Yokouchi Y, Noijiri Y, Barrie L A, Toom-Sauntry D, Machida T, Inuzuka Y, Akimoto H, Li H J, Fujinuma Y, Aoki S
National Institute for Environmental Studies, Tsukuba, Ibaraki, Japan.
Nature. 2000 Jan 20;403(6767):295-8. doi: 10.1038/35002049.
Methyl chloride (CH3Cl), the most abundant halocarbon in the atmosphere, has received much attention as a natural source of chlorine atoms in the stratosphere. The annual global flux of CH3Cl has been estimated to be around 3.5 Tg on the grounds that this must balance the loss through reaction with OH radicals (which gives a lifetime for atmospheric CH3Cl of 1.5 yr). The most likely main source of methyl chloride has been thought to be oceanic emission, with biomass burning the second largest source. But recent seawater measurements indicate that oceanic fluxes cannot account for more than 12% of the estimated global flux of CH3Cl, raising the question of where the remainder comes from. Here we report evidence of significant CH3Cl emission from warm coastal land, particularly from tropical islands. This conclusion is based on a global monitoring study and spot measurements, which show enhancement of atmospheric CH3Cl in the tropics, a close correlation between CH3Cl concentrations and those of biogenic compounds emitted by terrestrial plants, and OH-linked seasonality of CH3Cl concentrations in middle and high latitudes. A strong, equatorially located source of this nature would explain why the distribution of CH3Cl is uniform between the Northern and Southern hemispheres, despite their differences in ocean and land area.
氯甲烷（CH₃Cl）是大气中含量最丰富的卤代烃，作为平流层中氯原子的天然来源受到了广泛关注。据估计，全球每年的氯甲烷通量约为3.5太克，理由是这必须与通过与羟基自由基反应造成的损失相平衡（这使得大气中氯甲烷的寿命为1.5年）。人们一直认为氯甲烷最可能的主要来源是海洋排放，生物质燃烧是第二大来源。但最近的海水测量表明，海洋通量占估计的全球氯甲烷通量的比例不超过12%，这就引发了其余部分来自何处的问题。在此我们报告来自温暖沿海陆地，特别是热带岛屿大量排放氯甲烷的证据。这一结论基于一项全球监测研究和现场测量，这些研究和测量表明热带地区大气中氯甲烷增加，氯甲烷浓度与陆地植物排放的生物源化合物浓度密切相关，以及中高纬度地区氯甲烷浓度与羟基相关的季节性变化。这样一个位于赤道的强大此类来源可以解释为什么尽管南北半球海洋和陆地面积不同，但氯甲烷在两半球之间的分布却是均匀的。
