Measurements of stable carbon isotopic composition of ethane and propane over the western North Pacific and eastern Indian Ocean: A useful indicator of atmospheric transport process |
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| Authors: | Hideki Nara Sakae Toyoda " target="_blank">Naohiro Yoshida |
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| Institution: | (1) Department of Environmental Science and Technology, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, Yoshida Lab, S2-17, 4259, Nagatsuta, Mirori-ku, Yokohama Kanagawa, 226-8502, Japan;(2) Solution Oriented Research for Science and Technology (SORST), Japan Science and Technology Agency (JST), 4-1-8 Hon-cho, Kawaguchi Saitama, 332-0012, Japan;(3) Department of Environmental Chemistry and Engineering, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Yokohama 226-8502, Japan;(4) Frontier Collaborative Research Center, Tokyo Institute of Technology, 4259 Nagatsuta, Yokohama 226-8502, Japan |
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| Abstract: | Stable carbon isotopic composition of ethane and propane over the western North Pacific and eastern Indian Ocean between 31°N
and 26°S was investigated from February through March 2004. The isotopic composition of ethane ranged from −28 to −18‰ and
showed a gradual increase from north to south. Conversely, that of propane was between −31 and −24‰; it showed no systematic
latitudinal variation. Investigation of the ethane/propane ratio indicates that ethane and propane that originated from northern
mid-latitude countries in the eastern part of Eurasia were both transported into the western North Pacific region. However,
the results of the isotopic analyses indicate the contribution of oceanic emission to the atmospheric propane during transport,
although that contribution can not be discerned for ethane. A ship based stationary observation conducted in the western equatorial
North Pacific showed that the isotopic composition of ethane varied from −25 to −19‰ and showed clear systematic diurnal variation:
propane ranged between −32 to −26‰ and no such isotopic diurnal signal was observed. The diurnal variation for ethane is explained
by entrainment of free tropospheric air, whereas the variation for propane was influenced by oceanic emissions as well as
the entrainment. The contribution of oceanic emissions to the atmospheric propane inventory was considered from our isotopic
observation. Isotopic composition of dissolved propane is estimated to be less than −38‰, and the contribution up to 79% was
calculated when the isotopic composition of dissolved propane is assumed to be −40‰. Our study demonstrates that isotopic
analysis can be more useful than ratio-based analysis to improve our present understanding of transport processes, especially
for impact of the oceanic emissions on the atmospheric distribution of low level C2–C5 non-methane hydrocarbons such as propane in the remote marine atmosphere. |
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| Keywords: | maritime air non-methane hydrocarbons oceanic emissions stable carbon isotope ratio western North Pacific |
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