Klusteripopulaatioiden simuloiminen menetelmänä tutkia hiukkasmuodostusmekanismeja

Formation of aerosol particles from condensable vapors is a ubiquitous phenomenon in the atmosphere. Aerosols can affect regional and global climate, as well as visibility and human health. The work of this thesis contributes to the numerous efforts made to build understanding of atmospheric particle formation mechanisms. The focus is on the first molecular-level steps, where clustering of individual gas-phase molecules initiates the process, and the applied method is dynamic cluster population modeling. Sets of sub-2 nm molecular clusters are simulated in conditions relevant to the atmosphere or laboratory considering vapor production, external sinks for clusters and vapors, cluster collision and evaporation processes, and in some cases also ionization and recombination by generic ionizing species. Evaporation rates are calculated from the cluster formation free energies computed with quantum chemical methods. As sulfuric acid has been shown to be the key component in particle formation in most boundary layer locations, the majority of the work presented here concentrates on simulating sulfuric acid-containing clusters in the presence of potentially enhancing species, namely ammonia and amines. In laboratory experiments, these base compounds have been found to be capable of enhancing sulfuric acid driven particle formation to produce formation rates around the magnitude observed in the atmosphere. This result is reproduced by the cluster model. In this work, the performance of the modeling tools is validated against experimental data also by comparing simulated concentrations of charged sulfuric acid ammonia clusters to those measured with a mass spectrometer in a chamber experiment. Examination of clustering pathways in simulated sulfuric acid ammonia and sulfuric acid dimethylamine systems shows that the clustering mechanisms and the role of ions may be very different depending on the identity of the base. In addition to predictions related to cluster formation from different precursor vapors, the model is applied to study the effects of varying conditions on the qualitative behavior of a cluster population and quantities that have been deduced from measured cluster concentrations. It is demonstrated that the composition of the critical cluster corresponding to the maximum free energy along the growth pathway cannot be reliably determined from cluster formation rates by commonly used methods. Simulations performed using a simple model substance show that cluster growth rates determined from the fluxes between subsequent cluster sizes are likely to differ from the growth rates deduced from the time evolution of the concentrations as in experiments, with the difference depending on the properties of the substance as well as ambient conditions. Finally, the effect of hydration and base molecules on sulfuric acid diffusion measurement is assessed by mimicking an experimental setup. Applications of cluster population simulations are diverse, and the development of these types of modeling tools provides useful additions to the palette of theoretical approaches to probe clustering phenomena.Pienhiukkasten eli aerosolien muodostuminen höyryistä tiivistymällä on yleinen ilmiö ilmakehässä, ja nämä hiukkaset voivat vaikuttaa ilmastoon, näkyvyyteen ja terveyteen. Hiukkasmuodostusprosessin yksityiskohdat ovat kuitenkin huonosti tunnettuja. Tässä työssä on tutkittu molekyylitasolla tapahtuvaa hiukkasmuodostuksen ensivaihetta, jossa yksittäisten kaasumolekyylien klusteroituminen käynnistää prosessin. Tutkimuksessa sovellettu menetelmä on klusteripopulaatioiden dynamiikan mallintaminen. Halkaisijaltaan alle kahden nanometrin kokoisia klustereita on simuloitu ilmakehän tai laboratorioympäristön olosuhteissa ottaen huomioon höyryjen tuotto, klusterien ja höyryjen ulkoiset häviöt, klusterien väliset törmäys- ja haihtumisprosessit, ja joissakin tapauksissa myös ionisaatio- ja rekombinaatioprosessit, jotka aiheutuvat törmäyksistä pienten ionien kanssa. Klusterien haihtumisnopeudet on pääasiallisesti määritetty kvanttikemiallisilla metodeilla lasketuista klusterien muodostumisenergioista. Koska rikkihapon on osoitettu useimmiten olevan ilmakehän hiukkasmuodostuksen avainyhdiste, valtaosa työstä keskittyy rikkihappoa sisältävien klusterien simuloimiseen klusteroitumista mahdollisesti edesauttavien yhdisteiden, pääasiallisesti ammoniakin ja amiinien, läsnäollessa. Näiden emäsyhdisteiden lisäämisen rikkihappohöyryyn on laboratoriotutkimuksissa havaittu tuottavan suuruusluokaltaan ilmakehämittauksia vastaavia hiukkasmuodostusnopeuksia. Klusterimallinnuksen antamat tulokset sopivat yhteen näiden havaintojen kanssa. Tässä työssä mallinnuksen ja kokeellisten tulosten yhteensopivuutta on testattu myös vertailemalla simuloituja sähköisesti varattujen klusterien pitoisuuksia massaspektrometrimittauksiin. Klusterien muodostumisreittien tutkiminen simulaatioissa, joissa klusterit koostuvat rikkihaposta ja joko ammoniakista tai dimetyyliamiinista osoittaa, että klusteroitumismekanismit ja varattujen klusterien merkitys voivat olla hyvin erilaiset eri emäksille. Mallia soveltamalla tutkittiin myös erilaisten olosuhteiden vaikutusta klusterijoukon kvalitatiiviseen käytökseen ja suureisiin, joita voidaan määrittää mitatuista klusteripitoisuuksista. Työssä havainnollistetaan esimerkiksi, että kokeissa käytetyn menetelmän mukaisesti määritetyt klusterien kasvunopeudet saattavat erota merkittävästi effektiivisistä, klusterien välisiin hiukkasvoihin perustuvista kasvunopeuksista. Erot riippuvat sekä klusterien ominaisuuksista että ulkoisista olosuhteista. Simulaatioiden kautta arvioitiin myös vesi- ja emäsmolekyylien vaikutusta rikkihapon diffuusiomittaukseen jäljittelemällä koejärjestelyä. Klusteripopulaatiosimulaatioilla on monenlaisia sovelluksia, ja tämäntyyppisten mallinnustyökalujen kehittäminen tarjoaa hyödyllisen lisän molekyyliklusterien muodostumista tutkivien teoreettisten lähestymistapojen palettiin

