Published papers

Abstract: Clostridioides difficile is a spore−forming, obligate anaerobic bacillus commonly found in soil and the intestinal tracts of animals. It is a major cause of antibiotic−associated diarrhea (AAD) in humans. Our previous study suggested a possible environmental dissemination. To investigate whether atmospheric aerosols contribute to its spread, atmospheric aerosol samples were collected from ten geographic locations in East Asia, and the presence of C. difficile genomic sequences was assessed using 16S rRNA amplicon sequencing. Additionally, atmospheric PM2.5 samples were collected from two sites in Japan, and analyzed by 16S rRNA amplicon sequencing and selective culturing. C. difficile genomic sequences were detected in aerosols from 8 sampling sites, with relative abundances ranging from 0.03% to 9.88% of total prokaryotic reads. From PM2.5 samples, seven C. difficile strains were isolated, and classified into five genotypes. Three genotypes matched low−frequency clinical or environmental strains in Japan (0.1–0.6%), while two were previously unreported. Phylogenetic analysis showed that four genotypes were closely related to strains from North America, Europe, Africa, and Southeast Asia, respectively. These findings are consistent with the possibility that C. difficile may disperse globally via atmospheric aerosols. Successful colonization of C. difficile in the deposited environments likely depends on ecological adaptability.

Quantitative understanding of temporal distribution of submicron-charged particles is essential for estimating both the radiative forcing and the efficiency of particle deposition in human airways. To elucidate their behavior in the atmosphere, the number concentrations of charged and non-charged particles were measured at Noto Peninsula, Japan, to estimate the charge distribution of submicron atmospheric particles using a system that combines a parallel-plate particle separator and optical particle counters. The average charge number (pave) of particles and standard deviation (1σ) were obtained by fitting the particle charge distribution with a Gaussian distribution, representing the shift toward positive or negative and width broadening of the distribution, respectively. During summer, the particle charge distribution shifted toward the negative and the width increased. This change was likely due to collisions with negative cluster ions originating from the local vegetation and the Pacific Ocean under conditions of high negative polar conductivity in the atmosphere. During winter, the pave values were close to zero, and the width decreased. This change may arise from the balanced atmospheric electrical conductivity. The number concentrations of negatively charged and non-charged particles increased in summer and winter, respectively; their chemical compositions were primarily organic matter and sulfate, respectively. The pave values changed with the atmospheric electric field and atmospheric electrical conductivity ratio, indicating high production of submicron-sized charged particles owing to field charging. Our observation datasets are useful for constraining the submicron-sized charged particle concentration in numerical models.

Polycyclic aromatic sulfur heterocycles (PASHs) sulfur-containing analogues of polycyclic aromatic hydrocarbons (PAHs) are emerging atmospheric pollutants of concern due to their potential toxicity and environmental persistence. This study presents the first comparative analysis of PASHs in PM2.5 between two representative East Asian cities: Xi’an, China, and Yokohama, Japan. Ambient PM2.5 samples were collected from July 2022 to July 2023, and four PASHs compounds were quantified using gas chromatography tandem mass spectrometry (GC–MS/MS). Concentrations, seasonal variations, and emission sources were systematically analyzed. The results showed that the annual mean PASHs concentration in Xi’an (156.6 pg/mg) was significantly higher than that in Yokohama (102.6 pg/mg). Source apportionment using the Positive Matrix Factorization (PMF) model identified four major pollution sources in each city, revealing notable differences in source composition between the two countries. This study advances the understanding of the atmospheric behavior and origins of PASHs in East Asian urban environments and highlights cross-regional differences in their pollution profiles.

