Published papers

Reliable measurement of the size-resolved number concentration (size distribution) of solid particles dispersed in water or melted ice is of critical importance in many geoscientific observational studies. Because physical and chemical properties of particles can be measured more unambiguously and accurately in rarefied media (air) than in condensed media (water), particle measurement after aerosolization using a nebulizer is a significant method for the observation of solid particles dispersed in water. We propose a mathematical theory for estimating the original size distribution of solid particles in water from the measured size distribution of aerosolized particles. We assume that the size distribution of water droplets produced by a nebulizer is given. The size distribution of solid particles in water can be estimated by solving a system of nonlinear equations. The complexity in solving the equations increases rapidly with the computational resolution of particle size and the assumed maximum number of particles within each droplet. For such a system of equations, we found rigorous error bounds of a true solution using INTLAB, an interval arithmetic package. Our theoretical framework will be useful in many fields in geoscience as a fundamental scheme to quantify solid particles in water. In particular, an application of the proposed theoretical method is shown to be useful for the quantitative observations of the size distribution of black carbon particles in rainwater. (C) 2015 Elsevier Ltd. All rights reserved.

We developed an improved technique for measuring the size distribution of black carbon (BC) particles suspended in liquid water to facilitate quantitative studies of the wet deposition of BC. The measurement system, which consists of a nebulizer and a single-particle soot photometer, incorporates two improvements into the system that we developed earlier. First, we extended the upper limit of the detectable BC size from 0.9m to about 4.0m by modifying the photo-detector for measuring the laser-induced incandescence signal. Second, we introduced a pneumatic nebulizer (Marin-5) with a high extraction efficiency (approximate to 50.0%) that was independent of particle diameter up to 2.0m. For BC mass concentrations less than 70g L-1, we experimentally showed that the diameters of BC particles did not appreciably change during the Marin-5 extraction process, consistent with theoretical calculations. Finally, we demonstrated by laboratory experiments that the size distributions of ambient BC particles changed little during their growth into cloud droplets under supersaturation of water vapor. Using our improved system, we measured the size distributions of BC particles simultaneously in air and rainwater in Tokyo during summer 2014. We observed that the size distributions of BC particles in rainwater shifted to larger sizes compared with those observed in ambient air, indicating that larger BC particles in air were removed more efficiently by precipitation.Copyright (c) 2016 American Association for Aerosol Research

Quantitative simulation of an aerosol’s lifecycle by regional-scale and global-scale atmospheric models is mandatory for unbiased analysis and prediction of aerosol radiative forcing and climate change. Globally, aerosol deposition is dominated by the rainout process, which is mostly triggered by activation of aerosols to liquid droplets in supersaturated domains of precipitating clouds. However, the actual environmental supersaturation value that aerosols experience in precipitating clouds is difficult for models to predict, and it has never been constrained by observations; as a result, there is large uncertainty in atmospheric aerosol simulations. Here, by a particle-tracer analysis of 37 rainfall events in East Asia, near the largest source region of anthropogenic aerosols in the northern hemisphere, we observed that the environmental supersaturation actually experienced by the removed aerosols in precipitating clouds averaged 0.08 ± 0.03% and ranged from 0.03 to 0.2%. Simulations by a mixing-state-resolved global aerosol model showed that the simulated long-range transport efficiency and global atmospheric burden of black carbon aerosols can be changed by a factor of two or three as a result of a change in the environmental supersaturation in precipitating clouds within just 0.08 ± 0.03%. This result is attributable to the fact that the sensitivity of an aerosol’s rainout efficiency to environmental supersaturation is higher for the less-aged black carbon concentrated near source regions. Our results suggest that observational constraints of environmental supersaturation in precipitating clouds, particularly near source regions, are of fundamental importance for accurate simulation of the atmospheric burden of black carbon and other aerosols.