Guanidine : A Highly Efficient Stabilizer in Atmospheric New-Particle Formation

The role of a strong organobase, guanidine, in sulfuric acid-driven new-particle formation is studied using state-of-the-art quantum chemical methods and molecular cluster formation simulations. Cluster formation mechanisms at the molecular level are resolved, and theoretical results on cluster stability are confirmed with mass spectrometer measurements. New-particle formation from guanidine and sulfuric acid molecules occurs without thermodynamic barriers under studied conditions, and clusters are growing close to a 1:1 composition of acid and base. Evaporation rates of the most stable clusters are extremely low, which can be explained by the proton transfers and symmetrical cluster structures. We compare the ability of guanidine and dimethylamine to enhance sulfuric acid-driven particle formation and show that more than 2000-fold concentration of dimethylamine is needed to yield as efficient particle formation as in the case of guanidine. At similar conditions, guanidine yields 8 orders of magnitude higher particle formation rates compared to dimethylamine. Highly basic compounds such as guanidine may explain experimentally observed particle formation events at low precursor vapor concentrations, whereas less basic and more abundant bases such as ammonia and amines are likely to explain measurements at high concentrations.Peer reviewe

Temperature-Dependent Diffusion of H2SO4 in Air at Atmospherically Relevant Conditions : Laboratory Measurements Using Laminar Flow Technique

