From gas-phase oxidation to nanoparticles : a mass spectrometric approach

Atmospheric aerosols are small liquid or solid particles suspended in the air. These small particles have significant effects to our climate and health. Approximately half of the particles that grow into cloud condensation nuclei −size are primary particles and emitted directly into the atmosphere, whereas the other half are secondary particles which are formed in the atmosphere. In new particle formation, molecular clusters form from atmospheric low-volatility vapors by condensation and/or chemical reactions. Atmospheric oxidation is a key phenomenon enhancing atmospheric particle formation since oxidized compounds condense easier due to their lower vapor pressure. So far two oxidation processes have been identified as relevant for new particle formation: the oxidation of sulfur dioxide to sulfuric acid and oxidation of volatile organic compounds to highly oxygenated compounds. The most significant atmospheric oxidants have previously thought to be ozone, hydroxyl radical and nitrate radical. Recently the importance of stabilized Criegee intermediates in atmospheric oxidation has been brought into discussion. In this thesis, we used Chemical Ionization Atmospheric Pressure interface Time of Flight mass spectrometer together with different particle measurements in order to widen the understanding of the first steps of new particle formation. We also developed new mass spectrometric measurement techniques to fill the gaps in our current methods. We developed an indirect method to measure non- OH oxidants of sulfur dioxide to better understand the role of stabilized Criegee intermediates and other non-OH oxidants of sulfur dioxide in sulfuric acid formation. We also developed a new technique to determine concentration of ambient dimethylamine at sub-pptV-level. We used both of these new techniques to measure the ambient concentrations in Boreal forest, at SMEAR II station (Station for Measuring Ecosystem-Atmosphere Relations II, Hyytiälä, Finland). Furthermore, we measured new particle formation in different environments and in a chamber study and tried to identify the condensing vapors. We studied the ozonolysis of α-pinene, the most abundant monoterpene in the atmosphere, in controlled chamber measurements in order to be able to follow the formation of highly oxygenated organics and oxidation of sulfur dioxide purely by stabilized Criegee intermediates and compare the results with kinetic model results. We studied the new particle formation near an oil refinery and found that significantly large fraction of the growth during the new particle formation events was due to sulfuric acid condensation. In our studies at the Atlantic coast, we identified the molecular steps involved in new particle formation at iodine-rich environment and could follow the growth of molecular clusters by subsequent addition of iodic acid molecules. We also did field measurements in Arctic and Antarctic sites and showed that the occurrence of high iodic acid concentration is not limited only to coastal areas with macro algae beds. Keywords: mass spectrometry, atmospheric aerosols, low-volatility vapors, ozonolysis, new particle formationIlmakehän aerosolit