Experimental studies on nucleation and atmospheric aerosol particle formation down to the molecular level

Atmospheric aerosols have important effects on health and climate. An important source is the formation of aerosol particles from gas-phase precursors. In this thesis, the goal was to improve our understanding of how exactly this atmospheric particle formation proceeds. Attempts have been made to describe aerosol particle formation by classical nucleation theory. To test this theory, the heterogeneous nucleation of n-propanol vapor on 4 11 nm seed particles was investigated. The choice of seed particle material was found to determine if classical theories could be applied or not, probably because of material-specific inter-molecular interactions between the vapor and the seed particle. The classical theories fail to describe these interactions, which can be crucial in microscopic systems. The critical processes of atmospheric particle formation occur at sizes below 2 nm. In this thesis, novel techniques were employed to access this size range, primarily the atmospheric pressure interface time-of-flight (APi-TOF) mass spectrometer that can directly measure the composition of ions and ionic clusters up to a size of about 2 nm. APi-TOFs were employed at the CLOUD facility at CERN during experiments that focused on exploring particle formation from various systems of vapors. The results of the APi-TOF measurements were the key in revealing the detailed mechanisms of how clusters were initially formed by which vapors, and how these clusters grew to sizes > 2 nm. Clusters of sulfuric acid + ammonia and sulfuric acid + dimethylamine were shown to form and grow via strong hydrogen bonds. The APi-TOF measurements also showed that certain large monoterpene oxidation products, some of them very highly oxidized, can directly bind with bisulfate ions and with sulfuric acid molecules. The clusters then grow by the addition of more of these large oxidized organics and sulfuric acid molecules. Similarities with results from measurements in the boreal forest suggest that large oxidized organics indeed play a crucial role in ambient particle formation events. A light airplane was used to explore how the mechanisms of actual aerosol particle formation vary throughout the atmosphere above the boreal forest, from the canopy up into the free troposphere. They confirmed the extent of boundary layer new particle formation events, and showed indications of an important role of dynamical processes at the top of the boundary layer. Local enhancements of particle formation were observed in connection with clouds. This thesis goal was achieved chiefly by using state-of-the-art experimental techniques together with high-quality laboratory experiments as well as in the field, and by taking ambient measurements aloft. Hopes are that this work will prove to be an important contribution in advancing our knowledge of the detailed mechanisms of atmospheric aerosol particle formation.Pienhiukkasia on kaikkialla maapallon ilmakehässä. Niiden haitallisten terveysvaikutusten lisäksi ne voivat heijastaa auringon valoa takaisin avaruuteen ja toimia pilvien tiivistymisytiminä. Näin pienhiukkaset vaikuttavat jäähdyttävästi ilmastoon. Monet pienhiukkasista muodostuvat tietyistä kaasuista, joita on ilmakehässä hyvin pieninä pitoisuuksina. Tämän väitöskirjan päätavoitteena on parantaa ymmärrystä siitä, mistä kaasuista hiukkaset muodostuvat ja vastata yksityiskohtaisesti kysymykseen, miten hiukkasmuodostus tapahtuu alusta alkaen. Tutkimuksen ensimmäisessä vaiheessa tehtiin kokeita joilla tutkittiin klassisten hiukkasmuodostusteorioiden paikkansa pitävyyttä. Kokeilla saatiin selville, että klassiset teoriat voivat olla riittämättömiä, kun halutaan kuvata alle kymmenen nanometrin kokoisten hiukkasten tärkeitä prosesseja. Kuitenkin, pienhiukkaset jotka muodostuvat kaasuista ilmakehässä ovat aluksi alle kahden nanometrin kokoisia, ja ne koostuvat vain muutamista molekyyleistä. Tässä väitöskirjassa käytettiin uusia mittausmenetelmiä , jotta voidaan tutkia näitä hiukkasmuodostusprosesseja molekyylitasolla. Uutta massaspektrometriä (APi-TOF) käytettiin mittamaan molekyylien ja molekyyliklusterien kemiallista koostumusta. APi-TOF:it vietiin CLOUD-kammioon CERN-tutkimuskeskukseen, joka on tällä hetkellä edistyksellisin paikka tutkia, mitkä kaasut ja höyryt muodostavat pienhiukkasia. APi-TOF:illa saadut tutkimustulokset todellakin pystyivät paljastamaan mitkä kaasut muodostivat ja myöhemmin kasvattivat hiukkaset yli kahden nanometrin kokoihin. Erityisesti klusteroituneet rikkihapon ja emästen (esim. ammoniakki ja amiineja) molekyylit muodostavat ja kasvattavat hiukkasia, siten että ne tekevät vahvoja vetysidoksia happo- ja emäsmolekyylien välille. Lisäksi APi-TOF mittaukset paljastivat, että rikkihappo voi muodostaa sidoksia tiettyjen orgaanisten molekyylien kanssa, jotka ovat syntyneet esim. puiden erittämien yhdisteiden hapettuessa. (Nämä orgaaniset yhdisteet muodostavat tutun metsän tuoksun.) Vastaavia tuloksia saatiin myös ulkoilmamittauksissa havumetsävyöhykkeellä Etelä-Suomessa. Täten nuo hapettuneet orgaaniset yhdisteet näyttävät olevan merkittäviä pienhiukkasten muodostumisessa myös havumetsissä. Väitöskirja sisältää myös lentokonemittauksia Etelä-Suomen havumetsien yläpuolella. Näissä lentomittauksissa tutkittiin, kuinka hiukkasmuodostus muuttuu kun siirrytään puiden latvan tasolta neljän kilometrin korkeuteen. Nämä mittaukset näyttivät, että eri ilmakehän osien sekoittuminen toistensa kanssa saattaa olla tärkeää, koska ilman sekoittuminen voi mahdollistaa juuri hiukkasmuodostukselle suotuisat olosuhteet. Tämän väitöskirjan tavoitteet saavutettiin käyttäen huippumittaustekniikkaa luotettavissa laboratoriokokeissa ja ulkoilmamittauksissa, sekä vieden mittaukset myös yläilmoihin. Toivottavasti tämä työ tarjoaa merkittävän panoksen parantamaan ymmärrystämme mekanismeista, joilla ilmakehän hiukkaset muodostuvat

