-of-flight mass spectrometer. The negative ions mainly consisted of (H2SO4)0–3⚫HSO4-, (HNO3)0–2⚫NO3-, SO5-, (NH3)1–6⚫(H2SO4)3–7⚫HSO4-, malonic-acid-derived, and CHO / CHON⚫(HSO4- / NO3-) cluster ions. Their temporal variability exhibited distinct diurnal and seasonal patterns due to the changes in the corresponding neutral species' molecular properties (such as electron affinity and proton affinity) and concentrations resulting from the air masses arriving at CHC from different source regions. The positive ions were mainly composed of protonated amines and organic cluster ions but exhibited no clear diurnal variation. H2SO4–NH3 cluster ions likely contributed to the NPF process, particularly during the wet-to-dry transition period and the dry season, when CHC was more impacted by air masses originating from source regions with elevated SO2 emissions. Our study provides new insights into the chemical composition of atmospheric cluster ions and their role in new particle formation in the high-altitude mountain environment of the Bolivian Andes.

New particle formation (NPF) in the tropical free troposphere (FT) is a globally important source of cloud condensation nuclei, affecting cloud properties and climate. Oxidized organic molecules (OOMs) produced from biogenic volatile organic compounds are believed to contribute to aerosol formation in the tropical FT, but without direct chemical observations. We performed in-situ molecular-level OOMs measurements at the Bolivian station Chacaltaya at 5240 meters above sea level, on the western edge of Amazonia. For the first time, we demonstrate the presence of OOMs, mainly with 4-5 carbon atoms, in both gas-phase and particle-phase (in terms of mass contribution) measurements in tropical FT air from Amazonia. These observations, combined with air mass history analyses, indicate that the observed OOMs are linked to isoprene emitted from the rainforests hundreds of kilometers away. Based on particle-phase measurements, we find that these compounds can contribute to NPF, at least the growth of newly formed nanoparticles, in the tropical FT on a continental scale. Thus, our study is a fundamental and significant step in understanding the aerosol formation process in the tropical FT.

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.

Diterpenes (C20H32) are biogenically emitted volatile compounds that only recently have been observed in ambient air. They are expected to be highly reactive, and their oxidation is likely to form condensable vapors. However, until now, no studies have investigated gas-phase diterpene oxidation. In this paper, we explored the ozonolysis of a diterpene, ent-kaurene, in a simulation chamber. Using state-of-the-art mass spectrometry, we characterized diterpene oxidation products for the first time, and we identified several products with varying oxidation levels, including highly oxygenated organic molecules (HOM), monomers, and dimers. The most abundant monomers measured using a nitrate chemical ionization mass spectrometer were C19H28O8 and C20H30O5, and the most abundant dimers were C38H60O6 and C39H62O6. The exact molar yield of HOM from kaurene ozonolysis was hard to quantify due to uncertainties in both the kaurene and HOM concentrations, but our best estimate was a few percent, which is similar to values reported earlier for many monoterpenes. We also monitored the decrease in the gas-phase oxidation products in response to an increased condensation sink in the chamber to deduce their affinity to condense. The oxygen content was a critical parameter affecting the volatility of products, with four to five O atoms needed for the main monomeric species to condense onto 80 nm particles. Finally, we report on the observed fragmentation and clustering patterns of kaurene in a Vocus proton-transfer-reaction time-of-flight mass spectrometer. Our findings highlight similarities and differences between diterpenes and smaller terpenes during their atmospheric oxidation, but more studies on different diterpenes are needed for a broader view of their role in atmospheric chemistry.

Diterpenes (C20H32) are biogenically emitted volatile compounds that only recently have been observed in ambient air. They are expected to be highly reactive, and their oxidation is likely to form condensable vapors. However, until now, no studies have investigated gas-phase diterpene oxidation. In this paper, we explored the ozonolysis of a diterpene, ent-kaurene, in a simulation chamber. Using state-of-the-art mass spectrometry, we characterized diterpene oxidation products for the first time, and we identified several products with varying oxidation levels, including highly oxygenated organic molecules (HOM) monomers and dimers. The most abundant monomers measured using a nitrate chemical ionization mass spectrometer were C19H28O8 and C20H30O5, and dimers were C38H60O6 and C39H62O6. The exact molar yield of HOM from kaurene ozonolysis was hard to quantify due to uncertainties in both the kaurene and HOM concentrations, but our best estimate was a few percent, which is similar to values reported earlier for many monoterpenes. We also monitored the decrease of the gas-phase oxidation products in response to an increased condensation sink in the chamber to deduce their affinity to condense. The oxygen content was a critical parameter affecting the volatility of products, with 4–5 O-atoms needed for the main monomeric species to condense. Finally, we report on the observed fragmentation and clustering patterns of kaurene in a Vocus proton transfer reaction time-of-flight mass spectrometer. Our findings highlight similarities and differences between diterpenes and smaller terpenes during their atmospheric oxidation, but more studies on different diterpenes are needed for a broader view of their role in atmospheric chemistry.

