Hanna Vehkamäki

Direct Observations of Atmospheric Aerosol Nucleation

Aerosol Formation Most atmospheric aerosol particles result from a growth process that begins with atmospheric molecules and clusters, progressing to larger and larger sizes as they acquire other molecules, clusters, and particles. The initial steps of this process involve very small entities—with diameters of less than 2 nanometers—which have been difficult to observe. Kulmala et al. (p. 943; see the Perspective by Andreae) developed a sensitive observational protocol that allows these tiny seeds to be detected and counted, and they mapped out the process of aerosol formation in detail. Detailed aerosol measurements provide a consistent framework for the formation of particles from atmospheric gases. [Also see Perspective by Andreae] Atmospheric nucleation is the dominant source of aerosol particles in the global atmosphere and an important player in aerosol climatic effects. The key steps of this process occur in the sub–2-nanometer (nm) size range, in which direct size-segregated observations have not been possible until very recently. Here, we present detailed observations of atmospheric nanoparticles and clusters down to 1-nm mobility diameter. We identified three separate size regimes below 2-nm diameter that build up a physically, chemically, and dynamically consistent framework on atmospheric nucleation—more specifically, aerosol formation via neutral pathways. Our findings emphasize the important role of organic compounds in atmospheric aerosol formation, subsequent aerosol growth, radiative forcing and associated feedbacks between biogenic emissions, clouds, and climate.

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

Nucleation of aerosol particles from trace atmospheric vapours is thought to provide up to half of global cloud condensation nuclei. Aerosols can cause a net cooling of climate by scattering sunlight and by leading to smaller but more numerous cloud droplets, which makes clouds brighter and extends their lifetimes. Atmospheric aerosols derived from human activities are thought to have compensated for a large fraction of the warming caused by greenhouse gases. However, despite its importance for climate, atmospheric nucleation is poorly understood. Recently, it has been shown that sulphuric acid and ammonia cannot explain particle formation rates observed in the lower atmosphere. It is thought that amines may enhance nucleation, but until now there has been no direct evidence for amine ternary nucleation under atmospheric conditions. Here we use the CLOUD (Cosmics Leaving OUtdoor Droplets) chamber at CERN and find that dimethylamine above three parts per trillion by volume can enhance particle formation rates more than 1,000-fold compared with ammonia, sufficient to account for the particle formation rates observed in the atmosphere. Molecular analysis of the clusters reveals that the faster nucleation is explained by a base-stabilization mechanism involving acid–amine pairs, which strongly decrease evaporation. The ion-induced contribution is generally small, reflecting the high stability of sulphuric acid–dimethylamine clusters and indicating that galactic cosmic rays exert only a small influence on their formation, except at low overall formation rates. Our experimental measurements are well reproduced by a dynamical model based on quantum chemical calculations of binding energies of molecular clusters, without any fitted parameters. These results show that, in regions of the atmosphere near amine sources, both amines and sulphur dioxide should be considered when assessing the impact of anthropogenic activities on particle formation.

Heterogeneous Nucleation Experiments Bridging the Scale from Molecular Ion Clusters to Nanoparticles

Generation, investigation, and manipulation of nanostructured materials are of fundamental and practical importance for several disciplines, including materials science and medicine. Recently, atmospheric new particle formation in the nanometer-size range has been found to be a global phenomenon. Still, its detailed mechanisms are mostly unknown, largely depending on the incapability to generate and measure nanoparticles in a controlled way. In our experiments, an organic vapor (n-propanol) condenses on molecular ions, as well as on charged and uncharged inorganic nanoparticles, via initial activation by heterogeneous nucleation. We found a smooth transition in activation behavior as a function of size and activation to occur well before the onset of homogeneous nucleation. Furthermore, nucleation enhancement for charged particles and a substantial negative sign preference were quantitatively detected.

