Oceanic Reactive Carbon: Chemistry-Climate impacts (ORC3)

海洋有机碳(OC)气溶胶粒子已被提出通过改变海洋浅层积云的性质来对气候产生深远影响，但其来源尚不确定。这些气溶胶可由涉及气泡破裂机制的海洋喷雾直接产生，也可由产生半挥发性产品和气溶胶的生物挥发性有机化合物(BVOC)的排放和随后的氧化产生。有证据表明，我们目前的模型低估了海洋有机碳的次要成分。海洋萜类，特别是异戊二烯和单萜，是浮游植物产生的高活性BVOC，是二次有机气溶胶的首选。有限的观测显示，这些物种在偏远海洋大气的生物活跃区域高度集中，这意味着它们可能通过形成SOA在改变海洋云特性方面发挥作用。由于在偏远的MBL中缺乏观测，海洋萜类对SOA的贡献的清晰图景受到了阻碍。在估算全球排放量的“自下而上”方法和“自上而下”估算方法之间存在重大差异，前者以实验室为基础的浮游生物产生率按比例调整至全球海洋，后者按比例调整来源，以迫使模拟的大气浓度与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，这表明它们不能涵盖导致BVOC生产的全部海洋过程。然而，更高的自上而下的估计值也有很大的不确定性，因为它们只是根据MBL中有限数量的浓度测量得出的。我们的初步模式结果表明，重现观测到的海洋浓度所需的全球单萜源可能会使大片海洋区域上空作为云凝结核(CCN)的气溶胶数量增加多达两倍。这对于我们理解第一个气溶胶间接效应具有深远的意义，因为它改变了我们对背景自然气溶胶控制的理解。此外，海洋萜类具有高度的活性，因此可能会改变大气的氧化能力(从而改变温室气体甲烷的寿命)，特别是在大多数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，这是一种高度活性的物种，在距离海岸1000公里的偏远大气中观察到了这种物质。这种物种的寿命在几个小时左右，因此从陆地来源的运输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缩合产物的快速产生，这被认为是导致SOA形成的原因。然而，到目前为止，乙二醛在偏远海洋地区的存在尚不清楚。一种尚未探索的假说是较大的挥发性有机化合物(如萜类)的光氧化。大型海洋乙二醛来源对大气成分和气候的影响也有待测试。在这个项目中，我们将大幅增加海洋大气中单萜和异戊二烯的观测数据库，并评估现有的乙二醛和进行新的观测。我们将利用这些观测以及全球化学和气溶胶模型来量化海洋活性海洋碳对大气组成和气候的影响。更好地理解控制地球气候系统中本底气溶胶和大气氧化能力的自然过程是高度优先的，因为它们支持我们对气候和地球系统响应的人为影响的估计。迫切需要评估海洋活性挥发性有机化合物(VOCs)的来源，并对其对CCN、氧化能力和全球气候的重要性进行量化。

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.