Photosensitization: A novel pathway to SOA generation and property change in tropospheric particles

对流层气溶胶颗粒常常被以一种简单的方式描述和在模型中表示，认为它们是非挥发性和化学惰性的。这些假设最近受到了前线研究的挑战，根据这些研究，挥发性有机化合物(VOCs)和二次有机气溶胶(SOA)形成了一个系统，通过化学和动态过程在大气中演化。目前大气有机颗粒物物理化学中的一个关键问题是，基于实验室研究中现有参数的模型严重低估了大气颗粒物，并且没有充分考虑到大气中观察到的颗粒物的增长。环境和建模的SOA浓度之间的差异清楚地表明，其他重要的SOA源还没有被识别和表征。因此，为解释和缩小这一差距作出了重要努力。例如，研究表明，以前被认为挥发性太高而无法明显分配到颗粒相中的气态乙二醛，可以通过多相化学对SOA质量做出重大贡献。乙二醛和其他小的二羰基在VOC氧化过程中大量生成。这些气体的凝聚相汇确实能够解释模型中缺失的SOA质量的一个重要部分，通常被称为AQSOA。然而，观察表明，关于对流层SOA的形成仍然存在很大的不确定性--传统的AQSOA显然不能解释所有缺失的SOA。此外，多相过程也被证明在颗粒相中产生光吸收化合物。这种光吸收物种的形成可能会在气溶胶粒子内和/或在气体/粒子界面上诱导新的光化学过程。来自其他科学领域的大量关于有机分子中光致电荷或能量转移的文献已经存在。这种有机分子是芳烃、取代的羰基和/或含氮化合物--所有这些都普遍存在于对流层气溶胶中。因此，尽管水生光化学已经认识到其中几个加速溶解有机物降解的过程，但对大气颗粒中/上的这种过程知之甚少。因此，在光敏层中，建议研究对流层的光敏作用，因为它可能在光敏层的形成和老化中起重要作用。这种光敏化作用可能会引入新的化学途径，影响大气的化学成分，从而有助于弥补目前对大气化学成分的低估。该项目旨在通过结合不同的实验室活动来解决这些问题，这些活动的重点是相关光敏剂在不同阶段的三态化合物的化学及其在SOA处理中的作用。显然，需要对这些过程进行一线基础研究，以便能够评估其重要性。

Tropospheric aerosol particles have often been described and represented in models in a simplistic way considering them as non-volatile and chemically inert. Such assumptions were recently been challenged by frontline research, according to which volatile organic compounds (VOCs) and secondary organic aerosols (SOA) form a system that evolves in the atmosphere by chemical and dynamical processing. A current key issue concern in the physico-chemistry of atmospheric organic particulate matter is that the models based on available parameterizations from laboratory studies strongly underestimate SOA and do not adequately account for particle growth as it is observed in the atmosphere. The difference between ambient and modeled SOA concentrations clearly suggests that other significant SOA sources have not yet been identified and characterized. Important efforts were consequently made to explain and close this gap. For instance, it was shown that gaseous glyoxal, which was previously considered as too volatile to noticeably partition into the particulate phase, could significantly contribute to SOA mass through multiphase chemistry. Glyoxal, and other small dicarbonyls are formed in large amounts during VOC oxidation. Condensed phase sinks for these gases are indeed able to explain an important part of the missing SOA mass in models, often addressed as aqSOA. However, observations imply that there are still large uncertainties about the tropospheric SOA formation - conventional aqSOA apparently cannot explain all missing SOA. Furthermore, multiphase processes have also been shown to produce light absorbing compounds in the particle phase. The formation of such light absorbing species could induce new photochemical processes within the aerosol particles and/or at the gas/particle interface. A significant body of literature on photo-induced charge or energy transfer in organic molecules from other fields of science exists. Such organic molecules are aromatics, substituted carbonyls and/or nitrogen containing compounds - all ubiquitous in tropospheric aerosols. Therefore, while aquatic photochemistry has recognized several of these processes that accelerate degradation of dissolved organic matter, only little is known about such processes in/on atmospheric particles. Therefore, within PHOTOSOA it is suggested to study photosensitization in the troposphere as it may play a significant role in SOA formation and ageing. Such photosensitization may introduce new chemical pathways so far unconsidered impacting both the atmospheric chemical composition and can thus contribute to close the current SOA underestimation. This project aims at tackling such issues by combining different laboratory based activities focusing on the chemistry of triplet state compounds of relevant photosensitizers, in various phases and their role in SOA processing. Clearly, frontline basic research studies on such processes are needed in order to be able to assess their importance.