A Monte Carlo approach for determining cluster evaporation rates from concentration measurements

Evaporation rates of small negatively charged sulfuric acid-ammonia clusters are determined by combining detailed cluster formation simulations with cluster distributions measured in the CLOUD experiment at CERN. The analysis is performed by varying the evaporation rates with Markov chain Monte Carlo (MCMC), running cluster formation simulations with each new set of evaporation rates and comparing the obtained cluster distributions to the measurements. In a second set of simulations, the fragmentation of clusters in the mass spectrometer due to energetic collisions is studied by treating also the fragmentation probabilities as unknown parameters and varying them with MCMC. This second set of simulations results in a better fit to the experimental data, suggesting that a large fraction of the observed HSO4- and HSO4-center dot H2SO4 signals may result from fragmentation of larger clusters, most importantly the HSO4-center dot(H2SO4)(2) trimer.Peer reviewe

From cluster properties to concentrations and from concentrations to cluster properties

A large fraction of atmospheric aerosol particles are formed from condensable vapors in the air. This particle formation process has been observed to correlate in many locations with the sulfuric acid concentration, but the very first steps of cluster formation have remained beyond the reach of experimental investigation until recently. Charged clusters can now be detected and characterized starting from the smallest sizes and even neutral clusters consisting of only a few molecules can be detected, although their composition cannot be fully characterized. However, measuring the concentrations of different cluster types does not tell the full story of how the clusters were formed, and detailed simulations are needed in order to get a full understanding of the cluster formation pathways. Cluster formation is described by a set of nonlinear differential equations that cannot be solved analytically in any realistic situation. The best way to understand the complex behavior of cluster populations is by cluster kinetics simulations. The focus of this Thesis is on developing tools for simulating cluster formation, and using the simulation results to improve the detailed understanding of atmospheric aerosol particle formation. As sulfuric acid has been identified as the main driving force of cluster formation in many locations, it is also the main compound in the simulations of this Thesis. It cannot explain the observed atmospheric particle formation rates alone, and other possible participating species considered in this Thesis are ammonia, dimethylamine and water. In the first two papers of the Thesis, theoretical values are used for the collision and evaporation rates, and simulated cluster concentrations and formation rates are compared to experimental observations. The simulation results agree well with experimental findings from two very different studies. The third and fourth paper asses existing methods for interpreting cluster measurements and point out details that should be taken into account: the effect of dipole moments on chemical ionization of neutral molecules and clusters, and the conditions for the widely used nucleation theorem to be valid. The last paper introduces a new method for extracting cluster evaporation rates from measured cluster distributions.Suuri osa ilmakehän aerosolihiukkasista muodostuu ilmakehän kaasujen tiivistyessä pieniksi molekyyliryppäiksi. Tällaisen hiukkasmuodostuksen on havaittu monin paikoin olevan yhteydessä ilman rikkihappopitoisuuteen, mutta molekyyliryppäiden muodostumisen ensimmäisiä askeleita ei ole pystytty tutkimaan kokeellisesti kuin vasta aivan viime vuosina. Uusimmilla mittalaitteilla pystytään nykyään havaitsemaan ja tunnistamaan pienimmätkin varatut hiukkaset yksittäisistä molekyyli-ioneista ja muutaman molekyylin ryppäistä lähtien. Myös varaamattomat muutaman molekyylin suuruiset ryppäät pystytään havaitsemaan, mutta niiden koostumusta ei pystytä selvittämään täydellisesti. Kaikesta huolimatta pelkkä eri kokoisten molekyyliryppäiden lukumäärätiheyksien määrittäminen ei vielä kerro, miten ryppäät syntyivät, vaan hiukkasten muodostumisreittien selvittäminen vaatii yksityiskohtaista mallinnusta. Hiukkasmuodostusta voidaan kuvata epälineaaristen differentiaaliyhtälöiden ryhmällä, joka on liian monimutkainen ratkaistavaksi analyyttisesti missään todellisessa tilanteessa. Paras tapa tutkia molekyylirypäsjoukkojen käytöstä on mallintaa hiukkasjoukon aikakehitystä. Tässä väitöskirjatutkimuksessa on kehitetty työkaluja hiukkasmuodostuksen mallintamiseen, ja mallinnustulosten avulla on pyritty selkiyttämään käsitystä ilmakehän pienhiukkasten synnystä. Rikkihapon on päätelty olevan hiukkasmuodostuksen keskeinen lähtöaine monilla alueilla, ja se on mukana myös kaikissa tässä väitöskirjassa esitetyissä mallinnustuloksissa. Pelkkä rikkihappo ei kuitenkaan yksin muodosta hiukkasia niin tehokkaasti, että se riittäisi selittämään ilmakehässä mitattuja hiukkasmuodostusnopeuksia, ja tässä työssä tarkastellaan lähtöaineina rikkihapon lisäksi myös ammoniakkia, dimetyyliamiinia ja vettä. Väitöskirjan kahdessa ensimmäisessä artikkelissa mallinnetaan molekyylirypäsjoukkojen aikakehitystä käyttäen teoreettisia arvoja molekyyliryppäiden törmäys- ja haihtumisnopeuksille, ja saatuja hiukkasten lukumäärätiheyksiä ja muodostumisnopeuksia verrataan kokeellisiin havaintoihin. Artikkeleissa tarkastellut kokeet ovat hyvin erityyppisiä, mutta mallinnustulokset ovat molemmissa tapauksissa sopusoinnussa koetulosten kanssa. Kolmas ja neljäs artikkeli arvioivat kahta mittaustulosten tulkintaan yleisesti käytettyä menetelmää ja nostavat esiin yksityiskohtia, joiden suhteen on syytä olla varuillaan. Viimeinen artikkeli esittelee uuden menetelmän molekyyliryppäiden haihtumisnopeuksien määrittämiseksi ryppäiden lukumäärätiheyksien perusteella

