Discovering reasons for global atmospheric methane growth using deuterium isotopes

本研究旨在对甲烷中的氘/氢（D/H）同位素比值进行测量和建模，以约束全球甲烷收支的不确定性。测量将包括1)确定同位素源特征的实地活动；2)两个半球遥感站的时间序列；3)建立模型以提取全球预算和变化原因。大气中的甲烷正在迅速增加。自2007年以来，其混合比例上升了80 ppb（超过总负荷的4%）。2014年增长加速（13 ppb/年），此后一直保持在高位（7 - 10 ppb/年）。如此高的甲烷排放量增长出乎意料，是《巴黎协定》面临的最大挑战之一。自2007年以来甲烷重新增长并在2014年加速背后的原因尚不清楚。它是由排放增加引起的吗？如果是由哪些来源引起的，还是由甲烷的主要储存库OH的减少引起的？增长是气候变化的反馈吗？变暖导致变暖吗？还是人类活动的直接后果？单独的混合比例测量不足以解决甲烷收支问题，尽管地理焦点表明主要驱动因素是在热带和低北纬地区。同位素（12CH4、13CH4和12CH3D相对量的变化）识别和区分源和汇的变化。两个世纪以来，甲烷的碳含量越来越高，但自2007年以来，甲烷的碳含量开始“轻”化（碳含量更高）。c同位素的变化让我们了解了增长背后的主要驱动因素，但要充分了解年际变化和甲烷持续增长的原因，还需要更多的信息。最大的需求是测量h同位素，它提供了甲烷来源和汇的极其强大的区分。一项测量环境空气中甲烷氢同位素的新技术进步使该项目得以实施。2019年底安装了一套新的快速多样品高精度质谱系统，该系统从根本上降低了每个样品的测量成本，是NERC MOYA重点项目的主要目标。它将允许每年对数千个环境空气样本进行h同位素分析。目前，世界上只有很少的实验室进行这种具有挑战性的测量，并且源同位素特征和环境空气测量的时间序列很少。这项新工作将恢复一个全球半球网络，测量北极、中纬度北部、热带、中纬度南部和南极洲的时间序列。主要来源的D/H同位素特征将被表征：湿地、废物、生物质燃烧、化石燃料、反刍动物和水稻农业。实地活动将集中在热带非洲、东亚和南美，这些地区的甲烷排放量很高，但很少测量甲烷同位素特征。结果将给出源类型的区域源签名。建模将使用新的测量和来源特征来限制全球甲烷预算。将甲烷摩尔分数、13C/12C和D/H的时间序列测量与改进的来源特征相结合，将确定纬度梯度和时间趋势，使用UM-UKCA化学输送模型的数值模拟将使用D/H作为关键判别，以测试各种假设并确定甲烷上升的原因。新的快速多样品系统使我们能够从二维（混合比+ c同位素）到三维（添加h同位素）研究甲烷，这是解决甲烷预算问题的一个非常彻底的进步。了解甲烷上升的原因对于推动减缓政策以实现《巴黎协定》的目标至关重要。该项目将大大改善对全球甲烷预算的了解，并有助于制定稳定和减少甲烷所需的战略决策。

This proposal is to measure and model deuterium/hydrogen (D/H) isotope ratios in methane, to constrain the uncertainties in the global methane budget. Measurement will include 1) Field campaigns to determine isotopic source signatures; 2) time series from remote stations in both hemispheres; and 3) modelling to extract global budgets and causes of change. Atmospheric methane is growing rapidly. Its mixing ratio has risen 80 ppb (over 4% of total burden) since 2007. Growth accelerated in 2014 (13 ppb/yr) and has continued to be high since (7 to 10 ppb/yr). This high methane growth was unexpected and presents one of the greatest immediate challenges to the Paris Agreement. The reasons behind renewed methane growth since 2007 and acceleration in 2014 are not understood. Was it caused by increased emissions, and if so from which sources, or by declining OH, the main sink of methane? Is growth a feedback from climate change, the warming feeding warming? Or is it a direct consequence of human activities? Mixing ratio measurements alone are inadequate to solve the methane budget, though geographic foci indicate the main driving factors are in the tropics and low northern latitudes. Isotopologues (variations in the relative amounts of 12CH4, 13CH4 and 12CH3D) identify and discriminate between source and sink changes. After two centuries of becoming more 13C-rich, methane has shifted 'light' (more 12C-rich) since 2007. The C-isotope change gives insight into the main driving factors behind growth, but more information is needed to fully understand the reasons for interannual variability and continued methane growth. The greatest need is to measure H-isotopes, which provide extremely powerful discriminants of methane sources and sinks.A new technical advance in measuring H-isotopes in methane in ambient air permits this project. A new rapid multiple-sample high-precision mass spectrometric system, which radically cuts the per-sample cost of measurement was installed in late 2019 and was a major goal of NERC's MOYA highlight project. It will allow thousands of ambient air samples per year to be analysed for H-isotopes.Currently only very few labs worldwide make this challenging measurement and source isotopic signatures and time series of ambient air measurements are sparse. The new work will reinstate a global two-hemisphere network, measuring time series in the Arctic, northern mid-latitudes, tropics, southern mid-latitudes, and Antarctica. D/H isotopic signatures of the major sources will be characterised: wetlands, waste, biomass burning, fossil fuel, ruminants and rice agriculture. Field campaigns will focus on tropical Africa, East Asia and S America, with high emissions of methane, but very few measurements of methane isotopic signatures. Results will give regional source signatures for the source types.Modelling will use the new measurements and source signatures to constrain the global methane budget. Combining time series measurements of methane mole fraction and 13C/12C and D/H in methane with improved source signatures will determine latitudinal gradients and temporal trends, Numerical modelling using the UM-UKCA chemical transport model will use D/H as a key discriminant, to test the various hypotheses and identify the causes of methane's rise.The new rapid multi-sample system, which permits us to go from studying methane in 2D (mixing ratio + C-isotopes) to 3D (adding H-isotopes), is a very radical advance in solving the methane budget problem. Understanding why methane is rising is critical to driving mitigation policy to attain the Paris Agreement's goals. This project will lead to a major improvement in understanding the global methane budget, and help shape decisions on strategies needed to stabilise and reduce methane.