Photochemical Formation and Scavenging of Hydroxyl Radical in Rain and Dew Waters

Takemitsu ARAKAKI*, Takayuki MIYAKE, Michie SHIBATA and Hiroshi SAKUGAWA††

CREST, Japan Science and Technology Corporation, The Center for Forest Decline Studies; HiroshimaTechno-Plaza 308, 3-13-26 Kagamiyama, Higashi-Hiroshima-shi 739-0046 Japan
Graduate School of Biosphere Sciences, Hiroshima University; 1-7-1 Kagamiyama, Higashi-Hiroshima-shi 739-8521 Japan
†† Faculty of Integrated Arts and Sciences, Hiroshima University; 1-7-1 Kagamiyama, Higashi-Hiroshimashi-shi 739-8521 Japan

Photochemical formation and scavenging of hydroxyl radical (OH radical) was investigated using rain (n=69) and dew (n=26) waters collected during June, 1997 to June, 1998 in Higashi-Hiroshima city, Hiroshima prefecture. For the sources of OH, it was found that majority of OH radical was formed from sources other than nitric acid or nitrous acid photolysis in rain waters. For dew waters, almost all of the OH radical was formed from nitrite ion (NO2) photolysis. Scavenging rate constants (Σ(kscavenger, OH[scavenger])) for rain waters ranged from (0.2--1.6)×105 s−1 and the mean was 0.7×105 s−1. For dew waters, scavenging rate constants ranged from (2.1--4.6)×105 s−1 and the mean was 3.4×105 s−1. The corresponding lifetimes of OH radical for rain and dew waters were estimated to be on the average 20.6 and 3.2 μs, respectively. For rain waters, it was speculated that majority of the OH radical was scavenged by dissolved organic compounds. On the other hand, nitrous acid scavenged on the average 59% of OH radical in the dew waters studied here. Based on the aqueous-phase photochemical sources of OH in rain and dew waters and scavenging rate constants, steady-state OH radical concentrations were estimated to be approximately 1×10−15 M(1 M=1 mol dm−3) for both rain and dew waters.

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