Observational insights on the evolution of organic aerosol in the boreal environment

Aerosol particles, which are solid or liquid particles suspended in the air reduce air quality as well as influence Earth’s radiative balance through their direct and indirect interactions with solar radiation. The sensitivity of Earth’s climate to aerosol particles has remained elusive despite a wealth of studies conducted. Some of this uncertainty arises from the highly dynamic manner the physicochemical properties of aerosol particles evolve in the atmosphere. Recent advances in mass spectrometric measurement techniques have helped to assess the aerosol chemical (trans)formation altering many climate-relevant aerosol properties such as aerosol particle size, volatility and water affinity. The scope of my thesis is the formation and evolution of organic aerosol (OA) in the boreal environment. Boreal forests emit a wide variety of volatile organic compounds (VOCs), which can form secondary organic aerosol (SOA) after oxidation. Notably, the mechanisms behind the VOC to SOA conversion is not straightforward and depends highly on the type of VOC, oxidants and evnironmental conditions. This process can therefore be sensitive to the changes projected to take place within the boreal biome along with the changing climate. Changes regarding tree species can alter the composition and relative abundances of the emitted VOCs, which can further influence boreal SOA formation. Anthropogenic emissions can also affect natural VOC to SOA conversion and further SOA processing (aging) even in the pristine regions of the boreal biome. Due to attempts in air quality improvements in several locations worldwide, many anthropogenic species have shown declining trends with potential consequences in SOA formation and aging. The studies conducted within this thesis can be divided into two categories: 1) examining the accumulated OA composition and its seasonal dynamics in the boreal forest to understand the present state of boreal OA (field studies), and 2) zooming in on the early stages of SOA formation and evolution and the potential impacts of anthropogenic emissions on them (laboratory studies). The measurements were conducted via online mass spectrometry. The results highlight the importance of aged low-volatility oxygenated OA (LV-OOA) within the boreal environment throughout the year, with highly season-dependent sources. The LV-OOA production was found to be extremely sensitive to heat waves. In the laboratory, we examined the volatilities and fates of highly oxygenated organic molecules (HOMs), which form in boreal VOC oxidation. HOMs were shown to be primarily of low volatility and therefore good candidates as SOA precursors. In the presence of nitrogen oxides, which imply anthropogenic influence, HOMs of relatively higher volatilities were formed, potentially reducing HOM condensate formation. HOM condensate has been previously shown to be labile, and fragment quickly after formation. In our studies, this reaction was boosted in the presence of acidic aerosol particles. SOA formation was influenced by aerosol acidity also by significantly enhancing the SOA yield from semi- or intermediate volatility precursors and simultaneous oligomerisation reactions. The formation of high