Impacts of Criegee intermediate decomposition and reaction with water determined by direct measurements in ozonolysis reactions

根据DEFRA的数据，糟糕的空气质量每年给英国造成约150亿英镑的损失，这是由大气的化学成分决定的。了解由于生物或人为过程而排放到大气中的碳氢化合物（hc）和挥发性有机化合物（VOCs）的气相氧化对排放对氮氧化物（NOx = NO + NO2）、臭氧、甲烷寿命和二次有机气溶胶（SOA）形成的影响，从而对空气质量和气候变化的影响至关重要。一类重要的氧化反应是由臭氧引发的，涉及臭氧分解反应中不饱和挥发性有机化合物（包括人为和生物源）的氧化。这些反应长期以来被认为会产生反应性克里基中间体（CIs），并且在夜间和弱光条件下主导大气自由基的产生。2012年，利用实验室中的CIs光解源，首次对CI反应进行了直接动力学测量，结果表明，其反应活性远高于此前基于间接测量的预期。使用新发现的光解源进行的实验对我们对CI物种在大气中的作用的理解产生了怀疑，初步结果表明它们在SO2和NO2的氧化中起着增强的作用。然而，与水蒸气的竞争反应对CIs的大气影响至关重要。最简单的CI， ch220，已被证明与水二聚体反应迅速，但较大的CI与水（单体和二聚体）的反应受到的关注相对较少，并且没有温度依赖的动力学可用于大气模型中较大的物种。与水反应的产物将决定这些反应对大气的最终影响，这是高度不确定的。单分子分解反应也被强调为大型CI物种潜在的重要损失机制，关于这些反应的动力学或产物的信息很少。这项工作将解决动力学和产物的CI分解和反应与水的不确定性。此外，我们还将开发利用紫外/可见吸收光谱直接监测臭氧分解反应中CI物种的能力，从而能够直接确定臭氧分解反应中CI的产量，并在更现实的大气条件下研究CI化学。因此，本研究将解决有关在光解生产CI的实验中获得的动力学结果的适用性的问题。这项工作将减少Criegee中间体在大气命运和影响方面的重大不确定性，从而提高大气成分、空气质量和气候数值模拟的能力。

According to DEFRA poor air quality costs the UK ~£15billion per year, and is governed by the chemical composition of the atmosphere. Knowledge of the gas phase oxidation of hydrocarbons (HCs) and volatile organic compounds (VOCs) emitted into the atmosphere as a result of biogenic or anthropogenic processes is central to the impacts of emissions on NOx (NOx = NO + NO2), ozone, methane lifetimes, and formation of secondary organic aerosol (SOA), and thus on air quality and climate change.An important class of oxidation reactions are initiated by ozone, and involve the oxidation of unsaturated VOCs (including both anthropogenic and biogenic sources) in ozonolysis reactions. These reactions have long been postulated to produce reactive Criegee intermediates (CIs), and have been shown to dominate atmospheric radical production at night and in low light conditions. In 2012, the first direct kinetic measurements of CI reactions were made, using photolytic sources of CIs in the laboratory, with results indicating much higher reactivity than previously expected on the basis of indirect measurements. Experiments using the newly identified photolytic sources have cast doubt on our understanding of the role of CI species in the atmosphere, with initial results indicating an enhanced role in the oxidation of SO2 and NO2. However, the competing reaction with water vapour is critical to the atmospheric impacts of CIs.The simplest CI species, CH2OO, has been shown to react rapidly with the water dimer, but the reactions of larger CIs with water (both monomers and dimers) have received relatively little attention, and no temperature dependent kinetics are available for the larger species for use in atmospheric models. Products of the reactions with water will determine the ultimate atmospheric impacts of these reactions, and are highly uncertain. Unimolecular decomposition reactions have also been highlighted as potentially significant loss mechanisms for large CI species, with little information available regarding the kinetics or products of these reactions. This work will address the uncertainties in the kinetics and products of CI decomposition and reactions with water.Moreover, we will also develop capabilities for monitoring of CI species directly in ozonolysis reactions using UV/vis absorption spectroscopy, enabling the direct determination of CI yields from ozonolysis reactions and the investigation of CI chemistry under more realistic atmospheric conditions. This study will therefore address concerns regarding the applicability of kinetic results obtained in experiments in which CI are produced photolytically.This work will reduce the significant uncertainties in the atmospheric fate and impact of Criegee intermediates, leading to improvements in capabilities for numerical modelling of atmospheric composition, air quality and climate.