The toxicity of PM2.5has attracted interest and has been widely studied because of its adverse health effects. An in vitro assay exposing cultured cells to test particles was used to assess the biochemical response and evaluate PM2.5toxicity. The physicochemical properties of PM2.5suspensions, particularly particle size distribution, are important parameters that require accurate measurement to reliably assess toxicity. However, the conventional dynamic light scattering method has limited size resolution, making it unreliable for measuring the size distribution of particles with a broad size range. In this study, we present a novel method to accurately measure the size distribution of insoluble particles in PM2.5dispersed in a cell culture medium using aerosolization techniques at a submicrometer size range (100 nm to 1000 nm). This method includes sample pretreatment, aerosolization with a spraying device, and size distribution measurements of the aerosolized particles using an electrical mobility analyzing system. Sample pretreatment involves dialysis to remove dissolved nonvolatile components to minimize the formation of residual particles that may interfere with the size distribution measurements of insoluble particles. To validate the method, we measured the size distribution of the monodisperse polystyrene standard particles, including their mixtures, as well as the polydisperse silica standard particles with a known size distribution dispersed in cell culture medium. As a practical application, we demonstrated the measurement of ambient PM2.5samples collected from an urban atmosphere and a highway tunnel. Our method provides a novel approach for evaluating the size distribution of insoluble particles in cell culture medium.

Air pollution, particularly exposure to fine particulate matter (PM2.5), poses a substantial risk to human health. Diesel exhaust particles (DEPs), a major constituent of PM2.5, contain chemically reactive components that promote inflammation, oxidative stress, and immune dysfunction. Although the acute toxicity of PM2.5 and DEPs has been extensively studied, their effects under “sub-toxic” conditions—defined here as exposures that do not cause measurable cytotoxicity based on LDH release but still impair cellular function—remain poorly understood. This study investigated the impact of low-toxicity exposure to DEPs and PM2.5 on dendritic cell (DC) function using the human plasmacytoid DC-like cell line PMDC05. Cells exposed to DEPs or PM2.5 exhibited minimal cytotoxicity but accumulated intracellular particles, resulting in impaired endocytosis, phagocytosis, and interferon gene expression upon TLR7 stimulation. These functional impairments were not observed following TLR4 stimulation, suggesting a selective disruption of endolysosomal signalling. The findings demonstrate that DEPs and PM2.5 can impair innate immune responses without inducing cell death, likely through lysosomal overload and altered intracellular trafficking. This study identifies a non-cytotoxic pathway through which particulate air pollution may compromise antiviral immunity, thereby increasing susceptibility to infection in polluted environments. Strategies aimed at preserving lysosomal integrity and dendritic cell function may help mitigate the immunotoxic effects of airborne particles.

Ice-nucleating particles (INPs) play a pivotal role in cloud formation and precipitation, yet predicting their concentrations remains difficult due to limited understanding of their spatiotemporal variability and characteristics. Therefore, it is important to provide insights into how aerosol properties and meteorological parameters are related to INP concentrations. This study investigated INP concentrations, their heat sensitivity, particle chemical components, and meteorological parameters at urban (Yokohama) and rural (Noto) sites in Japan during Dust and Non-Dust periods. While no single key factor consistently explained INP concentrations, relatively strong associations were observed with factors implying particles such as mineral dust, heat-sensitive biological particles, and Pb-containing particles from coal and waste combustion, with variations depending on site and season. During the Dust period, INP concentrations at both sites were correlated with the mineral indicators (R > 0.57), though INPs above −15 °C remained low at Noto site (R = 0.49). In the Non-Dust period, mineral indicators cannot account for INP levels at either site (R < −0.31). Heat treatment-induced deactivation of INPs above −15 °C in the urban area suggested a close association between biological INPs and mineral particles. Although submicron INPs account for only a small fraction of total INPs at the surface (e.g., < 6.5 % at −12.5 °C), the particle types that contributed to the variability in INP concentrations were nevertheless supported previous qualitative findings. Our findings also highlight the complexity of quantitatively linking environmental factors that vary by region and time to INP concentration variations to characterize aerosol–cloud interactions.

