Antti Lauri

Fluxes of carbon dioxide and water vapour over Scots pine forest and clearing

The carbon dioxide and water vapour fluxes were measured by the eddy covariance (EC) technique from July to September 2000 at two closely located sites in southern Finland: over a 38-year-old pine forest and over a 5-year-old forest clearing. The night-time respiration was of the same magnitude at both sites. At day-time the pine forest was a strong sink but the clearing close to CO2 balance, indicating that CO2 uptake of ground vegetation over the clearing balanced the release from the soil. The shoot scale gas exchange measurements in combination with process-based modelling were used to evaluate the measured CO2 exchange of the forest ecosystem. The forest CO2 exchange was explained by soil respiration and photosynthesis of forest canopy, while the contribution of understory and ground vegetation CO2 exchange could be neglected. During the study period the forest was a net sink of CO2 and the clearing a source. The daily average uptake of CO2 by the forest was −2.4 and − 1.7 gm −2 per day in July–August and September periods, respectively; and average release by the clearing 4.0 and 2.5 g m −2 per day during the same periods. This shows that carbon losses 5 years after clear-cutting are substantial. The evapotranspiration (ET) was higher over the forest compared to clearing as a result of transpiration from the forest canopy. The difference in ET was small during the July–August period when precipitation frequently occurred.

Estimation of line tension and contact angle from heterogeneous nucleation experimental data

Using the classical nucleation theory corrected with line tension and experimental data of heterogeneous nucleation of n-nonane, n-propanol, and their mixture on silver particles of three different sizes, the authors were able to estimate the line tensions and the microscopic contact angles for the above mentioned systems. To do this they applied generalized Youngs equation for the line tension and calculated the interfacial tensions using Li and Neumanns equation [Adv. Colloid Interface Sci. 39, 299 (1992)]. It has been found that, for both unary and binary systems, the line tension is negative and the resulting microscopic contact angle derived from experimental nucleation data is most of the time larger than the macroscopic one. This is in contrast to earlier studies where the influence of line tension has not been accounted for. The values of the three phase contact line tension obtained in this way are of the same order of magnitude as the estimations for other systems reported in literature. The line tension effect also decreases considerably the nucleation barrier.

Interaction between SO2 and submicron atmospheric particles

In the atmosphere, oxidation of sulfur dioxide (SO2) to sulfate may occur in the gas phase, in cloud or fog droplets, or in the aerosol phase on the surface or inside aerosol particles. While aerosol phase reactions have been studied in the case of supermicron sea-salt and crustal particles, very few investigations regarding submicron particles are available. In this paper, the importance of aerosol phase sulfate production to the dynamics of submicron particle populations was examined. The investigation was based on model simulations and theoretical evaluations regarding potential SO2 oxidation reactions. None of the relatively well-quantified aqueous phase reactions was rapid enough to make small nuclei grow to cloud condensation nuclei (CCN) size within the particle lifetime in the lower troposphere. This is consistent with the few observations showing that the smallest atmospheric particles are enriched in organics rather than sulfate. The amount of submicron particulate matter could be enhanced significantly by certain aerosol phase reactions, but this is likely to require a particle population having a pH close to 7. Aerosol phase reactions could partly explain the apparently too low SO2-to-sulfate conversion rates predicted by several chemical transport models over polluted regions. In addition to the bulk aerosol phase, SO2-to-sulfate conversion might involve physical adsorption of SO2 or a compound reacting with it by the particle surface, or it could take place in a liquid surface layer that usually covers atmospheric particles. Reactions involving physical adsorption seem to have negligible influence on the dynamics of submicron atmospheric particle populations. Aerosol phase reactions worth future investigation are those occurring in particle surface layers and those occurring in cloud interstitial particles.

Two-component heterogeneous nucleation kinetics and an application to Mars

We develop a two-component heterogeneous nucleation model that includes exact calculation of the Stauffer-type [D. Stauffer, J. Aerosol Sci. 7, 319 (1976)] steady-state kinetic prefactor using the correct heterogeneous Zeldovich factor for a heterogeneous two-component system. The model, and a simplified version of it, is tested by comparing its predictions to experimental data for water-n-propanol nucleating on silver particles. The model is then applied to water-carbon dioxide system in Martian conditions, which has not been modeled before. Using the ideal mixture assumption, the model shows theoretical possibilities for two-component nucleation adjacent to the initial stages of one-component water nucleation, especially with small water vapor amounts. The numbers of carbon dioxide molecules in the critical cluster are small in the case of large water amounts (up to 300 ppm) in the gas phase, but larger when there is very little water vapor (1 ppm).

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.

