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Jan 7, 2010 - Impact of Low- and High-Oxidation. Diesel Particulate Filters on. Genotoxic Exhaust Constituents. NORBERT V. HEEB,* , †. PETER SCHMID,...
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Environ. Sci. Technol. 2010, 44, 1078–1084

Impact of Low- and High-Oxidation Diesel Particulate Filters on Genotoxic Exhaust Constituents N O R B E R T V . H E E B , * ,† P E T E R S C H M I D , † MARTIN KOHLER,† ERIKA GUJER,† MARKUS ZENNEGG,† DANIELA WENGER,† ADRIAN WICHSER,† ANDREA ULRICH,† URS GFELLER,‡ PETER HONEGGER,§ KERSTIN ZEYER,§ LUKAS EMMENEGGER,§ JEAN-LUC PETERMANN,| JAN CZERWINSKI,| THOMAS MOSIMANN,⊥ MARKUS KASPER,⊥ AND ANDREAS MAYER# Empa, Swiss Federal Laboratories for Materials Testing and Research, Laboratory for Analytical Chemistry, Laboratory for Solid State Chemistry and Catalysis, Laboratory for Air ¨ berlandstrasse 129, Pollution/Environmental Technology, U CH-8600 Du ¨ bendorf, Switzerland, UASB, University of Applied Sciences Biel, Laboratory for Exhaust Emission Control, Gwerdtstrasse 5, CH-2560 Nidau, Switzerland, Matter Engineering AG, Bremgarterstrasse 62, CH-5610 Wohlen, Switzerland and TTM, Technik Thermischer Maschinen, Fohrho¨lzlistr. 14b, CH-5443 Niederrohrdorf, Switzerland

Received June 30, 2009. Revised manuscript received November 20, 2009. Accepted December 3, 2009.

Diesel exhaust contains several genotoxic compounds that may or may not penetrate diesel particulate filters (DPFs). Furthermore, the DPF-supported combustion of soot and adsorbed compounds may lead to the formation of additional pollutants. Herein, we compare the impact of 14 different DPFs on emissions of known genotoxic compounds. During a four year period, these DPFs were tested on a heavy duty diesel engine, operated in the ISO 8178/4 C1 cycle. Integral samples, including gas-phase and particle-bound matter were taken. All DPFs were efficient wall-flow filters with solid particulate number filtration efficiencies η > 98%. On the basis of their CO, NO, and NO2 emission characteristics, two different filter families were distinguished. DPFs with high oxidation potential (hox, n ) 8) converted CO and NO besides hydrocarbons, whereas low oxidation potential DPFs (lox, n ) 6) did not support CO and NO oxidation but still converted hydrocarbons. LoxDPFs reduced NO2 from 1.0 ( 0.3 (engine-out) to 0.42 ( 0.11 g/kWh (η ) 0.59), whereas hox-DPFs induced a NO2 formation up to 3.3 ( 0.7 g/kWh (η ) -2.16). Emissions of genotoxic PAHs decreased for both filter families. Conversion efficiencies * Corresponding author: phone: +41 44 823 4257; fax: +41 44 823 4041; e-mail: [email protected] (N.V.H). † Swiss Federal Laboratories for Materials Testing and Research, Laboratory for Analytical Chemistry. ‡ Swiss Federal Laboratories for Materials Testing and Research, Laboratory for Solid State Chemistry and Catalysis. § Swiss Federal Laboratories for Materials Testing and Research, Laboratory for Air Pollution/Environmental Technology. | University of Applied Sciences Biel, Laboratory for Exhaust Emission Control. ⊥ Matter Engineering AG. # TTM, Technik Thermischer Maschinen. 1078