ovat ilman ja siinä leijuvien kiinteiden tai nestemäisten hiukkasten seos. Näillä pienhiukkasilla on merkittävä vaikutus ilmastoomme ja terveyteemme. Kun hiukkaset kasvavat noin 50 nanometrin kokoluokkaan, ne pystyvät toimimaan pilvipisaroiden tiivistymisytiminä. Noin puolet näistä ovat primäärisiä hiukkasia eli ne ovat päätyneet ilmakehään suoraan hiukkasina. Primäärisiä pienhiukkasia ovat esimerkiksi paloprosesseissa muodostuvat hiukkaset, meren suolahiukkaset ja katupöly. Puolet pilvipisaroiden tiivistymisytimistä muodostuu sekundäärisistä hiukkasista, jotka muodostuvat ilmakehän kaasuista. Pienhiukkasten muodostumista höyrystä tiivistymällä kutsutaan kaasu-hiukkas - muuntumaksi. Ilmakehän heikosti haihtuvien höyryjen tiivistyessä muodostuu molekyyliklustereita, jotka kasvavat jatkuvan tiivistymisen ja kemiallisten reaktioiden seurauksena. Ilmakehän hapetusreaktiot ovat tärkeitä hiukkasmuodostukselle, sillä hapettuneet yhdisteet tiivistyvät helpommin niiden matalamman höyrynpaineen ansiosta. Tähän mennessä on löydetty kaksi hapetusreaktiota, jotka ovat erityisen merkittäviä hiukkasmuodostukselle: rikkidioksidin hapettuminen rikkihapoksi ja haihtuvien orgaanisten yhdisteiden hapettuminen korkeasti hapettuneiksi molekyyleiksi (highly oxygenated molecules, HOM). Ilmakehän tärkeimpinä hapettajina on pidetty otsonia, hydroksyyliradikaalia ja nitraattiradikaalia. Viime aikoina stabiloidun Criegee välituotteen (stabilized Criegee intermediate, sCI) merkitys ilmakehän hapettajana on tuotu esiin. Tämän väitöskirjan tutkimuksissa käytimme kemialliseen ionisaatioon perustuvaa ilmanpaineliitäntäistä lentoaikamassaspektrometriä samanaikaisesti erilaisten hiukkasmittausleitteiden kanssa tavoitteenamme laajentaa hiukkasmuodostuksen ensimmäisten vaiheiden ymmärrystä. Väitöskirjatutkimuksessa kehitimme uusia massaspektrometrisia mittausmenetelmiä. Kehitimme mittalaitteen, jolla voimme mitata epäsuorasti muita rikkidioksidin hapettajia kuin hydroksyyliradikaalia, jotta voimme ymmärtää sCI:n ja mahdollisten muiden hapettajien merkityksen rikkihapon muodostuksessa. Kehitimme myös uuden menetelmän, jolla voimme mitata erittäin pieniä pitoisuuksia dimetyyliamiinia. Käytimme kumpaakin kehittämäämme menetelmää kenttämittauksissa SMEAR II asemalla, Hyytiälässä. Väitöskirjassa tutkimme hiukkasmuodostusta erilaisissa ympäristöissä kenttämittauksilla sekä kammiokokeilla ja selvitimme mitkä höyryt muodostavat pienhiukkasia erilaisissa olosuhteissa. Tutkimme otsonin ja α-pineenin, ilmakehän yleisimmän monoterpeenin, hapetusreaktiota ja reaktiotuotteiden muodostumista kammiomittauksissa Euroopan hiukkasfysiikan tutkimuskeskuksessa. Tutkimuksessa pystyimme seuraamaan HOM-yhdisteiden muodostumista ja rikkidioksidin hapettumista puhtaasti sCI-hapettajan kautta ja vertailemaan mitattuja pitoisuuksia kineettiseen malliin. Tämän lisäksi tutkimme hiukkasmuodostusta öljynjalostamon läheisyydessä, jossa määritimme, että huomattavan suuri osa hiukkasten kasvusta johtui rikkihapon tiivistymisestä. Tutkiessamme hiukkasmuodostusta Atlantin rannikolla pystyimme seuraamaan molekyyliklustereiden muodostumista jodihapoista molekyylitasolla. Kenttämittauksissamme Grönlannissa ja Etelämantereella olemme myös mitanneet korkeita pitoisuuksia jodihappoa