Molecular understanding of atmospheric particle formation from sulfuric acid and large oxidized organic molecules

Significance The formation of nanoparticles by condensable vapors in the atmosphere influences radiative forcing and therefore climate. We explored the detailed mechanism of particle formation, in particular the role of oxidized organic molecules that arise from the oxidation of monoterpenes, a class of volatile organic compounds emitted from plants. We mimicked atmospheric conditions in a well-controlled laboratory setup and found that these oxidized organics form initial clusters directly with single sulfuric acid molecules. The clusters then grow by the further addition of both sulfuric acid and organic molecules. Some of the organics are remarkably highly oxidized, a critical feature that enables them to participate in forming initial stable molecular clusters and to facilitate the first steps of atmospheric nanoparticle formation. Atmospheric aerosols formed by nucleation of vapors affect radiative forcing and therefore climate. However, the underlying mechanisms of nucleation remain unclear, particularly the involvement of organic compounds. Here, we present high-resolution mass spectra of ion clusters observed during new particle formation experiments performed at the Cosmics Leaving Outdoor Droplets chamber at the European Organization for Nuclear Research. The experiments involved sulfuric acid vapor and different stabilizing species, including ammonia and dimethylamine, as well as oxidation products of pinanediol, a surrogate for organic vapors formed from monoterpenes. A striking resemblance is revealed between the mass spectra from the chamber experiments with oxidized organics and ambient data obtained during new particle formation events at the Hyytiälä boreal forest research station. We observe that large oxidized organic compounds, arising from the oxidation of monoterpenes, cluster directly with single sulfuric acid molecules and then form growing clusters of one to three sulfuric acid molecules plus one to four oxidized organics. Most of these organic compounds retain 10 carbon atoms, and some of them are remarkably highly oxidized (oxygen-to-carbon ratios up to 1.2). The average degree of oxygenation of the organic compounds decreases while the clusters are growing. Our measurements therefore connect oxidized organics directly, and in detail, with the very first steps of new particle formation and their growth between 1 and 2 nm in a controlled environment. Thus, they confirm that oxidized organics are involved in both the formation and growth of particles under ambient conditions.

Ultrafine particle scavenging coefficients calculated from 6 years field measurements

Abstract Based on 6 years of outdoor measurements at a boreal forest site in Southern Finland, scavenging coefficients were calculated for aerosol particles having diameter between 10 and 510 nm . Median scavenging coefficients varied between 7×10−6 and 4×10 −5 s −1 in this size-range. The dependence of scavenging coefficients on rain intensity was studied, and the scavenging coefficients were parameterized as a function of particle size for particle diameters of 10– 500 nm and for rain intensities 0– 20 mm h −1 .

New parameterization of sulfuric acid-ammonia-water ternary nucleation rates at tropospheric conditions

[1] Recently, the classical theory of sulfuric acid-ammonia-water (H2SO4-NH3-H2O) nucleation was reinvestigated by including the effect of stable ammonium bisulfate formation into calculations. The predicted nucleation rates lowered by many orders of magnitude, bringing them close to agreement with the available experiments on H2SO4-NH3-H2O nucleation. However, because of complex thermodynamics involved, the theoretical calculations of nucleation rates are computationally demanding, and sometimes the theory breaks down at specific concentrations and temperatures. Here we present parameterized equations of ternary H2SO4-NH3-H2O nucleation rates, critical cluster sizes, and critical cluster compositions. Our parameterizations reduce the computing time of these values by a factor of 10 5 compared with the calculations with the full thermodynamic model. Also, our parameterizations provide reliable estimates for ternary nucleation rates in cases when the full theory fails in isolated points of the parameter space. The parameterized nucleation rates are accurate to one order of magnitude in nucleation rate. Because of their computational efficiency, our parameterizations are particularly suitable for large-scale models of atmosphere. They are valid for temperatures above 235 K, sulfuric acid concentrations 5 � 10 4 –10 9 cm � 3 , ammonia mixing ratios 0.1–1000 ppt, relative humidities 5%–95%, and nucleation rates over 10 � 5 cm � 3 s � 1 . At these conditions, no significant nucleation occurs above 295 K.