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.

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.

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.

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.

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.

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.

Highly-oxygenated Organic Molecules (HOM) are important contributors to Secondary Organic Aerosol (SOA) and New-Particle Formation (NPF) in the boreal atmosphere. This newly discovered class of molecules is efficiently formed from atmospheric oxidation of biogenic volatile organic compounds (VOC), such as monoterpenes, through a process called autoxidation. This process, in which peroxy-radical intermediates isomerize to allow addition of molecular oxygen, is expected to be highly temperature-dependent. Here, we studied the dynamics of HOM formation during alpha-pinene ozonolysis experiments performed at three different temperatures, 20 °C, 0 °C and −15 °C, in the Aarhus University Research on Aerosol (AURA) chamber. We found that the HOM formation, under our experimental conditions (50 ppb alpha-pinene, 100 ppb ozone), decreased considerably as temperature decreased, with molar yields dropping by around a factor of 50 when experiments were performed at 0 °C, compared to 20 °C. At −15 °C, the HOM signals were already close to the detection limit of the nitrate-based Chemical Ionization Atmospheric Pressure interface Time Of Flight (CI-APi-TOF) mass spectrometer used for measuring gas-phase HOM. Surprisingly, very little difference was seen in the mass spectral distribution of the HOM molecules of interest at 0 °C and 20 °C, with e.g. the ratios between the typical HOM products C10H14O7, C10H14O9, and C10H14O11 remaining fairly constant. The more oxidized species have undergone more isomerization steps, yet, at lower temperature, they did not decrease more than the less oxidized species. One possible explanation is be that the rate-limiting step forming these HOM occurs before the products become oxygenated enough to be detected by our CI-APi-TOF (i.e. typically seven or more oxygen atoms). The strong temperature dependence of HOM formation was observed under temperatures highly relevant for the boreal forest, but the exact magnitude of this effect in the atmosphere will be much more complex: the fate of peroxy-radicals is a competition between autoxidation (influenced by temperature and VOC type) and bimolecular termination pathways (influenced mainly by concentration of reaction partners). While the temperature influence is likely smaller in the boreal atmosphere than in our chamber, the magnitude and complexity of this effect clearly deserves more consideration in future studies in order to estimate the ultimate role of HOM on SOA and NPF under different atmospheric conditions.

Many of the particles in the troposphere are formed in situ but what fraction of all tropospheric particles do they constitute and how exactly are they made? Bianchi et al. report results from a high altitude research station. Roughly half of the particles were newly formed by the condensation of highly oxygenated multifunctional compounds. A combination of laboratory results field measurements and model calculations revealed that neutral nucleation is more than 10 times faster than ion induced nucleation that particle growth rates are size dependent and that new particle formation occurs during a limited time window.Science this issue p. 1109New particle formation (NPF) is the source of over half of the atmosphere’s cloud condensation nuclei thus influencing cloud properties and Earth’s energy balance. Unlike in the planetary boundary layer few observations of NPF in the free troposphere exist. We provide observational evidence that at high altitudes NPF occurs mainly through condensation of highly oxygenated molecules (HOMs) in addition to taking place through sulfuric acid–ammonia nucleation. Neutral nucleation is more than 10 times faster than ion induced nucleation and growth rates are size dependent. NPF is restricted to a time window of 1 to 2 days after contact of the air masses with the planetary boundary layer; this is related to the time needed for oxidation of organic compounds to form HOMs. These findings require improved NPF parameterization in atmospheric models.

