The formation of new particles in the atmosphere due to nucleation and the subsequent growth of these particles is an important process that regulates the number concentrations of aerosol particles. This process is linked to cloud processes and therefore to climate via the conversion of new particles to cloud condensation nuclei. In this article, recent field, laboratory, and modeling studies on the formation of new particles in the atmosphere are reviewed from the viewpoints of nucleation and the initial condensational growth caused by low-volatile gaseous precursors. The topics in this article include the nucleation involving sulfuric acid vapor, condensational growth of new particles caused by low-volatile organic and iodine compounds, and the contribution of ion-induced nucleation to particle formation.

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While mass transfer kinetics at liquid-vapor interfaces is of fundamental importance in heterogeneous atmospheric chemistry, quantitative understandings of mass transfer processes are still far from complete. Observed kinetics is governed by various factors associated with the gas and liquid phases and the interface, and accordingly precise analysis through decomposition of phenomenological rate into elementary steps is desirable for comprehensive understanding of the mass transfer processes. The present paper summarizes recent progress toward precise decomposition with the help of theoretical analyses, such as computational fluid dynamics (CFD) and molecular dynamics (MD) simulations. These simulation techniques are demonstrated to be quite powerful to directly calculate the contribution of each elementary kinetic step, which is not readily evaluated by other experimental means.

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Progresses and problems of laboratory studies of heterogeneous processes on solid aerosol surfaces are presented in this paper. Heterogeneous reactions on solid aerosol surfaces have attracted great attention because of its ability to affect the concentrations of OH radicals and ozone in the troposphere. Laboratory studies have been extensively carried out to measure the uptake coefficient of trace gases on surrogate surfaces of mineral dust aerosols. The uptake coefficients or the reaction probabilities are necessary in estimating the mass transfer processes around aerosols. Results of model simulations support the importance of laboratory studies of heterogeneous processes. The observed uptake coefficient should be corrected for the physical structures and surface area of the samples. As an example, recent papers reporting the reactions of nitrogen dioxide on solid surfaces are reviewed. Based on the recent studies, thin water films on solid aerosol particles may be quite important in the chemistry of the boundary layer in urban areas. Although heterogeneous reaction mechanisms have been proposed in some specific conditions, an understanding is required on the reaction mechanisms and reaction probabilities of nitrogen dioxide with coexistence water in a systematic manner. Finally problems and future directions on laboratory studies of heterogeneous aerosol chemistry are summarized.

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Modeling of heterogeneous reactions related to the stratospheric ozone depletion in the polar region is explained. The reactions are incorporated into the chemical transport model (CTM) of the Center for Climate System Research, University of Tokyo / National Institute for Environmental Studies (CCSR/NIES) . Thirteen heterogeneous reactions on liquid binary sulphate aerosols (LBA) , supercooled ternary solutions (STS) , nitric acid tri-hydrate (NAT) , and ice particles were taken into account. The CTM was applied to calculate the ozone destruction in the Arctic in 1995, 1996, and 1997, when a large amount of ozone destruction occurred. The calculated results were analyzed for studying the effects of bromine species on the ozone destruction. The results showed that the BrO-ClO catalytic cycle works efficiently in the stable Arctic polar vortex in the 1997 spring owing to the long-time coexistence of high concentrations of BrO and ClO. The situation is similar to the ozone destruction in the Antarctic in October and November.

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Atmospheric aerosol particles play an important role in atmospheric processes such as heterogeneous reactions and cloud formation, and climate system. Recently, the heterogeneous reaction on aerosol particles and their influences become one of growing interests. The reactions change the aerosol constituents and mixing states during the transport, and lead to changes in hygroscopic and optical properties. Recent analytical techniques and instruments provided new knowledge on the heterogeneous reactions and chemical/physical properties of aerosol particles. This manuscript reviews aerosol constituents, their mixing states and the contributions of heterogeneous reactions to chemical/physical properties on the basis of field aerosol observations.

