Observational constraints on the global organic aerosol budget

了解气候变化是当今科学面临的最重要挑战之一。大气颗粒物（气溶胶）通过吸收和散射阳光以及改变云的性质来影响地球的气候。来自人类活动的气溶胶（例如，来自汽车尾气和工业活动的温室气体（包括二氧化碳）是地球气候的主要冷却力，并在一定程度上抵消了工业时期的温室气体变暖。量化这种气溶胶冷却是更准确地预测气候变化的关键一步。在过去的几十年里，人们花了大量的精力试图更好地了解气溶胶如何影响气候。早期的努力主要集中在硫酸盐上，硫酸盐是一种重要的人为气溶胶。然而，在过去几年中，新的仪器已经允许对气溶胶的化学成分进行详细的评估。这些研究的一个令人惊讶的结果是有机碳气溶胶的重要性。事实上，在大气中的许多地方，人们发现有机碳气溶胶实际上主导了细（亚微米）颗粒的质量负荷。气候和大气气溶胶模型一直无法再现在大气中观测到的有机气溶胶的数量，模型预测不足的情况在污染地区最严重。有证据表明，模式的主要问题在于处理二次有机气溶胶：即大气中由气相前体形成的有机气溶胶。提高我们对SOA和有机气溶胶的理解将是具有挑战性的，因为它们的形成涉及许多1000个有机成分和10000个反应。然而，这是改善空气质量和气候预测的重要一步。另一个复杂因素来自放射性碳观测，它表明大气中的大部分有机碳气溶胶是现代的（非化石）。对这一观测结果的典型解释是，有机碳气溶胶的主要来源是生物（植被）源。如果是这样的话，预计在森林上空将发现最高浓度的有机碳气溶胶。然而，观测结果显示情况正好相反，污染地区的浓度较高，而森林地区的浓度则低得多。有人认为，这种明显的矛盾可能是由于在人为污染的情况下，生物源的有机蒸气更有效地转化为有机碳气溶胶。在最近的一项研究中，我们使用一个模型和有机碳观测表明，多达60%的全球有机碳气溶胶可以通过这种增强机制形成。如果这是真的，它对我们理解气溶胶气候强迫具有重大意义。在本提案中，我们将综合各种不同仪器对有机碳气溶胶的观测结果。不同的工具在提供的信息方面相互补充，共同为模型提供了严格和苛刻的测试。我们将使用这个数据库来评估一个全球气溶胶模型，并改善模型中有机气溶胶来源的处理。这将是迄今为止全球模式中对有机碳气溶胶最全面的测试。我们将测试是否有强有力的证据，人为污染的二次有机气溶胶的大幅增强。这项工作将大大提高我们对有机碳气溶胶的来源和控制过程的理解，并将有助于指导未来的现场和实验室测量。

Understanding climate change is one of the most important challenges facing science today. Atmospheric particles (aerosol) impact the Earth's climate through absorbing and scattering sunlight and through changing the properties of clouds. Aerosols from human activity (e.g., from car exhausts and industrial activity) are the dominant cooling forcing of the Earth's climate and have partly counteracted greenhouse gas warming over the industrial period. Quantifying this aerosol cooling is a critical step to making more accurate predictions of climate change.Over the past few decades substantial effort has been spent trying to better understand how aerosols impact climate. Much of this early effort focussed on sulfate which was known to be an important anthropogenic aerosol. However, in the past few years new instruments have allowed a detailed evaluation of the chemical composition of aerosol. A surprising result from these studies was the importance of organic carbon aerosol. In fact, in many places in the atmosphere it was found that organic carbon aerosol actually dominated the mass loading of fine (submicron) particles. Climate and atmospheric aerosol models have been unable to reproduce the amount of organic aerosol observed in the atmosphere with the model underprediction being greatest in polluted regions. Evidence points to the major problem with the models being in the treatment of secondary organic aerosol (SOA): that is organic aerosol that is formed in the atmosphere from gas-phase precursors. Improving our understanding of SOA and organic aerosol will be challenging because many 1000s of organic components and 10 000s reactions are involved in their formation. However, it is an essential step in improving both air quality and climate predictions. A further complication comes from radio-carbon observations that suggest that the majority of the organic carbon aerosol in the atmosphere is modern (non-fossil). The typical interpretation of this observation is that the dominant source of organic carbon aerosol is from biogenic (vegetation) sources. If this were the case it would be expected that the highest concentrations of organic carbon aerosol would be found over forests. However, observations show the opposite, with high concentrations over polluted regions and much lower concentrations over forests. It has been suggested that this apparent contradiction could be due to organic vapours from biogenic sources more efficiently converting to organic carbon aerosol in the presence of anthropogenic pollution. In a recent study we used a model and organic carbon observations to suggest that as much as 60% of global organic carbon aerosol could be formed through this enhancement mechanism. If this is true it has substantial implications for our understanding of aerosol climate forcing. In this proposal we will synthesise observations of organic carbon aerosol from a wide range of different instruments. The different instruments complement each other in the information they provide and together provide a rigorous and demanding test for the model. We will use this database to evaluate a global aerosol model and to improve the treatment of the sources of organic aerosol in the model. It will be the most comprehensive test of organic carbon aerosol in a global model to date. We will test whether there is strong evidence for a substantial enhancement of secondary organic aerosol by anthropogenic pollution. This work will greatly improve our understanding of the sources and processes controlling organic carbon aerosol and will help guide future field and laboratory measurements.