Bromoform and dibromomethane in the tropics: a 3-D model study of chemistry and transport
Bromoform and dibromomethane in the tropics: a 3-D model study of chemistry and transport
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DOI：
10.5194/acp-10-719-2010
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发表时间：
2009-08
影响因子：
6.3
通讯作者：
R. Hossaini;M. Chipperfield;B. Monge-Sanz;N. Richards;E. Atlas;D. Blake
中科院分区：
文献类型：
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作者：
R. Hossaini;M. Chipperfield;B. Monge-Sanz;N. Richards;E. Atlas;D. Blake
抽象。我们已经开发了一个详细的化学方案的短寿命源气体溴仿（CHBr3）和二溴甲烷（CH2Br2）的降解，并实施它在TOMCAT/SLIMCAT三维（3-D）化学传输模型（CTM）。CTM已被用来预测的两个源气体（SG）和11个有机产品气体（PG）的分布。这些对有机PG的首次全球计算表明，它们的丰度很小。最长寿命的有机PG是CBr2O和CHBrO，但它们的峰值对流层丰度相对于表面体积混合比（vmr）的SG是小于5%。我们计算出它们在热带地区的平均对流层寿命分别为~7天和~2天（由于光解作用）。因此，在先前的建模研究中，SG降解立即导致无机溴的假设似乎是合理的。我们比较了观察到的热带SG配置文件从一些飞机运动与各种模型实验。在热带对流层顶层（TTL），我们发现，CTM运行使用p水平（TOMCAT）和垂直风分析发散高估了丰富的CH2Br2，并在较小程度上CHBr3，虽然数据稀疏，比较是不确定的。在使用θ水平（SLIMCAT）和来自非绝热加热率的垂直运动的灵敏度运行中，TTL的一致性更好。两个模型版本中停留时间的轨迹估计显示SLIMCAT θ层版本中垂直输送较慢。在p层模式中，即使我们关闭对流，我们仍然发现大量的SG可能会到达冷点对流层顶;平流层源气注入（SGI）仅减少约16%的CHBr 3和约2%的CH2Br 2没有对流。总体而言，SG途径和PG途径对于溴向平流层上空（θ> 380 K）输送的相对重要性已得到评估。假设在TOMCAT中Bry的洗脱寿命为10天，我们发现来自CHBr 3的总Br的递送为0.72 pptv，其中约53%来自SGI。同样，对于CH2Br2，我们发现总溴值为1.69 pptv，其中约94%来自SGI。我们推断，这些物种的贡献~2.4 pptv的无机溴的平流层下部与SGI是主导途径。到TTL和通过TTL的传输速度越慢，这个估计值就越小。
Abstract. We have developed a detailed chemical scheme for the degradation of the short-lived source gases bromoform (CHBr3) and dibromomethane (CH2Br2) and implemented it in the TOMCAT/SLIMCAT three-dimensional (3-D) chemical transport model (CTM). The CTM has been used to predict the distribution of the two source gases (SGs) and 11 of their organic product gases (PGs). These first global calculations of the organic PGs show that their abundance is small. The longest lived organic PGs are CBr2O and CHBrO, but their peak tropospheric abundance relative to the surface volume mixing ratio (vmr) of the SGs is less than 5%. We calculate their mean local tropospheric lifetimes in the tropics to be ~7 and ~2 days (due to photolysis), respectively. Therefore, the assumption in previous modelling studies that SG degradation leads immediately to inorganic bromine seems reasonable. We have compared observed tropical SG profiles from a number of aircraft campaigns with various model experiments. In the tropical tropopause layer (TTL) we find that the CTM run using p levels (TOMCAT) and vertical winds from analysed divergence overestimates the abundance of CH2Br2, and to a lesser extent CHBr3, although the data is sparse and comparisons are not conclusive. Better agreement in the TTL is obtained in the sensitivity run using θ levels (SLIMCAT) and vertical motion from diabatic heating rates. Trajectory estimates of residence times in the two model versions show slower vertical transport in the SLIMCAT θ-level version. In the p-level model even when we switch off convection we still find significant amounts of the SGs considered may reach the cold point tropopause; the stratospheric source gas injection (SGI) is only reduced by ~16% for CHBr3 and ~2% for CH2Br2 without convection. Overall, the relative importance of the SG pathway and the PG pathway for transport of bromine to the stratospheric overworld (θ>380 K) has been assessed. Assuming a 10-day washout lifetime of Bry in TOMCAT, we find the delivery of total Br from CHBr3 to be 0.72 pptv with ~53% of this coming from SGI. Similary, for CH2Br2 we find a total Br value of 1.69 pptv with ~94% coming from SGI. We infer that these species contribute ~2.4 pptv of inorganic bromine to the lower stratosphere with SGI being the dominant pathway. Slower transport to and through the TTL would decrease this estimate.