エアロゾル研究 Vol. 32(2017) No. 3
秋

To investigate the pulmonary toxicity of various atmospheric aerosols or airborne engineered nanomaterials, cell-based in vitro studies that use air-liquid interface exposure systems and in vivo inhalation exposure studies that use laboratory animals by either nose-only exposure system or whole body exposure system are commonly employed. In this report, the feature of each exposure system is demonstrated.

Taking the scale of each exposure system into consideration, a target concentration of the airborne particles in the exposure system that is equivalent to the dose in human lungs in the environment was estimated using deposition efficiency, target surface area, and ventilation. If an adult man during light exercise inhales 100-nm spherical particles at the concentration of 15 μg/m³, which is an environmental standard value of PM_2.5 in Japan, the particle deposition rate in the lung is calculated to be 0.037 μg/m²/h. The concentrations that lead to the same deposition rate for cells in the air-liquid interface exposure system and rats in the inhalation exposure system are 1 μg/m³ and 4.6 μg/m³, respectively.

Risks of Nanoparticle should be assessed comprehensively by their physicochemical characteristics. However, such assessment has not been conducted because various physicochemical parameters and end points are involved. The relationship between one characteristic and the specific effect has to be analyzed in order to conduct sufficient risk assessment of nanomaterials. In this study, it was found that there was the quantitative relationship between the specific surface modification/size of nanoparticle and the endocytic uptake of alveolar epithelial cells. The results indicate that the amount of cellular uptake of nanoparticles is independent of the surface modification, but it depends on the particle size. The uptake rate is high in the order of COOH＞NH₂＞plain on the surface modification and faster as the particle size increases. It was suggested that uptake rate depends on the surface potential and the hydrophilicity.

Inhalation study is a gold standard study to estimate the harmful effect of respirable chemicals. Pulmonary inflammation, fibrosis and tumor are often used as endpoint for pulmonary toxicity of respirable chemicals. The main methods of aerosol generation are direct synthesis method, dry-based method and wet-based method, and the selection of generation method is necessary according to the chemicals. Experimental design should be included to avoid the concentration which is over the overload and prepare the observation period of which persistent inflammation, fibrosis and tumor in the lung could be estimated. The measurement of half times of respirable chemicals in the lung is important to judge to reach to the level of the overload. It is also useful to estimate the pulmonary toxicity of respirable chemicals especially for nanomaterials if the physico-chemical properties of nanomaterials are measured.

Ambient particulate matter (PM) consists of primary particles emitted directly from the source, and secondary particles formed by photo-oxidation reactions of volatile organic compounds and gases in the atmosphere, which are known as secondary organic aerosols (SOAs). Diesel exhaust (DE) is a major source of PM and a possible precursor of SOAs in the urban atmosphere. In our research institute, we generated SOAs by adding ozone to diesel exhaust, and have established a SOA inhalation exposure technique. Using the inhalation facility, we carried out an assessment of the toxicological effects of SOA exposure on the higher brain functions in mice. In this feature article, we shall discuss the results of our evaluation, through various animal studies, of the neurotoxicity observed after inhalation exposure to SOA.

Secondary organic aerosol (SOA) formation parameters of the two-product model for monoterpene ozonolysis are determined through the experiments conducted with five monoterpene hydrocarbons in this study and a reference study. The SOA mass yield increases at higher SOA concentrations or a lower temperature; however the dependence of SOA mass yield on temperature is relatively small for each hydrocarbon. The SOA mass yields are higher in the order of d-limonene, Δ³-carene, α-pinene and the other two compounds, β-pinene and sabinene, and depend on the number and position of the carbon double bond. At lower SOA concentrations and a higher temperature, SOA mass yields of monoterpene ozonolysis are higher than those of monoterpene photooxidation used in CMAQv4.7 (Carlton et al., 2010). It is expected that underestimation of SOA concentrations in non-heavily polluted areas calculated by CMAQv4.7 will be improved in some extent with substitution for SOA productivity for monoterpene ozonolysis obtained in this study.

In order to have a better understanding of Asian scale trans-boundary PM_2.5 contribution over western Japan and Japan Sea side area, source-receptor analysis based on the chemical transport model was conducted. We used GEOS Chem model and divided the Asian model domain into 15 major source regions, and the emission intensity sensitivity (source-receptor relationship) was examined. Long-term aerosol observation data at the Chikushi-campus of Kyushu University, Fukuoka, Japan, from December 2013 to March 2015 was used to validate the model performance. We also confirmed that the numerical model generally captured the observed time variation of PM_2.5 at remote islands (Miyakojima, Fukue, Oki, and Sado) quite well. We showed that the contribution of Japanese domestic emission to the annual averaged PM_2.5 concentration at Fukuoka was only 20%, and the other 80% of PM_2.5 was coming from outside of Japan (19.6% and 16.3% were coming from central north China region and Korea, respectively). Modeled source-receptor ratio in Fukuoka and the observed PM_2.5 concentration decrease in Beijing-Tianjin-Shandong area explained the recent decreasing trend of PM_2.5 (13% decrease from 2014 to 2016) at Fukuoka.