皇居の濠である千鳥が淵の堆積物中のPAHsの安定炭素同位体比の鉛直分布を報告している。表層に近づくにつれて自動車由来PAHsの特徴が強く観測されている。

Most studies of the properties of airborne radionuclides emitted from the Fukushima Daiichi Nuclear Power Plant have focused on the relatively early stages of the accident, and little is known about the characteristics of radiocesium in the long-term. In this study, we analyzed activity size distributions of airborne radiocesium collected over 5 months in Tsukuba, Japan. Radiocesium in the accumulation mode size range (0.1–2 μm in aerodynamic diameter) was overwhelming in the early aerosol samples and decreased with time, while that associated with coarse aerosols remained airborne. We examined the radiocesium adsorbed onto airborne soil particles, and found that the size dependence of 137Cs surface density adsorbed on soil particles was weak. That is, radiocesium was distributed homogeneously throughout the aerodynamic diameter range of 2.1–11 μm. This characteristic may be related to the reported structure of radiocesium-bearing soil particles collected from the ground, which consisted of an aggregate of specific clay minerals and other non-cesium adsorbing particles. The resuspension factors for the first two aerosol samples collected during late April and May 2011 were close to those in European cities in the months following the Chernobyl accident, despite different soil and weather conditions.

The severe social restrictions aiming at reducing the novel coronavirus disease (COVID-19) transmission during its outbreak significantly affected regional air quality. This study analyzed changes in polycyclic aromatic hydrocarbons (PAHs) concentrations in cyclone-collected powder form particulate matter (PM) samples collected in Yokohama, Japan, from 2018 to 2023. The annual mean ∑PAHs concentration decreased significantly from 20.7 ng/mg in 2019 to 13.2 ng/mg in 2020 after the outbreak. However, the main sources of PAHs remained stable and were dominated by incomplete fossil fuel combustion. A generalized additive model was utilized to analyze the statistical relationships of air pollutants, namely, aerosol particulate matter (PM2.5), ∑PAHs, ozone (O3), metal, and meteorological factors such as solar radiation and their impact on aerosol optical depth (AOD). AOD remote sensing data collected by the Terra/MODIS satellite reflects the degree of air turbidity and atmospheric aerosol loading. Despite the small sample size, the model fitting demonstrated a good fit and showed smoothing non-linear interaction effects among the target factors influencing secondary aerosol generation. In conclusion, intense solar radiation, high concentrations of oxidative gases, elevated levels of organic contaminants in aerosols, and the catalytic degradation of PAHs by transition metals contribute to increased atmospheric aerosol loads. This finding enhances the present understanding of the photochemical reaction processes of aerosol particles. However, longer-term observational studies are needed to confirm our findings due to the limited sample size in this study.

The volcano on Miyake Island first erupted in July 2000 and continuous emission of volcanic gas from the collapsed caldera has been observed from the middle of August 2000. The large volcanic emission of SO2 had a strong influence on Tokyo metropolitan area, which is located approximately 150 km north of Miyake Island. We measured major ions in precipitation and dry deposition samples which had been collected at five sampling sites (Yokohama, Kashiwa, Fujisawa, Yokosuka, and Hachioji) in the Tokyo metropolitan area for 12 years since 1990. We have evaluated quantitatively the impact of the volcanic SO2 gas emitted from Miyake Island on the Tokyo metropolitan area by comparing depositional ionic constituents in the volcanic degassing period (from September 2000 to August 2001) with those in the normal period of the past 10 years (September 1990 to August 2000). nss-SO42- concentrations in precipitation at the sampling sites in the Tokyo metropolitan area were 59.5-77.0 μeq/L during the degassing period, and 33.3-44.1 μeq/L during the normal period, respectively. The difference of nss-SO 42- concentrations between the two periods was statistically significant. In contrast, no significant differences were observed in the concentrations of the other major ions (NH4+, nss-Ca2+, Cl-, and NO3-) between the two periods. The impact of volcanic degassing from Miyake Island on the ionic concentrations in the precipitation of the Tokyo metropolitan area was seen only in the H+ and nss-SO42- concentrations. The annual wet deposition amount of volcanic nss-SO42- into the Tokyo metropolitan area has been quantitatively estimated. The annual wet deposition amounts were calculated as 701±277 Meq/year (22.4±8.9 kt SO2/year) on the total area of the Tokyo metropolitan area (14,000 km2). The wet deposition amount of nss-SO42- corresponds to only 0.15% of the total annual amount of volcanic SO2 (15 Mt/year) emitted from Miyake Island from September 2000 to August 2001. This estimation is consistent with a previous study which reported that 0.3% of the atmospheric discharge of SO2 from the volcano in Mt. Etna in Sicily was deposited by precipitation to the surrounding area (1200 km2). The annual wet deposition amount of volcanic SO2 to the Tokyo metropolitan area was at the same order of the magnitude (37±15%) as the annual anthropogenic SO2 amount (60 kt/year) emitted from the Tokyo metropolitan area. © 2004 Elsevier B.V. All rights reserved.