We report flow tube measurements of the effective sulfuric acid diffusion coefficient at ranges of different relative humidities (from similar to 4 to 70%), temperatures (278, 288 and 298 K) and initial H2SO4 concentrations (from 1 x 10(6) to 1 x 10(8) molecules.cm(-3)). The measurements were carried out under laminar flow of humidified air containing trace amounts of impurities such as amines (few ppt), thus representing typical conditions met in Earth's continental boundary layer. The diffusion coefficients were calculated from the sulfuric acid wall loss rate coefficients that were obtained by measuring H2SO4 concentration continuously at seven different positions along the flow tube with a chemical ionization mass spectrometer (CIMS). The wall loss rate coefficients and laminar flow conditions were verified with additional computational fluid dynamics (CFD) model FLUENT simulations. The determined effective sulfuric acid diffusion coefficients decreased with increasing relative humidity, as also seen in previous experiments, and had a rather strong power dependence with respect to temperature, around proportional to T-5.6, which is in disagreement with the expected temperature dependence of similar to T-1.75 for pure vapours. Further clustering kinetics simulations using quantum chemical data showed that the effective diffusion coefficient is lowered by the increased diffusion volume of H2SO4 molecules via a temperature-dependent attachment of base impurities like amines. Thus, the measurements and simulations suggest that in the atmosphere the attachment of sulfuric acid molecules with base molecules can lead to a lower than expected effective sulfuric acid diffusion coefficient with a higher than expected temperature dependence.Peer reviewe

What controls the observed size-dependency of the growth rates of sub-10 nm atmospheric particles?

The formation and growth of atmospheric particles involving sulfuric acid and organic vapors is estimated to have significant climate effects. To accurately represent this process in large-scale models, the correct interpretation of the observations on particle growth, especially below 10 nm, is essential. Here, we disentangle the factors governing the growth of sub-10 nm particles in the presence of sulfuric acid and organic vapors, using molecular-resolution cluster population simulations and chamber experiments. We find that observed particle growth rates are determined by the combined effects of (1) the concentrations and evaporation rates of the condensing vapors, (2) particle population dynamics, and (3) stochastic fluctuations, characteristic to initial nucleation. This leads to a different size-dependency of growth rate in the presence of sulfuric acid and/or organic vapors at different concentrations. Specifically, the activation type behavior, resulting in growth rate increasing with the particle size, is observed only at certain vapor concentrations. In our model simulations, cluster-cluster collisions enhance growth rate at high vapor concentrations and their importance is dictated by the cluster evaporation rates, which demonstrates the need for accurate evaporation rate data. Finally, we show that at sizes below similar to 2.5-3.5 nm, stochastic effects can importantly contribute to particle population growth. Overall, our results suggest that interpreting particle growth observations with approaches neglecting population dynamics and stochastics, such as with single particle growth models, can lead to the wrong conclusions on the properties of condensing vapors and particle growth mechanisms.Peer reviewe

Identification of molecular cluster evaporation rates, cluster formation enthalpies and entropies by Monte Carlo method

We address the problem of identifying the evaporation rates for neutral molecular clusters from synthetic (computer-simulated) cluster concentrations. We applied Bayesian parameter estimation using a Markov chain Monte Carlo (MCMC)algorithm to determine cluster evaporation/fragmentation rates from known cluster distributions, assuming that the clustercollision rates are known. We used the Atmospheric Cluster Dynamic Code (ACDC) with evaporation rates based on quantumchemical calculations to generate cluster distributions for a set of electrically neutral sulphuric acid and ammonia clusters. We then treated these concentrations as synthetic experimental data, and tested two approaches for estimating the evaporation rates. First we have studied a scenario where at one single temperature time-dependent cluster distributions are measured before thesystem reaches a time-independent steady-state. In the second scenario only steady-state cluster distributions are measured, butat several temperatures. This allowed us to use multiple sets of concentrations at different temperatures. Additionally, in thelatter case the evaporation rates were represented in terms of cluster formation enthalpies and entropies which were considered to be free parameters. This reparametrization reduced the number of unknown parameters, since several evaporation ratesdepend on the same cluster formation enthalpy and entropy values. We show that in the second setting, even if only two temperatures were used, the temperature-dependent steady-state dataoutperforms the first setting for parameter identification. We can thus conclude that for experimentally determining evaporationrates, cluster distribution measurements at several temperatures are recommended over time-dependent measurements at one temperature.Peer reviewe

New particle formation from sulfuric acid and amines : Comparison of monomethylamine, dimethylamine, and trimethylamine