molecular weight oligomers significantly reduced SOA volatility. However, the difference observed between the ambient LV-OOA and fresh laboratory SOA composition underlines the importance of photochemical aging needed for the formation of the highly oxidised ambient OA.Ilmakehän aerosolihiukkaset ovat kiinteitä tai nestemäisiä hiukkasia ilmassa. Pienestä koostaan huolimatta niillä on suuria vaikutuksia elämiimme: ne huonontavat merkittävästi ilmanlaatua sekä vaikuttavat ilmastoon vuorovaikuttaessaan säteilyn kanssa. Säteilyvuorovaikutus voi tapahtua joko suoraan, missä tapauksessa aerosolihiukkanen sirottaa tai absorboi säteilyä. Epäsuorat vaikutukset liittyvät aerosolihiukkasten rooliin pilvipisaroiden tiivistymisytiminä: jos tiivistymisytimiä on paljon, niiden kautta muodostuneet pilvet heijastavat auringonsäteilyä ja siten viilentävät ilmastoa tehokkaammin kuin tapauksessa, jossa tiivistymisytimiä olisi tarjolla vain vähän. Vaikka aerosolihiukkasten ilmastovaikutuksia on tutkittu laajalti, niihin liittyy suuria epävarmuuksia. Osa tästä epävarmuudesta johtuu siitä, että aerosolihiukkasten ominaisuudet ovat hyvin muutosherkkiä ilmakehässä. Massaspektrometria on osoittautunut hyväksi työkaluksi arvioimaan erityisesti hiukkasten kemiallisia muutoksia, joiden tiedetään vaikuttavan moniin hiukkasten ilmaston kannalta tärkeisiin ominaisuuksiin, kuten niiden kokoon, haihtuvuuteen sekä niiden kykyyn absorboida vettä. Tässä tutkielmassa perehdyn orgaanisen aerosolin muodostumiseen ja kemialliseen kehitykseen pohjoisella havumetsävyöhykkeellä. Havumetsien kasvillisuus tuottaa laajalti erilaisia haihtuvia orgaanisia yhdisteitä, jotka voivat ilmakehässä hapettuessaan muodostaa sekundääristä orgaanista aerosolia (SOA). SOA-muodostus riippuu monista tekijöistä, kuten siitä minkälainen haihtuva orgaaninen yhdiste on kyseessä tai ilmakehän olosuhteista, kuten esimerkiksi lämpötilasta. Näiden seikkojen vuoksi SOA-muodostus pohjoisella havumetsävyöhykkeellä voi muuttua samalla kun ilmasto näillä leveysasteilla muuttuu. Ilmastonmuutoksen on arvioitu aiheuttavan muutoksia pohjoisen havumetsän kasvillisuuteen, mikä voi vaikuttaa muun muassa haihtuvien orgaanisten yhdisteiden suhteellisiin osuuksiin. Myös ihmiskunnan aiheuttamat päästöt voivat vaikuttavat SOA-muodostukseen sekä sen kemialliseen kehitykseen. Täten esimerkiksi päästörajoitukset vaikuttavat tulevaisuuden SOA pitoisuuksiin ja ominaisuuksiin yhdessä ilmastonmuutoksen kanssa. Näillä muutoksilla on mahdollisia vaikutuksia sekä paikalliseen että globaaliin ilmastoon. Tämän väitöskirjan tutkimukset voidaan jakaa kahteen osaan, joista ensimmäisessä tutkin orgaanisen aerosolin koostumusta ja vuodenaikaisvaihtelua suomalaisessa havupuumetsässä hyödyntäen kahdeksanvuotista aikasarjaa. Koska aerosolihiukkaset elävät ilmakehässä noin viikon verran, nämä tulokset edustavat täten erityisesti ilmakehään kerääntynyttä orgaanista aerosolia. Jotta ymmärtäisimme paremmin myös orgaanisen aerosolin elämänkaaren alkumetrejä, tutkielman toisessa osassa tutkin SOA-muodostusta ja transformaatiota laboratoriossa, jossa aerosolihiukkasten keskimääräinen elinaika oli noin tunnin verran. Tutkielman tulokset korostivat boreaalisen SOA-muodostuksen ja kemiallisen kehityksen herkkyyttä erityisesti hellejaksoille ja ihmiskunnan päästöille

