Mass Spectrometry Studies on Vapours Forming Atmospheric Aerosol Particles

Aerosol particles present in the atmosphere can affect climate, visibility and human health. Low-volatility vapours form a large fraction of aerosol particles through gas-to-particle conversion. Our knowledge of chemical composition of low-volatility vapours has greatly improved in recent years with the development of more sensitive analytical tools. It is now widely accepted that in addition to sulphuric acid (SA), bases (e.g., ammonia and amines) and ions, which have been identified already a decade ago, highly oxygenated organic molecules (HOM) are crucial for the first steps of particle formation and growth. The main goal of this thesis was to identify which organic and inorganic vapours contribute to atmospheric particle formation in different environments. In this thesis, I aimed to 1) determine the role of HOM and SA in forming clusters and particles in the boreal forest; 2) identify the aerosol precursor vapours in a megacity; and 3) determine HOM composition and yield in laboratory oxidation of selected compounds emitted from these two environments. The primary tool that I used for investigating the vapours and clusters was an atmospheric pressure interface time-of-flight mass spectrometer (APi-TOF). It was applied to detect either natural ions or, when equipped with a nitrate chemical ionisation, electrically neutral vapours. To achieve the aims of the thesis, we conducted measurements at a boreal forest station in Finland and in Shanghai, China. In addition, we performed laboratory experiments in a flow reactor and an atmospheric simulation chamber. In the boreal forest, we observed that neutral HOM and SA concentrations influenced the composition of natural negatively charged clusters. Specifically, the ratio between HOM and SA controlled which chemical pathway initiated charged particle formation at this site. In contrast, by comparing our Shanghai observations to laboratory studies, we could conclude that SA-dimethylamine clustering initiated the formation of particles and their initial growth. In the laboratory experiments, we studied HOM formation in oxidation of sesquiterpenes and aromatics, which are emitted from the boreal forest and human activity, respectively. Both experiments showed that HOM formed at high yields. In aromatic oxidation, multi-step oxidation reactions were very important in HOM formation. The results of this thesis increased our understanding of vapours that participate in secondary aerosol formation in the atmosphere. Finally, the results underlined the utmost importance of combining ambient investigations with laboratory experiments in atmospheric science.Ilmakehän pienhiukkaset vaikuttavat ilmastoon, näkyvyyteen sekä terveyteen. Suuri osa näistä pienhiukkasista muodostuu heikosti haihtuvista höyryistä kaasu-hiukkasmuunnoksen kautta. Tiedämme näistä höyryistä huimasti enemmän kuin ennen viime vuosien mittalaitekehityksen ansiosta. Rikkihapon, emästen ja ionien, joiden tiedettiin vaikuttavan hiukkasmuodostukseen jo vuosikymmen sitten, lisäksi nykyään tiedetään, että myös korkeasti hapettuneet orgaaniset molekyylit (engl. highly oxygenated organic molecules, HOM) ovat tärkeitä hiukkasten muodostumisen ja kasvun kannalta. Tässä väitöskirjassa tutkin, mitkä orgaaniset ja epäorgaaniset höyryt ottavat osaa hiukkasten muodostumiseen erilaisissa ympäristöissä. Tässä väitöskirjatyössä pyrin 1) selvittämään HOM-yhdisteiden ja rikkihapon vaikutuksen molekyyliryppäiden ja pienhiukkasten muodostukseen havumetsän yllä, 2) määrittämään pienhiukkasia muodostavat aineet suurkaupungissa, ja 3) määrittämään laboratoriokokeissa HOM-yhdisteiden koostumuksen ja muodostumistehokkuuden näitä ympäristöjä edustavista yhdisteistä. Pääasiallinen väitöskirjassa käytetty mittalaite oli ilmanpaineliitäntäinen lentoaikamassaspektrometri (engl. atmospheric pressure interface time-of-flight mass spectrometer, APi-TOF). Sitä käytettiin sekä luonnollisesti varautuneiden höyryjen ja molekyylirypäiden havaitsemiseen, että, kemiallisen ionisaation suulakkeella varustettuna, sähköisesti neutraalien höyryjen havaitsemiseen. Käytimme näitä mittalaitteita suomalaisessa havumetsässä, sekä Shanghain megakaupungissa Kiinassa. Näiden kenttämittausten lisäksi käytimme samoja laitteita laboratoriokokeissa, niin virtausputkimittauksissa kuin ilmakehää simuloivassa kammiossakin. Havaitsimme, että havumetsässä mitatut, neutraalit HOM-yhdisteet ja rikkihappo vaikuttivat myös sähköisesti varattujen ionien koostumukseen. HOM-rikkihappo-suhde määritti, mikä mekanismi aloitti varattujen hiukkasten muodostumisen. Shanghaissa havaitsimme, laboratoriotuloksiin vertaamalla, että rikkihappo-dimetyyliamiini-ryvästyminen aloitti hiukkasmuodostuksen. Laboratoriossa tutkimme seskviterpeenien ja aromaattisten yhdisteiden muodostamia HOM-yhdisteitä. Näistä ensimmäistä pääsee ilmaan havumetsistä, ja toista ihmistoimien kautta. Havaitsimme kummastakin muodostuvan reippaasti HOM-yhdisteitä, ja että aromaattisten yhdisteiden tapauksessa monivaiheinen hapetus oli erittäin tärkeää yhdisteiden muodostumiselle. Väitöskirjan tulokset lisäävät ymmärrystämme höyryistä, jotka muodostavat pienhiukkasia ilmakehässä. Ne myös osoittavat kenttä- ja laboratoriomittauksien yhdistämisen tärkeyden ilmakehätieteissä

