Understanding chlorine chemistry in polluted and clean atmospheres

在过去的十年中，对流层氯化学已被证明在清洁和受污染的大气中都发挥着重要作用。在这个项目中，你将通过在百慕大和英国进行观测，并使用数值模型，探索氯化学在产生和去除空气污染物中的作用，这是英美联合漂白项目的一部分。氯化学在平流层臭氧消耗中起着关键作用。然而，直到最近，人们才认为它在对流层中并不重要。然而，在过去的10年里，这种观点发生了变化。在污染地区，夜间ClNO2的生成导致高活性Cl自由基的产生，而Cl自由基可以驱动臭氧和颗粒物的产生（https://www.nature.com/articles/ngeo177?proof=t），在某些地方，氯本身可以构成颗粒物的很大一部分（https://www.nature.com/articles/s41561-020-00677-x）。在清洁地区，氯自由基可以从大气中去除甲烷和其他碳氢化合物，破坏臭氧（https://acp.copernicus.org/articles/19/3981/2019/）。氯以多种方式排放到大气中：主要是海盐，其次是天然和人造氯碳的排放、燃烧（尤其是煤）和工业过程。自工业化前以来，人为源的增加已导致对流层中氯的质量显著增加（https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2021GL093808）。尽管有这些先前的工作，关于氯化学仍然存在一些“大问题”：氯自由基在碳氢化合物和甲烷的氧化中是否起着全球性的重要作用？这对甲烷同位素比率有何影响？我们知道以海盐形式排放的氯变成气相氯的比例吗？氯化学如何影响区域氧化？氯在人为气溶胶中的作用是什么？人为排放的氯化合物有哪些？本项目将主要通过使用数值模式探索其中一些过程，但也将作为国际和国家实地运动的一部分收集新的观测结果。GEOS-Chem模型（https://www.geos-chem.org）提供了大气化学的计算表示。它是一个开源模型，被世界各地的大量团体（哈佛大学、美国宇航局、麻省理工学院、北京大学、加州理工学院等）使用。Evans小组在使用这种建模工具来提供高质量的科学研究方面有着长期的经验。该模型描述了我们对控制大气成分的化学、物理和生物过程的最佳估计，但它需要根据观测结果进行评估，以确保其稳健性，探索失败，做出新的过程水平发展，并探索这些过程对空气污染和气候变化的影响。除了利用GEOS-Chem模型发展氯化学的理论表征外，学生还将参与实地收集清洁和污染环境中的观察结果。

About the ProjectOver the last decade, tropospheric chlorine chemistry has been shown to play an important role in both the clean and polluted atmosphere. In this project, you will explore the role of chlorine chemistry in creating and removing air pollutants by making observations at Bermuda and in the UK, and using numerical models, as part of the joint UK-US BLEACH project.Chlorine chemistry plays a pivotal role in stratospheric ozone depletion. However, until recently it was not thought to be important in the troposphere. Over the last 10 years though, this perspective has changed. In polluted regions, the nighttime generation of ClNO2 leads to the generation of highly reactive Cl radicals which can drive the production of ozone and particulate matter (https://www.nature.com/articles/ngeo177?proof=t) and chlorine itself can make up a significant fraction of particulate matter in some locations (https://www.nature.com/articles/s41561-020-00677-x). In clean regions, chlorine radicals can remove methane and other hydrocarbons from the atmosphere and destroys ozone (https://acp.copernicus.org/articles/19/3981/2019/). Chlorine is emitted into the atmosphere in a number of ways: sea-salt dominants but emissions of natural and man-made chloro-carbons, combustion (notably of coal) and industrial processes. Increases in the anthropogenic sources have led to a significant increase in the mass of chlorine in the troposphere since the preindustrial (https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2021GL093808). Despite this previous work, there are still some "big questions" about chlorine chemistry: Does the chlorine radical play a globally significant role in the oxidation of hydrocarbons and methane? How does this impact methane isotopic ratios? Do we understand the fraction of chlorine emitted as sea salt that becomes gas-phase chlorine species? How does chlorine chemistry impact regional oxidation? What is the role of chlorine in anthropogenic aerosol? What are the anthropogenic emissions of chlorine compounds? This project will explore some of these processes primarily through the use of numerical models but also by collecting new observations as part of international and national field campaigns.The GEOS-Chem model (https://www.geos-chem.org) provides a computational representation of the chemistry of the atmosphere. An open-source model, it is used by a large number of groups around the world (Harvard, NASA, MIT, PKU, CalTech etc). The Evans group has long experience of working with this modelling tool to deliver high-quality science. The model describes our best estimate for the chemical, physical and biological processes controlling the composition of the atmosphere, but it needs to be evaluated against observations to ensure its robustness, to explore failures, to make new process level developments and to explore the impact of these processes on air pollution and climate change. As well as developing out theoretical representation of chlorine chemistry with the GEOS-Chem model, the student will also be involved in going into the field to collect observations in both clean and polluted environments.