Impact of Ocean Acidification on the Air-Sea Exchange of Trace Gases

随着大气中二氧化碳的持续上升，海水的pH值将逐渐变得不那么碱性。随着本世纪的进展，这种影响几乎肯定会越来越明显。它很可能影响海洋生物，从珊瑚到微小的浮游生物，特别是那些以碳酸钙为主要成分形成结构的生物。对非碳酸盐分泌生物可能产生的影响更难预测。在这篇博士论文中，学生将研究pH值降低对海洋微生物产生微量气体（二甲基硫化物及其前体二甲基硫代丙酸酯和一系列自然产生的有机卤素气体）的影响。这些化合物具有挥发性，可以穿过海气界面。在大气中，它们的氧化产物可导致新粒子的形成或现有粒子的增长到云凝结核的大小。正因为如此，它们可以影响云的形成，从而影响气候。此外，这些气体在控制大气的氧化能力方面发挥着重要作用，例如，通过破坏臭氧，从而影响空气质量。学生将研究pH值变化对微生物产生气体的影响。为了量化高二氧化碳条件下气体通量变化的影响，学生将把当前排放量和未来排放量的估计输入到最先进的大气海洋边界层一维模型（MISTRA模型）中。这将有助于评估由ph引起的海气微量气体通量变化对颗粒形成和大气氧化能力的潜在影响。我们提出的研究该系统的方法是通过使用浮动和固定的中生态系统进行实地工作，其中调整pH以模拟未来的条件，以及使用pH改变的天然海水和特定生物培养的体外研究。此外，由于越来越多的证据表明海水的pH值降低会导致海洋有机物的类型和组成发生重大变化，学生将测试这是否会导致气体通过海气界面的传输速率发生变化。这将在中观研究中通过添加微量的六氟化硫和3-氦对示踪剂来完成，在实验的时间过程中，其浓度比的变化将给出空气-海洋气体交换率的估计。如果这种影响（以前从未被测试过）被证明是显著的，那么其影响将远远不止于这里提出研究的气体，例如海洋对人为二氧化碳的吸收速率和其他与气候有关的气体的海气通量的计算。最后，学生将使用一维模型来评估ph诱导的海气交换中微量气体对颗粒形成和空气质量的重要性。这里提出的建议将为NERC/Defra海洋酸化研究计划增加一个额外的维度，不仅研究海水pH值变化对海洋过程的一个重要影响，而且还将海洋效应与更广泛的气候和大气化学领域联系起来。

As atmospheric carbon dioxide continues to rise the pH of seawater will get progressively less alkaline. The effect of this will almost certainly become progressively more apparent as the century progresses. It is likely to affect marine organisms from corals to microscopic plankton, particularly those that form their structures with calcium carbonate as a major component. The possible effects on non-carbonate secreting organisms are harder to predict. In this Ph.D. proposal the student will study the effect of decreasing pH on the production of trace gases (dimethyl sulphide and its precursor dimethylsulphoniopropionate and a range of naturally produced organo-halogen gases) by marine microorganisms. These compounds are volatile and can cross the air-sea interface. In the atmosphere their oxidation products can lead to new particle formation or growth of existing particles to cloud condensation nuclei size. Because of this they can affect the formation of clouds and hence climate. In addition, these gases play important roles in controlling the oxidation capacity of the atmosphere by, for example, destruction of ozone and so affect air quality. The student will examine the effect of pH change on the biogenic production of the gases by microbes. In order to quantify the implications of changed gas fluxes under high CO2 conditions, the student will input estimates of current day emissions as well as future emissions into a state-of-the-art one-dimensional model of the atmospheric marine boundary layer (the MISTRA model). This will allow assessment of the potential impact of pH-induced changes in air-sea fluxes of trace gases on particle formation and the oxidation capacity of the atmosphere. The approach we propose to studying this system is by a mixture of field work using floating and fixed mesocosms in which the pH is adjusted to mimic future conditions and in vitro studies using pH-altered natural seawaters and cultures of specific organisms. In addition, since there is growing evidence that lowered pH of seawater leads to significant changes in the type and composition of marine organic matter, the student will test whether this brings about alteration in the rate of transfer of gases across the air-sea interface. This will be done in the mesocosm studies by adding minute amounts of the tracer pair sulphur hexafluoride and 3-helium, changes in whose concentration ratio over the time-course of the experiments will give estimates of air-sea gas exchange rates. If this effect (which has never been tested previously) proves significant then the implications would be far wider than just for the gases proposed for study here, e.g. on rates of man-made carbon dioxide uptake by the oceans and calculation of air-sea fluxes of other climate relevant gases. Finally, the student will use a one-dimensional model to assess the importance of pH-induced changes in the air-sea exchange of trace gases for particle formation and air quality. What is proposed here would add an additional dimension to the NERC/Defra Ocean Acidification research programme by not only studying one important impact of seawater pH change on processes in the oceans but also by linking effects in the oceans to the wider fields of climate and atmospheric chemistry.