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

点击翻译按钮获取中文摘要

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.