New particle formation (NPF) events have been observed all around the world and are known to be a major source of atmospheric aerosol particles. Here we combine 20 years of observations in a boreal forest at the SMEAR II station (Station for Measuring Ecosystem-Atmosphere Relations) in Hyytiälä, Finland, by utilizing previously accumulated knowledge, and by focusing on clear-sky (non-cloudy) conditions. We first investigated the effect of cloudiness on NPF and then compared the NPF event and non-event days during clear-sky conditions. In this comparison we considered, for example, the effects of calculated particle formation rates, condensation sink, trace gas concentrations and various meteorological quantities. The formation rate of 1.5 nm particles was calculated by using proxies for gaseous sulfuric acid and oxidized products of low volatile organic compounds. As expected, our results indicate an increase in the frequency of NPF events under clear-sky conditions. Also, focusing on clearsky conditions enabled us to find a clear separation of many variables related to NPF. For instance, oxidized organic vapors showed higher concentration during the clear-sky NPF event days, whereas the condensation sink (CS) and some trace gases had higher concentrations during the non-event days. The calculated formation rate of 3 nm particles showed a notable difference between the NPF event and non-event days during clear-sky conditions, especially in winter and spring. For spring time, we are able to find a threshold value for the combined values of ambient temperature and CS, above which practically no clear-sky NPF event could be observed. Finally, we present a probability distribution for the frequency of NPF events at a specific CS and temperature.

International audience; The magnitude of aerosol radiative forcing caused by anthropogenic emissions depends on the baseline state of the atmosphere under Q:11 pristine preindustrial conditions. Measurements in the European Organisation for Nuclear Research (CERN) CLOUD chamber show that particle formation in atmospheric conditions can occur solely from Q:12 biogenic vapors. Here, we evaluate the potential effect of this source of particles on preindustrial cloud condensation nucleus (CCN) concentrations and aerosol cloud radiative forcing over the industrial period.Model simulations show that the pure biogenic particle formation mechanism has a much larger relative effect on CCN concentrations in the preindustrial atmosphere than in the present atmosphere because of the lower aerosol concentrations. Consequently, preindustrial cloud albedo is increased more than under present day conditions, and therefore, the cooling forcing of anthropogenic aerosols is reduced. Q:13 The mechanism increases CCN concentrations by 20–100% over a large fraction of the preindustrial lower atmosphere, and the magnitude of annual global mean radiative forcing caused by changes of cloud albedo since 1750 is reduced by 0.22 W m−² (27%) to −0.60 W m−².Model uncertainties, relatively slow formation rates, and limited available ambient measurements make it difficult to establish the significance of a mechanism that has its dominant effect under preindustrial conditions. Our simulations predict more particle formation in the Amazon than is observed. However, the first observation of pure organic nucleation has now been reported for the free troposphere. Given the potentially significant effect on anthropogenic forcing, effort should be made to better understand such naturally driven aerosol processes.

Homogeneous nucleation and subsequent cluster growth leads to the formation of new aerosol particles in the atmosphere(1). The nucleation of sulfuric acid and organic vapours is thought to be responsible for the formation of new particles over continents(1,2), whereas iodine oxide vapours have been implicated in particle formation over coastal regions(3-7). The molecular clustering pathways that are involved in atmospheric particle formation have been elucidated in controlled laboratory studies of chemically simple systems(2,8-10), but direct molecular-level observations of nucleation in atmospheric field conditions that involve sulfuric acid, organic or iodine oxide vapours have yet to be reported(11). Here we present field data from Mace Head, Ireland, and supporting data from northern Greenland and Queen Maud Land, Antarctica, that enable us to identify the molecular steps involved in new particle formation in an iodine-rich, coastal atmospheric environment. We find that the formation and initial growth process is almost exclusively driven by iodine oxoacids and iodine oxide vapours, with average oxygen-to-iodine ratios of 2.4 found in the clusters. On the basis of this high ratio, together with the high concentrations of iodic acid (HIO3) observed, we suggest that cluster formation primarily proceeds by sequential addition of HIO3, followed by intracluster restructuring to I2O5 and recycling of water either in the atmosphere or on dehydration. Our study provides ambient atmospheric molecular-level observations of nucleation, supporting the previously suggested role of iodine-containing species in the formation of new aerosol particles(3-7,12-18), and identifies the key nucleating compound.

