Tomoaki Okuda and colleagues at Keio University, Japan, report that measuring the charging states of atmospheric particles offers accurate data for formulating advanced models on the effects of particulates for human health. The findings are published in the journals Aerosol Science and Technology and Atmospheric Environment. 

The Keio Research Highlights website offers more details about this and other recent research being conducted by researchers at Keio University including the potential of molecular materials for high-yield solar energy conversion by Taku Hasobe and colleagues.

Measuring the charging states of atmospheric particles offers accurate data for formulating advanced models on the effects of particulates for human health

Inhalation of particulate matter is one of the top ten risks to human health according to the World Health Organization (WHO). In 2016, as many as 4.2 million people died due to the effects of particulates in the air. With these alarming statistics, it is not surprising that there have been increased calls for greater awareness and monitoring of the air for potentially deadly particulates.

«The health risks of inhaling particles less than 2.5 micrometers, so-called PM2.5, are well documented with environmental agencies monitoring particle-concentrations and issuing warnings to the public about dangerous levels of PM2.5 in the air,» says environmental scientist Tomoaki Okuda, an associate professor at the Department of Applied Chemistry, Keio University. «However, recent reports indicate that singly charged particles are about six times more likely to be adsorbed into the respiratory tract than uncharged particles. So, an accurate assessment of the effect of air-borne particles on human health requires a much deeper understanding of not only the size of particles, but also their physico-chemical properties including charge and surface structure. My research is focused on determining the charging state of particles in the air, which, surprisingly, is not well understood.»

Okuda and colleagues developed K-MACS (Fig. 1, Refs, [1,2]) as an inexpensive and versatile system to continuously separate and quantify the charging state of atmospheric particles ranging in size from 300 nm to 500 nm for short durations over a period of 12 months. «We studied this range of particles because they have a larger concentration in the air,» says Okuda. Measurements of these particles were taken at Keio University’s Yagami Campus from April 2017 to February 2018.

The two main findings were that the charging state of the atmospheric particles was different from the results of theoretical studies to-date (Fig. 2), and that the particle charging state fluctuated markedly with changing atmospheric conditions (water vapor amount and entrained air mass) due to seasonal variations [3].

«These results offer new insights into the changes of the charging state of particles,» says Okuda. «They will play an important role in the accurate evaluation of the effects of atmospheric particles on human health on a global scale.»


Related research conducted by Tomoaki Okuda

Okuda conducted a theoretical assessment of the performance of a parallel plate particle separator (K-MACS) instrument designed to measure the charging state of PM2.5 particles. Results yielded an optimal voltage to maximize the types of particles [2]. This research addresses the question of what happens to aerosol particles in humid conditions, such as haze. A discrete element model showed a correlation between relative humidity and the amount of water contained in the particles that were found to be charged negatively or positively [4].


Fig. 1
Illustration and photograph of the Keio Measurement system of Aerosol Charging State (K-MACS) ©Keio University



Fig. 2
(Left) Charge distributions of measured (green) and theoretically calculated particles.
(Right) Variation of the volumetric humidity versus charged particles fraction.
©Keio University



  1. Tomoaki Okuda, Yuma Gunji & I.W. Lenggoro, Measurement of the electrostatic charging state of individual particles in ambient aerosol using Kelvin Probe Force Microscopy, Earozoru Kenkyu 30, 190-197 (2015) | article (Japanese language only)

  1. Takuto Yonemichi, Koji Fukagata, Kentaro Fujioka & Tomoaki Okuda, Numerical simulation of parallel-plate particle separator for estimation of charge distribution of PM2.5, Aerosol Science and Technology (2019).

  1. Ayumi Iwata, et al., Seasonal variation in atmospheric particle electrostatic charging states determined using a parallel electrode plate device, Atmospheric Environment, 203, 62-69 (2019).

  1. Yuanping He et al., Atmospheric humidity and particle charging state on agglomeration of aerosol particles, Atmospheric Environment, 197, 141-149 (2019).


Research by Taku Hasobe and colleagues: Exciton fission for high-yield solar energy conversion

Quantifying multi-exciton generation for solar energy conversion in molecular materials

In molecular materials, the absorption of one photon can result in the formation of a singlet exciton ― a state formed by an electron and a hole bound by their electrostatic attraction and with opposite spins. In a process called singlet fission, the singlet exciton splits into two triplet excitons, that is, two excitons in which the electron and hole have their spins pointing in the same direction. The triplet excitons are initially strongly correlated, but they can be separated, and the electrons they carry can then be transferred to other molecules.

This complex process is very promising for applications in solar energy conversion, because it opens up the opportunity of reaching very high power conversion efficiencies, as the absorption of a single photon results in the generation of multiple electrical charges (i.e., multi-exciton generation).

Singlet fission can happen within a single molecule in systems of two molecular units covalently linked by an organic bridge (the process in this case is called intramolecular singlet fission). This is advantageous because we can quantitatively evaluate the photophysical processes and parameters such as kinetic constants and quantum yields. Singlet fission theoretically enables the performance of the sequential photoenergy conversion process starting from the singlet state and leading to electron transfer with the radical ion pair quantum yield approaching 200%. However, the quantitative two-electron transfer process through singlet fission has yet to be reported.

Shunta Nakamura, Hayato Sakai, and Taku Hasobe of Keio University, together with colleagues from Kobe University and Tempere University of Technology (Finland) have quantitatively characterized a sequential process involving intramolecular singlet fission and intermolecular two-electron transfer by using two tetracene-based molecules linked by a biphenyl unit (Tet-BP-Tet) as the singlet fission and electron donor system, and chloranil as the electron acceptor system.

Read more



Fig 3 Sequential photoenergy conversion process from singlet fission to intermolecular electron transfer in this study. ©Keio University


Shunta Nakamura, Hayato Sakai, Hiroki Nagashima, Yasuhiro Kobori, Nikolai V. Tkachenko, and Taku Hasobe, Quantitative Sequential Photoenergy Conversion Process from Singlet Fission to Intermolecular Two-Electron Transfers Utilizing Tetracene Dimer. ACS Energy Letters 4, 26−31 (2019).

DOI: 10.1021/acsenergylett.8b01964



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