Peroxide concentrations measured over a rural site in the Hokuriku district, coastal region of the Sea of Japan, central Japan, during early summer using a helicopter

Atmospheric concentrations of peroxides (hydrogen peroxide (H 2 O 2 ) and methyl hydroperoxide (MHP)), formaldehyde (HCHO), ozone (O 3 ), and sulfur dioxide (SO 2 ) were measured using a helicopter over Imizu City, Toyama Prefecture, central Japan in early summer (May and June) from 2022 to 2024. The impact of transboundary air pollution was relatively small during the observation days. H 2 O 2 concentrations were higher in the high-altitude atmosphere than on the ground and were sufficiently high relative to SO 2 , similar to the H 2 O 2 concentrations previously measured in mid-summer (August), suggesting that the potential for sulfur dioxide oxidation is high over central Japan during warm seasons. HCHO concentrations in the high-altitude atmosphere were high due to the influence of air pollution from domestic urban areas. The measurement results in this study were compared with the results of a series of helicopter observations previously conducted over Toyama Prefecture, and the characteristics of the oxidizing capacity in the atmosphere over central Japan are discussed.