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varied for individual PAHs and were lower for lox- (η ) 0.31-0.87) than for hox-DPFs (η ) 0.75-0.98). Certain nitroPAHs were formed indicating that nitration is an important step along PAH oxidation. For example, 1-nitronaphthalene emissions increased from 11 to 17 to 21 µg/L without, with lox-, and hox-DPFs respectively, whereas 2-nitronaphthalene emissions decreased from 25 to 19 to 4.7 µg/L. In contrast to our expectations, the nitration potential of lox-DPFs was higher than the one of hox-DPFs, despite the intense NO2 formation of the latter. The filters converted most genotoxic PAHs and nitroPAHs and most soot particles, acting as carriers for these compounds. Hox-DPF exhaust remains oxidizing and therefore is expected to support atmospheric oxidation reactions, whereas lox-DPF exhaust is reducing and consuming oxidants such as ozone, when mixed with ambient air.

Introduction Diesel Exhaust, a Source of Genotoxic Compounds. Longterm exposure to diesel exhaust induces various forms of cancer (1, 2). Several polycyclic aromatic hydrocarbons (PAHs) and nitro-PAHs found in diesel exhaust are genotoxic, either acting as mutagens or carcinogens proliferating the development of cancer (3, 4). In several steps, these PAHs are metabolically activated in the cytoplasm of cells by various cytochrome P450-dependent enzymes to form epoxides and hydroxylated PAHs (5). The aryl hydrocarbon receptor, a ligand-inducible transcription factor, actively transports PAHs into the cell nucleus (6). Some of these metabolically activated PAHs bind to DNA and interfere with transcription and regulation processes of cells. Diesel exhaust also contains large numbers of soot nanoparticles, typically 1012 to 1013 particles/m3 exhaust, besides these genotoxic compounds or their precursors. Acting like Trojan horses, these inhalable particles may transport genotoxic compounds across the alveolar membrane. The translocation of synthetic nanoparticles smaller than 200 nm across the alveolar membrane and across walls of red blood cells was reported lately (7). Technologies for Soot Filtration and Combustion. Heavyduty diesel engines are used on roads, rails, and waterways for transportation of goods and people. Increasingly, passenger cars and light-duty vehicles are operated with diesel engines and perspectives for the U.S. market also point in this direction. Furthermore, diesel engines are used off roads, in farming-, and construction-machinery, and mining equipment. Diesel particulate filters (DPFs) are considered as the key technology to detoxify diesel exhaust. Their impact may be comparable to the one of three-way catalysts introduced to gasoline vehicles in the 1970’s. Over the last years, a variety of different DPF technologies were developed, differing in catalyst and substrate materials and regeneration strategies. Several DPF families can be distinguished, for example porous or fibrous substrates coated with catalysts, or uncoated structures, accumulating so-called fuel-borne catalysts (FBCs), and filters relying on active regeneration strategies such as burners. Highly efficient wall-flow filters, which force the entire exhaust through porous substrates, are now available, but open-structured filters with low filtration efficiencies have been commercialized too. The filtration of solid particles requires a properly designed filter media, offering optimal flow conditions to support particle impactation and, more importantly, diffusive particle adsorption. Well designed filters reach excellent solid particle number filtration efficiencies above 99% (8). To avoid an increase of exhaust back pressure, complete filter regenera10.1021/es9019222

