Secondary Effects of Catalytic Diesel Particulate Filters: Conversion of PAHs versus Formation of Nitro-PAHs

Abstract

Diesel particulate filters (DPFs) are a promising technology to detoxify diesel exhaust. However, the secondary combustion of diesel soot and associated compounds may also induce the formation of new pollutants. Diesel soot is rated as carcinogenic to humans and also acts as a carrier for a variety of genotoxic compounds such as certain polycyclic aromatic hydrocarbons (PAHs) or nitrated PAHs (nitro-PAHs). Furthermore, diesel exhaust contains considerable amounts of nitric oxide (NO), which can be converted to more powerful nitrating species like nitrogen dioxide (NO2), nitric acid (HNO3), and others. This mix of compounds may support nitration reactions in DPFs. Herein we report effects of two cordierite-based, monolithic, wall-flow DPFs on emissions of genotoxic PAHs and nitro-PAHs and compare these findings with those of a reporter gene bioassay sensitive to aryl hydrocarbons (AHs). Soot combustion was either catalyzed with an iron- or a copper/iron-based fuel additive (fuel-borne catalysts). A heavy duty diesel engine, operated according to the 8-stage ISO 8178/4 C1 cycle, was used as test platform. Emissions of all investigated 4- to 6-ring PAHs were reduced by about 40-90%, including those rated as carcinogenic. Emissions of 1- and 2-nitronaphthalene increased by about 20-100%. Among the 3-ring nitro-PAHs, emissions of 3-nitrophenanthrene decreased by about 30%, whereas 9-nitrophenanthrene and 9-nitroanthracene were found only after DPFs. In case of 4-ring nitro-PAHs, emissions of 3-nitrofluoranthene, 1-nitropyrene, and 4-nitropyrene decreased by about 40-60% with DPFs. Total AH-receptor (AHR) agonist concentrations of diesel exhaust were lowered by 80-90%, when using the iron- and copper-based DPFs. The tested PAHs accounted for < 1% of the total AHR-mediated response, indicating that considerable amounts of other aryl hydrocarbons must be present in filtered and unfiltered exhaust. We conclude that both DPFs detoxified diesel exhaust with respect to total aryl hydrocarbons, including the investigated carcinogenic PAHs, but we also noticed a secondary formation of selected nitro-PAHs. Nitration reactions were found to be stereoselective with a preferential substitution of hydrogen atoms at peri-positions. The stereoisomers obtained are related to combustion chemistry, but differ from those formed upon atmospheric nitration of PAHs.

Isolation and identification of new vasodilative substances in diesel exhaust particles

“…Many automobile companies either in Japan or European countries are competitively developing modern Diesel cars equipped with different types of exhaust treatment such as a particulate trap/filter to detoxify diesel exhaust. Swiss researchers very recently investigated the secondary effects of catalytic diesel particulate filters on the coversion of polyaromatic hydrocarbons (PAHs) versus formation of nitro-polyaromatic hydrocarbons (nito-PAHs), whereby PAHs were reduced by using filters, while they noticed the formation of selected nitro-PAHs (Norbert V. Heeb et al 2008). Although our present investigation was performed on the DEP collected from an engine without using a filter, we have demonstrated significant amounts of vasodilative nitro compounds are consisted in DEP, and their hazardous effects on human health cannot be ignored because of the vast amount of DEP emitted into the environment.…”

Functional group composition of ambient and source organic aerosols determined by tandem mass spectrometry

“…The carbonyl functionalisation rate is twice lower, ranging from 0.3 to 0.5% (Table 2). Compared to the other samples, the large nitro content may be linked to the high temperatures of the combustion processes occuring in the vehicle engines (Matthews, 1980;Eberhardt et al, 2003) and can be partially attributed to nitro-PAHs, previously identified in diesel exhaust OA (Heeb et al, 2008). The low content in functional groups containing oxygen atoms (carbonyl and carboxylic acid) is also consistent with the moderate water soluble organic carbon (WSOC) fraction measured, which accounts for 20% of the OC measured inside the tunnel (El Haddad et al, 2009) and with the hydrophobic propreties of fresh vehicular exhaust soot particles previously described in the literature (Weingartner et al, 1997).…”

Analytical Methodologies for the Control of Particle-Phase Polycyclic Aromatic Compounds from Diesel Engine Exhaust

“…The majority of experiments are performed on chassis dynamometers under predetermined engine load; a heated hose transfers the exhaust from the tailpipe to full flow exhaust dilution tunnels, and therefore, the exhaust gases are collected at low temperature after being mixed and homogenised with the ambient air (with moderate dilution ratios ranging from 6 to 14) in the dilution tunnel, which is connected to a constant volume sampling system (CVS technique) [45,46]. Chassis dynamometers can also be equipped with dilution systems such as critical-flow Venturi dilution tunnels [47], ejection dilutors [48], or rotating disk diluters [49], but undiluted exhaust can also be directly collected through a sampling probe inserted inside the tailpipe, which is a less conventional method [50].…”