低温科学 = Low Temperature Science;第68巻

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大気エアロゾル中の低分子ジカルボン酸の分布と安定炭素同位体比

河村, 公隆

Permalink : http://hdl.handle.net/2115/45169

Abstract

低分子ジカルボン酸の安定炭素同位体比(δ13C)の測定法について説明し, 海洋エアロゾル試料中のジカルボン酸のδ13C値の特徴とそれから得られる大気化学情報解読の可能性について解説する. 本研究では赤道域を含む外洋大気中でシュウ酸など低分子ジカルボン酸が生物起源有機物の安定炭素同位体比にくらべ極めて高いδ13C 値を示すことを見つけた. 本稿では, その結果について概説するとともに, 低分子ジカルボン酸の安定炭素同位体比を有機エアロゾルの光化学的変質(エイジング)のトレーサーとして使える可能性についても考察する.
Stable carbon isotopic composition (δ13C) has been studied for C2-C9 diacids in the remote marine aerosols (n=32) collected from the Pacific Ocean including tropics (34°N-14°S, 140°E-150°W). Their isotopic compositions showed a diverse variation with the highest values in oxalic (C2) acid (range: +4.1 to +15.3‰,av.+10.3‰), followed by malonic (C3) acid (-11.2 to -3.1‰, av.-7.3‰) and succinic (C4) acid (-29.9 to -11.2‰, av.-17.6‰). The isotope ratios generally decrease with an increase in their carbon chain length, showing the lowest value of azelaic (C9) acid (av. -22.4‰). Although a significant enrichment of 13C in oxalic acid was found in all the remote marine aerosol samples, highest δ13C values were observed in the samples collected in the equatorial Pacific. We found that the δ13C values sharply increased as a function of ambient temperature and relative abundance of diacid-carbon in TC at a latitudinal transect in the western (0-35°N) and central equatorial Pacific (18°S to 20°N). This study suggests that photochemically induced degradation may be responsible to the enrichment of 13C in the C2-C4 diacids in the remote marine atmosphere. We propose that δ13C values of small diacids can be used as potential tracer of photochemical aging of organic aerosols.

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