Evidence for a major missing source in the global chloromethane budget from stable carbon isotopes
Evidence for a major missing source in the global chloromethane budget from stable carbon isotopes
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DOI：
10.5194/acp-19-1703-2019
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发表时间：
2019-02-08
影响因子：
6.3
通讯作者：
Zetzsch, Cornelius
中科院分区：
文献类型：
--
作者：
Bahlmann, Enno;Keppler, Frank;Zetzsch, Cornelius
氯甲烷(CH3Cl)是活性氯进入平流层的最重要的自然输入，对平流层臭氧消耗的贡献率约为16%。由于氯氟烃人为排放的逐步淘汰，CH3Cl将在很大程度上控制未来平流层氯的水平。热带雨林通常被认为是最强大的单一CH3Cl源，贡献了全球每年约4000至5000 Gg(1 GG=10(9)g)排放量的一半以上。这一来源显示出独特的碳同位素指纹，使同位素研究成为改善其大气收支的一个有希望的工具。应用碳同位素来更好地限制CH3Cl的大气收支，需要关于主要汇过程的动力学同位素效应的可靠信息：与对流层中的OH和Cl的反应。我们在3500dm(3)的烟雾室中进行了CH3Cl的光化学降解实验，以确定CH3Cl与OH和Cl反应的碳同位素效应(epsilon=k(13)C/k(12)C-1)。对于CH_3Cl与OH的反应，我们测得的epsilon值为(-11.2+/-0.8)‰(n=3)，对于与Cl的反应，我们测得的epsilon值为(-10.2+/-0.5)‰(n=1)，比以前报道的小5~6倍。我们较小的同位素效应得到了强有力的支持，因为以前报道的对流层C-13(CH3Cl)值与对流层混合比中OH驱动的季节周期没有任何显著的季节协同变化。将这些碳同位素效应的新值应用到全球CH3Cl预算中，使用一个简单的双半球盒子模式，我们得到了热带雨林CH3Cl源为(670+/-200)GGa(-1)，比以前的估计要小得多。对以前自下而上的估计进行了修订，使用地上生物量而不是雨林面积，有力地支持了这一较低的估计。最后，我们的结果提出了(1530+/-200)gGa(-1)的一个巨大的未知CH3Cl源。
Chloromethane (CH3Cl) is the most important natural input of reactive chlorine to the stratosphere, contributing about 16% to stratospheric ozone depletion. Due to the phase-out of anthropogenic emissions of chlorofluorocarbons, CH3Cl will largely control future levels of stratospheric chlorine.The tropical rainforest is commonly assumed to be the strongest single CH3Cl source, contributing over half of the global annual emissions of about 4000 to 5000 Gg (1 Gg = 10(9) g). This source shows a characteristic carbon isotope fingerprint, making isotopic investigations a promising tool for improving its atmospheric budget. Applying carbon isotopes to better constrain the atmospheric budget of CH3Cl requires sound information on the kinetic isotope effects for the main sink processes: the reaction with OH and Cl in the troposphere. We conducted photochemical CH3Cl degradation experiments in a 3500 dm(3) smog chamber to determine the carbon isotope effect (epsilon = k(13)C/k(12)C - 1) for the reaction of CH3Cl with OH and Cl. For the reaction of CH3Cl with OH, we determined an epsilon value of (- 11.2 +/- 0.8)parts per thousand (n = 3) and for the reaction with Cl we found an epsilon value of (-10.2 +/- 0.5)parts per thousand (n = 1), which is 5 to 6 times smaller than previously reported. Our smaller isotope effects are strongly supported by the lack of any significant seasonal covariation in previously reported tropospheric delta C-13 (CH3Cl) values with the OH-driven seasonal cycle in tropospheric mixing ratios.Applying these new values for the carbon isotope effect to the global CH3Cl budget using a simple two hemispheric box model, we derive a tropical rainforest CH3Cl source of (670 +/- 200) Gg a(-1), which is considerably smaller than previous estimates. A revision of previous bottom-up estimates, using above-ground biomass instead of rainforest area, strongly supports this lower estimate. Finally, our results suggest a large unknown CH3Cl source of (1530 +/- 200) Gg a(-1).