Formation of low-volatility aerosol precursor molecules and clusters in the atmosphere

We live in a world full of aerosols and witness their existence constantly. Changes in visibility, road dust and pollen filling the air in the spring time and even dosing some medicines are all related to aerosols. The most important aspect for this thesis is however, the formation of aerosol precursor molecules and clusters and their possible effects on aerosol properties. Different types of aerosols e. g. organic and inorganic, ice and pollen, biogenic and anthropogenic, when acting as cloud condensation nuclei (CCN), can change the optical properties of clouds and thus have different climate effects via changes in precipitation or cloud cover. Also the mechanism how the small cloud seeds are formed can have a major effect on the cloud properties. Clouds reflect and scatter radiation cooling the atmosphere but to this day aerosol effects still form the largest uncertainty in estimates of the climate of the future. Low-volatility compounds in the ambient air are the most important components in both aerosol formation and their growth to sizes that can affect cloud properties such as their reflectivity. The vapours in the atmosphere form around half of the first precursors of aerosol particles via nucleation, still molecular in size. The rest is released directly into the air e.g. by the traffic or from the oceans as sea salt. Neutral molecular size precursor substituents are difficult to detect because they lack electric charge and their concentrations rarely exceed the detection limits of the used measurement instruments. This is one of the reasons why the first steps of nucleation process are still partly unsolved after decades of research. This thesis concentrates first in development of selective methods and ultrasensitive instrumentation for the detection of acidic aerosol precursor molecules and clusters. These compounds include sulphuric acid, which is known to be highly important precursor for new particle formation. This thesis presents the first ambient measurements with the new instrumentation and even though sulphuric acid was detected in relative high concentrations during a particle formation event, no ambient cluster formation was yet observed. The same instrumentation was further used in an ultraclean chamber experiment were sulphuric acid and dimethyl amine formed growing clusters and they were detected from the smallest clusters all the way up to ~2 nm size. These compounds are likely to dominate the new particle formation with low-volatility organic compounds. Tens of thousands of volatile organic compounds (VOC) are emitted in the atmosphere by terrestrial vegetation, marine environments and anthropogenic sources, making the search for the meaningful compounds for new particle formation extremely difficult. In this thesis a new group of extremely low-volatility organic compounds (ELVOC) from terpene oxidation are introduced. They form a large source of secondary organic aerosol (SOA) and might help explaining the former big gap between the measured and the modelled SOA loads in the atmosphere. Formation of these compounds from RO2 radicals via rapid autoxidation process to stable molecules is also studied in details in laboratory experiments. This thesis also utilizes a global modelling framework, where the measurement results are applied. According to this thesis, ELVOC in particular are in crucial role when estimating SOA and CCN formation in the air. All together this thesis is a comprehensive study of both organic and inorganic low-volatility precursor molecules and clusters in the atmosphere, from their origin, formation mechanisms, and measurement techniques to their possible effects on climate now and in the future.Ilman aerosoleja, eli ilmassa leijuvia nestemäisiä tai kiinteitä hiukkasia, ei taivaalle muodostu pilviä. Aerosoleilla on niiden alkuperästä riippuen huomattavia vaikutuksia ilmastoon ja ihmisten terveyteen. Tämän väitöskirjan tarkoituksena on laajentaa tietoa aerosolien muodostumismekanismeista ilmakehän tiivitymiskykyisistä kaasuista. Tätä tarkoitusta varten kehitettiin ensin laite tiivistymiskykyisten kaasumaisten yhdisteiden havaitsemiksi ilmasta ja sen jälkeen tutkittiin boreaalissa metsässä tapahtuvaa pienhiukkasmuodostusta käyttäen hyväksi sekä kenttä- että laboratoriokokeita. Tiivistymiskykyiset, alhaisen höyrynpaineen kaasut ovat päätekijä aerosolinmuodostuksessa, koska ne eivät määritelmän mukaan viihdy kaasufaasissa vaan pyrkivät tiivistymään kohdattuaan pinnan tai toisen molekyylin. Näistä tiivistymiskykyisistä kaasumolekyyleistä tunnetuin on rikkihappo (H2SO4). Yksinään rikkihappo ei pysty hiukkasia muodostamaan, koska rikkihapon ja veden muodostamat molekyyliryppäät haihtuvat ilmakehän rajakerroksessa. Onkin ehdotettu, että molekyyliryppäät tarvitsevatkin stabiloivia aineksia pitämään muodostuvat hiukkaset kasassa. Näiksi ns. liimamolekyyleiksi on ehdotettu happojen vastakohtaa, emäksiä, kuten amiineja jotka päätyvät ilmakehään eloperäisen materiaalien aminohappojen hajotessa maaperässä tai vaihtoehtoisesti kasvillisuuden vapauttamia orgaanisia yhdisteitä, jotka hapettuessaan muuttuvat tiivistymiskykyisiksi. Tutkimus toteutettiin Helsingin Yliopistossa ja läheisessä yhteistyössä ulkomaisten yhteistyökumppaneiden kanssa. Kehitetty mittalaite sijoitettiin keskisuomalaiseen havumetsään, jossa hiukkasmuodostuksen aikana havaittiin korkeita rikkihappopitoisuuksia. Laboratoriokokeissa todettiin myöhemmin että rikkihapporyppäät stabiloituvat amiinien kanssa ulkoilman kaltaisissa olosuhteissa ja näin ollen kasvavat isommiksi. Metsän vapauttamien yhdisteiden osuutta hiukkasmuodostukseen tutkittiin myös ja väitöskirjan suurimpiin saavutuksiin kuuluukin ennen kaasufaasissa havaitsemattomien äärimmilleen hapettuneiden orgaanisten yhdisteiden havaitseminen. Tulevaisuuden tavoitteena on osoittaa metsien ehkäisevän tehokkaasti ilmaston lämpiämistä näiden tiivistymiskykyisten höyryjen muodostamien aerosolien kautta

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

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

Aerosol Nucleation and Growth in a Mixture of Sulfuric Acid / Alpha-Pinene Oxidation Products at the CERN CLOUD Chamber

The role of α-pinene in aerosol nucleation and growth was investigated using the CERN CLOUD chamber, a nano scanning mobility particle sizer (nanoSMPS) and several condensation particle counters (CPCs) with different diameter cut-offs. Different oxidation conditions for α-pinene - OH⋅ vs. ozone oxidation - were considered to investigate their contributions to particle nucleation and growth. Results from the latest CERN experiment from fall 2012 (CLOUD 7) are presented