Heterogeneous Nucleation onto Monoatomic Ions: Support for the Kelvin-Thomson Theory.

In this study, the process of heterogeneous nucleation is investigated by coupling a high-resolution differential mobility analyser (DMA) to an expansion-type condensation particle counter, the size-analyzing nuclei counter (SANC). More specifically, we measured the activation probabilities of monoatomic ions of both polarities by using n-butanol as condensing liquid. All seed ions were activated to grow into macroscopic sizes at saturation ratios well below the onset of homogeneous nucleation, showing for the first time that the SANC is capable of detecting sub-nanometer sized, atomic seed ions. The measured onset saturation ratios for each ion were compared to the Kelvin-Thomson (KT) theory. Despite the fact that certain dependencies of activation behaviour on seed ion properties cannot be predicted by the KT theory, it was found that with a simple adjustment of the n-butanol molecular volume (9-15 % lower compared to bulk properties) good agreement with experimental results is achievable. The corresponding density increase may result from the dipole-charge interaction. This study thus offers support for the application of the KT model for heterogeneous, ion-induced nucleation studies at the sub-nanometer level.

A unifying identity for the work of cluster formation in heterogeneous and homogeneous nucleation theory.

A unifying identity is derived relating the reversible work of cluster formation (W) and its molecular number content (n) and surface work (Φ) components, each ratioed to the corresponding values for a spherical capillary drop of critical size in classical nucleation theory. The result is a relationship that connects these ratios: fW = -2fN + 3fS, where fW = W/W*CNT, fN = n/n*CNT, and fS = Φ/Φ*CNT. Shown to generalize two early thermodynamic relationships of Gibbs, the new result is demonstrated here for Fletcher's model of heterogeneous nucleation, resulting in a unified treatment of condensation on flat and curved substrates and smooth passage to the homogeneous limit. Additional applications are made to clusters of non-critical as well as critical size and to a molecular-based extension of classical nucleation theory based on the Kelvin relation. The new identity serves as a consistency check on complicated theoretical expressions and numerical calculations and can be used to guide the construction of theory and interpretation of measurements.

Temperature Dependence in Heterogeneous Nucleation with Application to the Direct Determination of Cluster Energy on Nearly Molecular Scale.

A re-examination of measurements of heterogeneous nucleation of water vapor on silver nanoparticles is presented here using a model-free framework that derives the energy of critical cluster formation directly from measurements of nucleation probability. Temperature dependence is correlated with cluster stabilization by the nanoparticle seed and previously found cases of unusual increasing nucleation onset saturation ratio with increasing temperature are explained. A necessary condition for the unusual positive temperature dependence is identified, namely that the critical cluster be more stable, on a per molecule basis, than the bulk liquid to exhibit the effect. Temperature dependence is next examined in the classical Fletcher model, modified here to make the energy of cluster formation explicit in the model.  The contact angle used in the Fletcher model is identified as the microscopic contact angle, which can be directly obtained from heterogeneous nucleation experimental data by a recently developed analysis method. Here an equivalent condition, increasing contact angle with temperature, is found necessary for occurrence of unusual temperature dependence. Our findings have immediate applications to atmospheric particle formation and nanoparticle detection in condensation particle counters (CPCs).

Comparison of the SAWNUC model with CLOUD measurements of sulphuric acid-water nucleation.

Binary nucleation of sulphuric acid-water particles is expected to be an important process in the free troposphere at low temperatures. SAWNUC (Sulphuric Acid Water Nucleation) is a model of binary nucleation that is based on laboratory measurements of the binding energies of sulphuric acid and water in charged and neutral clusters. Predictions of SAWNUC are compared for the first time comprehensively with experimental binary nucleation data from the CLOUD chamber at European Organization for Nuclear Research. The experimental measurements span a temperature range of 208-292 K, sulphuric acid concentrations from 1·106 to 1·109 cm-3, and distinguish between ion-induced and neutral nucleation. Good agreement, within a factor of 5, is found between the experimental and modeled formation rates for ion-induced nucleation at 278 K and below and for neutral nucleation at 208 and 223 K. Differences at warm temperatures are attributed to ammonia contamination which was indicated by the presence of ammonia-sulphuric acid clusters, detected by an Atmospheric Pressure Interface Time of Flight (APi-TOF) mass spectrometer. APi-TOF measurements of the sulphuric acid ion cluster distributions ( (H2SO4)i·HSO4- with i = 0, 1, ..., 10) show qualitative agreement with the SAWNUC ion cluster distributions. Remaining differences between the measured and modeled distributions are most likely due to fragmentation in the APi-TOF. The CLOUD results are in good agreement with previously measured cluster binding energies and show the SAWNUC model to be a good representation of ion-induced and neutral binary nucleation of sulphuric acid-water clusters in the middle and upper troposphere.

Near-unity mass accommodation coefficient of organic molecules of varying structure.

