Tiivistymiskykyisten höyryjen muodostuminen monoterpeenien hapetuksessa

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

Eri reaktiokanavien vaikutus erittäin heikosti haihtuvien orgaanisten yhdisteiden muodostumiseen kammiokokeissa

Aerosolihiukkaset vaikuttavat maapallon ilmastoon pääosin viilentävästi. Niiden syntyprosessit ovat kuitenkin vielä osittain hämärän peitossa. Haihtuvien orgaanisten yhdisteiden hapetustuotteiden tiedetään vaikuttavan hiukkasten kasvuun ilmaston kannalta merkityksellisen kokoisiksi. On myös mahdollista että jotkut hapetustuotteista voivat muodostaa kokonaan uusia hiukkasia. Hapetustuotteista tehokkaimpia hiukkasten muodostuksessa ovat niin sanotut erittäin heikosti haihtuvat orgaaniset yhdisteet, eli ELVOC-yhdisteet. Niiden muodostuminen on havaittu vasta hiljattain, mutta ne ovat hyvin tärkeässä roolissa hiukkasten muodostuksen kannalta. ELVOC-yhdisteitä syntyy muun muassa monoterpeenien, kuten alfa-pineenin, hapetuksessa. Niiden oletetaan syntyvät hapetusreaktioissa muodostuvien RO2-radikaalien itsehapetuksen tuloksena. Näin ollen yhdisteiden syntyyn vaikuttaa se, miten RO2-radikaalit terminoituvat ja muodostavat suljetun kuoren yhdisteitä. Pyrin tutkielmassani selvittämään, miten eri hapettimet ja RO2-radikaalien terminaatiotavat vaikuttavat ELVOC-yhdisteiden muodostukseen. Mittasin ELVOC-muodostusta PNNL:n ympäristökammiossa nitraatti-CI-APi-TOF-massaspektrometrillä. Tutkin eri olosuhteissa muodostuneista ELVOC-monomeereistä ja -dimeereistä mitattua signaalia. Tämän lisäksi analysoin monomeerispektrejä tarkemmin, tutkien muun muassa tuotteiden hapetuksen tasoa. Kokeissa havaitsin, että otsonihapetus synnyttää odotetusti ELVOC-yhdisteitä OH-hapetusta tehokkaammin. Kammiossa syntyi huomattava määrä suhteellisen vähän hapettuneita tuotteita, joita ei olla havaittu aiemmissa kammiokokeissa. Niitä on kuitenkin havaittu kenttämittauksissa, ja ne syntyvät luultavasti HO2-radikaalien terminoidessa RO2-radikaalit ennen kuin ne saavuttavat korkean happi-hiili-suhteen. Näin HO2-radikaali voisi vähentää hyvin hapettuneiden yhdisteiden tuottoa. ELVOC-yhdisteet vaikuttivat hapettuvan pidemmälle, kun kammioon lisättiin typen oksideja. ELVOC-dimeerien muodostus myös väheni odotettua vähemmän typen oksidien vaikutuksesta. Voikin olla, että pieninä pitoisuuksina NO-radikaalit RO2-radikaalien terminoimisen sijasta muodostavat niistä RO-radikaaleja ja tätä kautta edistävät niiden hapetusta. Kokonaisuutena tutkimus onnistui tavoitteissaan. Havaituista hapetustuotteista valtaosan synty kyettiin selittämään aiemmassa tutkimuksessa kuvatuilla mekanismeilla. Joidenkin tuotteiden syntytapa jäi kuitenkin mysteeriksi. Näin ollen myös jatkotutkimukselle jäi paljon tilaa

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

Gas-to-Particle Partitioning of Cyclohexene- and alpha-Pinene-Derived Highly Oxygenated Dimers Evaluated Using COSMOtherm

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 alpha-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.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

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

Long-term analysis of clear-sky new particle formation events and nonevents in Hyytiälä

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 Hyytiala, Finland, by building on 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 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 x 10(-5) x 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.Peer reviewe

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