The basis and recent developments of molecular simulation to investigate homogeneous vapor-liquid nucleation are reviewed. Nucleation is an important process in many research areas. Classical nucleation theory, which is based on a simple liquid droplet model, is widely used as a practical method to estimate nucleation rates. There is a considerable discrepancy between rates estimated by the theory and experimental values. The failure of theory is thought to be caused by the incorrectness of the liquid droplet model. Nucleation is controlled by small molecular clusters, and it is inappropriate to assume such small clusters as macroscopic droplets. Molecular simulation can provide direct microscopic insight of the processes and useful information for the cause of failure in prediction of the classical nucleation theory. In this paper we outline and discuss the basis, the recent development and some problems in such molecular approaches.
Experimental investigations are not sufficient to understand the aerosol formation process, because aerosol generation and growth take place via simultaneous generation of monomers, nucleation, coagulation, condensation of vapor and sintering. Therefore, modeling of individual aerosol formation processes is especially important in realizing the whole aerosol formation process. In this article, fundamentals of modeling of aerosol kinetics and transport using bin method and moment method are described.
Morphology control of aerosol nanoparticles is an important subject in industrial application and the modeling of formation process of nonspherical particles is a key to predict the size and shape of products. In this paper, we overview the recent research trend on the modeling of generation process of nonspherical aerosol particles. Then the basics parameters for describing shape of agglomerates are presented, and various models to predict evolution of size and morphology of nonspherical particles via gas phase process are explained. Furthermore, recent attempts to incorporate the effects of agglomerate structures in the modeling of general dynamic (population balance) equation are described.
Recent progress in modeling particle deposition onto vegetation and the issues in validating these models are described in the present paper. Comparisons of size-dependent deposition velocity between calculations and observations may not be sufficient to validate individual modeled processes incorporated in the particle deposition model. The differences of environmental factors between measurements and calculations should be taken into account to validate the modeled processes depending on particle size ranges. Numerical simulations using detailed one-dimensional atmosphere-soil-vegetation model including particle deposition onto vegetation (SOLVEG) developed by the authors was carried out. The model reproduced the measured turbulent fluxes over the coniferous forest canopies for large (> 1 µm) and fine (< several 100 nm) particles. For the sub-micron particles, however, our model underestimated the measured particle deposition velocity obtained from various campaigns. Electrostatic deposition, thermo- and diffusio-phoretic deposition, micro-roughness such as cilium at the leaf surface, and particle growth under humid environment could affect particle deposition in this size range. Vegetation parameters such as Leaf Area Index (LAI) and leaf width also have a significant effect on the deposition velocity. This effect should be incorporated into future parameterizations of particle deposition velocity onto vegetation.
Recently, we can employ higher-performance computers like the supercomputer to calculate more complex processes related to aerosol life cycles in a global scale than ever. These processes are important to properly estimate aerosol effects on the radiation and cloud fields. In this paper, the state-of-the-art implementations to global aerosol models are represented in terms of three aspects: (1) interactions between aerosols and gases are treated in unified chemistry-aerosol models (2) aerosol size distributions can be predicted in global models, (3) treatments of aerosol mixing state are also improved under the treatments of aerosol dynamics such as condensation and coagulation. Progress in computer brings deeper understanding of nature through more comprehensive calculations.
Recently, waste disposal facilities are constructed as closed systems (CS disposal facilities), because of consideration on the influence to the surrounding environment. In these facilities, gaseous compornents such as methane, carbon dioxide, and hydrogen sulfite, are emitted from the waste. Large ventilation is required to ensure the safety of personnel working in the facilities by keeping the gas concentrations below the admitted level. However, it is difficult to evaluate the total emission rate of these gases, and the actual air exchange rate may be different from the set value because of natural ventilation and channeling of air flow in large space such as CS disposal facilities. The authors have developed a method to evaluate both emission rate and air exchange rate through the measurement of gas concentration change. Furthermore, the effect of natural ventilation caused by outside wind and that caused by temperature difference were compared. Finally, the possibility of environmental improvement by natural ventilation was studied.
We describe the design and performance of an aerosol spectrometer that simultaneously measures size-dependent concentration and chemical composition of volatile organic nanoparticles ranging from 10 to 470 nm. The spectrometer consists of a differential mobility analyzer (DMA) for size classification and a gas chromatography mass spectrometer (GC-MS) for composition analysis. The size-classified particles of two kinds of hydrocarbons, an aliphatic hydrocarbon and aromatic hydrocarbon, were directly introduced into the GC-MS after vaporization of the particles. The relationships between the concentrations of classified particles measured with an aerosol electrometer and the peak area of major fragment ions measured with the GC-MS were given to show the detection limit of our spectrometer. For a particle size of 330 nm, the lower detection limit of the spectrometer for particle concentration was approximately 7.5×104 particles cm-3.
The scavenging ratio of sea-salt components for the winter monsoon was presented on the coastal region of the Sea of Japan. Concentrations of sodium in air and precipitation were analyzed from four monitoring networks, CRIEPI, NPG, NIES/ADORC, and CRIEPI during the period from 1987 to 2008. On the basis of the monitoring data collected, we analyzed (i) seasonal variation and horizontal distribution of sea-salt concentration in air and precipitation, (ii) statistical properties of scavenging ratio on a regional scale, (iii) relationship between scavenging ratio and precipitation intensity, (iv) wet scavenging coefficient on a local scale, and (v) temporal variation of sodium concentration associated with the winter monsoon.