Concentrations, molecular compositions, and compound-specific stable carbon isotopic compositions (δ13C) of polycyclic aromatic hydrocarbons (PAHs) in gasoline exhaust particles (GEPs) and diesel exhaust particles (DEPs) were investigated in this study. δ13C of PAHs in GEPs ranged from -13.3‰ to -26.8‰, and that in DEPs ranged from -21.7‰ to -26.3‰. The interspecies δ13C variations in each sample were 5.3 ± 2.2‰ in GEPs and 2.6 ± 1.3‰ in DEPs. PAHs in GEPs show larger interspecies δ13C variation than those in DEPs; hence, a degree of carbon isotopic fractionation during the conversion from fuel to PAH seems to be larger in gasoline engines than that in diesel engines. Pyrene series PAHs, which consist of only hexagonal rings, in almost all GEP samples show strong negative correlation between the H/C ratio and δ13C whereas fluoranthene series PAHs, which contain a pentagonal ring, show less systematic isotopic behavior in GEP samples. A kinetic isotope effect in thermal cracking of organic macromolecules may be minor for PAH formation in vehicle engines. We suggest that the isotopic trend of pyrene series in GEPs can be explained by a thermodynamic isotope effect, and that disturbance from isotopic equilibrium may cause a weak correlation between the isotopic behavior and the H/C ratio among the interspecies PAHs.

マレーシアにおける大気浮遊粒子中のPAHsの安定炭素同位体比を報告している。マレーシア都市域では、木材燃焼よりも自動車由来PAHsの寄与が大きいと結論づけている。

石油による海洋汚染が深刻な問題となっているマレーシア沿岸域で、その石油が放出された元を追跡した。HopaneやPAHsといった有機化合物を指標として放出源を考察している。

It would be convenient if we could achieve multi-component (organic and inorganic species) analysis using just one quartz fiber filter (QFF) as an aerosol collection medium. In general, QFF have been used for analyzing carbonaceous materials in aerosols. This study shows a nondestructive, rapid, and simple method using EDXRF (Energy Dispersive X-Ray Fluorescence spectrometry) that has secondary targets and three-dimensional polarization optics for analyzing inorganic multi-elements in aerosols collected on QFFs. Multi-element analysis can be achieved by using EDXRF coupled with fundamental parameter (FP) quantification, as speed of up to 900 s (15 min) per sample. The EDXRF-FP method shows good repeatability, with generally less than 5% variation when measuring each analyzed element. While the elemental concentrations in the blank QFFs in any lots examined in this study could be considered to be lower than the actual samples, the blank level of Ni cannot be ignored. The EDXRF-FP results for each element of the aerosols collected on QFF agreed well with ICP-MS and ICP-AES results. The temporal variations of the selected elements in aerosols obtained by EDXRF-FP showed very good agreement with those obtained by ICP-MS and AES, and thus this method is suitable for elucidating day-to-day variations of multi-elements in aerosols. © Taiwan Association for Aerosol Research.