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In order to clarify the behavior of bioaerosols diffusing in the atmosphere with KOSA (yellow sand) , we attempted to identify the bioaerosols in the atmospheric mixed layer over the KOSA source region, Dunhuang, China. Bioaerosols were collected at 50-100 m (using a tethered balloon) and at 10 m above the ground, and microorganisms, as the source of bioaerosols, were collected from desert sand. The bioaerosols were cultivated immediately after the collection at the Dunhuang City meteorological station. 20 strains of isolates were grouped into three major categories according to the morphology and physiological characteristics: Gram-positive rod (15 strains) , Gram-negative rod (3 strains) , and Gram-positive cocci (2 strains) . The 500-bp 16S rDNA partial sequences of 7 strains isolated from the samples collected 50-100 and 10 m above the ground showed identity. Bioaerosols appear to rise in the atmospheric mixed layer from near the ground to the upper ground with KOSA. The results of a homology search by 16S rDNA or 18S rDNA sequences of isolates in DNA databases (GenBank, DDBJ, and EMBL) revealed that bioaerosols in the atmospheric mixed layer over the KOSA source region contain Bacillus cereus (50-100 and 10 m above the ground) , Rhodosporidium sphaerocarpum (50-100 m above the ground) , Candida parapsilosis (10 m above the ground) , Pantoea agglomerans (10 m above the ground) , and Enterobacter endosymbiont (10 m above the ground) and that microorganisms from the sand were Bacillus subtilis and Bacillus atrophaeus. Although Bacillus cereus isolated from the samples collected 50-100 and 10 m above the ground was a kind of food spoilage or pathogenic bacterium, our findings indicate that the KOSA bioaerosol diffusion may influence human and natural environments.

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Decomposition characteristics of fifteen different poly-cyclic aromatic hydrocarbons (PAHs) in the ambient particles by soft X-rays were experimentally investigated by irradiating the soft X-rays onto ambient particulates collected on silica fibrous filters. Influences of irradiating duration, distance between a soft X-ray tube and filter sample and energy of soft X-rays on the decomposed fraction of PAHs were discussed. Properties of PAHs such as boiling temperature and molecular band gap energy ΔE were related to the decomposed fraction of each PAH.
PAHs with 2-6 aromatic rings were decomposed by the soft X-ray irradiation but a maximum decomposed fraction was found to exist. The maximum decomposed fraction and decomposing rate constant increased with the irradiation energy of soft X-rays. The maximum decomposed fraction seems to decrease for PAHs with 4-aromatic rings such as Chrysene and increase for ΔE > ∼ 7.7 J.

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In exposure dose assessment, the ICRP (International Commission on Radiological Protection) Publication 66 human respiratory tract model has been used extensively beyond its original purpose which was for radiation protection. The ICRP models are historically based on Caucasians. In this study, the applicability of the ICRP Publication 66 model to Japanese was examined. Aerosol deposition to respiratory tracts was calculated using a computer code, LUDEP (LUng Dose Evaluation Programme) , developed by the NRPB (National Radiological Protection Board), UK. The aerosol deposition efficiencies of Japanese were shown to be almost the same as those of Caucasians. It seemed that there was no problem in the applicability of the ICRP model. But distinctive differences were found in aerosol depositions per person and per body-weight. For example, the maximum deposition per unit time of adult male Japanese in the AI (Alveolar-Interstitial) region at light exercise was approximately 80 % of that of Caucasians.

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The pulverized coal burner developed by the previous work (Kimoto et al., J. Japan Inst. Energy, 78, 404-415 (1999)) is capable of reducing the emissions of both NOx and unburned carbon in fly ash by improving the combustion stability at a low particle load. It is equipped with a function of enhancing the concentration of pulverized coal at the burner nozzle. However, the mechanisms of thickening the pulverized coal particles are not well understood because of the complicated structure of the burner nozzle. In the present work, the mechanisms of the pulverized coal thickening at the burner nozzle are investigated through a CFD simulation using Fluent 6.2 (Fluent, Inc.) . As a result, it was shown that the strong swirl given at the inlet of burner nozzle is restrained not only by the deflector angles attached to the inner wall of burner cylinder but by the concentration control ring installed in the burner nozzle. It was also found that the thickening of pulverized coal is achieved by reducing the distance between the control ring and the burner outlet because the pulverized coal particles tend to diffuse towards the center of burner nozzle by the turbulence generated by the control ring.

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