Why is Lower Stratospheric Ozone Not Recovering?

平流层臭氧的耗尽使更大剂量的有害太阳紫外线(UV)辐射到达平流层表面，导致人类皮肤癌和白内障增加，并造成其他影响，如对作物的损害。臭氧还影响地球的辐射平衡，尤其是平流层下部的臭氧消耗对气候产生了重要的影响。虽然大多数长期消耗臭氧层物质(如氯氟烃)现在受《联合国蒙特利尔议定书》管制，其丰度正在缓慢下降，但围绕臭氧层恢复速度仍存在重大不确定性。虽然在平流层上层和南极探测到了恢复的迹象，但中低纬度的平流层下层却并非如此。事实上，与预期相反，这个极地外平流层下部的臭氧一直在减少(自1998年以来下降了5%)。造成这一现象的原因(S)尚不清楚，但推测的原因包括大气动力学的变化或短期活性碘和氯物种日益丰富。我们将利用全面的模拟研究和对低平流层中短寿命氯物质的新的有针对性的观察来调查这种持续消耗的原因。虽然《蒙特利尔议定书》控制了长寿命消耗臭氧层物质的生产，但卤化极短寿命物质(VSLS，寿命和6个月)的情况并非如此，因为人们认为它们不会足够丰富或不够持久，不足以产生影响。最近的观察表明情况并非如此，大气中几种气体(CH2Cl2、CHCl3)的丰度显著增加，这主要是由于亚洲排放量的增加。2021年夏季，以日本为基地的美国大型飞机行动将提供重要的新信息，说明这些短命物种的排放如何通过亚洲夏季风(ASM)到达平流层。UEA将通过在台湾和马来西亚进行有针对性的地面观测来补充ACCLIP活动，这将有助于限制氯的排放。这些观测将与利兹和兰开斯特的详细和全面的3-D模拟研究相结合，这两个国家在研究大气中的氯和碘方面拥有世界领先的专业知识和工具。该模型将使用离线化学传输模型(CTM)和化学-气候耦合模型(CCM)，前者是解释观测的理想工具，后者是研究化学动力学反馈和未来预测所必需的。关于重力分离如何影响气体的新观测将被用来检验对大气环流变化的模型描述。CTM也将被用在一种“反向”模式中，以追溯人为VSLS的观测结果到它们的地理来源区域。这些模型将被用来量化短期氯和碘进入平流层的通量，并确定它们对平流层低层臭氧趋势的影响。将使用CTM来量化动态变化的影响，并使用CCM来确定这种影响的驱动因素。模型结果将使用用于从观测中得出臭氧减少趋势的相同统计模型进行分析，包括动态线性模型(DLM)。总体而言，模型实验的结果将被综合成对低平流层臭氧正在减少的理解。然后，这些信息将被用来改进未来对臭氧如何演变的预测，这将在专家合作伙伴的协作下提供给政策制定进程(《蒙特利尔议定书》)。该项目的结果将为今后的国际评估提供重要信息，例如气象组织/环境署和气专委的报告。

Depletion of stratospheric ozone allows larger doses of harmful solar ultraviolet (UV) radiation to reach the surface leading to increases in skin cancer and cataracts in humans and other impacts, such as crop damage. Ozone also affects the Earth's radiation balance and, in particular, ozone depletion in the lower stratosphere (LS) exerts an important climate forcing. While most long-lived ozone-depleting substances (ODSs, e.g. chlorofluorocarbons, CFCs) are now controlled by the United Nations Montreal Protocol and their abundances are slowly declining, there remains significant uncertainty surrounding the rate of ozone layer recovery. Although signs of recovery have been detected in the upper stratosphere and the Antarctic, this is not the case for the lower stratosphere at middle and low latitudes. In fact, contrary to expectations, ozone in this extrapolar lower stratosphere has continued to decrease (by up to 5% since 1998). The reason(s) for this are not known, but suggested causes include changes in atmospheric dynamics or the increasing abundance of short-lived reactive iodine and chlorine species. We will investigate the causes of this ongoing depletion using comprehensive modelling studies and new targeted observations of the short-lived chlorine substances in the lower stratosphere.While the Montreal Protocol has controlled the production of long-lived ODSs, this is not the case for halogenated very short-lived substances (VSLS, lifetimes <6 months), based on the belief that they would not be abundant or persistent enough to have an impact. Recent observations suggest otherwise, with notable increases in the atmospheric abundance of several gases (CH2Cl2, CHCl3), due largely to growth in emissions from Asia. A major US aircraft campaign based in Japan in summer 2021 will provide important new information on how these emissions of short-lived species reach the stratosphere via the Asian Summer Monsoon (ASM). UEA will supplement the ACCLIP campaign by making targeted surface observations in Taiwan and Malaysia which will help to constrain chlorine emissions.The observations will be combined with detailed and comprehensive 3-D modelling studies at Leeds and Lancaster, who have world-leading expertise and tools for the study of atmospheric chlorine and iodine. The modelling will use an off-line chemical transport model (CTM), ideal for interpreting observations, and a coupled chemistry-climate model (CCM) which is needed to study chemical-dynamical feedbacks and for future projections. Novel observations on how gases are affected by gravitational separation will be used to test the modelled descriptions of variations in atmospheric circulation. The CTM will also be used in an 'inverse' mode to trace back the observations of anthropogenic VSLS to their geographical source regions. The models will be used to quantify the flux of short-lived chlorine and iodine species to the stratosphere and to determine their impact on lower stratospheric ozone trends. The impact of dynamical variability will be quantified using the CTM and the drivers of this determined using the CCM. The model results will be analysed using the same statistical models used to derive the decreasing trend in ozone from observations, including the Dynamical Linear Model (DLM). Overall, the results of the model experiments will be synthesised into an understanding of the ongoing decrease in lower stratospheric ozone. This information will then be used to make improved future projections of how ozone will evolve, which will feed through to the policy-making process (Montreal Protocol) with the collaboration of expert partners. The results of the project will provide important information for future international assessments e.g. WMO/UNEP and IPCC reports.