Homogeneous nucleation and subsequent cluster growth leads to the formation of new aerosol particles in the atmosphere1. Nucleation of sulphuric acid and organic vapours is thought to be responsible for new particle formation over continents1,2 while iodine oxide vapours have been implicated in particle formation over coastal regions3–7. Molecular clustering pathways involved in atmospheric particle formation have been elucidated in controlled laboratory studies of chemically simple systems2,8–10. But no direct molecular-level observations of nucleation in atmospheric field conditions involving either sulphuric acid, organic or iodine oxide vapours have been reported to date11. Here we report field data from Mace Head, Ireland and supporting data from northern Greenland and Queen Maud Land, Antarctica that allow for the identification of the molecular steps involved in new particle formation in an iodine-rich, coastal atmospheric environment. We find that the formation and initial growth process is almost exclusively driven by iodine oxoacids and iodine oxide vapours with average resulting cluster O:I ratios of 2.4. Based on the high O:I ratio, together with observed high concentrations of iodic acid, HIO3, we suggest that cluster formation primarily proceeds by sequential addition of iodic acid HIO3, followed by intra-cluster restructuring to I2O5 and recycling of water in the atmosphere or upon drying. Overall, our study provides ambient atmospheric molecular-level observations of nucleation, supporting the previously suggested role of iodine containing species in new particle formation3–7, 12–18, and identifies the key nucleating compound.

Atmospheric aerosols and their effect on clouds are thought to be important for anthropogenic radiative forcing of the climate, yet remain poorly understood1. Globally, around half of cloud condensation nuclei originate from nucleation of atmospheric vapours2. It is thought that sulfuric acid is essential to initiate most particle formation in the atmosphere3,4, and that ions have a relatively minor role5. Some laboratory studies, however, have reported organic particle formation without the intentional addition of sulfuric acid, although contamination could not be excluded6,7. Here we present evidence for the formation of aerosol particles from highly oxidized biogenic vapours in the absence of sulfuric acid in a large chamber under atmospheric conditions. The highly oxygenated molecules (HOMs) are produced by ozonolysis of α-pinene. We find that ions from Galactic cosmic rays increase the nucleation rate by one to two orders of magnitude compared with neutral nucleation. Our experimental findings are supported by quantum chemical calculations of the cluster binding energies of representative HOMs. Ion-induced nucleation of pure organic particles constitutes a potentially widespread source of aerosol particles in terrestrial environments with low sulfuric acid pollution.

Secondary organic aerosols (SOA) forms a major fraction of organic aerosols in the atmosphere. Knowledge of SOA properties that affect their dynamics in the atmosphere is needed for improving climate models. By combining experimental and modeling techniques, we investigated the factors controlling SOA evaporation under different humidity conditions. Our experiments support the conclusion of particle phase diffusivity limiting the evaporation under dry conditions. Viscosity of particles at dry conditions was estimated to increase several orders of magnitude during evaporation, up to 109 Pa s. However, at atmospherically relevant relative humidity and time scales, our results show that diffusion limitations may have a minor effect on evaporation of the studied α-pinene SOA particles. Based on previous studies and our model simulations, we suggest that, in warm environments dominated by biogenic emissions, the major uncertainty in models describing the SOA particle evaporation is related to the volatility of SOA constituents., published version, peerReviewed

Oxidized organic compounds are expected to contribute to secondary organic aerosol (SOA) if they have sufficiently low volatilities. We estimated saturation vapor pressures and activity coefficients (at infinite dilution in water and a model water-insoluble organic phase) of cyclohexene- and á-pinene-derived accretion products, "dimers", using the COSMOtherm19 program. We found that these two property estimates correlate with the number of hydrogen bond-donating functional groups and oxygen atoms in the compound. In contrast, when the number of H-bond donors is fixed, no clear differences are seen either between functional group types (e.g., OH or OOH as H-bond donors) or the formation mechanisms (e.g., gas-phase radical recombination vs liquid-phase closed-shell esterification). For the cyclohexene-derived dimers studied here, COSMOtherm19 predicts lower vapor pressures than the SIMPOL.1 group-contribution method in contrast to previous COSMOtherm estimates using older parameterizations and nonsystematic conformer sampling. The studied dimers can be classified as low, extremely low, or ultra-low-volatility organic compounds based on their estimated saturation mass concentrations. In the presence of aqueous and organic aerosol particles, all of the studied dimers are likely to partition into the particle phase and thereby contribute to SOA formation. publishedVersion