 2010 American Chemical Society

Published on Web 01/07/2010

tion is important. Soot combustion is accelerated by transition-, noble-, and rare earth-metals or their oxides. Two strategies are currently used: FBCs or catalytic coatings. FBC precursors are added to diesel fuel, either suspended in colloidal form or dissolved as metal-organic compounds. The active catalyst is formed during combustion and deposited in the filter. Alternatively, the catalyst material is coated directly on the filter substrate. At temperatures >500 °C, soot is oxidized by oxygen, or by reactive oxygen of certain metal oxides. Some catalysts convert nitric oxide (NO) to nitrogen dioxide (NO2), which is a powerful oxidizing agent also supporting the soot oxidation. In other words, we are now confronted with a rapidly increasing variety of different DPF technologies. It was thus our intention to assess benefits and risks of various DPFs in a comprehensive and comparable manner. Cost, durability, and robustness of DPFs are relevant, when introducing such technologies, but from a toxicological and environmental point of view their impact on emissions of solid particles and genotoxic compounds are important too (9-11). In this context, several test procedures (VERT procedures), now available as a Swiss standard (12), were developed. They include testing of (i) filtration efficiency, (ii) durability, (iii) impact on genotoxic compounds, and (iv) risks for secondary poisoning. Herein, we compare the effects of 14 DPFs on emissions of major exhaust components and on genotoxic compounds. We hypothesized that DPFs (i) lower PAH and nitro-PAH emissions, assuming that both classes of compounds are adsorbed, or (ii) convert PAHs to nitro-PAHs, if nitration chemistry is supported, or (iii) induce the de novo formation of both PAHs and nitro-PAHs via soot decomposition and subsequent nitration. Thus, depending on the contributions of these processes, DPFs would either reduce or increase the genotoxicity of diesel exhaust. With the exception of one filter, all tested DPFs were VERT-approved (13), indicating that they all reach solid particle number filtration efficiencies >98%. The impact of these filters on genotoxic compounds is discussed below.

Experimental Section Engine, Test Cycle, Fuels, Lubricants. All engine tests were carried out on a heavy duty diesel engine with direct fuel injection (Liebherr, type 914 T, 6.11 L, 4 cylinders, 105 kW, Bulle, Switzerland). The engine was operated in the 8-stage ISO 8178/4 C1 test cycle as described in detail in the supporting material (Figure S1 of the Supporting Information). Over four years, four commercial low-sulfur diesel fuels (SN 181190-1:2000, class D) with sulfur contents of 33 ( 32 mg/kg were used. Further details on fuels and lubricants are given in Table S1 of the Supporting Information. Particulate Filters, Catalysts, Test Configurations. Table S2 of the Supporting Information reports data on filter characteristics, catalyst-type, classification based on their oxidation potential and conversion of major exhaust constituents. The different test configurations performed with coated and FBC-based DPFs are described in the supporting material together with engine and filter preconditioning procedures. Exhaust Sampling, Workup, Chemical Analysis. Sampling and analyses of the major exhaust constituents and the genotoxic compounds have been described in detail elsewhere (9, 10, 14). In accordance with EN 1948-1 (15), typically 5-7 m3 of undiluted exhaust was collected with the all-glass sampling arrangement shown in Figure S2 of the Supporting Information and is described in detail in the supporting material. The examined samples include compounds present in the gas, liquid, and particle phases and represent emissions at steady-state and at transient operation. Analyses of genotoxic compounds were performed by gas chromatog-

raphy and high-resolution mass spectrometry. Experimental conditions and reference materials used for quantification are also given as Supporting Information. Quality Assurance, Recovery Rates, Nitration Artifacts. Nitration of PAHs can occur during combustion and in DPFs, but also during sampling and cleanup. The latter two would be considered as unwanted artifacts. To test the extent of nitration during sampling and cleanup, 13C-labeled naphthalene, phenanthrene, and pyrene were spiked to the device prior to sampling and recoveries for these compounds and the extent of 13C-labeled nitro-PAH formation during sampling and cleanup were determined. Recovery rates and nitration artifacts are discussed later on and are given in detail as Supporting Information as well.

