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

全球氢经济正在迅速增长。随着氢气使用量的增加，泄漏到大气中是不可避免的，大气中的氢气含量也会上升。人们对氢在大气中的生命周期的许多方面知之甚少，这使得向氢转变的环境后果存在很大的不确定性。土壤微生物从大气中去除大量但不确定的氢（50-80%）。大气化学通过与羟基自由基（OH）反应除去其余部分。氢含量的增加会消耗OH，延长甲烷的寿命。氢氧化还产生对流层臭氧和平流层水蒸气。因此，氢气是一种间接的温室气体（GHG）。对平流层臭氧的进一步影响和氧化剂的变化将影响气溶胶和云。氢排放将如何影响模型中所有这些过程的表示仍处于起步阶段。该项目将提高我们的全球建模能力，评估未来的影响，并确定和减少与氢使用相关的不确定性。目前大多数全球大气氢模型规定了氢和甲烷的表层混合比例，而不是增加排放量。该项目将开发UKESM模型（已经有甲烷排放）的版本，以包括氢的表面通量（排放和沉积），将通过地面站点的观测，飞机数据和积雪记录进行调整和评估。我们将使用两种化学方案-一种标准方案和另一种更全面描述氧化剂的方案-以探索化学表征对于量化氢的影响有多重要。我们还将开发另一种英国模型（STOCHEM），该模型还代表氢的同位素异构体，从而进一步限制工艺评估。我们将与来自世界各地的其他几个建模团队协调我们的建模工作，以探索模型的多样性。我们将分析不同氢气泄漏量的模拟，并详细量化这如何影响全球氢气预算，以及对甲烷，臭氧和平流层水蒸气的影响。分析模型预算条款和影响的范围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泄漏的位置和季节-我们预计会有与沉积到土壤中的氢的比例有关的重要差异（例如，对半球、陆地/海洋比例和土壤性质的依赖性）和氧化剂的水平（例如，热带/高纬度，夏季/冬季）。我们将综合我们的结果和对不确定性的分析，以产生对气候指标的全面定量评估（例如，全球变暖潜势、全球温度潜势和有效辐射强迫）。我们将把这些关于氢的新知识纳入FaIR模型，这是一种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）能够探索广泛的未来氢气情景，包括例如：（i）氢气使用抵消其他温室气体排放的程度;（ii）来自世界不同位置的不同水平的氢气泄漏;（iii）大气化学代表性的差异;以及（iv）氢气最终用途的差异（例如，氢燃烧可能伴随NOx排放，这也影响氧化剂）。除了作为一个媒介，简单地传达我们的新的建模结果的影响，政策界，公平也将使我们能够迅速协调与其他资助项目在此呼吁，即主题B（不同的氢的土壤汇的表示）和主题C（未来的情景）。

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).