Long-term measurements of black carbon (BC) aerosols by filter-based absorption photometers with a heated inlet (COSMOS) in different regions have been useful in elucidating spatial variations and radiative impacts of BC. Evaluations of mass concentrations of BC (M-BC) measured by the COSMOS have been made by our previous studies through comparisons with other measurement techniques. However, how variations in the microphysical properties of BC and the co-existing light scattering aerosols affect the COSMOS measurements should be evaluated in more detail. In this study, we assessed these potential effects under various field environments in the Arctic and in the East Asia. From the slopes of the correlation plots between the M-BC values measured by the COSMOS and a single-particle soot photometer, the average accuracy of the COSMOS was estimated as similar to 10% in the M-BC range 1-3000 ng m(-3). On an hourly basis, the estimated sensitivity of the COSMOS M-BC values to the changes in the BC size distributions was less than 10%, within the typical variabilities of BC size at individual observation sites. The COSMOS measurements depended little on the mixing states of BC and the concentrations of co-existing light scattering aerosols, except in the maritime air masses of East Asia, where the relative abundance of sea salt to BC was very high. The M-BC measured by COSMOS also well agreed with elemental carbon measurements. Our results demonstrate the high reliability of COSMOS measurements under various environments. Copyright (c) 2019 American Association for Aerosol Research

Carbonaceous aerosol particles emitted from biomass burning (BB) have a large impact on the global climate. In particular, tarball particles (TBs), which are spherical organic aerosol particles, account for a large proportion of aerosol particles from BB. In this study, we collected aerosol particles over the western North Pacific and analyzed them using transmission electron microscopy with energy-dispersive X-ray spectroscopy (TEM-EDX) to reveal their shape and composition. We detected TBs and organic carbon particles originating from Siberian forest fires. To the best of our knowledge, this is the first case in which a large number of TBs have been found over the Pacific Ocean far from the BB source. The spherical shapes of the TBs were maintained even after long-range transport. In addition, our individual analysis of TBs showed that the size and composition of TBs differ depending on the air mass origin. The occurrence and microphysical properties of TBs are important to accurately evaluate the impact of TBs on climate. Our results imply that TBs can be transported to the Arctic and have an influence on radiative forcing over the ocean and in the Arctic.

The global limit on the sulfur content of ship fuel was reduced from 3.50% to 0.50% in January 2020 to reduce ship emissions of SO2 and particulate matter. We conducted observational campaigns before and after the new global limit was introduced to detect changes in coastal air quality. We measured ambient concentrations of SO2 and CO2 ship plumes on shore with the sniffing method under the Kanmon Bridge over the Kanmon Straits between Honshu and Kyusyu Islands, Japan, for several weeks in August to September in 2019 and 2020. The fuel sulfur content (FSC) estimated from our measurements mainly varied from 0.50% to 3.00% in 2019, whereas the range narrowed to 0.10% to 0.40% in 2020, showing that all the ships complied. The mean FSC in 2020 was reduced to 16% of that in 2019, which was consistent with the reduction in the ambient SO2 concentration. Sakurai et al. (2021) estimated that after the 2020 global limit was brought in, SO2 emissions from ships were reduced to 24% of their previous values by assuming that all ships have a FSC of 0.50%. Our results indicate the 2020 global limit led to much greater reductions in SO2 emissions from ships than expected.

Here, we used a modified single-particle soot photometer (SP2) coupled with a concentric pneumatic nebulizer to measure the size-resolved number and mass concentrations of light-absorbing iron oxide aerosols (FeOx) in liquid water (C NFeOx and C MFeOx, respectively). The SP2 could selectively detect individual FeOx particles in mixed wüstite–fullerene soot laboratory samples and melted Arctic snow samples. The nebulizer efficiency for FeOx particles was about 50% within the 70–650 nm diameter range, as derived from the ratio of the volume of ammonium sulfate before and after extraction by the nebulizer and the size-resolved transmission efficiency in the nebulizer–SP2 sampling line. Uncertainty from the boundary lines empirically drawn to discriminate the scatterplots of FeOx and black carbon in the mixed wüstite–fullerene soot suspensions and snow samples was approximately 3.0% and 10%, respectively. Overall uncertainty in total C NFeOx and C MFeOx (220–1400 nm) was approximately 19% and 18%, respectively. After storage at 4 °C for 16 months, the FeOx particle size distributions in melted Arctic snow had remained stable, and C NFeOx and C MFeOx had changed by less than 19% and 1.0%, on average, respectively. Most of the FeOx on dust particles measured by this system was estimated to be in the diameter range smaller than 1000 nm, considering the nebulizer efficiency for dust particles. The high accuracy of the C NFeOx and C MFeOx measurements will help to improve our quantitative understanding of the wet deposition of FeOx and provide more accurate estimates of the effects of FeOx on snow surface albedo.