New particle formation (NPF) events have been observed all around the world and are known to be a major source of atmospheric aerosol particles. Here we combine 20 years of observations in a boreal forest at the SMEAR II station (Station for Measuring Ecosystem–Atmosphere Relations) in Hyytiälä, Finland, by building on previously accumulated knowledge and by focusing on clear-sky (noncloudy) conditions. We first investigated the effect of cloudiness on NPF and then compared the NPF event and nonevent days during clear-sky conditions. In this comparison we considered, for example, the effects of calculated particle formation rates, condensation sink, trace gas concentrations and various meteorological quantities in discriminating NPF events from nonevents. The formation rate of 1.5 nm particles was calculated by using proxies for gaseous sulfuric acid and oxidized products of low volatile organic compounds, together with an empirical nucleation rate coefficient. As expected, our results indicate an increase in the frequency of NPF events under clear-sky conditions in comparison to cloudy ones. Also, focusing on clear-sky conditions enabled us to find a clear separation of many variables related to NPF. For instance, oxidized organic vapors showed a higher concentration during the clear-sky NPF event days, whereas the condensation sink (CS) and some trace gases had higher concentrations during the nonevent days. The calculated formation rate of 3 nm particles showed a notable difference between the NPF event and nonevent days during clear-sky conditions, especially in winter and spring. For springtime, we are able to find a threshold equation for the combined values of ambient temperature and CS, (CS (s−1) > −3.091 × 10−5 × T (in Kelvin) + 0.0120), above which practically no clear-sky NPF event could be observed. Finally, we present a probability distribution for the frequency of NPF events at a specific CS and temperature., published version, peerReviewed

This paper presents an introduction to the Southern Hemisphere High Altitude Experiment on Particle Nucleation and Growth (SALTENA). This field campaign took place between December 2017 and June 2018 (wet to dry season) at Chacaltaya (CHC), a GAW (Global Atmosphere Watch) station located at 5,240 m MSL in the Bolivian Andes. Concurrent measurements were conducted at two additional sites in El Alto (4,000 m MSL) and La Paz (3,600 m MSL). The overall goal of the campaign was to identify the sources, understand the formation mechanisms and transport, and characterize the properties of aerosol at these stations. State-of-the-art instruments were brought to the station complementing the ongoing permanent GAW measurements, to allow a comprehensive description of the chemical species of anthropogenic and biogenic origin impacting the station and contributing to new particle formation. In this overview we first provide an assessment of the complex meteorology, airmass origin, and boundary layer–free troposphere interactions during the campaign using a 6-month high-resolution Weather Research and Forecasting (WRF) simulation coupled with Flexible Particle dispersion model (FLEXPART). We then show some of the research highlights from the campaign, including (i) chemical transformation processes of anthropogenic pollution while the air masses are transported to the CHC station from the metropolitan area of La Paz–El Alto, (ii) volcanic emissions as an important source of atmospheric sulfur compounds in the region, (iii) the characterization of the compounds involved in new particle formation, and (iv) the identification of long-range-transported compounds from the Pacific or the Amazon basin. We conclude the article with a presentation of future research foci. The SALTENA dataset highlights the importance of comprehensive observations in strategic high-altitude locations, especially the undersampled Southern Hemisphere.

While the role of highly oxygenated molecules (HOMs) in new particle formation (NPF) and secondary organic aerosol (SOA) formation is not in dispute, the interplay between HOM chemistry and atmospheric conditions continues to draw significant research attention. During the Influence of Biosphere-Atmosphere Interactions on the Reactive Nitrogen budget (IBAIRN) campaign, profile measurements of neutral HOM molecules below and above the forest canopy were performed for the first time in the boreal forest SMEAR II station during September 2016. The HOM concentrations and composition distributions below and above the canopy were similar, supporting a well-mixed boundary layer approximation during daytime. However, much lower HOM concentration were frequently observed at ground level due to the formation of a shallow decoupled layer below the canopy attached to the forest floor. Near ground HOMs were influenced by the changes in the precursors and oxidants, and enhancement of the loss on surfaces in this layer, while the HOMs above the canopy top were not significantly affected. Our findings also illustrate that near-ground HOM measurements conducted in strong stably stratified conditions might only be representative of a small fraction of the entire nocturnal boundary layer. This might, in turn, influence the growth of newly formed particles and SOA formation below the canopy where a large majority of measurements are typically conducted.