Simulation of ion-induced nucleation in the CLOUD chamber

A comparison between the binary Sulphuric Acid Water NUCleation model SAWNUC and CLOUD results is presented. Comparison includes direct comparison with a battery of particle counters of various counting efficiencies and APi-TOF charged cluster distribution. A good agreement is found for nucleation rates at various temperatures

Estimation of sulfuric acid concentration using ambient ion composition and concentration data obtained with atmospheric pressure interface time-of-flight ion mass spectrometer

Sulfuric acid (H2SO4, SA) is the key compound in atmospheric new particle formation. Therefore, it is crucial to observe its concentration with sensitive instrumentation, such as chemical ionisation (CI) inlets coupled to atmospheric pressure interface time-of-flight (APi-TOF) mass spectrometers. However, there are environmental conditions for which and physical reasons why chemical ionisation cannot be used, for example in certain remote places or during flight measurements with limitations regarding chemicals. Here, we propose a theoretical method to estimate the SA concentration based on ambient ion composition and concentration measurements that are achieved by APi-TOF alone. We derive a theoretical expression to estimate the SA concentration and validate it with accurate CI-APi-TOF observations. Our validation shows that the developed estimate works well during daytime in a boreal forest (R-2 = 0.85); however, it underestimates the SA concentration in, e.g. the Antarctic atmosphere during new particle formation events where the dominating pathway for nucleation involves sulfuric acid and a base (R-2 = 0.48).Peer reviewe

Comparing secondary organic aerosol (SOA) volatility distributions derived from isothermal SOA particle evaporation data and FIGAERO-CIMS measurements

