Seasonal charge distributions of submicron atmospheric particles in Yokohama, Japan.

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 (p ave) and the standard deviation (1 σ), derived from the charge distribution of the submicron particles, were within 15%. The monthly median values of 1 σ increased in summer and decreased in winter and correlated with the water vapor amount and wind speed. The 1 σ 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 p ave 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. • We observed charge distribution of submicron particles at Yokohama, Japan. • Charge distribution of submicron particles was expanded in summer. • Particle charge distribution was controlled by the water vapor amount. • Non-charged particle fraction value was consistent with a previous study. • Lightning events changed the particle polarity ratio and charge distribution. [ABSTRACT FROM AUTHOR]