Charge distribution measurements are required to understand the spatiotemporal distribution of the number concentrations of submicron atmospheric particles that affect radiative forcing and particle deposition in human airways. The number concentrations of non-charged and charged particles within the 0.3–0.5 μm diameter (D) range were measured at Keio University in Yokohama, Japan, from June 2022 to January 2023 by combining a parallel-pate particle separator and optical particle counters to investigate critical parameters controlling the charging state of submicron atmospheric particles. The measurement uncertainties in the average charge number per particle (pave) and the standard deviation (1σ), derived from the charge distribution of the submicron particles, were within 15%. The monthly median values of 1σ increased in summer and decreased in winter and correlated with the water vapor amount and wind speed. The 1σ values in summer and winter, derived from the seasonally averaged charge distributions of particles, were close to those from the theoretically calculated charge distribution of particles within 0.387–0.5 μm D range and with D = 0.3 μm, respectively, suggesting that the observed particle charge distributions approached the stationary charge distribution for the effective D. In summer, the frequent transport of water molecules and ions from the Pacific Ocean causes efficient collisions between multiple ions and submicron particles with a larger effective D, which may expand the charge distribution of particles. The polarity ratio, the concentration of positively charged particles relative to that of negatively charged particles, was almost unity, indicating the well-balanced charge polarity of the submicron atmospheric particles. The polarity ratio and pave changed significantly during lightning events, indicating that the atmospheric particle charge balance broke. Our findings show that the charge distribution of submicron atmospheric particles can be partly controlled by meteorological parameters (e.g., absolute humidity) and the microphysical properties of the particles.

Ice cores can provide long-term records of refractory black carbon (rBC), an important aerosol species closely linked to the climate and environment. However, previous studies of ice cores only analyzed rBC particles with a diameter of < 500 nm, which could have led to an underestimation of rBC mass concentrations. Information on the size distribution of rBC particles is very limited, and there are no Arctic ice core records of the temporal variation in rBC size distribution. In this study, we applied a recently developed improved technique to analyze the rBC concentration in an ice core drilled at the SIGMA-D site in northwestern Greenland. The improved technique, which uses the modified Single-Particle Soot Photometer (SP2) and a high-efficiency nebulizer, widens the measurable range of rBC particle size. For high-resolution continuous analyses of ice cores, we developed a continuous flow analysis (CFA) system. Coupling of the improved rBC measurement technique with the CFA system allows accurate high-resolution measurements of the size distribution and concentration of rBC particles with a diameter between 70 nm and 4 µm, with minimal particle losses. Using this technique, we reconstructed the size distributions and the number and mass concentrations of rBC particles during the past 350 years. On the basis of the size distributions, we assessed the underestimation of rBC mass concentrations measured using the conventional SP2s. For the period 2003–2013, the underestimation of the average mass concentration would have been 12 %–31 % for the SIGMA-D core.

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.

Aerosol composition and mixing state influence its ability to form cloud droplets and ice crystals and to scatter and absorb sunlight, all of which affect its impact on climate. In this study, aerosol samples were collected from different altitudes, ranging from the sea surface to ∼ 8000 m, over the ocean in the western North Pacific in the summer of 2022 using an aircraft and a research vessel. The samples were classified into three periods based on the sampled air parcel sources: ocean and desert (period 1), Siberian Forest biomass burning event (period 2), and their mixtures (period 3). Measurements of particle composition using transmission electron microscopy with energy-dispersive X-ray spectrometry revealed that samples from period 1 had high sea salt and mineral dust fractions, whereas samples from period 2 had high fractions of potassium-bearing particles with organics and black carbon. Samples from period 3 showed influences of both sea spray and biomass burning. During periods 1 and 3, the sea salt fractions increased as the samples were collected at lower altitudes. The compositions of biomass burning and sea spray were mixed at individual particles, with higher fractions of Na and K during period 1 and period 2, respectively, than in other periods. Our analysis of individual particles revealed a wide range of compositions and mixing states of particles, which depend on the aerosol source, size, and altitude. These factors need to be considered when evaluating aerosol composition and mixing state, both of which affect aerosol climate effects.

