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

对流层（大气层最低层~12公里）的化学性质在气候变化、空气质量退化和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对流层（和平流层）化学的当前观点，科学中的不确定性对空气污染和气候预测产生影响。有限的观测制约因素，加上缺乏适当的建模工具，导致在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反应方面的作用方面存在很大的不确定性。与合作者一起，我们已经表明，活性卤素在决定对流层的组成方面发挥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，并显著改变了大气对人为扰动的反应。我们发现，O3和碘化物（I-）在海洋表面之间的反应驱动了大部分的大气碘排放，并表明这一过程已导致大气碘在20世纪后半叶的一些地区增加了两倍，由于人为O3的增加，这意味着碘驱动的O3损失现在比过去更活跃。然而，到目前为止，由于缺乏对机制的理解，对卤素在整个世纪的影响的模拟没有考虑到海洋表面的任何潜在变化。我们的团队构建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年内，由于海洋酸化，硝化率降低高达44%的情况下，该模型预测某些地区的地表[I-]将翻一番。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，并建议需要一种综合方法来充分了解碘的影响。将[I-]的知识转化为海-气碘排放及其对大气影响的预测也是高度不确定的，因为缺乏对环境代表性浓度的测量，以及碘通量对[O3]和[I-]的复杂额外依赖性，水侧湍流混合和表面活性剂/有机材料。SEA是来自领先的地球系统科学研究所的大气和海洋科学家与地球化学家之间的多学科合作。我们建议采用新的技术和想法来解决碘的地球化学循环中的主要不确定性，以解决我们的关键假设，即全球变化将在未来世纪推动大气碘排放的显着变化，这将影响空气质量和气候。最终，该项目将为环境变化与活性卤素对空气质量、生态系统和气候变化的影响之间的反馈提供变革性的新知识。

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.