Chemical Characteristics of Cloud Water and Sulfate Production Under Excess Hydrogen Peroxide in a High Mountainous Region of Central Japan
2021
Kagawa, Masako | Katsuta, Nagayoshi | Ishizaka, Yutaka
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, NOₓ, 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.
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