Quantifying methane emissions in remote tropical settings: a new 3D approach

这项提议的目的是开发一种可行的方法来量化非洲偏远的季节性热带湿地环境中的甲烷排放，从而对增长的原因施加限制。NERC/Met Office FAAM飞机计划于2019年初部署到乌干达：该提案基于部署提供的机会，通过在赞比亚刚果和赞比西河流域的主要季节性南部热带湿地增加一个分遣队。经过一段时间的稳定后，大气甲烷在2007年开始迅速上升，2014年及随后几年的增长速度加快。与此同时，甲烷的同位素含量也出现了明显的变化，碳-13的消耗越来越少。我们不知道甲烷增长和同位素变化发生的原因。自2007年以来，大部分增长发生在热带和亚热带地区。关于甲烷负担的增长和同位素变化的一个假设是，热带湿地和牲畜的甲烷排放量正在迅速增加，这是对热带扩张和伴随气象变暖的降水增加的响应。另一种假设是，主要碳汇OH的显著下降延长了甲烷的寿命。因此，确定具有代表性的大型季节性非洲湿地甲烷排放的通量和同位素特征是一个关键要求，使模型能够更好地处理热带纬向带，从而能够理解甲烷之谜。非洲热带湿地以纸莎草等富含碳-13的C4植物为主，自给牛也以C4植物为食。与温带和凉爽气候源相比，非洲湿地和牛排放到空气中的生物甲烷具有非常不同的同位素特征，富含13C。非洲湿地甲烷的同位素特征在某种程度上与一些化石燃料的排放重叠。为了有效地使用同位素来区分全球预算中的来源类型（例如化石燃料或湿地和牛），迫切需要获得关于非洲生物源排放的区域同位素特征的更好信息。自2007年以来，甲烷的增长速度如此之快，以至于已经远远超出了联合国《巴黎气候变化协定》所规定的预期水平。报告的国家排放清单与大气估计之间存在很大差异：如果要了解预算，就迫切需要编制更好的热带清单。为了解决这些问题，我们打算：在赞比亚湿地进行FAAM（机载大气测量设施）现场部署，作为我们计划于2019年1月在乌干达部署MOYA的南部“附加项目”，以测量来自非常广泛的赞比亚湿地和Bangweulu湖和Mweru湖周围的农业区以及Kafue和Zambesi盆地的雨季温室气体通量。飞行将在主要湿地源区的下风处进行，量化雨季高峰期的排放羽流。以帮助解决“排放在增加还是OH汇在减少？”在辩论中，实地活动将用于确定d13C[CH4]和dD[CH4]的同位素特征，这些特征是在赞比亚C4湿地的排放中测量的。缺乏对热带排放同位素特征的测量是成功进行全球逆建模的一个关键障碍（Turner等人，2017；Rigby等人，2017）。3. 开发低成本的方法来量化排放通量和确定偏远热带环境主要来源的同位素特征，包括湿地、牛和生物质燃烧。特别是，将测试一种简单的无人机（UAV）和气球袋取样方法。改进对当地排放发明者的“自下而上”评估，与赞比亚和津巴布韦的当地同事合作建立国家甲烷排放清单，从而支持《巴黎协定》的意图。

The purpose of this proposal is to develop a viable methodology to quantify methane emissions in remote seasonal tropical wetland settings in Africa, and thus to place constraints on the causes of growth. The NERC/Met Office FAAM aircraft is scheduled to be deployed to Uganda in early 2019: this proposal builds on the opportunity that deployment offers, by adding a detachment to the major seasonal southern tropical wetlands in Zambia's Congo and Zambesi river drainages. Atmospheric methane began rising rapidly in 2007, after a period of stability, and the growth rate then accelerated in 2014 and subsequent years. Concurrently with the rise, the methane burden has shown a marked isotopic shift, becoming more C-13 depleted. We do not know why the methane growth and isotopic shift is happening. Much of the growth since 2007 has been in the tropics, and sub-tropics. One hypothesis for the growth and isotopic shift in the methane burden is that methane emissions from tropical wetlands and cattle are rapidly increasing, as a response to the tropical expansion and increased precipitation that has accompanied meteorological warming. An alternative hypothesis is that a marked decline in OH, the main sink, has lengthened methane's lifetime. Thus determining the flux and isotopic signatures of methane emissions from large representative seasonal African wetlands is a key requirement, enabling models to have better treatment of tropical latitudinal zones, so that the methane puzzle can be understood.African tropical wetlands are dominated by C-13 rich C4 plants such as papyrus, and subsistence cattle also graze C4 plants. In comparison to temperate and cool climate sources, biogenic methane emitted to the air from African wetlands and cattle has a very different isotopic signature, richer in 13C. This isotopic signature of methane from African wetlands overlaps somewhat with some fossil fuel emissions. To use isotopes effectively to differentiate between source types in the global budget (e.g. fossil fuels or wetlands and cattle) it is urgent that much better information is obtained on the regional isotopic signatures of African biogenic emissions. Methane growth since 2007 has been so rapid that methane is already far from its expected pathway under the UN Paris Agreement on Climate Change. There is a large discrepancy between reported national emissions inventories and atmsopheric estimates: in particular there is an urgent need for better tropical inventories if the budget is to be understood.To address these problems, we intend:1. To conduct a FAAM (Facility for Airborne Atmospheric Measurement) field deployment over Zambian wetlands, as a southern 'add-on' to our planned MOYA deployment to Uganda in January 2019, to measure wet-season greenhouse gas fluxes from the very extensive Zambian wetlands and farming areas around Lakes Bangweulu and Mweru, and in the Kafue and Zambesi basins. Flights would be carried out downwind of major wetland source regions, quantifying emission plumes at the height of the wet season.2. To help solve the 'are emissions rising or is the OH sink falling?' debate, on-ground campaigns will be used to determine d13C[CH4] and dD[CH4] isotopic signatures, measured in emissions from Zambian C4 wetlands. The lack of measurement of isotopic signatures of tropical emissions is a crucial impediment to successful global inverse modelling (Turner et al., 2017; Rigby et al., 2017). 3. To develop low-cost ways of quantifying emission fluxes and determining isotopic signatures from major sources in remote tropical settings, including wetlands, cattle and biomass burning. In particular, a simple drone (UAV) and balloon bag-sampling methodology will be tested.4. To improve 'bottom-up' assessment of local emission inventors, working with local colleagues in Zambia and Zimbabwe to construct national methane emission inventories, and thus support the intentions of the Paris Agreement.