This study aimed to elucidate the seasonal characteristics and potential sources of humic-like substances (HULIS) in PM_2.5, with a focus on separating humic acid (HA) and fulvic acid (FA). Daily PM_2.5 samples were collected continuously over five years (2013–2018) in Kazo, a rural site in Saitama prefecture, Japan, and grouped by season. Water-soluble components were extracted and analyzed using the DEAE-UV method, alongside measurements of ionic species, elemental composition, carbon fractions, and levoglucosan. FA concentrations were higher in autumn and winter, while HA showed less seasonal fluctuation, resulting in elevated HULIS in the cold seasons. It was suggested that FA and HA were influenced by open biomass burning, with certain influences from fossil fuel combustion. In comparison to those, it was suggested that the influences of marine source and photochemical secondary production to seasonal characteristics of HULIS were relatively small. The findings highlight biomass burning as a key driver of seasonal HULIS variability and underscore the need for broader, long-term monitoring to better understand their formation and environmental impacts.

Electrostatic atomization generates fine water droplets, known as electrostatic atomized water particles (EAWP), which have attracted attention for their air purification effects. Although scanning mobility particle sizers (SMPS) are widely used for nanoparticle measurements, the diffusion behavior and number density of EAWP are difficult to evaluate using conventional sampling-based techniques because of their nanoscale diameter and temporally varying generation rate. In this study, to visualize EAWP, ethanol vapor was condensed onto EAWP to increase their diameter into optically detectable sizes. The diffusion behavior of ethanol condensed droplets was visualized using LED light, and their number density was measured by a laser sheet. The diffusion behavior changed in three stages: cylindrical, domed, and homogeneous distribution. As the droplet number increased, gas-phase velocity variations led to the formation of a stagnation point. Therefore, analyses of droplet terminal settling velocity and number density were restricted to the cylindrical distribution. The terminal settling velocity of ethanol condensed droplets was measured by a laser sheet, and their diameter was estimated to be in the range of 1.5×10⁻⁵ to 2.2×10⁻⁵ m (“15–22 µm”) based on a balance between gravity and Stokes’ law of drag. Furthermore, the generation rate was estimated to be on the order of 10³ particles/s.