Concurrently with greenhouse gases, humankind has been emitting aerosol particles and their precursors into the atmosphere. These solid or liquid particles, tiny enough to float in the air, cause adverse health effects as well as a net cooling effect on the Earth's climate, counteracting part of the warming caused by greenhouse gases. The magnitude of this effect is uncertain, leading to uncertainties in projections of future climate. One of the main causes for the uncertainty is our lacking knowledge of the natural, pre-industrial aerosol particles. A major source of aerosol particles is the oxidation of volatile organic compounds (VOCs). VOCs are emitted into the atmosphere in large quantities, with biogenic emissions dominating globally over anthropogenic ones. In the atmosphere, VOCs such as monoterpenes, the main group of VOCs emitted by the boreal forests, undergo oxidation reactions, producing vapours of lower volatility. Part of the products condense on pre-existing aerosol particles, or may even form new particles altogether. The conversion of monoterpenes into condensible vapours is the main topic of this thesis. In this thesis, I aimed to 1) determine which oxidants are important for monoterpene oxidation in the context of new particle formation, 2) quantify the volatilities of a group of VOC oxidation products, highly oxygenated organic molecules (HOMs), and 3) develop new data analysis methods to gain new insights into the formation of condensible vapours. To address these aims, I utilized mass spectrometric methods for measuring VOCs and their oxidation products, in both field and laboratory conditions. First, we found that oxidation of monoterpenes by the hydroxyl radical was likely very important for the growth of newly formed particles. Our results also suggest that multi-generation oxidation reactions are important. Second, we found that monoterpene-derived HOMs are predominantly of low volatility, though also semi-volatile behavior was observed when the HOMs contained eight or less oxygen atoms. Our estimates for the volatilities lie between earlier parametrizations and recent computations. Finally, we developed a new data analysis method for mass spectrometric measurements, based on a novel factorization technique. Our method efficiently uses the high resolution information in the measured spectra, avoiding many of the time consuming and subjective procedures commonly used. It also allowed us to separate new HOM formation processes that could not be found using traditional methods. Ilmastoa lämmittävien kasvihuonekaasujen lisäksi ihmiskunta on päästänyt ilmaan myös pien- eli aerosolihiukkasia ja niiden esiasteita. Toisin kuin kasvihuonekaasut, pienhiukkaset vaikuttavat kokonaisuutena ilmastoa viilentävästi. Ne ovatkin kumonneet osan kasvihuonekaasujen lämmitysvaikutuksesta. Ilmastovaikutustensa lisäksi pienhiukkasilla on kuitenkin myös haitallisia terveys- ja näkyvyysvaikutuksia. Pienhiukkasten ilmastovaikutukset tunnetaan verrattain huonosti: tämä epävarmuus heijastuu tulevaisuuden ilmaston ennustamiseen. Suuri osa ilmakehän pienhiukkasista saa alkunsa haihtuvista orgaanisista yhdisteistä (engl. VOC-yhdisteet). Näitä yhdisteitä vapautuu ilmaan myös ihmistoiminnan seurauksena, mutta luonnolliset lähteet, erityisesti metsät, ovat globaalisti suurimpia päästöjen lähteitä. Ilmakehässä VOC-yhdisteet hapettuvat ja muodostavat heikommin haihtuvia höyryjä. Nämä hapetustuotteet voivat tiivistyä jo olemassa olevien pienhiukkasten pinnalle, tai jopa muodostaa kokonaan uusia hiukkasia. Väitöskirjassani tutkin miten erään VOC-yhdisteiden ryhmän, monoterpeenien, hapetuksessa muodostuu tiivistymiskykyisiä yhdisteitä. Monoterpeenit ovat maailmanlaajuisesti erittäin merkittävä pienhiukkaslähde, ja erityisen tärkeitä ne ovat pohjoisella havumetsävyöhykkeellä. Käytin väitöskirjassani massaspektrometrisia mittauksia sekä ympäröivästä ilmakehästä että laboratoriosta. Selvitin, mitkä eri hapettimet ovat tärkeitä monoterpeenihapetukselle vuorokauden ja vuoden eri aikoina, ja miten monoterpeenihapetus vaikuttaa juuri muodostuneiden pienhiukkasten kasvuun eteläisessä Suomessa sijaitsevalla mittausasemalla. Määritin myös erään vastikään löydetyn VOC-hapetustuotteiden ryhmän, korkeasti hapettuneiden orgaanisten yhdisteiden (engl. HOM-yhdisteiden) haihtuvuusjakauman: haihtuvuuksien tunteminen on olennaista, jotta voimme selvittää, miten HOM-yhdisteet vaikuttavat hiukkasmuodostukseen. Lisäksi, jotta massaspektrometridatan analyysi olisi suoraviivaisempaa ja vähemmän subjektiivista, kehitin uuden data-analyysimenetelmän. Menetelmä pystyy hyödyntämään tehokkaasti datassa olevaa informaatiota, vaatimatta etukäteistietoa mittauksen kohteesta. Pääpiirteissään väitöskirjan tulokset ovat linjassa aiemman tiedon kanssa siitä, että monoterpeenien hapetus on merkittävä osatekijä hiukkasmuodostuksessa.