Atmospheric aerosol particles have a significant effect on global climate, air quality, and consequently human health. Condensation of organic vapors is a key process in the growth of nanometer-sized particles to climate relevant sizes. This growth is very sensitive to the mass accommodation coefficient α, a quantity describing the vapor uptake ability of the particles, but knowledge on α of atmospheric organics is lacking. In this work, we have determined α for four organic molecules with diverse structural properties: adipic acid, succinic acid, naphthalene, and nonane. The coefficients are studied using molecular dynamics simulations, complemented with expansion chamber measurements. Our results are consistent with α = 1 (indicating nearly perfect accommodation), regardless of the molecular structural properties, the phase state of the bulk condensed phase, or surface curvature. The results highlight the need for experimental techniques capable of resolving the internal structure of nanoparticles to better constrain the accommodation of atmospheric organics.

Oxidation products of biogenic emissions contribute to nucleation of atmospheric particles.

Atmospheric new-particle formation affects climate and is one of the least understood atmospheric aerosol processes. The complexity and variability of the atmosphere has hindered elucidation of the fundamental mechanism of new-particle formation from gaseous precursors. We show, in experiments performed with the CLOUD (Cosmics Leaving Outdoor Droplets) chamber at CERN, that sulfuric acid and oxidized organic vapors at atmospheric concentrations reproduce particle nucleation rates observed in the lower atmosphere. The experiments reveal a nucleation mechanism involving the formation of clusters containing sulfuric acid and oxidized organic molecules from the very first step. Inclusion of this mechanism in a global aerosol model yields a photochemically and biologically driven seasonal cycle of particle concentrations in the continental boundary layer, in good agreement with observations.

Quantitative characterization of critical nanoclusters nucleated on large single molecules.

First order phase transitions involve nucleation, formation of nanoscale regions of a new phase within a metastable parent phase. Using the heterogeneous nucleation theorem we show how clusters formed by nucleation on single molecules evolve from the gas phase and determine the critical size beyond which condensation starts to form aerosol particles. Our experiments reveal the activation of molecules into droplets to happen via formation of critical clusters substantially larger than the seed molecule. The nanosized critical clusters were found to be well predicted by the Kelvin-Thomson relation pointing directly to the key step in the phase transition.

Experiments on the temperature dependence of heterogeneous nucleation on nanometer-sized NaCl and Ag particles.

Experimental investigations on the activation of NaCl and Ag aerosol particles by heterogeneous nucleation of n-propanol vapor at well-defined vapor saturation ratios are presented. Particular emphasis is placed on the temperature dependence of this process from -11 to +14 °C. Aerosols are generated in a tube furnace and electrostatically classified at mean geometric mobility equivalent diameters between 3.6 and 11 nm. Activation probabilities are measured by means of expansion chamber experiments, and onset n-propanol saturation ratios are subsequently determined. The experiments with Ag particles do not produce any unexpected results. The results for NaCl particles, however, show a temperature trend of the onset saturation ratios that is opposite to that predicted by classical nucleation theory. This stresses the important role that surface properties play in heterogeneous nucleation processes. By tentatively assuming a temperature-dependent contact angle, we are able to theoretically reproduce this reversed temperature trend. In addition, the shrinkage of NaCl condensation particles is investigated for varying amounts of n-propanol vapor, and contact angle measurements are performed at temperatures ranging from -7 to +30 °C.

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.

Heterogeneous multicomponent nucleation theorems for the analysis of nanoclusters.

In this paper we present a new form of the nucleation theorems applicable to heterogeneous nucleation. These heterogeneous nucleation theorems allow, for the first time, direct determination of properties of nanoclusters formed on pre-existing particles from measured heterogeneous nucleation probabilities. The theorems can be used to analyze the size (first theorem) and the energetics (second theorem) of heterogeneous clusters independent of any specific nucleation model. We apply the first theorem to the study of small water and n-propanol clusters formed at the surface of 8 nm silver particles. According to the experiments the size of the two-component critical clusters is found to be below 90 molecules, and only less than 20 molecules for pure water, less than 300 molecules for pure n-propanol. These values are drastically smaller than the ones predicted by the classical nucleation theory, which clearly indicates that the nucleating clusters are too small to be quantitatively described using a macroscopic theory.

Homogeneous nucleation rates of 1-pentanol.

We have measured isothermal homogeneous nucleation rates J for 1-pentanol vapor in two different carrier-gases, argon, and helium, using a two-valve nucleation pulse chamber. The nucleation rates cover a range of 10(5)

Comment on "Postnucleation droplet growth in supersaturated gas with arbitrary vapor concentration" [J. Chem. Phys. 120, 10455 (2004)].

We provide the overview of the papers dealing with the growth of the liquid droplet. The purpose is to put the paper "Postnucleation droplet growth in supersaturated gas with arbitrary vapor concentration" [V. Pines, M. Zlatkowski, and A. Chait, J. Chem. Phys. 120, 10455 (2004)] in the right historical context. The original paper and this comment point the correct condensation formula to the atmospheric science community.

Mass and thermal accommodation during gas-liquid condensation of water.

In this Letter we report, for the first time, direct and simultaneous determinations of mass and thermal accommodation coefficients for water vapor condensation in air, based on the observation of droplet growth kinetics in an expansion cloud chamber. Our experiments exclude values below 0.85 for the thermal and below 0.4 for the mass accommodation coefficients at temperatures ranging from 250 to 290 K. Both coefficients are likely to be 1 for all studied conditions. Previously available experimental data on the mass accommodation coefficient for water span about 3 orders of magnitude. Our results provide new and firm insight to cloud microphysics and consequently to the global radiative balance.