Aerosolihiukkasten kemiallisen koostumuksen vuodenaikaisvaihtelu pohjoisessa havumetsässä

Ilmakehän pienhiukkaset, eli aerosolihiukkaset, vaikuttavat maapallon säteilypakotteeseen riippuen hiukkasen kemiallisesta koostumuksesta. Kemiallinen koostumus ohjaa yli 50 nanometristen hiukkasten taipumusta joko sirottaa tai absorboida auringonsäteilyä. Toisaalta hiukkaset voivat myös osallistua pilvenmuodostukseen, jos ne ovat koostumukseltaan kyllin hapettuneita ja siksi pystyvät sitomaan ympärilleen vesimolekyylejä. Ilman aerosolihiukkasia maapallo olisi varmasti paljon lämpimämpi, sillä sekä suora säteilyn sirottaminen että epäsuora sirottaminen pilvien kautta ovat tärkeitä ilmakehän viilennysmekanismeja. Auringonsäteilyn määrä ja siten pintalämpötilat ohjaavat ilmakehän pienhiukkasten pitoisuuksia ihmisten ja luonnon kautta. Hiukkasten primäärilähteitä ja lähtöaineita on valtava kirjo, joka luo laajan hiukkasten fysiokemiallisten ominaisuuksien kokoelman. Tässä työssä esitellään tutkimus aerosolihiukkasten kemiallisen koostumuksen vuodenaikaisvaihtelusta SMEAR II -asemalla, Etelä-Suomessa, jossa lämpötilan vuodenaikaisvaihtelu on suurta. Työhön liittyvissä mittauksissa hyödynnettiin massaspektrometriaa ja in situ -suodatinmittauksia. Aerosolikemiamittaukset kuuluvat SMEAR II -aseman rutiinimittauksiin, ja niitä on tehty jatkuvasti vuodesta 2012 lähtien. Tässä tutkielmassa analysoinnin kohteena ovat vuoden 2014 neljä termistä vuodenaikaa, jotka sijoittuivat tarkemmin ajanjaksolle 23.1. - 27.10.2014. Tutkimuksen analyysimenetelmät todettiin hyödyllisiksi ja niitä sovelletaan tulevaisuudessa koko nelivuotisen aikasarjan (2012 - 2015) analyysiin. Työssä todettiin aerosolihiukkasten koostuvan kesällä pääasiassa luontoperäisistä orgaanisista yhdisteistä (~77 %) ja talvella enimmäkseen epäorgaanisista yhdisteistä (~54 % ), jotka olivat kaukokulkeumaa kaupungeista. Sulfaatti dominoi tasaisesti epäorgaanista massaa vuodenajasta riippumatta. Ilmakehän hapetuskapasiteetti heijastui kesällä orgaanisen aerosolin haihtuvuustasoon madaltaen sitä huomattavasti. Kesällä puolihaihtuvalla orgaanisella aerosolilla ja nitraatilla oli selkeä vuorokausisykli, jossa valtaosa massasta oli hiukkasfaasissa yöllä ja katosi kaasufaasiin keskipäivällä. Vastaavanlaista käyttäytymistä ei orgaanisilla aerosolityypeillä ollut havaittavissa talvella, mikä johtui toisaalta kylmemmistä lämpötiloista ja toisaalta myös orgaanisen aerosolin erilaisesta koostumuksesta. Epäorgaanisten yhdisteiden vuorokausisyklien tulkinta oli haastavaa, johtuen kaukokulkeuman ajallisten vaihteluiden runsaudesta, joka tulevaisuuden laajemmassa analyysissä saadaan minimoitua

Experimental investigation into the volatilities of highly oxygenated organic molecules (HOMs)

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.Peer reviewe

Chemical Characterization of Gas- and Particle-Phase Products from the Ozonolysis of alpha-Pinene in the Presence of Dimethylamine

Amines are recognized as key compounds in new particle formation (NPF) and secondary organic aerosol (SOA) formation. In addition, ozonolysis of a-pinene contributes substantially to the formation of biogenic SOAs in the atmosphere. In the present study, ozonolysis of a-pinene in the presence of dimethylamine (DMA) was investigated in a flow tube reactor. Effects of amines on SOA formation and chemical composition were examined. Enhancement of NPF and SOA formation was observed in the presence of DMA. Chemical characterization of gas and particle-phase products by high-resolution mass spectrometric techniques revealed the formation of nitrogen containing compounds. Reactions between ozonolysis reaction products of a-pinene, such as pinonaldehyde or pinonic acid, and DMA were observed. Possible reaction pathways are suggested for the formation of the reaction products. Some of the compounds identified in the laboratory study were also observed in aerosol samples (PM1) collected at the SMEAR II station (Hyytiala, Finland) suggesting that DMA might affect the ozonolysis of a-pinene in ambient conditions.Peer reviewe

Formation of highly oxygenated organic molecules from chlorine-atom-initiated oxidation of alpha-pinene

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 (C1NO(2)) 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.Peer reviewe

Resolving anthropogenic aerosol pollution types - deconvolution and exploratory classification of pollution events