A number of investigations have examined the impact of the use of biodiesel on the emissions of carbon dioxide and regulated emissions, but limited information exists on the chemical composition of particulate matter from diesel engines burning biodiesel blends. This study examines the composition of diesel particulate matter (DPM) emissions from a commercial agriculture tractor burning a range of biodiesel blends operating under a load that is controlled by a power take off (PTO) dynamometer. Ultra-low sulfur diesel (ULSD) fuel was blended with soybean and beef tallow based biodiesel to examine fuels containing 0% (B0), 25% (B25), 50% (B50), 75% (B75), and 100% (B100) biodiesel. Samples were then collected using a dilution source sampler to simulate atmospheric dilution. Diluted and aged exhaust was analyzed for particle mass and size distribution, PM2.5 particle mass, PM2.5 organic and elemental carbon, and speciated organic compounds. PM2.5 mass emissions rates for the B25, B50, and B75 soybean oil biodiesel mixtures had 20%-30% lower emissions than the petroleum diesel, but B100 emissions were about 40% higher than the petroleum diesel. The trends in mass emission rates with the increasing biodiesel content can be explained by a significant decrease in elemental carbon (EC) emissions across all blending ranges and increasing organic carbon (OC) emissions with pure biodiesel. Beef tallow biodiesel blends showed similar trends. Nevertheless, it is important to note that the study measurements are based on low dilution rates and the OC emissions changes may be affected by ambient temperature and different dilution conditions spanning micro-environments and atmospheric conditions. The results show that the use of biodiesel fuel for economic or climate change mitigation purposes can lead to reductions in PM emissions and a co-benefit of EC emission reductions. Detailed speciation of the OC emissions were also examined and are presented to understand the sensitivity of OC emissions with respect to biodiesel fuel blends. Copyright © American Association for Aerosol Research.

Emission rates and composition of known hazardous air pollutants in the exhaust gas from a commercial agriculture tractor, burning a range of biodiesel blends operating at two different load conditions were investigated to better understand the emission characteristics of biodiesel fuel. Ultra-Low Sulfur Petroleum Diesel (ULSD) fuel was blended with soybean oil and beef tallow based biodiesel to examine fuels containing 0% (B0), 50% (B50) and 100% (B100) soybean oil based biodiesel, and 50% (B50T) and 100% (B100T) beef tallow biodiesel. Samples were collected using a dilution source sampler to simulate atmospheric dilution. Particulate matter and exhaust gases were analyzed for carbonyls, Volatile Organic Compounds (VOCs), and Polycyclic Aromatic Hydrocarbons (PAHs) to determine their respective emission rates. This analysis is focused on the emissions of organic compounds classified by the US EPA as air toxics and include 2,2,4 trimethylpentane, benzene, toluene, ethylbenzene, m-, p- and o-xylene, formaldehyde, acetaldehyde and methylethyl ketone. Emission rates of 2,2,4 trimethylpentane, toluene, ethylbenzene, m-, p- and o-xylene decreased more than 90% for B50, B100 and B100T blends; decreases in emission rates of benzene, formaldehyde and acetaldehyde were more modest, producing values between 23 and 67%, and methyl ethyl ketone showed decreases not exceeding 7% for the studied biodiesel blends. PAHs emission rates were reduced by 66% for B50, 84% for B100, and by 89% for B100T. The overall emissions of toxic organic compounds were calculated and expressed as benzene equivalents. The largest contributors of toxic risk were found to be formaldehyde and acetaldehyde. Reductions in formaldehyde emissions were 23% for B50 and 42% for B100 soybean, and 40% for B100T beef tallow compared to B0. Similarly, acetaldehyde reductions were 34% for B50 and 53% for B100 soybean biodiesel and 42% for B100T beef tallow biodiesel. © 2011 Elsevier Ltd.

To date, areas contaminated by radionuclides discharged from the Fukushima Dai-ichi nuclear power plant accident have been mapped in detail. However, size of the radionuclides and their mixing state with other aerosol components, which are critical in their removal from the atmosphere, have not yet been revealed. We measured activity size distributions of 134Cs and 137Cs in aerosols collected 47 days after the accident at Tsukuba, Japan, and found that the activity median aerodynamic diameters of 134Cs and 137Cs in the first sample (April 28-May 12) were 0.54 and 0.53 μm, respectively, and those in the second sample (May 12-26) were both 0.63 μm. The activity size distributions of these radiocesium were within the accumulation mode size range and almost overlapped with the mass size distribution of non-sea-salt sulfate aerosol. From the analysis of other aerosol components, we found that sulfate was the potential transport medium for these radionuclides, and resuspended soil particles that attached radionuclides were not the major airborne radioactive substances at the time of measurement. This explains the relatively similar activity sizes of radiocesium measured at various sites during the Chernobyl accident. Our results can serve as basic data for modeling the transport/deposition of radionuclides. © 2012 American Chemical Society.