Heterogeneous nucleation as a potential sulphate‐coating mechanism of atmospheric mineral dust particles and implications of coated dust on new particle formation

[1] The plausibility of heterogeneous conucleation of water, sulphuric acid, and ammonia as a pathway leading to soluble coating of atmospheric mineral dust is investigated. In addition, the effect of such sulphate-coated dust on the formation and growth of atmospheric aerosol particles is addressed. The simulated new particle formation mechanism is ternary nucleation of water, sulphuric acid, and ammonia vapors, while in the condensational growth process the effect of condensable organic vapor is also studied. The results indicate that soluble coating of dust by heterogeneous nucleation can occur at atmospheric sulphuric acid concentrations. In addition, the simulations show that homogeneous ternary nucleation and subsequent growth are decoupled. Although observed (or even higher) dust concentrations are unable to inhibit new particle formation, coated dust particles acting as condensation and coagulation sinks can prevent the growth of newly formed particles to detectable sizes. This is particularly true in desert areas, where organic vapor concentrations are low. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0365 Atmospheric Composition and Structure: Troposphere—composition and chemistry; 3210 Mathematical Geophysics: Modeling; KEYWORDS: aerosol, mineral dust, soluble coating, heterogeneous nucleation, particle formation and growth, condensation sink

Connection between the virial equation of state and physical clusters in a low density vapor

We carry out Monte Carlo simulations of physical Lennard-Jones and water clusters and show that the number of physical clusters in vapor is directly related to the virial equation of state. This relation holds at temperatures clearly below the critical temperatures, in other words, as long as the cluster-cluster interactions can be neglected--a typical assumption used in theories of nucleation. Above a certain threshold cluster size depending on temperature and interaction potential, the change in cluster work of formation can be calculated analytically with the recently proposed scaling law. The breakdown of the scaling law below the threshold sizes is accurately modeled with the low order virial coefficients. Our results indicate that high order virial coefficients can be analytically calculated from the lower order coefficients when the scaling law for cluster work of formation is valid. The scaling law also allows the calculation of the surface tension and equilibrium vapor density with computationally efficient simulations of physical clusters. Our calculated values are in good agreement with those obtained with other methods. We also present our results for the curvature dependent surface tension of water clusters.

Pan Eurasian Experiment (PEEX) - A research initiative meeting the grand challenges of the changing environment of the Northern Pan-Eurasian Arctic-Boreal areas

The Pan-Eurasian Experiment (PEEX) is a new multidisciplinary, global change research initiative focusing on understanding biosphere-ocean-cryosphere-climate interactions and feedbacks in Arctic and boreal regions in the Northern Eurasian geographical domain. PEEX operates in an integrative way and it aims at solving the major scientific and society relevant questions in many scales using tools from natural and social sciences and economics. The research agenda identifies the most urgent large scale research questions and topics of the land-atmosphere-aquatic-anthropogenic systems and interactions and feedbacks between the systems for the next decades. Furthermore PEEX actively develops and designs a coordinated and coherent ground station network from Europe via Siberia to China and the coastal line of the Arctic Ocean together with a PEEX-modeling platform. PEEX launches a program for educating the next generation of multidisciplinary researcher and technical experts. This expedites the utilization of the new scientific knowledge for producing a more reliable climate change scenarios in regional and global scales, and enables mitigation and adaptation planning of the Northern societies. PEEX gathers together leading European, Russian and Chinese research groups. With a bottom-up approach, over 40 institutes and universities have contributed the PEEX Science Plan from 18 countries. In 2014 the PEEX community prepared Science Plan and initiated conceptual design of the PEEX land-atmosphere observation network and modeling platform. Here we present the PEEX approach as a whole with the specific attention to research agenda and preliminary design of the PEEX research infrastructure.

Comparison between the classical theory predictions and molecular simulation results for heterogeneous nucleation of argon

We have performed Monte Carlo simulations of homogeneous and heterogeneous nucleations of Lennard-Jones argon clusters. The simulation results were interpreted using the major concept posing a difference between the homogeneous and heterogeneous classical nucleation theories-the contact parameter. Our results show that the multiplication concept of the classical heterogeneous nucleation theory describes the cluster-substrate interaction surprisingly well even for small molecular clusters. However, in the case of argon nucleating on a rigid monolayer of fcc(111) substrate at T=60 K, the argon-substrate atom interaction being approximately one-third as strong as the argon-argon interaction, the use of the classical theory concept results in an underestimation of the heterogeneous nucleation rate by two to three orders of magnitude even for large clusters. The main contribution to this discrepancy is induced by the failure of the classical theory of homogeneous nucleation to predict the energy involved in bringing one molecule from the vapor to the cluster for clusters containing less than approximately 15 molecules.

The molecular approach to heterogeneous nucleation

A molecular approach to heterogeneous nucleation has been developed. The expressions for the equilibrium cluster distribution, the reversible work of the cluster formation, and the nucleation rate have been derived. Two separate statements for the work of formation were formulated. If the equilibrium cluster distribution is normalized on the monomer concentration near the substrate surface, the reversible work of formation is expressed by DeltaG(het) (I) = (F(n) (het)-F(n) (hom))-(F(1) (het)-F(1) (hom)) + DeltaG(hom) where F(n) (het) and F(n) (hom) are the Helmholtz free energies of a cluster interacting with a substrate and a cluster not interacting with the substrate, respectively. If the equilibrium cluster distribution is normalized on the monomer concentration far from the substrate surface, the work of cluster formation is given by DeltaG(het) (II) = (F(n) (het)-F(n) (hom)) + DeltaG(hom). The former expression corresponds to the approach of the classical heterogeneous nucleation theory. The cluster partition function appears to be dependent on the location of a virtual plane, which separates the volume, where the interaction of the clusters with the substrate is effective from the one where interaction is negligible. Our Monte Carlo simulations have shown that the dependence is rather weak and thus the location of the plane is not very important. According to the simulations the variation of the plane position in the range from 20 to 50 Angstroms does not lead to a considerable change of the heterogeneous nucleation rate.