Oceanic Reactive Carbon: Chemistry-Climate impacts (ORC3)

海洋有机碳（OC）气溶胶粒子通过改变浅海层积云的性质对气候产生深远影响，但其来源高度不确定。这些气溶胶可能是由海上的浪花直接产生的，包括气泡破裂机制，或由生物源性挥发性有机化合物（BVOC）的排放和随后的氧化产生的半挥发性产品和气溶胶。有证据表明，我们目前的模式低估了海洋OC的次级成分。海洋萜烯，特别是异戊二烯和单萜烯，是浮游植物产生的高活性BVOC，是二次有机气溶胶（SOA）的主要候选者。有限的观测显示，这些物种在遥远海洋大气生物活跃区域的浓度很高，这意味着它们可能通过SOA的形成在改变海洋云特性方面发挥作用。由于缺乏对远程MBL的观察，对海洋萜烯对SOA的贡献的清晰描绘受到了阻碍。估算全球排放量的“自下而上”方法和“自上而下”估算方法之间存在着重大差异，前者是根据实验室的浮游生物光合速率按比例计算全球海洋，后者是按比例计算源以迫使模拟和观测到的大气浓度相匹配。自下而上的方法通常比自上而下的方法小10-1000倍，这表明它们不能捕捉到产生双挥发性有机化合物的全部海洋过程。然而，高得多的自上而下的估计值也存在很大的不确定性，因为它们仅来自MBL中有限数量的浓度测量。我们的初步模式结果表明，重现观测到的海洋浓度所需的量级的全球单萜烯源可能会使充当云凝结核（CCN）的气溶胶数量在大的海洋区域增加两倍。这对于我们理解第一种气溶胶间接效应具有深远的意义，因为它改变了我们对自然气溶胶背景控制的理解。此外，海洋萜烯具有高活性，因此可能改变大气的氧化能力（从而改变温室气体甲烷的寿命），特别是在大部分甲烷被破坏的海洋热带地区。海洋SOA的另一个潜在来源是乙二醛，这是一种在距离海岸1000公里远的大气中观察到的高度活性物质。这一物种的生命周期约为几小时，因此从陆地来源的迁移不能解释它在遥远的海洋大气中的存在。乙二醛的氧化导致过氧自由基和可冷凝产物的快速产生，这被认为是导致SOA形成的原因。然而，乙二醛在偏远海洋地区的存在至今仍无法解释。一个尚未被探索的假设是较大的挥发性有机化合物如萜烯的光氧化。大量的海洋乙二醛来源对大气成分和气候的影响也有待检验。在本项目中，我们将大幅增加海洋大气中单萜烯和异戊二烯的观测数据库，并对乙二醛的现有观测进行评估和新的观测。我们将利用这些观测资料以及全球化学和气溶胶模型来量化海洋活性碳对大气成分和气候的影响。提高对地球气候系统中控制本底气溶胶和大气氧化能力的自然过程的理解是高度优先的，因为它们是我们对气候和地球系统响应的人为影响的估计的基础。目前迫切需要评估海洋中活性挥发性有机化合物（VOCs）的来源，并量化其对CCN、氧化能力和全球气候的重要性。

Oceanic organic carbon (OC) aerosol particles have been proposed to exert a profound effect on climate through modification of the properties of shallow marine stratocumulus clouds, yet their sources are highly uncertain. These aerosol may be generated directly by sea spray involving a bubble bursting mechanism, or by the emission and subsequent oxidation of biogenic VOC (BVOC) that produce semivolatile products and aerosol. There is evidence that the secondary component of marine OC is underpredicted by our current models. Oceanic terpenes, especially isoprene and the monoterpenes, are highly reactive BVOC produced by phytoplankton, and are prime candidates for secondary organic aerosol (SOA). Limited observations show high concentrations of these species over biologically active regions of the remote marine atmosphere, which implies that they may play a role in modifying marine cloud properties through SOA formation. A clear picture of the contribution of marine terpenes to SOA is hampered by a lack of observations in the remote MBL. Substantial differences exist between "bottom-up" methods of estimating global emissions, where lab-based photoplankton production rates are scaled to the global oceans, and "top-down" estimates, where the source is scaled to force a match between modelled and observed atmospheric concentrations. The bottom-up methods are generally a factor of 10-1000 smaller than top-down methods, suggesting that they do not capture the full range of marine processes giving rise to BVOC production. The much higher top-down estimates however are also subject to significant uncertainty since they are derived from only a limited quantity of concentration measurements in the MBL. Our preliminary model results suggest that a global monoterpene source of the magnitude required to reproduce observed marine concentrations may increase the aerosol number acting as cloud condensation nuclei (CCN) by up to a factor of two over large oceanic regions. This has profound significance for our understanding of the first aerosol indirect effect, since it alters our understanding of controls on the background natural aerosol. In addition, marine terpenes are highly reactive, and therefore may alter the atmospheric oxidising capacity (and therefore the lifetime of the greenhouse gas methane), especially in the marine tropics where most methane is destroyed. Another potential source of marine SOA is glyoxal, a highly reactive species which has been observed in the remote atmosphere many 1000s km away from the coasts. This species has a lifetime on the order of hours and so transport from terrestrial sources cannot explain its presence in the remote marine atmosphere. Oxidation of glyoxal leads to the rapid production of peroxy radicals and condensable products which are believed to lead to the formation of SOA. However the presence of glyoxal in remote marine regions is so far unexplained. One as yet unexplored hypothesis is photo-oxidation of larger VOCs such as terpenes. The implications of a large marine glyoxal source for atmospheric composition and climate has also yet to be tested.In this project we will substantially increase the observational database of monoterpenes and isoprene in the marine atmosphere and evaluate existing and make new observations of glyoxal. We will use these observations along with global models of chemistry and aerosol to quantify the impact of marine reactive oceanic carbon on atmospheric composition and climate.Improved understanding of natural processes controlling background aerosol and atmospheric oxidative capacity in the Earth's climate system is of high priority, since they underpin our estimates of man-made impacts on climate and the Earth system response. There is an urgent need to evaluate the marine sources of reactive volatile organic compounds (VOCs) and to quantity their importance for CCN, oxidative capacity and global climate.