Over the last two decades, atmospheric sulfur dioxide (SO₂) concentrations have decreased in air-polluted regions. However, this decrease was accompanied by a rise in cloud water acidity (pH), which remains below 5. With this change, gas-phase hydrogen peroxide (H₂O₂) exceeded SO₂ in most of these regions. These conditions where SO₂ < H₂O₂ occurred at Mt. Norikura (2770 m.a.s.l.) during the 1900s. Therefore, to reveal the aqueous-phase oxidation of SO₂ by H₂O₂, the present study investigated the inorganic and organic major ions and Se concentrations in cloud water in addition to aerosols and concentrations of gas species including O₃, NO_x, and SO₂ collected in cloud events at Mt. Norikura during the summer of 1999. Backward air trajectory analyses indicated that the (NH₄)₂SO₄ and trace (NH₄)HSO₄ aerosols originated from industrial and metropolitan areas in southwest Japan. The cloud water pH was between 3.6 and 4.4. The aqueous-phase SO₄²⁻/NO₃⁻ ratio (1.2 ± 0.6) was lower than that of the early 1990s (2.2) and 1960s (> 10) in our observation site, which was due to power plant restrictions in Japan since the 1970s. The ion species concentrations in cloud water indicated that cloud acidification resulted from dissolution of gaseous HNO₃ and SO₂, whereas gaseous hydrochloric acid and organic acid had a minor contribution to the acidification. Significant losses of Cl⁻ and Mg²⁺ were observed in some of the cloud water. The excess value of non-sea-salt sulfate (nss-SO₄²⁻) over NH₄⁺ in cloud water implies the in-cloud oxidation of gaseous SO₂ to aqueous SO₄²⁻. A Se tracer technique was used to conduct in-situ measurements of in-cloud SO₄²⁻ production. The results showed that the in-cloud production varied in a range between 7 ± 2 and 41 ± 14%. This temporal variation might be due to ambient SO₂ concentrations based on Henry’s law.

在过去的二十年中，在空气污染的地区，大气中的二氧化硫（SO ₂）浓度有所降低。但是，这种下降伴随着浊水酸度（pH）的上升，该上升仍保持在5以下。随着这种变化，在大多数这些区域中，气相过氧化氢（H ₂ O ₂）超过了SO ₂。SO ₂ <H ₂ O _2的这些条件发生在Mt。1900年代的Norikura（2770马斯拉）。因此，揭示了H ₂ O ₂对SO ₂的水相氧化，本研究调查了云水中除浮质外的气溶胶以及包括O ₃，NO _x和SO ₂在内的气体物种的浓度，还研究了云水中的无机和有机主离子以及Se的浓度。1999年夏季的Norikura。向后的空气轨迹分析表明（NH ₄）₂ SO ₄和痕量（NH ₄）HSO ₄气溶胶起源于日本西南部的工业和大都市地区。浊水的pH在3.6至4.4之间。将水相SO ₄ ^2- / NO ₃ ^-在我们的观测地点，该比率（1.2±0.6）低于1990年代初（2.2）和1960年代（> 10），这是由于1970年代以来日本的发电厂限制。云水中的离子种类浓度表明，云状酸化是由气态HNO ₃和SO ₂溶解引起的，而气态盐酸和有机酸对酸化的贡献较小。的Cl显著损失^-和Mg ²⁺中的一些云水的观察。云水中的非海盐硫酸盐（nss-SO ₄ ^2-）相对于NH ₄ ^+而言过高，意味着气态SO ₂在云中氧化为SO ₄水溶液²⁻。Se示踪剂技术用于进行云中SO ₄ ^2-产生的原位测量。结果表明，云内产量在7±2到41±14％的范围内变化。该时间变化可能是由于基于亨利定律的环境SO ₂浓度。