An integrated observational study of aerosols and clouds was carried out in summer 2022 over the western North Pacific (off the eastern coast of Hokkaido, Japan) using both an aircraft and a research vessel for the first time in this area. The aim of this experiment was to characterize the physical and chemical properties of aerosols and to evaluate the contributions of anthropogenic and natural emissions to cloud condensation nuclei (CCN) and ice-nucleating particle (INP) concentrations. The experiment also aims to examine the vertical structure of low-level clouds, which have high radiative effects, over this region. Here, we present an overview of the experiment and some preliminary results. Onboard the research vessel, suspended particulate matter in seawater was sampled, and the size distributions of dominant particle types (e.g., diatom fragments and organic matter) were determined. By analyzing atmospheric aerosols sampled from aircraft, we derived the number size distributions of dust-like particles in the air far from source regions. The aerosol size distributions, as measured from both the vessel and aircraft, suggested that most of the aerosols that could act as CCN were of anthropogenic origin: The upper-bound estimate of the sea spray aerosol (SSA) contribution to the CCN concentration at altitudes below 1200 m was 23%. Numerical model simulations generally reproduced the observed aerosol number concentrations. The simulations supported the inference from the observation that most CCN over the study area were of anthropogenic origin. Using this model, SSA emissions were estimated to account for 9% of the cloud radiative effect. Cloud observations from aircraft revealed that low-level clouds tended to have an adiabatic vertical structure under cold air advection, whereas under warm air advection, they occasionally had a layered structure. Such diverse vertical structures of low-level clouds are not generally seen in the subtropics. The integrated observations carried out in this study provided a basic picture of aerosols and clouds over the study area in summer. Further research is needed to elucidate the roles of aerosols in cloud formation and the radiation budget under various meteorological conditions within the framework of atmosphere–ocean interactions. (Figure presented.)

Air pollutants are primarily transported from midlatitude emission regions in winter and early spring, leading to elevated concentrations of aerosols, including black carbon (BC), in the Arctic, a phenomenon known as Arctic haze. The Weather Research and Forecasting model coupled with chemistry is used to investigate potential causes of uncertainties in modeling Arctic BC for winter 2014. The model captures observed variability in BC at surface sites, reproducing BC concentrations at Zeppelin but showing a low bias at Tiksi, Alert, and Utqiaġvik/Barrow. The influence of removal processes on model BC biases is explored by switching off dry or wet deposition. Wet deposition, during transport in the North Atlantic storm track, and locally over Svalbard, dominates BC removal at Zeppelin, while wet removal in the Pacific storm track influences BC at Alert and Utqiaġvik/Barrow. Dry removal over Asian source regions and Alaska affects BC at Utqiaġvik/Barrow, and is larger than wet removal at Tiksi due to the proximity of local/regional anthropogenic sources. Regional runs over northern Alaska in late January show improved simulated BC compared to observations at Utqiaġvik/Barrow, in part, due to better resolution of removal processes, and local/regional emissions. Sensitivity runs also show that regional Alaskan sources, notably from the North Slope of Alaska oil fields, may be contributing 30%–50%, on average, to observed BC at Utqiaġvik/Barrow in January and February 2014, with the remainder from outside the region. These findings highlight the importance of local Arctic emissions, and the need for improved emission inventories in the Arctic.

Mineral dust accounts for a significant fraction of the atmospheric aerosol burden, impacting climate through its interactions with radiation and clouds. However, limited observations of dust particles in the free troposphere hinder our understanding of its spatial distribution and climatic impacts. In this study, aerosols were collected on filters using aircraft over the western North Pacific at 0.2–8 km altitudes in summer 2022. Individual water-insoluble particles dispersed in water were analyzed using a complex amplitude sensor (CAS), which enabled classifying particle type, sizing, and counting from the particle's complex amplitude data. During the eight observation flights, the number and mass concentrations of dust were 1.2 ± 1.0 cm−3 and 0.6 ± 0.4 μg m−3 (mean ± 1σ), respectively, in the 0.3–2.5 μm diameter range, which were 2–3 orders of magnitude lower than previously reported values near the major dust sources in East Asia. A comparison with the total aerosol data from an optical particle counter onboard aircraft suggested that dust dominated aerosols larger than 0.8 μm, consistent with electron microscopy analyses. The observed dust size distributions implied more efficient removal of larger dust during transport. Since our samples were collected at −18°C to 21°C, the free tropospheric dust could contribute to ice formation in clouds. Our observation using the CAS revealed microphysical properties of dust over the western North Pacific when no distinct Asian dust events were observed near the ground, aiding in the validation of models that assess the climatic impacts of dust.