Soft X-ray Atmospheric Pressure Photoionization in Liquid Chromatography–Mass Spectrometry

Soft X-ray atmospheric pressure photoionization (soft X-ray APPI) as an ionization method in liquid chromatography-mass spectrometry (LC-MS) is presented. The ionization mechanism was examined with selected test compounds in the negative ion mode, using soft X-ray APPI source emitting 4.9 keV photons. Test compounds with an acidic group were ionized by a proton transfer reaction, producing deprotonated molecules ([M-H](-)), whereas compounds having positive electron affinity were ionized by a charge exchange reaction, producing negative molecular ions (M-.). Soft X-ray APPI does not require a dopant to achieve high ionization efficiency, which is an advantage compared with vacuum ultraviolet APPI with 10 eV photons, in which a dopant is needed to improve ionization efficiency. The energy of the soft X-ray photons is in the keV range, which is high enough to displace a valence electron and often also inner shell electrons from LC eluents and atmospheric gases, initiating an efficient ionization process in the negative ion mode.Peer reviewe

Molecular Steps of Neutral Sulfuric Acid and Dimethylamine Nucleation in CLOUD

We have run a set of experiments in the CLOUD chamber at CERN, Switzerland, studying the effect of dimethylamine (DMA) on sulfuric acid (SA)-water nucleation using a nitrate based Chemical Ionization Atmospheric Pressure ionization Time-Of-Flight Mass Spectrometer (CI-APi-TOF). Experiment was designed to produce neutral high m/z SA-DMA clusters in close to atmospherically relevant conditions to be detected and characterized by the CI-APi-TOF. We aimed in filling up the gap in measurement techniques from molecular level up to climatically relevant aerosol particles and thus improve our understanding of the role of sulfuric acid and DMA in atmospheric nucleation

Chemistry of stabilized Criegee intermediates in the CLOUD chamber

In atmospheric conditions the oxidation of sulphur dioxide to sulphuric acid in gas phase has been considered to be determined by the concentration of hydroxyl radical. Recently the significance of stabilized Criegee intermediate as an oxidizer of sulphuric acid has been brought out. In this study we investigated the oxidation of sulphur dioxide in the CLOUD chamber in conditions where the hydroxyl radical was removed. The concentration of formed sulphuric acid was measured with a chemical ionization atmospheric pressure interface time-of-flight mass spectrometer and it was compared with the calculated yield of sulphuric acid

Evidence for Diverse Biogeochemical Drivers of Boreal Forest New Particle Formation

New particle formation (NPF) is an important contributor to particle number in many locations, but the chemical drivers for this process are not well understood. Daytime NPF events occur regularly in the springtime Finnish boreal forest and strongly impact aerosol abundance. In April 2014 size-resolved chemical measurements of ambient nanoparticles were made using the Time-of-Flight Thermal Desorption Chemical ionization Mass Spectrometer and we report results from two NPF events. While growth overall was dominated by terpene oxidation products, newly formed 20-70nm particles showed enhancement in apparent alkanoic acids. The events occurred on days with rapid transport of marine air, which correlated with low background aerosol loading and higher gas phase methanesulfonic acid levels. These results are broadly consistent with previous studies on Nordic NPF but indicate that further attention should be given to the sources and role of non-terpenoid organics and the possible contribution of transported marine compounds in this process. Plain Language Summary Clouds are an enormously important part of the climate system because they control the radiation entering and leaving the Earth. Clouds form as water condenses onto small particles called cloud condensation nuclei. These particles can be directly emitted from the Earth's surface, like sea spray, for example, or they can form in the atmosphere out of precursor gases. We have measured the composition of these atmosphere-formed particles to understand better how this process works in the Nordic boreal forest. We found that a diverse mix of processes and molecules are likely involved, possibly including the transport of materials from the ocean. While these results will ultimately lead to a better understanding of ocean-land-cloud interactions, they currently indicate that more work is needed to learn the processes involved.Peer reviewe

Nucleation of H_2SO_4 and oxidized organics in CLOUD experiment

The research of atmospheric new particle formation has proceeded lately as the role of sulphuric acid has been established. Still, the roles of other atmospheric compounds in nucleation remain largely unclear. To clarify the first steps of atmospheric new particle formation extensive nucleation experiments were performed in CLOUD chamber in 2012. Especially the role of oxidations products of α-pinene was studied in detail. The experiments provided new information about the part of oxidized organics in nucleation

Measurement report : Long-term measurements of aerosol precursor concentrations in the Finnish subarctic boreal forest