Mass spectrometric measurements commonly yield data on hundreds of variables over thousands of points in time. Refining and synthesizing this raw data into chemical information necessitates the use of advanced, statisticsbased data analytical techniques. In the field of analytical aerosol chemistry, statistical, dimensionality reductive methods have become widespread in the last decade, yet comparable advanced chemometric techniques for data classification and identification remain marginal. Here we present an example of combining data dimensionality reduction (factorization) with exploratory classification (clustering), and show that the results cannot only reproduce and corroborate earlier findings, but also complement and broaden our current perspectives on aerosol chemical classification. We find that applying positive matrix factorization to extract spectral characteristics of the organic component of air pollution plumes, together with an unsupervised clustering algorithm, k -means C C, for classification, reproduces classical organic aerosol speciation schemes. Applying appropriately chosen metrics for spectral dissimilarity along with optimized data weighting, the source-specific pollution characteristics can be statistically resolved even for spectrally very similar aerosol types, such as different combustion-related anthropogenic aerosol species and atmospheric aerosols with similar degree of oxidation. In addition to the typical oxidation level and source-driven aerosol classification, we were also able to classify and characterize outlier groups that would likely be disregarded in a more conventional analysis. Evaluating solution quality for the classification also provides means to assess the performance of mass spectral simi-larity metrics and optimize weighting for mass spectral variables. This facilitates algorithm-based evaluation of aerosol spectra, which may prove invaluable for future development of automatic methods for spectra identification and classification. Robust, statistics-based results and data visualizations also provide important clues to a human analyst on the existence and chemical interpretation of data structures. Applying these methods to a test set of data, aerosol mass spectrometric data of organic aerosol from a boreal forest site, yielded five to seven different recurring pollution types from various sources, including traffic, cooking, biomass burning and nearby sawmills. Additionally, three distinct, minor pollution types were discovered and identified as amine-dominated aerosols.Peer reviewe

Measurement report : Molecular composition and volatility of gaseous organic compounds in a boreal forest - from volatile organic compounds to highly oxygenated organic molecules

The molecular composition and volatility of gaseous organic compounds were investigated during April- July 2019 at the Station for Measuring Ecosystem - Atmosphere Relations (SMEAR) II situated in a boreal forest in Hyytiala, southern Finland. In order to obtain a more complete picture and full understanding of the molecular composition and volatility of ambient gaseous organic compounds (from volatile organic compounds, VOCs, to highly oxygenated organic molecules, HOMs), two different instruments were used. A Vocus proton-transfer-reaction time-of-flight mass spectrometer (Vocus PTR-ToF; hereafter Vocus) was deployed to measure VOCs and less oxygenated VOCs (i.e., OVOCs). In addition, a multi-scheme chemical ionization inlet coupled to an atmospheric pressure interface time-of-flight mass spectrometer (MION API-ToF) was used to detect less oxygenated VOCs (using Br- as the reagent ion; hereafter MION-Br) and more oxygenated VOCs (including HOMs; using NO3- as the reagent ion; hereafter MION-NO3). The comparison among different measurement techniques revealed that the highest elemental oxygen-to-carbon ratios (O : C) of organic compounds were observed by the MION-NO3 (0.9 +/- 0.1, average +/- 1 standard deviation), followed by the MION-Br (0.8 +/- 0.1); lowest O : C ratios were observed by Vocus (0.2 +/- 0.1). Diurnal patterns of the measured organic compounds were found to vary among different measurement techniques, even for compounds with the same molecular formula, suggesting contributions of different isomers detected by the different techniques and/or fragmentation from different parent compounds inside the instruments. Based on the complementary molecular information obtained from Vocus, MION-Br, and MION-NO3, a more complete picture of the bulk volatility of all measured organic compounds in this boreal forest was obtained. As expected, the VOC class was the most abundant (about 53.2 %), followed by intermediate-volatility organic compounds (IVOCs, about 45.9 %). Although condensable organic compounds (low-volatility organic compounds, LVOCs; extremely low volatility organic compounds, ELVOCs; and ultralow-volatility organic compounds, ULVOCs) only comprised about 0.2 % of the total gaseous organic compounds, they play an important role in new particle formation as shown in previous studies in this boreal forest. Our study shows the full characterization of the gaseous organic compounds in the boreal forest and the advantages of combining Vocus and MION API-ToF for measuring ambient organic compounds with different oxidation extents (from VOCs to HOMs). The results therefore provide a more comprehensive understanding of the molecular composition and volatility of atmospheric organic compounds as well as new insights into interpreting ambient measurements or testing/improving parameterizations in transport and climate models.Peer reviewe

The effect of clouds and precipitation on the aerosol concentrations and composition in a boreal forest environment