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

Comparing simulated and experimental molecular cluster distributions

Peer reviewe

The charging of neutral dimethylamine and dimethylamine-sulphuric acid clusters using protonated acetone

Peer reviewe

Experimental particle formation rates spanning tropospheric sulfuric acid and ammonia abundances, ion production rates, and temperatures

Binary nucleation of sulfuric acid and water as well as ternary nucleation involving ammonia are thought to be the dominant processes responsible for new particle formation (NPF) in the cold temperatures of the middle and upper troposphere. Ions are also thought to be important for particle nucleation in these regions. However, global models presently lack experimentally measured NPF rates under controlled laboratory conditions and so at present must rely on theoretical or empirical parameterizations. Here with data obtained in the European Organization for Nuclear Research CLOUD (Cosmics Leaving OUtdoor Droplets) chamber, we present the first experimental survey of NPF rates spanning free tropospheric conditions. The conditions during nucleation cover a temperature range from 208 to 298K, sulfuric acid concentrations between 5x10(5) and 1x10(9)cm(-3), and ammonia mixing ratios from zero added ammonia, i.e., nominally pure binary, to a maximum of -1400 parts per trillion by volume (pptv). We performed nucleation studies under pure neutral conditions with zero ions being present in the chamber and at ionization rates of up to 75ion pairs cm(-3)s(-1) to study neutral and ion-induced nucleation. We found that the contribution from ion-induced nucleation is small at temperatures between 208 and 248K when ammonia is present at several pptv or higher. However, the presence of charges significantly enhances the nucleation rates, especially at 248K with zero added ammonia, and for higher temperatures independent of NH3 levels. We compare these experimental data with calculated cluster formation rates from the Atmospheric Cluster Dynamics Code with cluster evaporation rates obtained from quantum chemistry.Peer reviewe

Molecular understanding of sulphuric acid-amine particle nucleation in the atmosphere

4 pages 359-363 in the print version, additional 7 pages online.Peer reviewe

The effect of acid-base clustering and ions on the growth of atmospheric nano-particles

The growth of freshly formed aerosol particles can be the bottleneck in their survival to cloud condensation nuclei. It is therefore crucial to understand how particles grow in the atmosphere. Insufficient experimental data has impeded a profound understanding of nano-particle growth under atmospheric conditions. Here we study nano-particle growth in the CLOUD (Cosmics Leaving OUtdoors Droplets) chamber, starting from the formation of molecular clusters. We present measured growth rates at sub-3 nm sizes with different atmospherically relevant concentrations of sulphuric acid, water, ammonia and dimethylamine. We find that atmospheric ions and small acid-base clusters, which are not generally accounted for in the measurement of sulphuric acid vapour, can participate in the growth process, leading to enhanced growth rates. The availability of compounds capable of stabilizing sulphuric acid clusters governs the magnitude of these effects and thus the exact growth mechanism. We bring these observations into a coherent framework and discuss their significance in the atmosphere.Peer reviewe