Non-LTE dust nucleation in sub-saturated vapors

We use the kinetic theory of nucleation to explore the properties of dust nucleation in sub-saturated vapors. Due to radiation losses, the sub-critical clusters have a smaller temperature compared to their vapor. This alters the dynamical balance between attachment and detachment of monomers, allowing for stable nucleation of grains in vapors that are sub-saturated for their temperature. We find this effect particularly important at low densities and in the absence of a strong background radiation field. We find new conditions for stable nucleation in the n-T phase diagram. The nucleation in the non-LTE regions is likely to be at much slower rate than in the super-saturated vapors. We evaluate the nucleation rate, warning the reader that it does depend on poorly substantiated properties of the macro-molecules assumed in the computation. On the other hand, the conditions for nucleation depend only on the properties of the large stable grains and are more robust. We finally point out that this mechanism may be relevant in the early universe as an initial dust pollution mechanism, since once the interstellar medium is polluted with dust, mantle growth is likely to be dominant over non-LTE nucleation in the diffuse medium.Comment: 8 pages, 8 figures, accepted for publication in MNRA

Exploring the chemical fate of the sulfate radical anion by reaction with sulfur dioxide in the gas phase

Peer reviewe

Microcanonical Determination of the Interface Tension of Flat and Curved Interfaces from Monte Carlo Simulations

The investigation of phase coexistence in systems with multi-component order parameters in finite systems is discussed, and as a generic example, Monte Carlo simulations of the two-dimensional q-state Potts model (q=30) on LxL square lattices (40<=L<=100) are presented. It is shown that the microcanonical ensemble is well-suited both to find the precise location of the first order phase transition and to obtain an accurate estimate for the interfacial free energy between coexisting ordered and disordered phases. For this purpose, a microcanonical version of the heatbath algorithm is implemented. The finite size behaviour of the loop in the curve describing the inverse temperature versus energy density is discussed, emphasizing that the extrema do not have the meaning of van der Waals-like "spinodal points" separating metastable from unstable states, but rather describe the onset of heterophase states: droplet/bubble evaporation/condensation transitions. Thus all parts of these loops, including the parts that correspond to a negative specific heat, describe phase coexistence in full thermal equilibrium. However, the estimates for the curvature-dependent interface tension of the droplets and bubbles suffer from unexpected and unexplained large finite size effects which need further study.Comment: submitted to special issue "Liquid Matter" of Journal of Physics C: Condensed Matter on occasion of the 8th Liquid Matter Conference held Sept. 6-10, 2011 in Vienna, Austri

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

Sulfuric acid is generally considered one of the most important substances taking part in atmospheric particle formation. However, in typical atmospheric conditions in the lower troposphere, sulfuric acid and water alone are unable to form particles. It has been suggested that strong bases may stabilize sulfuric acid clusters so that particle formation may occur. More to the point, amines - strong organic bases - have become the subject of interest as possible cause for such stabilization. To probe whether amines play a role in atmospheric nucleation, we need to be able to measure accurately the gas-phase amine vapour concentration. Such measurements often include charging the neutral molecules and molecular clusters in the sample. Since amines are bases, the charging process should introduce a positive charge. This can be achieved by, for example, using chemical ionization with a positively charged reagent with a suitable proton affinity. In our study, we have used quantum chemical methods combined with a cluster dynamics code to study the use of acetone as a reagent ion in chemical ionization and compared the results with measurements performed with a chemical ionization atmospheric pressure interface time-of-flight mass spectrometer (CI-APi-TOF). The computational results indicate that protonated acetone is an effective reagent in chemical ionization. However, in the experiments the reagent ions were not depleted at the predicted dimethylamine concentrations, indicating that either the modelling scheme or the experimental results - or both - contain unidentified sources of error.Peer reviewe

Methane sulfonic acid-enhanced formation of molecular clusters of sulfuric acid and dimethyl amine

Over oceans and in coastal regions, methane sulfonic acid (MSA) is present in substantial concentrations in aerosols and in the gas phase. We present an investigation into the effect of MSA on sulfuric acid- and dimethyl amine (DMA)-based cluster formation rates. From systematic conformational scans and well-tested ab initio methods, we optimise the structures of all MSA_<i>x</i> (H₂SO₄)_yDMA_<i>z</i> clusters where <i>x</i> + <i>y</i> &leq; 3 and <i>z</i> &leq; 2. The resulting thermodynamic data are used in the Atmospheric Cluster Dynamics Code, and the effect of MSA is evaluated by comparing ternary MSA–H₂SO₄–DMA cluster formation rates to binary H₂SO₄–DMA cluster formation rates. Within the range of atmospherically relevant MSA concentrations, we find that MSA may increase cluster formation rates by up to 1 order of magnitude, although typically, the increase will be less than 300 % at 258 K, less than 100 % at 278 K and less than 15 % at 298 K. The results are rationalised by a detailed analysis of the main growth paths of the clusters. We find that MSA-enhanced clustering involves clusters containing one MSA molecule, while clusters containing more than one MSA molecule do not contribute significantly to the growth

An improved parameterization for sulfuric acid-water nucleation rates for tropospheric and stratospheric conditions

In this paper we present parameterized equations for calculation of sulfuric acid-water critical nucleus compositions, critical cluster radii and homogeneous nucleation rates for tropospheric and stratospheric conditions. The parameterizations are based on a classical nucleation model. We used an improved model for the hydrate formation relying on ab initio calculations of small sulfuric acid clusters and on experimental data for vapor pressures and equilibrium constants for hydrate formation. The most rigorous nucleation kinetics and the thermodynamically consistent version of the classical binary homogeneous nucleation theory were used. The parameterized nucleation rates are compared with experimental ones, and at room temperature and relative humidities above 30% they are within experimental error. At lower temperatures and lower humidities the agreement is somewhat poorer. Overall, the values of nucleation rates are increased compared to a previous parameterization and are within an order of magnitude compared with theoretical values for all conditions studied. The parameterized equations will reduce the computing time by a factor 1/500 compared to nonparameterized nucleation rate calculations and therefore are in particular useful for large- scale models. The parameterized formulas are valid at temperatures between 230.15 K and 305.15 K, relative humidities between 0.01% and 100%, and sulfuric acid concentrations from 10(4) to 10(11) cm(-3). They can be used to extrapolate the classical results down to 190 K. The parametrization is limited to cases where nucleation rates are between 10(-7) and 10(10) cm(-3) s(-1), and the critical cluster contains at least four molecules