Aerosol particles form in the atmosphere via the clustering of certain atmospheric vapors. After growing into larger particles by the condensation of low-volatility gases, they can affect the Earth's climate by scattering light and acting as cloud condensation nuclei (CCN). Observations of low-volatility aerosol precursor gases have been reported around the world, but longer-term measurement series and any Arctic data sets showing seasonal variation are close to nonexistent. Here, we present similar to 7 months of aerosol precursor gas measurements performed with a nitrate-based chemical ionization atmospheric pressure interface time-of-flight (CI-APi-TOF) mass spectrometer. We deployed our measurements similar to 150 km north of the Arctic Circle at the SMEAR I (Station for Measuring Ecosystem-Atmosphere Relations) continental Finnish subarctic field station, located in the Varrio strict nature reserve. We report concentration measurements of the most common compounds related to new particle formation (NPF): sulfuric acid (SA), methane sulfonic acid (MSA), iodic acid (IA) and the total concentration of highly oxygenated organic molecules (HOMs). At this remote measurement site, SA originates from both anthropogenic and biological sources and has a clear diurnal cycle but no significant seasonal variation. MSA shows a more distinct seasonal cycle, with concentrations peaking in the summer. Of the measured compounds, IA concentrations are the most stable throughout the measurement period, except in April during which time the concentration of IA is significantly higher than during the rest of the year. Otherwise, IA has almost identical daily maximum concentrations in spring, summer and autumn, and on NPF event or non-event days. HOMs are abundant during the summer months and low in the autumn months. Due to their low autumn concentrations and high correlation with ambient air temperature, we suggest that most HOMs are products of biogenic emissions, most probably monoterpene oxidation products. NPF events at SMEAR I happen under relatively low-temperature (1-8 degrees C) conditions, with a fast temperature rise in the early morning hours as well as lower and decreasing relative humidity (RH, 55% vs. 80 %) during NPF days compared with non-event days. NPF days have clearly higher global irradiance values (similar to 450 m(-2) vs. similar to 200 m(-2) and about 10 ppbv higher ozone concentrations than non-event days. During NPF days, we have, on average, higher SA concentrations, peaking at noon; higher MSA concentrations in the afternoon; and slightly higher IA concentration than during non-event days. In summary, these are the first long-term measurements of aerosol-forming vapors from SMEAR I in the subarctic region, and the results of this work will help develop an understanding of atmospheric chemical processes and aerosol formation in the rapidly changing Arctic.Peer reviewe

On the relation between apparent ion and total particle growth rates in the boreal forest and related chamber experiments

The understanding of new particle formation and growth processes is critical for evaluating the role of aerosols in climate change. One of the knowledge gaps is the ion-particle interaction during the early growth process, especially in the sub-3 nm range, where direct observations are sparse. While molecular interactions would imply faster growth rates of ions compared to neutral particles, this phenomenon is not widely observed in the atmosphere. Here, we show field measurements in the boreal forest indicating a smaller apparent growth rate of the ion population compared to the total particles. We use aerosol dynamics simulations to demonstrate that this effect can be caused by the changing importance of ion-induced nucleation mechanisms during the day. We further compare these results with chamber experiments under similar conditions, where we demonstrate that this effect critically depends on the abundance of condensable vapors and the related strength of ion-induced nucleation. Our results imply that atmospheric ion growth rate measurements below 3 nm need to be evaluated very carefully as they do not represent condensational growth alone but are influenced by ion-particle population interactions.Peer reviewe

How Do Amines Affect the Growth of Recently Formed Aerosol Particles

Growth rates of recently born nanometer-scale particles were measured during the CLOUD experiments at CERN. Combining the data from several recently developed measurement techniques allowed us to follow the growth of the particles starting from molecules to molecular clusters and finally to climatically relevant particles. We studied the binary system with sulphuric acid and water, and the ternary systems with ammonia or dimethylamine added to the chamber, both in purely neutral situation, and with ionization from cosmic rays or the CERN particle beam

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