Tropospheric halogen chemistry: Reaction mechanisms, processes and global impacts

大约20年前，人们发现活性卤素化合物（碘、氯和溴）会导致北极对流层最低处的臭氧突然流失。与此同时，在对流层的许多其他地方也发现了活性卤素，主要是在海洋边界层，但也在盐湖上空，在火山的羽流中，在自由对流层，甚至在大陆的中部。对流层中活性卤素的来源似乎主要是天然的，大多数与海水或盐沉积物中所含的卤化物有关，科学界在测量这些化合物以及了解其释放和转化过程方面取得了很大进展。非常详细的过程模型已经成功地再现了复杂的化学反应，其中包括气相中的反应，气溶胶颗粒和云滴中的反应，这就是为什么我们称之为多相化学。与实地数据的比较表明，活性卤素对臭氧破坏的贡献通常为30-50%（例如在佛得角观测站）。然而，很少有全球模式包括对流层中的活性卤素。模型通常必须对源做出粗略的假设，并采用简化的反应机制，以使其在计算上可行，以执行全球模型运行。最近的另一个发现是，氯原子可以在热带地区造成高达15%的甲烷化学损失;这种损失不包括在任何气候模型中。在许多大陆环境中，已经发现了数百万亿分之一（ppt）的氯，这表明氯化学也可能与那里有关。甲烷和对流层臭氧都是强温室气体，本项目的目标是通过彻底重新审视反应机制，提供在全球对流层中遇到的各种情景的过程模型中经过测试的简化反应机制，以及通过开发活性卤素释放的参数化，来加强全球模型的理论基础。这项工作的成果将纳入最先进的全球化学-气溶胶模型，以量化活性卤素化学对臭氧破坏和产生、甲烷破坏以及气溶胶颗粒的形成和增长的全球影响。此外，我们将比较当前的情景与工业化前的情景，以确定人为污染物对活性卤素释放的重要性。这是由于许多卤素释放机制涉及酸性，有些与氮氧化物有关。人为活动增加了大气酸度和氮氧化物浓度。该项目汇集了在几乎所有对流层相关领域具有对流层卤素化学长期经验的UEA小组和在包括卤素化学在内的全球建模方面具有非常强大记录的利兹小组。这个项目非常及时，因为在过去几年中，已经有几个数据集可用，并且正在收集更多数据集，使我们能够在比几年前更大的规模上测试我们的模型预测。鉴于对对流层化学和气候的潜在巨大影响，该项目的相关性很大。

Around two decades ago reactive halogen compounds (iodine, chlorine and bromine) were found to cause sudden ozone loss in the lowest part of the troposphere in the Arctic. In the meantime reactive halogens were also found in many other parts of the troposphere, mainly in the marine boundary layer but also over salt lakes, in the plumes of volcanoes, in the free troposphere and even in the middle of the continents. The sources for reactive halogens in the troposphere appear to be mainly natural, mostly linked to halides contained in sea water or salt deposits.The scientific community has made great progress in the measurement of these compounds and also in the understanding of the underlying release and transformation processes. Very detailed process models have been successful in reproducing the intricate chemistry which involves reactions in the gas phase, in and on aerosol particles as well as cloud droplets, which is why we refer to this as multiphase chemistry. Comparisons with field data show that the contribution of reactive halogens to ozone destruction is often on the order of 30-50% (e.g. at the Cape Verde observatory). However very few global models include reactive halogens in the troposphere. The models that do usually have to make crude assumptions regarding the sources and have to employ a reduced reaction mechanism to make it computationally feasible to perform global model runs. Another recent discovery is that chlorine atoms can contribute up to 15% to the chemical loss of methane in the tropics; this loss is not included in any of the climate models. In many continental settings several hundred parts per trillion (ppt) of chlorine have been found indicating that chlorine chemistry can be relevant there as well. It is important to stress that methane and tropospheric ozone are strong greenhouse gases.In this project we aim to strengthen the theoretical foundation for global models by thoroughly revisiting the reaction mechanisms, providing reduced reaction mechanisms that have been tested in process models for a variety of scenarios encountered in the global troposphere and by developing parameterisations for the release of reactive halogens. The outcomes from this work will be included in a state-of-the-art global chemistry-aerosol model in order to quantify the global impacts of reactive halogen chemistry on ozone destruction and production, methane destruction as well as the formation and growth of aerosol particles. Furthermore, we will compare current day scenarios with preindustrial scenarios in order to establish the importance of anthropogenic pollutants for the release of reactive halogens. This is motivated by the fact that many halogen release mechanism involve acidity and some are linked to nitrogen oxides. Anthropogenic activity has increased both atmospheric acidity and nitrogen oxide concentrations.This project brings together the UEA group with a long-standing experience in tropospheric halogen chemistry in virtually all tropospherically relevant areas and the Leeds group with a very strong track record in global modelling including halogen chemistry. This project is very timely as in the last few years several data sets have become available and more are being collected that allow us to test our model predictions on a much larger scale than possible just a few years ago. Given the potentially large impacts on tropospheric chemistry and climate the relevance of this project is significant.