An improved model for hydrate formation in sulfuric acid–water nucleation

The formation of sulfuric acid–water hydrates in the vapor phase and nucleation rates of sulfuric acid–water clusters are investigated. The result of ab initio calculations and experimental data related to hydrates are utilized to improve the description of sulfuric acid–water hydration and nucleation in atmospheric conditions. The nucleation rates are obtained using the most rigorous nucleation kinetics and the thermodynamically consistent version of the classical nucleation model. The improvements increase the predicted nucleation rates compared to previous models. The predicted nucleation rates are compared with experimental ones, and they are in most cases within experimental errors. Some experimental evidence suggests that the present model gives a more realistic dependence of nucleation rate on relative humidity and sulfuric acid concentration than the earlier versions of the theory.

An improved model for ternary nucleation of sulfuric acid-ammonia-water

A revised homogeneous ternary nucleation model in H2O–H2SO4–NH3 vapors is presented. The model is based on a self-consistent version of classical nucleation theory with a rigorous treatment of nucleation kinetics. The calculation of equilibrium vapor pressures is completely revised and the effect of H2O–H2SO4 hydration is considered in detail. Compared to earlier models, the new model is able to predict nucleation rates over a wider range of temperatures and trace gas concentrations. A considerable dependence on relative humidity is found. The critical clusters corresponding to significant nucleation rates typically contain less than ten molecules and consist almost exclusively of H2SO4 and NH3.

Atmospheric Cluster Dynamics Code: a flexible method for solution of the birth-death equations

The Atmospheric Cluster Dynamics Code (ACDC) is presented and explored. This program was created to study the first steps of atmospheric new particle formation by examining the formation of molecular clusters from atmospherically relevant molecules. The program models the cluster kinetics by explicit solution of the birth–death equations, using an efficient computer script for their generation and the MATLABode15s routine for their solution. Through the use of evaporation rate coefficients derived from formation free energies calculated by quantum chemical methods for clusters containing dimethylamine or ammonia and sulphuric acid, we have explored the effect of changing various parameters at atmospherically relevant monomer concentrations. We have included in our model clusters with 0–4 base molecules and 0–4 sulfuric acid molecules for which we have commensurable quantum chemical data. The tests demonstrate that large effects can be seen for even small changes in different parameters, due to the non-linearity of the system. In particular, changing the temperature had a significant impact on the steady-state concentrations of all clusters, while the boundary effects (allowing clusters to grow to sizes beyond the largest cluster that the code keeps track of, or forbidding such processes), coagulation sink terms, non-monomer collisions, sticking probabilities and monomer concentrations did not show as large effects under the conditions studied. Removal of coagulation sink terms prevented the system from reaching the steady state when all the initial cluster concentrations were set to the default value of 1 m −3, which is probably an effect caused by studying only relatively small cluster sizes.

Liquid-drop formalism and free-energy surfaces in binary homogeneous nucleation theory

Three different derivations of the classical binary nucleation theory are considered in detail. It is shown that the derivation originally presented by Wilemski [J. Chem. Phys. 80, 1370 (1984)] is consistent with more extensive derivations [Oxtoby and Kashchiev, J. Chem. Phys. 100, 7665 (1994)]; Debenedetti, Metastable Liquids: Concepts and Principles (Princeton University Press, Princeton, 1996) if and only if the assumption is made that the surface of tension of the binary nucleus coincides with the dividing surface specified by the surface condition ∑nsivli=0, where the nsi denote surface excess numbers of molecules of species i, and the v’s are partial molecular volumes. From this condition, it follows that (1) the surface tension is curvature independent and (2) that the nucleus volume is V=∑nlivli=∑givli, where the nli are the numbers of molecules in the uniform liquid phase of the droplet model encompassed by the surface of tension, and the gi are the total molecular occupation numbers contained by...