Topic A. Hydrogen Emissions: Constraining The Earth system Response (HECTER)

全球氢气经济正在快速增长。随着氢气使用量的增加，泄漏到大气中是不可避免的，大气中的氢气水平将会上升。氢的大气生命周期的许多方面都知之甚少，这给这种转向氢气的环境后果带来了很大的不确定性。土壤微生物从大气中去除大量但不确定的氢气(50%-80%)。大气化学通过与羟基自由基(OH)反应除去其余部分。不断上升的氢气会消耗氢气中的氢气，延长甲烷的使用寿命。氢氧化还会产生对流层臭氧和平流层水蒸气。通过这种方式，氢起到了间接温室气体(GHG)的作用。对平流层臭氧和氧化剂的变化还有进一步的影响，这将影响气溶胶和云层。氢气排放将如何影响模型中的所有这些过程还处于初级阶段。这个项目将提高我们的全球建模能力，评估未来的影响，并确定和减少与氢气使用相关的不确定性。大多数目前的全球大气氢气模型规定了氢气和甲烷的表层混合比率，而不是增加排放。该项目将开发UKESM模型的版本(已经有甲烷排放)，以包括氢气的表面通量(排放和沉积)，这些通量将通过来自地面站点的观测、飞机数据和冰层记录进行调整和评估。我们将使用两个化学方案--一个标准方案和另一个更全面地描述氧化剂--以探索化学表示对于量化氢的影响有多重要。我们还将开发另一个UK模型(STOCHEM)，该模型另外表示氢的同位素异构体，进一步限制了工艺评估。我们将与来自世界各地的其他几个模特儿小组协调我们的模特儿工作，以探索模型的多样性。我们将分析不同氢泄漏量的模拟，并详细量化这如何影响全球氢收支，以及由此对甲烷、臭氧和平流层水蒸气的影响。对模型预算条款和影响范围的分析将使我们能够确定模型之间的共性和差异，从而确定不确定的过程，例如导致不同氢寿命的过程。进一步的模型实验将探索影响如何取决于氢气泄漏的地点和季节-我们预计与土壤中沉积的氢气的比例(例如，对半球、陆地/海洋的比例和土壤性质的依赖)和氧化剂水平(例如，热带/高纬度、夏季/冬季)有关的重要差异。我们将综合我们的结果和不确定性分析，对与氢气相关的气候指标(例如，全球变暖潜力、全球温度潜力和有效辐射强迫)进行全面的定量评估。我们将把这种关于氢的新知识纳入公平模型，该模型是用于分析一系列未来情景的政策工具。这将使我们(和政策制定者)能够探索未来广泛的氢气情景，例如：(1)氢的使用在多大程度上抵消了其他温室气体的排放；(2)来自世界不同地点的不同程度的氢泄漏；(3)大气化学表示的差异；以及(4)氢终端使用的差异(例如，氢燃烧可能伴随着NOx排放，这也影响氧化剂)。除了作为一种媒介，将我们的新模型结果的含义简单地传达给政策界之外，FIRE还将允许我们在这次通话中与其他获得资助的项目迅速协调，即主题B(氢气土壤汇的不同表示)和主题C(未来情景)。

A global hydrogen economy is growing rapidly. As hydrogen usage increases, leakage to the atmosphere is inevitable, and atmospheric hydrogen levels will rise. Many aspects of hydrogen's atmospheric life cycle are poorly understood, placing large uncertainties on the environmental consequences of this shift to hydrogen. Soil microbes remove a large but uncertain proportion (50-80%) of hydrogen from the atmosphere. Atmospheric chemistry removes the rest, through reaction with the hydroxyl radical (OH). Rising levels of hydrogen thus deplete OH, lengthening methane's lifetime. Hydrogen oxidation also generates tropospheric ozone and stratospheric water vapour. In this way, hydrogen acts as an indirect greenhouse gas (GHG). There are further impacts on stratospheric ozone and changes in oxidants that will affect aerosols and clouds. The representation of how hydrogen emissions will affect all these processes in models is in its infancy.This project will improve our global modelling capabilities, assess future impacts, and identify and reduce uncertainties associated with hydrogen use. Most current global atmospheric hydrogen models prescribe surface layer mixing ratios of hydrogen and methane, rather than adding emissions. This project will develop versions of the UKESM model (already with methane emissions) to include surface fluxes (emissions and deposition) of hydrogen that will be tuned and evaluated with observations from surface sites, aircraft data, and firn ice records. We will use two chemistry schemes - a standard scheme and another with a more comprehensive description of oxidants - in order to explore how important the representation of chemistry is for quantifying hydrogen's impacts. We will also develop another UK model (STOCHEM), which additionally represents the isotopomers of hydrogen, adding further constraints on process evaluation. We will co-ordinate our modelling efforts with several other modelling groups from around the world in order to explore model diversity. We will analyse simulations with different hydrogen leakage amounts and quantify in detail how this affects the global hydrogen budget, and the resultant impacts on methane, ozone and stratospheric water vapour. Analysis of the range of model budget terms and impacts will allow us to identify commonality and differences between models, and hence identify uncertain processes, such as processes that lead to different hydrogen lifetimes. Further model experiments will explore how impacts depend upon the location and season of hydrogen leakage - we expect there to be important differences related to the proportion of hydrogen deposited to soils (e.g., dependence on hemisphere, proportion of land/ocean, and soil properties) and levels of oxidants (e.g., tropics/high-latitudes, summer/winter).We will synthesize our results and analysis of uncertainty to produce a comprehensive quantitative assessment of climate metrics (e.g., Global Warming Potential, Global Temperature Potential, and Effective Radiative Forcing) associated with hydrogen. We will incorporate this new knowledge about hydrogen into the FaIR model, which is a policy tool used for analysing a range of future scenarios. This will allow us (and policymakers) to explore a wide range of future hydrogen scenarios, including for example: (i) the extent to which hydrogen use offsets other GHG emissions; (ii) different levels of hydrogen leakage, from different world locations; (iii) differences in the representation of atmospheric chemistry; and (iv) differences in hydrogen end usage (e.g., hydrogen combustion may be accompanied by NOx emissions, which also affect oxidants). As well as being a medium to simply communicate the implications of our new modelling results to the policy community, FaIR will also allow us to co-ordinate rapidly with the other funded projects within this call, i.e. Topic B (different representations of hydrogen's soil sink) and Topic C (future scenarios).