The volatility distribution of the organic compounds present in secondary organic aerosol (SOA) at different conditions is a key quantity that has to be captured in order to describe SOA dynamics accurately. The development of the Filter Inlet for Gases and AEROsols (FIGAERO) and its coupling to a chemical ionization mass spectrometer (CIMS; collectively FIGAERO-CIMS) has enabled near-simultaneous sampling of the gas and particle phases of SOA through thermal desorption of the particles. The thermal desorption data have been recently shown to be interpretable as a volatility distribution with the use of the positive matrix factorization (PMF) method. Similarly, volatility distributions can be inferred from isothermal particle evaporation experiments when the particle size change measurements are analyzed with process-modeling techniques. In this study, we compare the volatility distributions that are retrieved from FIGAERO-CIMS and particle size change measurements during isothermal particle evaporation with process-modeling techniques. We compare the volatility distributions at two different relative humidities (RHs) and two oxidation conditions. In high-RH conditions, where particles are in a liquid state, we show that the volatility distributions derived via the two ways are similar within a reasonable assumption of uncertainty in the effective saturation mass concentrations that are derived from FIGAERO-CIMS data. In dry conditions, we demonstrate that the volatility distributions are comparable in one oxidation condition, and in the other oxidation condition, the volatility distribution derived from the PMF analysis shows considerably more high-volatility matter than the volatility distribution inferred from particle size change measurements. We also show that the Vogel-Tammann-Fulcher equation together with a recent glass transition temperature parametrization for organic compounds and PMF-derived volatility distribution estimates are consistent with the observed isothermal evaporation under dry conditions within the reported uncertainties. We conclude that the FIGAERO-CIMS measurements analyzed with the PMF method are a promising method for inferring the volatility distribution of organic compounds, but care has to be taken when the PMF factors are analyzed. Future process-modeling studies about SOA dynamics and properties could benefit from simultaneous FIGAERO-CIMS measurements.Peer reviewe

Characterization of positive clusters in the CLOUD nucleation experiments

The mechanism of new particle formation (NPF) events is still poorly understood. The CLOUD experiment is studying at which conditions this process occurs. During the CLOUD 7 campaign (fall 2012) the evolution of the nucleation rate with different reagents (sulphuric acid, ammonia, dimethylamine and α-pinene) was tested. Here we investigate the composition of freshly formed positive ions during the nucleation process, which allows us to understand the formation mechanisms of these clusters. The time evolution of the clusters is shown demonstrating the correlation between the formation of these clusters and the nucleation process

Secondary Organic Aerosol Formation from Healthy and Aphid-Stressed Scots Pine Emissions.

One barrier to predicting biogenic secondary organic aerosol (SOA) formation in a changing climate can be attributed to the complex nature of plant volatile emissions. Plant volatile emissions are dynamic over space and time, and change in response to environmental stressors. This study investigated SOA production from emissions of healthy and aphid-stressed Scots pine saplings via dark ozonolysis and photooxidation chemistry. Laboratory experiments using a batch reaction chamber were used to investigate SOA production from different plant volatile mixtures. The volatile mixture from healthy plants included monoterpenes, aromatics, and a small amount of sesquiterpenes. The biggest change in the volatile mixture for aphid-stressed plants was a large increase (from 1.4 to 7.9 ppb) in sesquiterpenes-particularly acyclic sesquiterpenes, such as the farnesene isomers. Acyclic sesquiterpenes had different effects on SOA production depending on the chemical mechanism. Farnesenes suppressed SOA formation from ozonolysis with a 9.7-14.6% SOA mass yield from healthy plant emissions and a 6.9-10.4% SOA mass yield from aphid-stressed plant emissions. Ozonolysis of volatile mixtures containing more farnesenes promoted fragmentation reactions, which produced higher volatility oxidation products. In contrast, plant volatile mixtures containing more farnesenes did not appreciably change SOA production from photooxidation. SOA mass yields ranged from 10.8 to 23.2% from healthy plant emissions and 17.8-26.8% for aphid-stressed plant emissions. This study highlights the potential importance of acyclic terpene chemistry in a future climate regime with an increased presence of plant stress volatiles

Airborne measurements of aerosols and carbon dioxide during a prescribed fire experiment at a boreal forest site

Peer reviewe

Measurements of cluster ions using a nano radial DMA and a particle size magnifier in CLOUD

We built a new instrumental setup for measuring ion distributions in the size range [1.3-6] nm. The implementation of an high transmission inlet increased the total transmission efficiency to more than 6% at 1.47 nm mobility equivalent diameter, allowing the detection of ions at atmospheric concentrations. The size resolution of our measurements is as high as 6. We characterized the instrument in the laboratory and carried out measurements during the CLOUD7 campaign. We compared the results obtained with the Neutral cluster and Air Ion spectrometer finding very good agreement