Middle Atmosphere Processes and Lifetime Evaluation for ODSs and GHGs (MAPLE)

排放到大气中的气体可以持续多年甚至几个世纪。气体被去除的速率由其所谓的寿命决定。因此，要了解人类活动排放的污染物气体等的影响，必须准确了解其在大气中的寿命。然而，目前许多关键污染气体的已知寿命存在很大的不确定性。消耗臭氧层物质（ODS），例如氯氟烃（CFC），是长寿命源气体，在平流层中分解释放氯和溴。根据《蒙特利尔议定书》，这些物种的排放已被逐步淘汰，臭氧层预计将在未来 50-100 年内恢复。然而，这种回收率将取决于这些气体及其仍在排放的替代品的大气寿命。事实上，目前这些大气寿命存在很大的不确定性，这些大气寿命用于未来卤素负荷的所有模型预测（通过预测的表面混合比模型边界条件）。例如，在 WMO 和 IPCC 评估中，主要的氯氟碳 CFC-11 的大气寿命为 45 年，尽管其他研究表明寿命长达 60 年。最近建立的世界气候研究计划 (WCRP) 平流层过程及其在气候中的作用 (SPARC) 项目认识到了这一关键的不确定性，该项目旨在重新评估这些 ODS 及其替代品（例如氢氟碳化合物、HFC）的寿命，使用最先进的 3D 化学气候模型 (CCM) 中的最新实验室数据。这些物种也是高效的温室气体（GHG），其已知的大气寿命的变化将改变对其影响气候变化的估计（通过其全球变暖/温度潜力（GWP/GTP）来衡量）。该项目将确保英国化学气候模型（UKCA）全面参与WCRP/SPARC重新评估。寿命估计直接影响模型对未来臭氧恢复的预测。先前关于臭氧层恢复的 CCM 研究使用了基于旧寿命估计和简单盒模型的未来表面 ODS 浓度预测。因此，未来臭氧变化的主要驱动因素，即平流层氯和溴负荷，受到粗略的时间依赖性边界条件的约束。通过消除这一约束并在主要 ODS 的排放通量表面边界条件下运行 CCM，可以获得更真实的臭氧恢复率表示，并允许模型本身预测未来十年氯和溴的去除量。我们将在这个项目中执行这些模拟。具有很长寿命（数百到数千年）的源气体太稳定，无法通过分解影响平流层臭氧，但它们总是有效的温室气体。对于高层大气（中间层）的这些气体损失过程，通常被忽视或非常粗暴地处理，可能会显着缩短其大气寿命，从而减少其估计的气候影响。此类气体的三个示例是 NF3、CFC-115 和 SF6。我们已经确定，这些气体与存在于上层中间层的金属原子（Fe、Na 和 Mg）的反应可能是一个重要的附加汇，并与莱曼-α 光解和其他反应竞争。我们将在实验室中评估这些汇的速率。对于所有研究的气体，我们将通过重新计算其 GWP/GTP 值，对其气候影响进行新的、改进的估计。该项目将使用英国的核心对流层-平流层化学-气候模型 (UKCA)。所进行的测试和开发工作将为英国研究人员社区带来改进且经过更彻底测试的模型

Gases emitted into the atmosphere can persist for many years or even centuries. The rate at which a gas is removed is determined by its so-called lifetime. Therefore, to understand the impact of, for example, pollutant gases emitted by human activity, it is essential to have an accurate knowledge of their atmospheric lifetimes. However, there is currently large uncertainty in the known lifetime of many key pollutant gases.Ozone-depleting substances (ODS), such as chlorofluorocarbons (CFCs), are long-lived source gases which decompose in the stratosphere to release chlorine and bromine. Under the Montreal Protocol the emissions of these species have been phased out and the ozone layer is expected to recover over the next 50-100 years. However, the rate of this recovery will depend on the atmospheric lifetime of the these gases and their replacements which are still being emitted. In fact, there is currently significant uncertainty in these atmospheric lifetimes, which are used in all model predictions of future halogen loadings (via predicted surface mixing ratio model boundary conditions). For example, a major chlorofluorocarbon CFC-11 has a quoted atmospheric lifetime of 45 years in WMO and IPCC assessments, although other studies suggest a lifetime of up to 60 years.This key uncertainty has been recognised by the recent establishment of a World Climate Research Program (WCRP) Stratospheric Processes and their Role in Climate (SPARC) project to re-evaluate the lifetimes of these ODS and their replacements (such as hydrofluorocarbons, HFCs) using up-to-date laboratory data in state-of-the-art 3-D chemistry-climate models (CCMs). These species are also efficient greenhouse gases (GHGs) and changes to their known atmospheric lifetime will change estimates of how they will affect climate change (as measured by their global warming/temperature potential (GWP/GTP)). This project will ensure full participation of the UK's chemistry-climate model (UKCA) in the WCRP/SPARC re-evaluation.Lifetime estimates directly affect model predictions of future ozone recovery. Previous CCM studies of the recovery of the ozone layer have used projected future surface ODS concentrations based on old lifetime estimates and a simple box model. Therefore, the major driver of future ozone change, the stratospheric chlorine and bromine loading, has been constrained with crude time-dependent boundary conditions. A more realistic representation of the rate of ozone recovery can be obtained by removing this constraint and running the CCMs with emission flux surface boundary conditions for major ODSs, and allow the model itself to predict the future decadal removal of chlorine and bromine. We will perform these simulations within this project.Source gases with very long lifetimes (many hundreds to thousands of years) are too stable to affect stratospheric ozone by decomposition but they are invariably potent GHGs. For these gases loss processes in the upper atmosphere (mesosphere), which are usually ignored or treated very crudely, could significantly reduce their atmospheric lifetime, thereby decreasing their estimated climate impact. Three examples of such gases are NF3, CFC-115, and SF6. We have identified that the reactions of these gases with metallic atoms (Fe, Na and Mg) which are present in the upper mesosphere could be an important additional sink and compete with Lyman-alpha photolysis and other reactions. We will evaluate the rates of these sinks in the laboratory.For all of the gases studied, we will produce new, improved estimates of their climate impact by recalculating their GWP/GTP values.This project will use the UK's core tropospheric-stratospheric chemistry-climate model (UKCA). The testing and development work performed will lead to an improved, and more thoroughly tested, model for the UK community of researchers