Aerosol number concentrations and trace gases (SO 2 , O 3 , HCHO, and H 2 O 2 ) were measured over Imizu City, Toyama Pre fecture, Japan on 5 August 2020, from a helicopter. The concentra tions of fine particles (0.3−0.5 μm) were high at ground level and at an altitude of approximately 1200 m due to the volcanic plume from Nishinoshima. However, concentrations of aerosol particles were low at an altitude of approximately 2400 m. The volcanic plume did not significantly affect the lower free troposphere. High concentrations of SO 2 were also observed. The H 2 O 2 concentra tions (0.27−0.56 ppb) were much lower than those during previous observations in the summer (2.0−6.7 ppb) and significantly lower than the SO 2 concentrations (1.8−3.3 ppb). The concentrations of HCHO (2.1−2.5 ppb) were higher than those of H 2 O 2 . The oxida tion of S(IV) to H 2 SO 4 in cloud water might have been suppressed over the studied area.