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。气体被移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气体等的影响，就必须准确了解其在大气中的寿命。臭氧消耗物质（ODS），如氯氟烃（CFCs），是长寿命的源气体，在平流层中分解释放氯和溴。根据《蒙特利尔议定书》，这些物种的排放已逐步停止，预计臭氧层将在今后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物在大气中的寿命。事实上，这些大气寿命目前存在很大的不确定性，这些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，据气象组织和气候小组的评估，一种主要的氟氯化碳CFC-11在大气中的寿命为45年，尽管其他研究表明臭氧消耗物质的寿命长达60年，但最近建立的世界气候研究计划平流层过程及其在气候中的作用项目已经认识到这一关键的不确定性，该项目旨在重新评估这些臭氧消耗物质及其替代品的寿命（如氢氟碳化物、氢氟碳化合物），使用最先进的三维化学气候模型（CCM）中的最新实验室数据。这些物种也是有效的温室气体（GHG），其已知大气寿命的变化将改变对它们如何影响气候变化的估计（根据其全球变暖/温度潜力（GWP/GTP））。这一项目将确保联合王国的化学-气候模型充分参与世界气候方案/臭氧再评价。《集束弹药公约》以前关于臭氧层恢复的研究采用了根据旧的寿命估计和一个简单的箱型模型预测的未来表面臭氧消耗物质浓度。因此，未来臭氧变化的主要驱动因素-平流层的氯和溴含量-受到了粗略的时间性边界条件的限制。通过消除这一限制，并使用主要消耗臭氧层物质的排放通量表面边界条件运行国家标准模型，可以更真实地表示臭氧恢复率，并使模型本身能够预测未来十年的氯和溴去除情况。我们将在该项目中进行这些模拟。具有很长寿命（数百至数千年）的源气体太稳定，不会通过分解影响平流层臭氧，但它们总是强效的温室气体。这些气体在高层大气（中层）的损失过程通常被忽视或被非常粗略地对待，但这可能会大大缩短其在大气中的寿命，从而减少其估计的气候影响。这类气体的三个例子是NF 3、CFC-115和SF6。我们已经确定，这些气体与金属原子（铁，钠和镁），这是目前在上层中间层的反应可能是一个重要的额外的汇和竞争莱曼α光解和其他反应。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它们的GWP/GTP值来对其气候影响进行新的改进估计。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一个改进的、更彻底的测试模型

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