Why is Lower Stratospheric Ozone Not Recovering?

平流层臭氧的耗竭使更大剂量的有害太阳紫外线（UV）辐射到达地表，导致人类皮肤癌和白内障的增加，并造成其他影响，如农作物受损。臭氧还影响地球的辐射平衡，特别是平流层下层的臭氧耗竭产生了重要的气候强迫。虽然大多数长寿命消耗臭氧物质，如氯氟化碳、氟氯化碳，目前已受到《联合国蒙特利尔议定书》的控制，其丰度正在缓慢下降，但臭氧层恢复的速度仍有很大的不确定性。虽然在平流层上层和南极已经发现了恢复的迹象，但在中低纬度的平流层下层却没有这种情况。事实上，与预期相反，极外平流层下层的臭氧继续减少（自1998年以来减少了5%）。其原因尚不清楚，但可能的原因包括大气动力学的变化或短寿命活性碘和氯的丰度增加。我们将利用综合模拟研究和对平流层下部短寿命氯物质的新目标观测来调查这种持续耗竭的原因。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，但对于卤化极短寿命物质（VSLS，寿命<6个月）却没有这样做，因为人们认为它们的数量或持久性不足以产生影响。最近的观测结果表明并非如此，大气中几种气体（CH2Cl2和CHCl3）的丰度显著增加，这主要是由于亚洲排放的增加。2021年夏天，美国将在日本进行一次大型飞机飞行活动，届时将提供有关这些短命物种的排放物如何通过亚洲夏季风（ASM）到达平流层的重要新信息。东英吉利大学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运动，这将有助于限制氯的排放。这些观测结果将与利兹大学和兰开斯特大学详细而全面的三维建模研究相结合，这两所大学在研究大气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输运模型（CTM）和耦合化学-气候模型（CCM），前者非常适合解释观测结果，后者用于研究化学动力学反馈和未来预测。关于气体如何受引力分离影响的新观测结果将用于检验对大气环流变化的模拟描述。CTM还将以“反向”模式用于追溯对人为VSLS的观测到其地理来源区域。这些模式将用于量化短寿命氯和碘物种进入平流层的通量，并确定它们对平流层低层臭氧趋势的影响。动态变率的影响将使用CTM进行量化，并使用CCM确定其驱动因素。将使用从观测中得出臭氧减少趋势的统计模型，包括动态线性模型（DLM），对模型结果进行分析。总的来说，模式实验的结果将被综合成对平流层下层臭氧持续减少的理解。然后，这些资料将用于改进今后臭氧将如何演变的预测，这些预测将在专家伙伴的合作下，提供给决策过程（《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资料，例如气象组织/环境规划署和政府间气候变化专门委员会的报告。

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.