2026 Volume 41 Issue 2 Pages 151-162
Size-segregated aerosol samples were collected on Mt. Fuji with a cascade impactor during daytime and nighttime from 29 July to 2 August 2013 (first period) and from 19 to 21 August 2013 (second period). Analysis of chemical components and the Community Multiscale Air Quality Modeling System indicated that in the first period, the air masses were transported from East Asia. In the second period, an eruption plume from Sakurajima was detected. NH4+ and SO42− were the major ions in the particle size range 2.5–10 µm during the second period. The slope of a linear regression of non-sea-salt (nss)-SO42− versus NH4+ was nearly unity, particularly during the first period; however, there was an excess of nss-SO42− in coarse aerosols in the second period. In the second period, the abundance of sulfate increased in the 2.5–10 µm size range during both daytime and nighttime. Traditional formation mechanisms of (NH4)2SO4 in coarse particles (neutralization of the surface of coarse acidic particles by gaseous NH3, or coagulation of fine (NH4)2SO4 particles onto the surfaces of coarse particles) do not apply to the Sakurajima eruption because (NH4)2SO4 condensed on the surfaces of volcanic ash particles. Moreover, when (NH4)2SO4 was captured in fog droplets, the main sulfate peak was at 2.5–10 µm. The formation mechanisms of sulfates appear to differ between coarse (2.5–10 µm) and finer particles (1–2.5 µm) derived from volcanic aerosols and aerosols transported from Asia.