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

Efficient alkane oxidation under combustion engine and atmospheric conditions

Efficient autoxidation of organic compounds typically requires that they possess double bonds or oxygen-containing moieties, which is why alkanes were thought to contribute little to atmospheric organic aerosol formation. Here, mass spectrometry shows significant autoxidation of alkanes under both atmospheric and combustion conditions. Oxidation chemistry controls both combustion processes and the atmospheric transformation of volatile emissions. In combustion engines, radical species undergo isomerization reactions that allow fast addition of O-2. This chain reaction, termed autoxidation, is enabled by high engine temperatures, but has recently been also identified as an important source for highly oxygenated species in the atmosphere, forming organic aerosol. Conventional knowledge suggests that atmospheric autoxidation requires suitable structural features, like double bonds or oxygen-containing moieties, in the precursors. With neither of these functionalities, alkanes, the primary fuel type in combustion engines and an important class of urban trace gases, are thought to have minor susceptibility to extensive autoxidation. Here, utilizing state-of-the-art mass spectrometry, measuring both radicals and oxidation products, we show that alkanes undergo autoxidation much more efficiently than previously thought, both under atmospheric and combustion conditions. Even at high concentrations of NOX, which typically rapidly terminates autoxidation in urban areas, the studied C-6-C-10 alkanes produce considerable amounts of highly oxygenated products that can contribute to urban organic aerosol. The results of this inter-disciplinary effort provide crucial information on oxidation processes in both combustion engines and the atmosphere, with direct implications for engine efficiency and urban air quality.Peer reviewe

Efficient alkane oxidation under combustion engine and atmospheric conditions

Oxidation chemistry controls both combustion processes and the atmospheric transformation of volatile emissions. In combustion engines, radical species undergo isomerization reactions that allow fast addition of O2. This chain reaction, termed autoxidation, is enabled by high engine temperatures, but has recently been also identified as an important source for highly oxygenated species in the atmosphere, forming organic aerosol. Conventional knowledge suggests that atmospheric autoxidation requires suitable structural features, like double bonds or oxygen-containing moieties, in the precursors. With neither of these functionalities, alkanes, the primary fuel type in combustion engines and an important class of urban trace gases, are thought to have minor susceptibility to extensive autoxidation. Here, utilizing state-of-the-art mass spectrometry, measuring both radicals and oxidation products, we show that alkanes undergo autoxidation much more efficiently than previously thought, both under atmospheric and combustion conditions. Even at high concentrations of NOX, which typically rapidly terminates autoxidation in urban areas, the studied C6–C10 alkanes produce considerable amounts of highly oxygenated products that can contribute to urban organic aerosol. The results of this inter-disciplinary effort provide crucial information on oxidation processes in both combustion engines and the atmosphere, with direct implications for engine efficiency and urban air quality

Oxidation product characterization from ozonolysis of the diterpene ent-kaurene

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.Peer reviewe

Detecting and Characterizing Particulate Organic Nitrates with an Aerodyne Long-ToF Aerosol Mass Spectrometer

Particulate organic nitrate (pON) can be a major part of secondary organic aerosol (SOA) and is commonly quantified by indirect means from aerosol mass spectrometer (AMS) data. However, pON quantification remains challenging. Here, we set out to quantify and characterize pON in the boreal forest, through direct field observations at Station for Measuring Ecosystem Atmosphere Relationships (SMEAR) II in Hyytia''la'', Finland, and targeted single precursor laboratory studies. We utilized a long time-of-flight AMS (LToF-AMS) for aerosol chemical characterization, with a particular focus to identify CxHyOzN+ ("CHON+") fragments. We estimate that during springtime at SMEAR II, pON (including both the organic and nitrate part) accounts for similar to 10% of the particle mass concentration (calculated by the NO+/NO2+ method) and originates mainly from the NO3 radical oxidation of biogenic volatile organic compounds. The majority of the background nitrate aerosol measured is organic. The CHON+ fragment analysis was largely unsuccessful at SMEAR II, mainly due to low concentrations of the few detected fragments. However, our findings may be useful at other sites as we identified 80 unique CHON+ fragments from the laboratory measurements of SOA formed from NO3 radical oxidation of three pON precursors (beta-pinene, limonene, and guaiacol). Finally, we noted a significant effect on ion identification during the LToF-AMS high-resolution data processing, resulting in too many ions being fit, depending on whether tungsten ions (W+) were used in the peak width determination. Although this phenomenon may be instrument-specific, we encourage all (LTOF-) AMS users to investigate this effect on their instrument to reduce the possibility of incorrect identifications.Peer reviewe

Factors controlling the evaporation of secondary organic aerosol from alpha-pinene ozonolysis

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 10(9)Pas. 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 -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.Peer reviewe

Measurement report : The influence of traffic and new particle formation on the size distribution of 1–800 nm particles in Helsinki – a street canyon and an urban background station comparison

Peer reviewe