Contribution of very short-lived substances to stratospheric bromine loading: uncertainties and constraints
Contribution of very short-lived substances to stratospheric bromine loading: uncertainties and constraints
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DOI：
10.5194/acp-13-1203-2013
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发表时间：
2013-01-01
影响因子：
6.3
通讯作者：
Sinnhuber, B. -M.
中科院分区：
文献类型：
--
作者：
Aschmann, J.;Sinnhuber, B. -M.
在平流层溴含量的量化方面，极短寿命物质仍然是一个主要的不确定因素。短寿命源气体进入平流层的主要障碍之一通常被认为是热带对流层顶（TTL）中由于脱水而损失的无机溴（Br-y）。我们使用的三维化学传输模型，包括一致的参数化对流传输和一个全面的化学方案，调查相关的过程的敏感性计算。该模型考虑了两种最重要的溴VSLS，溴仿（CHBr 3）和二溴甲烷（CH 2Br 2）。有机溴源气体以及由此产生的模式中的无机溴的配置文件与现有的观测结果是一致的。与其有机前体相反，Br-y被认为具有关于沉降液体或冷冻颗粒的显著吸附能力，因此在其上升通过TTL期间完整源气体的分数是关键因素。我们发现，源气体注入是进入平流层的主要途径，约50%的CHBr 3和94%的CH 2Br 2能够克服冷点对流层顶在约17 km的高度，调制的年际变化的垂直输送效率。事实上，我们的灵敏度计算表明，CHBr 3的源气体注入的程度是高度敏感的对流和大规模的上升的强度，相比之下，修改的光解或通过OH的破坏产生一个显着较小的响应。原则上，这同样适用于CH 2Br 2，尽管由于其较长的寿命，其响应性相当低。我们确定的下一个重要方面是，从短寿命源的可用Br-y的分区明显地从HBr转移，根据我们目前的知识状态，HBr是Br-y家族中唯一有效吸附在冰粒上的成员。这种影响是由非常有效的非均相反应在冰面上，减少HBr/Br-y分数低于15%，在热带对流层顶。在这些情况下，由于模型中的脱水，Br-y没有显著损失，VSLS完全贡献于平流层溴。此外，我们进行了几次灵敏度计算，以测试这一结果的鲁棒性。如果忽略非均相化学，则HBr/Br-y分数超过50%，并且从VSLS清除约10%的溴。在HOBr也被有效吸附在冰上的假设下，脱水对Br-y去除起次要作用，因为非均相反应改变了Br-y的分配平衡，有利于HOBr。在这种情况下，从VSLS中去除高达12%的溴。即使在极端和不现实的情况下，吸附在冰粒上的物质被瞬间除去，溴的最大损失也不超过25%。假设对流上升气流中溴短寿命源气体的体积分数为万亿分之六（pptv），这一数值得到了观测数据的支持，我们发现VSLS对平流层溴的贡献最有可能在4.5-6 pptv的范围内。
Very short-lived substances (VSLS) still represent a major factor of uncertainty in the quantification of stratospheric bromine loading. One of the major obstacles for short-lived source gases in contributing to the stratosphere is generally thought to be loss of inorganic bromine (Br-y) in the tropical tropopause layer (TTL) due to dehydration. We use sensitivity calculations with a three-dimensional chemistry transport model comprising a consistent parametrization of convective transport and a comprehensive chemistry scheme to investigate the associated processes. The model considers the two most important bromine VSLS, bromoform (CHBr3) and dibromomethane (CH2Br2). The organic bromine source gases as well as the resulting profile of inorganic bromine in the model are consistent with available observations. In contrast to its organic precursors, Br-y is assumed to have a significant sorption capacity regarding sedimenting liquid or frozen particles thus the fraction of intact source gases during their ascent through the TTL is a critical factor. We find that source gas injection is the dominant pathway into the stratosphere, about 50% of CHBr3 and 94% of CH2Br2 is able to overcome the cold point tropopause at approximately 17 km altitude, modulated by the interannual variability of the vertical transport efficiency. In fact, our sensitivity calculations indicate that the extent of source gas injection of CHBr3 is highly sensitive to the strength of convection and large-scale ascent; in contrast, modifying the photolysis or the destruction via OH yields a significantly smaller response. In principle, the same applies as well to CH2Br2, though it is considerably less responsive due to its longer lifetime. The next important aspect we identified is that the partitioning of available Br-y from short-lived sources is clearly shifted away from HBr, according to our current state of knowledge the only member of the Br-y family which is efficiently adsorbed on ice particles. This effect is caused by very efficient heterogeneous reactions on ice surfaces which reduce the HBr/Br-y fraction below 15% at the tropical tropopause. Under these circumstances there is no significant loss of Br-y due to dehydration in the model, VSLS contribute fully to stratospheric bromine. In addition, we conduct several sensitivity calculations to test the robustness of this result. If heterogeneous chemistry is ignored, the HBr/Br-y fraction exceeds 50% and about 10% of bromine from VSLS is scavenged. Dehydration plays a minor role for Br-y removal under the assumption that HOBr is efficiently adsorbed on ice as well since the heterogeneous reactions alter the partitioning equilibrium of Br-y in favor of HOBr. In this case, up to 12% of bromine from VSLS is removed. Even in the extreme and unrealistic case that adsorbed species on ice particles are instantaneously removed the maximum loss of bromine does not exceed 25 %. Assuming 6 parts per trillion by volume (pptv) of bromine short-lived source gases in convective updrafts, a value that is supported by observational data, we find a most likely contribution of VSLS to stratospheric bromine in the range of 4.5-6 pptv.