The roles and impacts of refractory black carbon (rBC), an important aerosol species affecting Earth's radiation budget, are not well understood owing to a lack of accurate long-term observations. To study the temporal changes in rBC since the pre-industrial period, we analyzed rBC in an ice core drilled in northwestern Greenland. Using an improved technique for rBC measurement and a continuous flow analysis (CFA) system, we obtained accurate and high-temporal-resolution records of rBC particle size and mass/number concentrations for the past 350 years. Number and mass concentrations, which both started to increase in the 1870s associated with the inflow of anthropogenically derived rBC, reached their maxima in the 1910s-1920s and then subsequently decreased. Backward-trajectory analyses suggest that North America was likely the dominant source region of the anthropogenic rBC in the ice core. The increase in anthropogenic rBC shifted the annual concentration peaks of rBC from summer to winter-early spring. After rBC concentrations diminished to pre-industrial levels, the annual peak concentration of rBC returned to the summer. We found that anthropogenic rBC particles were larger than biomass burning rBC particles. By separating the rBC in winter and summer, we reconstructed the temporal variations in rBC that originated from biomass burning, including the period with large anthropogenic input. The rBC that originated from biomass burning showed no trend in increase until the early 2000s. Finally, possible albedo reductions due to rBC are discussed. Our new data provide key information for validating aerosol and climate models, thereby supporting improved projections of future climate and environment.

Chronic obstructive pulmonary disease (COPD) is induced by inhalation of toxic substances such as cigarettes and air pollution. Dry powder inhalers (DPIs) are the primary treatment for these diseases. However, they have some problems, such as residuals in a capsule caused by electrostatic force before reaching the human lungs. This study investigated the particle tribocharging mechanism in a DPI using a tandem differential mobility analyzer (TDMA) and a combined discrete element method and computational fluid dynamics (DEM-CFD) approach. In the TDMA experiment, the charging state of the particles changed from negative to positive charge in the DPI device fabricated by the 3D printer. This is because tribocharging is caused by particle–particle collisions and particle–wall collisions. In the numerical simulation, particle–wall collisions occurred more frequently than particle–particle collisions. Therefore, the particle–wall collisions change the charging state of the particle in the DPI device. These results suggest that collisions between particles and walls of the device cause the particles to become charged, leading to a decrease in their deposition in the deeper regions of the lungs. Moreover, the large turbulence kinetic energy of the airflow in the DPI device caused particle–wall collisions because the particles were widely dispersed in the DPI device. These results suggest that optimum turbulence kinetic energy is necessary to reduce particle aggregation and improve the delivery efficiency of DPIs to the human lungs.

COSMOS is a filter-based absorption photometer for long-term observations of black carbon (BC) mass concentrations (MBC) in the atmosphere. In environments with very high MBC, such as in South Asia, rapid accumulation of BC on the filter tape results in a rapid decrease of filter transmittance (Tr), leading to more frequent filter advances and instrument downtime for changing the filter rolls if the standard threshold setting of Tr (0.7 or 0.8) is applied. Thus, for long-term observations in such environments, correction factors are needed to allow for accurate determination of MBC values, even after large decreases of Tr. Here, we compared MBC measurements made by two COSMOS instruments with different Tr threshold settings and derived two kinds of correction factors that allow for the derivation of MBC at very low Tr while maintaining reasonable accuracy. We found that reliable MBC measurements are possible down to Tr ≈ 0.23, depending on the average MBC level. The Tr-dependent corrections reflect the effects of two processes: enhancement of absorption of BC by multiple scattering inside the filter, and overlapping of BC accumulated on the filter. The correction factors reported here enable long-term measurements of BC by COSMOS under a wider range of conditions while reducing maintenance costs and the frequency of observation interruptions.

This paper reviews studies of atmospheric climate forcers, namely, greenhouse gases (GHGs) and aerosols, and their impacts on radiation and clouds in the Arctic during the Arctic Challenge for Sustainability II (ArCS II) project conducted between 2020 and 2024. In GHG research, we measured atmospheric mixing ratios of CO2, CH4, and N2O and their isotope ratios, as well as O2/N2 ratios from the ground, ship (research vessel Mirai), and commercial aircraft over the northern high latitudes. We showed that the rapid increase of CH4 mixing ratios after ∼2018 could be attributed to elevated microbial CH4 emissions. The increase of N2O after ∼2011 was attributable to an increase of N2O emissions from soil treated with chemical fertilizer. In aerosol research, we provided a scientific basis for constructing unified black carbon (BC) datasets in the Arctic and estimated the contributions to Arctic BC from biomass-burning and anthropogenic sources in various regions. We showed the abundance of highly active ice nucleating particles (INPs) increased with rising surface temperatures above 0 °C and evaluated impacts of Arctic dust that serve as INP on clouds. We found that marine biota were likely the sources of cloud condensation nuclei and INPs over the remote Arctic Ocean during periods of high biological activity.