Atmospheric aerosol particle concentrations are strongly affected by various wet processes, including below and in-cloud wet scavenging and in-cloud aqueous-phase oxidation. We studied how wet scavenging and cloud processes affect particle concentrations and composition during transport to a rural boreal forest site in northern Europe. For this investigation, we employed air mass history analysis and observational data. Long-term particle number size distribution (similar to 15 years) and composition measurements (similar to 8 years) were combined with air mass trajectories with relevant variables from reanalysis data. Some such variables were rainfall rate, relative humidity, and mixing layer height. Additional observational datasets, such as temperature and trace gases, helped further evaluate wet processes along trajectories with mixed effects models. All chemical species investigated (sulfate, black carbon, and organics) exponentially decreased in particle mass concentration as a function of accumulated precipitation along the air mass route. In sulfate (SO4) aerosols, clear seasonal differences in wet removal emerged, whereas organics (Org) and equivalent black carbon (eBC) exhibited only minor differences. The removal efficiency varied slightly among the different reanalysis datasets (ERA-Interim and Global Data Assimilation System; GDAS) used for the trajectory calculations due to the difference in the average occurrence of precipitation events along the air mass trajectories between the reanalysis datasets. Aqueous-phase processes were investigated by using a proxy for air masses travelling inside clouds. We compared air masses with no experience of approximated in-cloud conditions or precipitation during the past 24 h to air masses recently inside non-precipitating clouds before they entered SMEAR II (Station for Measuring Ecosystem-Atmosphere Relations). Significant increases in SO4 mass concentration were observed for the latter air masses (recently experienced non-precipitating clouds). Our mixed effects model considered other contributing factors affecting particle mass concentrations in SMEAR II: examples were trace gases, local meteorology, and diurnal variation. This model also indicated in-cloud SO4 production. Despite the reanalysis dataset used in the trajectory calculations, aqueous-phase SO4 formation was observed. Particle number size distribution measurements revealed that most of the in-cloud SO4 formed can be attributed to particle sizes larger than 200 nm (electrical mobility diameter). Aqueous-phase secondary organic aerosol (aqSOA) formation was non-significant.Peer reviewe

A novel approach for simple statistical analysis of high-resolution mass spectra

Recent advancements in atmospheric mass spectrometry provide huge amounts of new information but at the same time present considerable challenges for the data analysts. High-resolution (HR) peak identification and separation can be effort- and time-consuming yet still tricky and inaccurate due to the complexity of overlapping peaks, especially at larger mass-to-charge ratios. This study presents a simple and novel method, mass spectral binning combined with positive matrix factorization (binPMF), to address these problems. Different from unit mass resolution (UMR) analysis or HR peak fitting, which represent the routine data analysis approaches for mass spectrometry datasets, binPMF divides the mass spectra into small bins and takes advantage of the positive matrix factorization's (PMF) strength in separating different sources or processes based on different temporal patterns. In this study, we applied the novel approach to both ambient and synthetic datasets to evaluate its performance. It not only succeeded in separating overlapping ions but was found to be sensitive to subtle variations as well. Being fast and reliable, binPMF has no requirement for a priori peak information and can save much time and effort from conventional HR peak fitting, while still utilizing nearly the full potential of HR mass spectra. In addition, we identify several future improvements and applications for binPMF and believe it will become a powerful approach in the data analysis of mass spectra.Peer reviewe

Insights into atmospheric oxidation processes by performing factor analyses on subranges of mass spectra

Our understanding of atmospheric oxidation chemistry has improved significantly in recent years, greatly facilitated by developments in mass spectrometry. The generated mass spectra typically contain vast amounts of information on atmospheric sources and processes, but the identification and quantification of these is hampered by the wealth of data to analyze. The implementation of factor analysis techniques have greatly facilitated this analysis, yet many atmospheric processes still remain poorly understood. Here, we present new insights into highly oxygenated products from monoterpene oxidation, measured by chemical ionization mass spectrometry, at a boreal forest site in Finland in autumn 2016. Our primary focus was on the formation of accretion products, i.e., dimers. We identified the formation of daytime dimers, with a diurnal peak at noontime, despite high nitric oxide (NO) concentrations typically expected to inhibit dimer formation. These dimers may play an important role in new particle formation events that are often observed in the forest. In addition, dimers identified as combined products of NO3 and O3 oxidation of monoterpenes were also found to be a large source of low-volatility vapors at night. This highlights the complexity of atmospheric oxidation chemistry and the need for future laboratory studies on multi-oxidant systems. These two processes could not have been separated without the new analysis approach deployed in our study, where we applied binned positive matrix factorization (binPMF) on subranges of the mass spectra rather than the traditional approach where the entire mass spectrum is included for PMF analysis. In addition to the main findings listed above, several other benefits compared to traditional methods were found.Peer reviewe