Results and Discussion A heavy duty diesel engine (6.11 L, 105 kW) was used as platform for all filter tests. Mean engine load (55.7 ( 0.4 kW, n ) 66), fuel consumption (231.9 ( 2.6 g/kWh, n ) 66), and amount of exhaust (556 ( 22 m3/kWh, n ) 66) varied within less than 4% during the four year period. In first priority, DPFs were evaluated for potential applications in construction machinery, where extended engine operation at few working points is prevailing. The ISO 8178/4 C1 test cycle, valid for such machinery, reflects this. The torque-load diagram and typical exhaust temperatures are given as Supporting Information (Figure S2). This cycle is not considered as the most representative for on-road applications. However, wall-flow DPFs were found to be comparably efficient with respect to solid particle removal both under transient and steady-state conditions. DPF Classification Based on Oxidation Potential. Table S2 of the Supporting Information lists some properties of the tested DPFs of which seven had coated substrates and seven were used in combination with FBCs. All DPFs were wallflow filters efficiently removing particles by 99.43 ( 0.62% (8). In other words, the different filter concepts were not distinguishable with respect to their filtration efficiency. However, when comparing their emissions of volatile compounds, remarkable differences were noticed, which were used to classify the filters according to the chemistry they supported rather than to the components they were madeof. We used certain exhaust constituents as marker compounds to probe for the oxidation potential of a given DPF. All DPFs can be assigned to two families with respect to their CO conversion efficiency (Table S2 of the Supporting Information). Six filters did not convert CO (η ) -0.12), whereas the others supported CO oxidation (η ) 0.90, Figure 1). The latter are therefore classified as filters with high oxidation potential (hox), and the former have a lower oxidation potential (lox). It is noteworthy that FBC-DPFs (Figure 1, filled circles) and coated DPFs (open diamonds) are found in both filter families. The oxidation potential also affects the hydrocarbon (HC) conversion. Median HC emissions decreased from 0.37 to 0.27 to 0.05 g/kWh without, with lox-, and hox-DPFs respectively, corresponding to conversion efficiencies of η ) 0.27 and 0.86. Large effects were noticed for NO2 emissions, which decreased from 1.0 to 0.4 g/kWh when comparing engine-out and lox-DPF data (η ) 0.59), but increased to 3.3 g/kWh for hox-DPFs (η ) -2.16). NO emissions of 8.2 (η ) -0.03) and 6.4 g/kWh (η ) 0.20) were noticed for lox- and hox-DPFs, which compares with engine-out emissions of 8.0 g/kWh. This indicates that hox-DPFs not only oxidize CO to CO2 but also converted NO to NO2, whereas lox-DPFs neither support CO nor NO oxidation but consumed some of the engine-out NO2. No significant effects were observed when comparing NOxand fuel consumption data (Figure 1). On average, 231.9 ( VOL. 44, NO. 3, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Effects of DPFs on emissions of major exhaust constituents and on fuel consumption. Box plots display median, 10%-, 90%-percentiles, minimum, and maximum emission factors (g/kWh) for CO, HC, NOx, NO, NO2, and consumed fuel without (ref), with low(lox), and high-oxidation potential (hox) DPFs. Median conversion efficiencies are given. Negative values indicate a net formation, positive a net conversion. Conversion efficiencies (η) of individual DPFs are plotted versus their CO conversion (ηCO). Coated (]) and fuel-borne catalyst DPFs (•) are distinguished. 2.6 (n ) 37) and 233.0 ( 2.4 (n ) 29) g fuel/kWh were consumed without and with DPFs. In summary, hox-DPFs efficiently removed reducing agents such as CO and hydrocarbons but form considerable amounts of oxidants such as NO2, which is a toxic irritant. Lox-DPFs do not affect CO levels, still remove hydrocarbons, and consume most of the engine-out NO2. Depending on the application, filters from one or the other family are more appropriate. At Swiss workplaces, more stringent air quality limits have to be fulfilled for NO2 than for NO (6 vs 30 mg/m3). Impact on Genotoxic Exhaust Constituents. DPFs are expected to alter the genotoxicity of diesel exhaust. We hypothesize that DPFs remove both PAHs and nitro-PAHs, or convert PAHs to nitro-PAHs, or support the formation of both PAHs and nitro-PAHs. When considering the distinct impact of lox- and hox-DPFs on NO and NO2, we questioned if they also affect nitration chemistry differently. 1080

