Effect of after-treatment systems on organic particulate matter emissions in diesel engine exhaust

Diesel engine exhausts were analyzed downstream of a diesel particulate filter (DPF) and a selective catalytic reactor (SCR) to verify their effect on carbon particulate matter. An array of chemical, physical and spectroscopic techniques (Gas chromatography coupled with mass spectrometry (GC-MS), mobility analyzer, UV-visible absorption and fluorescence spectroscopy) was applied for characterizing the organic particulate matter (PM, constituted of polycyclic aromatic hydrocarbons (PAH), heavy aromatic compounds, soot) in the exhaust. The engine was operated in "half-load" (50% of the accelerator pedal position, representing the more common condition for engine in urban traffic) and "full-load" (100% of the accelerator pedal position, representing the best performance of the engine operation) conditions, at the same engine speed (2000 rpm). In "half-load" condition, soot formation is enhanced with respect to "full-load" one, but it was found that in this last condition the after-treatment systems are less efficient in the soot abatement just due to the much lower soot amount . Indeed, soot concentration in the exhaust after the DPF was about 40% higher in the "full-load" condition with respect to "half-load", and did not undergo any significant further concentration decrease after SCR, in both conditions. By contrast, PAH concentration after DPF was found to be much higher in the "half-load" (3 times) with respect to "full-load" and a further decrease of about 30% was found after the SCR. Also the heavy aromatic compounds, with molecular weights over the detectability range of GC-MS (300 u), were mitigated by SCR. Therefore, SCR resulted not effective in causing a further soot reduction, whereas it was able to largely reduce PAH and heavy aromatics emissions, especially in a lower temperature condition, when combustion efficiency is worse, as in the "half-load" case. Moreover, SCR system seems to support an agglomeration of particles, with beneficial effect on the harmfulness to human health.