HR: 0800h
AN: A51C-0784    [Abstracts]
TI: The budget of atmospheric methyl chloride using stable carbon isotopic mass-balance approach
AU: * Komatsu, D D
AF: Earth and Planetary Sciences, Graduate School of Science, Hokkaido University, N10 W8, Kita-ku, sapporo, 060-0810 Japan
AU: Tsunogai, U
AF: Earth and Planetary Sciences, Graduate School of Science, Hokkaido University, N10 W8, Kita-ku, sapporo, 060-0810 Japan
AU: Yamaguchi, J
AF: Earth and Planetary Sciences, Graduate School of Science, Hokkaido University, N10 W8, Kita-ku, sapporo, 060-0810 Japan
AU: Nakagawa, F
AF: Earth and Planetary Sciences, Graduate School of Science, Hokkaido University, N10 W8, Kita-ku, sapporo, 060-0810 Japan
AU: Yokouchi, Y
AF: National Institute for Environmental Studies, 16-2 Onogawa, Tsukuba, 305-8506 Japan
AU: Nojiri, Y
AF: National Institute for Environmental Studies, 16-2 Onogawa, Tsukuba, 305-8506 Japan
AB: The atmospheric budget of methyl chloride (CH$_{3}$Cl), an ozone-depleting gas, is highly uncertain as yet. Although biomass burning, tropical plant, and ocean have been identified as the major sources, each contribution to atmospheric CH$_{3}$Cl is not well understood. In recent studies, much more contributions from tropical plant are suggested to balance the atmospheric CH$_{3}$Cl budget. An isotope mass-balance approach that utilizes differences in the stable carbon isotopic compositions ($\delta$$^{13}$C) of CH$_{3}$Cl in sources can provide useful clues to determine atmospheric budget. Previous studies revealed that, while atmospheric CH$_{3}$Cl exhibited $\delta$$^{13}$C values around -36 $\permil$VPDB, the representative source materials, biomass burning and tropical plant, exhibited substantially lower $\delta$$^{13}$C values of -38~-68 $\permil$VPDB, and -62~-73 $\permil$VPDB respectively. The discrepancy is too large to be explained by the kinetic isotope effect (KIE) during the removal of atmospheric CH$_{3}$Cl, so that we must assume the other significant CH$_{3}$Cl source that have rather $^{13}$C-enriched $\delta$$^{13}$C value. In recent study, we developed a new analytical system, which enable us to determine the $\delta$$^{13}$C values of CH$_{3}$Cl using continuous-flow isotope ratio MS (CF-IRMS), even when a sample contain substantial amounts of unsaturated hydrocarbons. In this study, we determined the $\delta$$^{13}$C values of CH$_{3}$Cl from ocean for which we have no available $\delta$$^{13}$C data. In addition, we determined those from biomass burning and tropical plants to reconfirm the reported $\delta$$^{13}$C values from them. To determine the $\delta$$^{13}$C values of CH$_{3}$Cl from ocean, we analyzed extracted gases from surface water on both coastal ocean in Japan and open ocean in NW Pacific. To determine the $\delta$$^{13}$C values of CH$_{3}$Cl from biomass burning, we analyzed biomass burning exhaust emitted from pinewood (C3 plant), rice (C3 plant), and maize (C4 plant). To determine the $\delta$$^{13}$C values of CH$_{3}$Cl from tropical plant, we analyzed gas samples emitted from tropical ferns and mangroves. In conclusion, our isotopic mass-balance calculation for CH$_{3}$Cl suggested that it is difficult to assume the studied C3 tropical plants as major CH$_{3}$Cl source. We must consider ocean and/or biomass burning as the major CH$_{3}$Cl sources, together with large average KIE of more than 5$\permil$ for the removal of atmospheric CH$_{3}$Cl. If such large average KIE would be difficult to assume, we must assume the other un-identified $^{13}$C-enriched sources (such as C4-plant burning) as an alternative major CH$_{3}$Cl source.
DE: 4870 Stable isotopes
DE: 1040 Isotopic composition/chemistry
DE: 0315 Biosphere/atmosphere interactions
DE: 0322 Constituent sources and sinks
DE: 0365 Troposphere--composition and chemistry
SC: Atmospheric Sciences [A]
MN: 2004 AGU Fall Meeting