Iodine sea-air emissions and atmospheric impacts in a changing world (I-SEA)

对流层（大气最低~12公里）的化学成分在气候变化、空气质量退化和生物地球化学循环中起着关键作用。在过去的几十年里，我们对对流层化学复杂性的理解有了很大的发展。最近的发展之一是卤素（Cl, Br， I）化学。卤素原子过程可以从根本上挑战当前对流层（和平流层）化学的观点，科学上的不确定性会对空气污染和气候预测产生影响。有限的观测约束，加上缺乏适当的模拟工具，在（为数不多的）计算卤素对区域或全球尺度的影响及其在改变地球系统对人为扰动的响应方面的作用方面造成了很大的不确定性。与合作者一起，我们已经表明，活性卤素在确定对流层的组成中起着重要而普遍的作用。在卤素中，碘对对流层臭氧（O3）循环的影响最为深远，并显著改变大气对人为扰动的响应。我们发现，海洋表面O3与碘化物（I-）之间的反应驱动了大气中大部分碘的排放，并表明这一过程导致20世纪下半叶某些地区大气中碘的含量增加了三倍，这意味着现在由碘驱动的O3损失比过去更加活跃。然而，由于缺乏对机制的理解，到目前为止，对卤素在21世纪的影响的模拟还没有考虑到海洋表面I-的任何潜在变化。我们的团队建立了第一个海洋碘循环模型，发现表面碘分布主要受生物生产力、硝化速率、混合层深度和平流的影响。事实上，在未来20 - 30年，由于海洋酸化，硝化率将降低44%的情况下，该模型预测，某些地区的表面[I-]将增加一倍（由于细菌I-氧化减少）。这一结果表明气候引起的海洋变化与大气空气质量和气候之间存在新的耦合关系，并表明需要采用综合方法来充分了解碘的影响。将有关[I-]的知识转化为对海洋-空气碘排放及其对大气的影响的预测也非常不确定，因为缺乏对具有环境代表性浓度的测量，而且碘通量除了对[O3]和[I-]、对水侧湍流混合以及对表面活性剂/有机物质的复杂附加依赖之外，还依赖于碘通量。I-SEA是大气和海洋科学家以及来自领先的地球系统科学研究所的地球化学家之间的多学科合作。我们建议引入新的技术和想法来解决碘的生物地球化学循环中的主要不确定性，以解决我们的关键假设，即全球变化将推动未来一个世纪大气碘排放的重大变化，这将影响空气质量和气候。最终，该项目将提供关于环境变化与反应性卤素对空气质量、生态系统和气候变化影响之间反馈的变革性新知识。

The chemistry of the troposphere (lowest ~12 km of the atmosphere) plays a critical role in climate change, air quality degradation and biogeochemical cycling. Our understanding of the complexity of tropospheric chemistry has developed immensely over the last decades. One of the more recent developments is halogen (Cl, Br, I) chemistry. Halogen atom processes can fundamentally challenge current perspectives of tropospheric (and stratospheric) chemistry, and the uncertainty in the science generates impacts on air pollution and climate predictions. Restricted observational constraints, coupled to a lack of suitable modelling tools, translate into large uncertainties in (the few) calculations of the impact of halogens on regional or global scales, and their role in modifying the response of the Earth system to anthropogenic perturbations. Together with collaborators, we have shown that reactive halogens play a significant and pervasive role in determining the composition of the troposphere. Of the halogens, iodine has the most profound impact on tropospheric ozone (O3) cycling, and significantly modifies the atmospheric response to anthropogenic perturbations. We identified that the reaction between O3 and iodide (I-) at the ocean surface drives the majority of atmospheric iodine emissions and showed that this process has resulted in a tripling of atmospheric iodine in some regions over the latter half of the 20th century due to increased anthropogenic O3, meaning that iodine-driven O3 loss is more active now than in the past. However, simulations of the impacts of halogens through the 21st century have so far made no account of any potential changes in surface ocean I-, due to a lack of mechanistic understanding. Our team have constructed the first model of marine iodine cycling and find that the surface iodide distribution is impacted primarily by biological productivity, nitrification rates, mixed layer depth and advection. Indeed, under the scenario where nitrification rates are reduced by up to 44% in the next 20 - 30 years due to ocean acidification, the model predicts a doubling of surface [I-] in some regions (due to decreased bacterial I- oxidation).This result indicates a new coupling between climate-induced oceanographic changes and atmospheric air quality and climate, and suggests the need for an integrated approach to fully understand the impacts of iodine. Translating knowledge of [I-] into predictions of sea-air iodine emissions and their resulting impacts on the atmosphere is also highly uncertain due to a lack of measurements at environmentally representative concentrations and complex additional dependencies of iodine fluxes, over and above on [O3] and [I-], on water-side turbulent mixing and on surfactants/organic material.I-SEA is a multidisciplinary collaboration between atmospheric and marine scientists and geochemists from leading Earth System science institutes. We propose to bring new technology and ideas to address major uncertainties in the biogeochemical cycling of iodine in order to address our key hypothesis, that global change will drive significant changes in atmospheric iodine emissions over the coming century which will impact on air quality and climate. Ultimately the project will provide transformative new knowledge of the feedbacks between environmental change and the impact of reactive halogens on air quality, ecosystems and climate change.