Amines are bases that originate from both anthropogenic and natural sources, and they are recognized as candidates to participate in atmospheric aerosol particle formation together with sulfuric acid. Monomethylamine, dimethylamine, and trimethylamine (MMA, DMA, and TMA, respectively) have been shown to enhance sulfuric acid-driven particle formation more efficiently than ammonia, but both theory and laboratory experiments suggest that there are differences in their enhancing potentials. However, as quantitative concentrations and thermochemical properties of different amines remain relatively uncertain, and also for computational reasons, the compounds have been treated as a single surrogate amine species in large-scale modeling studies. In this work, the differences and similarities of MMA, DMA, and TMA are studied by simulations of molecular cluster formation from sulfuric acid, water, and each of the three amines. Quantum chemistry-based cluster evaporation rate constants are applied in a cluster population dynamics model to yield cluster concentrations and formation rates at boundary layer conditions. While there are differences, for instance, in the clustering mechanisms and cluster hygroscopicity for the three amines, DMA and TMA can be approximated as a lumped species. Formation of nanometer-sized particles and its dependence on ambient conditions is roughly similar for these two: both efficiently form clusters with sulfuric acid, and cluster formation is rather insensitive to changes in temperature and relative humidity. Particle formation from sulfuric acid and MMA is weaker and significantly more sensitive to ambient conditions. Therefore, merging MMA together with DMA and TMA introduces inaccuracies in sulfuric acid-amine particle formation schemes.Peer reviewe

Role of base strength, cluster structure and charge in sulfuric-acid-driven particle formation

In atmospheric sulfuric-acid-driven particle formation, bases are able to stabilize the initial molecular clusters and thus enhance particle formation. The enhancing potential of a stabilizing base is affected by different factors, such as the basicity and abundance. Here we use weak (ammonia), medium strong (dimethylamine) and very strong (guanidine) bases as representative atmospheric base compounds, and we systematically investigate their ability to stabilize sulfuric acid clusters. Using quantum chemistry, we study proton transfer as well as intermolecular interactions and symmetry in clusters, of which the former is directly related to the base strength and the latter to the structural effects. Based on the theoretical cluster stabilities and cluster population kinetics modeling, we provide molecular-level mechanisms of cluster growth and show that in electrically neutral particle formation, guanidine can dominate formation events even at relatively low concentrations. However, when ions are involved, charge effects can also stabilize small clusters for weaker bases. In this case the atmospheric abundance of the bases becomes more important, and thus ammonia is likely to play a key role. The theoretical findings are validated by cluster distribution experiments, as well as comparisons to previously reported particle formation rates, showing a good agreement.Peer reviewe

Growth of atmospheric clusters involving cluster-cluster collisions : comparison of different growth rate methods

We simulated the time evolution of atmospheric cluster concentrations in a one-component system where not only do clusters grow by condensation of monomers, but cluster-cluster collisions also significantly contribute to the growth of the clusters. Our aim was to investigate the consistency of the growth rates of sub-3aEuro-nm clusters determined with different methods and the validity of the common approach to use them to estimate particle formation rates. We compared the growth rate corresponding to particle fluxes (FGR), the growth rate derived from the appearance times of clusters (AGR), and the growth rate calculated based on irreversible vapor condensation (CGR). We found that the relation between the different growth rates depends strongly on the external conditions and the properties of the model substance. The difference between the different growth rates was typically highest at the smallest, sub-2aEuro-nm sizes. FGR was generally lower than AGR and CGR; at the smallest sizes the difference was often very large, while at sizes larger than 2aEuro-nm the growth rates were closer to each other. AGR and CGR were in most cases close to each other at all sizes. The difference between the growth rates was generally lower in conditions where cluster concentrations were high, and evaporation and other losses were thus less significant. Furthermore, our results show that the conventional method used to determine particle formation rates from growth rates may give estimates far from the true values. Thus, care must be taken not only in how the growth rate is determined but also in how it is applied.Peer reviewe

Comparing simulated and experimental molecular cluster distributions

Peer reviewe