Modelling the budget of middle atmospheric water vapour isotopes
Modelling the budget of middle atmospheric water vapour isotopes
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模拟中层大气水蒸气同位素的预算
DOI：
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发表时间：
2006
期刊：
影响因子：
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通讯作者：
T. Röckmann
中科院分区：
文献类型：
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作者：
A. Zahn;P. Franz;C. Bechtel;J. Grooß;T. Röckmann
应用一维化学模型研究了平流层和中间层水汽的稳定氢(D)和稳定氧同位素（17o、18o）组成。在对流层，这种同位素组成是由“物理”分馏效应决定的，即相变（例如在云形成期间）、扩散过程（例如在海洋蒸发期间）和气团混合。由于这些过程水蒸气进入平流层首先显示同位素消逝在D / H与海水,这~ 5倍的18 O / 16 O,其次是mass-dependently分馏(MDF),即同位素比值的变化17 O / 16 O ~ 0.52倍的18 O / 16 O .相比之下,在平流层和中间层“化学分馏机制,生产的H O由于甲烷的氧化,H 2 O通过HO x的家庭,同位素交换反应大大提高了从对流层输入的水蒸气中的同位素比率。该模式合理地预测，相对于对流层顶，中间层中D/H的稳定同位素比值总体上提高了~25%，17 O/ 16 O的稳定同位素比值提高了~8.5%，18 O/ 16 O的稳定同位素比值提高了~14%。h2o中的17o / 16o和18o / 16o比值表明，从储层o2或o3接收O原子的HO x的相对比例。整个中层大气,MDF O 2是氧原子的主要捐赠者纳入哦,因此HO 2和H 2 O .在同温层已知mass-independent分馏(MIF)信号在O 3是在第一步转移到没有x家庭,只有在第二步HO x和H 2 O . CO 2相比,O (1 D)在这个MIF转移只扮演一个次要角色。我们计算中的主要不确定性来自于同位素交换反应速率系数和动力学同位素分馏因子的不精确量化。
A one-dimensional chemistry model is applied to study the stable hydrogen (D) and stable oxygen isotope ( 17 O, 18 O) composition of water vapour in stratosphere and mesosphere. In the troposphere, this isotope composition is determined by "physical' fractionation effects, that are phase changes (e.g. during cloud formation), diffusion processes (e.g. during evaporation from the ocean), and mixing of air masses. Due to these processes water vapour entering the stratosphere first shows isotope depletions in D/H relative to ocean water, which are ~5 times of those in 18 O/ 16 O, and secondly is mass-dependently fractionated (MDF), i.e. changes in the isotope ratio 17 O/ 16 O are ~0.52 times of those of 18 O/ 16 O. In contrast, in the stratosphere and mesosphere "chemical' fractionation mechanisms, that are the production of H O due to the oxidation of methane, re-cycling of H 2 O via the HO x family, and isotope exchange reactions considerably enhance the isotope ratios in the water vapour imported from the troposphere. The model reasonably predicts overall enhancements of the stable isotope ratios in H 2 O by up to ~25% for D/H, ~8.5% for 17 O/ 16 O, and ~14% for 18 O/ 16 O in the mesosphere relative to the tropopause values. The 17 O/ 16 O and 18 O/ 16 O ratios in H 2 O are shown to be a measure of the relative fractions of HO x that receive the O atom either from the reservoirs O 2 or O 3 . Throughout the middle atmosphere, MDF O 2 is the major donator of oxygen atoms incorporated in OH and HO 2 and thus in H 2 O. In the stratosphere the known mass-independent fractionation (MIF) signal in O 3 is in a first step transferred to the NO x family and only in a second step to HO x and H 2 O. In contrast to CO 2 , O( 1 D) only plays a minor role in this MIF transfer. The major uncertainty in our calculation arises from poorly quantified isotope exchange reaction rate coefficients and kinetic isotope fractionation factors.