Formation of Nitrated Polycyclic Aromatic Hydrocarbons (NPAHs) in Combustion Engine Exhaust Studied by Laser Photofragmentation

硝化多环芳烃(NPAHs)及其母体多环芳烃(PAHs)可能是对人类健康危害最大的非生物类物质。在很大程度上，环境空气中三分之一以上的诱变潜力可归因于NPAHs。NPAH是在NO2存在的情况下在燃烧过程中形成的。内燃机尾气中的NPAH尤其危险，因为它们附着在烟尘颗粒上，烟尘颗粒充当运输工具，穿过肺泡膜进入生物体，并穿过红血球的壁。柴油机微粒过滤器(DPF)中使用的一些策略利用了过量的NO2。这导致了关于废气后处理系统中NPAHs可能从头形成的激烈讨论。然而，与其他空气污染物相比，关于污染物的发生、形成和汇的知识要零散和不完整得多。造成这一差距的原因是半挥发性、不稳定性和现有测量技术缺乏敏感性。二十多年来，在这些方面中的大多数方面没有取得重大进展。在废气中，基于过滤器的采样和随后的分析特别容易产生伪影。在这里，我们建议通过(1)提供NPAHs和PAHs的快速、无伪影测量方法来促进对NPAHs的了解，这将允许(2)在流动反应器中模拟汽车排气系统的受控条件下研究NPAH在气溶胶颗粒物上的NPAH的形成、降解和吸附，(3)NPAHs和PAHs对内燃机排气系统的直接、无伪影的研究。这项要求很高的挑战将进行激光光碎化(PF)。PF是光对化学键的选择性断裂。选择性由所选波长定义，对应于特定的光子能量，其等于化学结合能。在我们的NPAH检测的情况下，产生的碎片、硝基和剩余的PAH将通过它们的光学发射、当碎片在激发态产生时被检测到，或者通过碎片离子产生的电流来检测。虽然PF的基本原理几十年前就已为人所知，但我们相信，通过新的仪器，这项技术可以成功地应用于高灵敏度在线NPAH检测这一具有挑战性的任务的常规应用。

Nitrated Polycyclic Aromatic Hydrocarbons (NPAHs), together with its parent compound PAHs, are probably the abiotic class of substances which is most harmful for human health. A large part, more than one third of the mutagen potential of ambient air is attributable to NPAHs. NPAHs are formed during combustion in the presence of NO2. NPAHs in combustion engine exhaust are particularly hazardous due to their attachment to soot particles, which act as transport vehicles across the alveolar membrane into the organism and across walls of red blood cells. Some strategies used in Diesel particulate filters (DPF) make use of excess NO2. This led to intense discussions about possible de-novo formation of NPAHs within an exhaust aftertreatment system. However, compared to other air pollutants, knowledge of occurrence, formation, and sinks is much more fragmentary and incomplete. The reason for this gap is semivolatility, instability, and lack of sensitivity of existing measuring techniques. For more than two decades, no major progress was achieved with regard to most of these aspects. In exhaust gas, filter-based sampling and subsequent analysis is particularly prone to artifact formation. Here, we propose to advance knowledge of NPAHs by (1) providing a fast, artifact-free measurement method for NPAHs and PAHs, which will allow to (2) study NPAH formation, degradation and sorption of NPAH from PAH on aerosol particulate matter under controlled conditions in a flow reactor, simulating an automotive exhaust system, (3) direct, artifact-free study of NPAHs and PAHs on the exhaust system of a combustion engine. This demanding challenge will be performed laser-photofragmentation (PF). PF is the selective fragmentation of a chemical bond by light. Selectivity is defined by the selected wavelength, corresponding to a specific photon energy, which is equal to the chemical binding energy. The generated fragments, in our case of NPAH detection, the nitro group and the remaining PAH, will be detected either by their optical emission, when the fragments are generated in the excited state, or by the current due to fragment ions. Although the fundamentals of PF are known for decades, we believe that by novel instrumentation, this techniques can successfully be implemented for the routine application of the challenging task of a highly sensitive on-line NPAH detection.