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Figure 2 represents the chemical structures of all investigated PAHs and nitro-PAHs. Compounds 3-8 are classified as carcinogenic to humans by the WHO (3), whereas pyrene (1) and fluoranthene (2) are both precursors for nitropyrenes and nitrofluoranthenes, of which several isomers for example 14 and 15 are classified as direct-acting mutagens (4). Box plots in Figure 3 display median, 10%-, 90%percentile, minimum, and maximum emission factors in g/Liter consumed fuel for PAHs 1-8 in configurations without (ref, engine-out) and with lox- and hox-DPFs. As a general trend, all DPFs lowered the emissions of the investigated PAHs. Conversion efficiencies varied, depending on both PAH volatility and reactivity against oxidants such as NO2 (Table S3 of the Supporting Information). For example, pyrene (1) and fluoranthene (2) are both 4-ring PAHs with identical elemental composition (C16H10) but different vapor pressures of 6.0 × 10-4 and 1.2 × 10-3 Pa (25 °C). Pyrene and fluoranthene conversion efficiencies of 0.70 and 0.31 were noticed for lox-DPFs. The lower vapor pressure and with it the lower volatility of pyrene explains the stronger retention in the filter compared to fluoranthene. Furthermore, the reactivity of a given PAH against the various oxidizing agents is important, as can be noticed for chrysene (3) and benz(a)anthracene (4), which have the same composition (C18H12) but different vapor pressures of 8.5 × 10-7 and 2.8 × 10-5 Pa (25 °C). Lox-DPFs converted chrysene (3) with 70% efficiency only, even though its vapor pressure is considerably lower than the one of benz(a)anthracene (4), which was removed with 87% efficiency. We assume that chrysene (3) must be less reactive than benz(a)anthracene (4). In other words, volatile and inert PAHs are not removed as efficient as nonvolatile and highly reactive PAHs. Volatility and reactivity also influence the conversion in hox-DPFs and the same trends were observed in principle (Figure 3). However, because these filters, on average, were more efficient, effects on individual PAHs were less pronounced. Consequently, we expect that the average PAH pattern of diesel vehicle emissions will change with increasing DPF use. Pyrene/fluoranthene ratios decreased form 2.6 to 1.1 to 0.6 without, with lox-, and hox-DPFs respectively, and chrysene/benz(a)anthracene ratios increased from 1.8 to 4.2 and 2.3. Similar effects were noticed when introducing catalytic converters to gasoline-fueled vehicles. With increasing shares of three-way-catalyst vehicles, benzene/ alkylbenzene ratios increased (16, 17). It was noticed that alkylbenzenes are converted more efficiently in TWCs than benzene (18, 19); the latter can even form de novo under certain conditions (20, 21). Figure 3 also shows PAH conversion efficiencies (ηPAH) for individual DPFs against their CO conversion (ηCO). Efficiencies varied but were always positive. Thus soot combustion in DPFs does not support PAH formation, as hypothesized. Most lox-DPFs relied on FBCs (•), whereas coated DPFs (]) dominated the hox-DPF family. However, conversion efficiencies of a given DPF did not primarily depend on the way catalysts are deposited on the substrate but depend more on its oxidation potential. DPFs with highest oxidation potentials or in other words with highest CO and NO conversion and strongest NO2 formation also showed highest PAH conversion efficiencies (Figure 3). PAHs seem to be more susceptible to oxidation processes, than other hydrocarbons, which were converted with lower efficiencies (Figure 1). DPF-Induced Nitration Chemistry. Of the investigated nitro-PAHs 9-16 (Figure 2), 3-nitrofluoranthene (14), and 1-nitropyrene (15) are both direct-acting mutagens in the Ames Salmonella typhimurium TA98 reversion assay (22). The latter is also a precursor for 1,3-, 1,6-, and 1,8dinitropyrenes, which are among the most potent directacting mutagens known (22). Nitro-PAHs can form from precursor PAHs via nitration in the gas, liquid, or solid phases.

FIGURE 2. Chemical structures of investigated aryl hydrocarbons, pyrene (1), fluoranthene (2), chrysene (3), benz(a)anthracene (4), benzo(b)fluoranthene (5), benzo(k)fluoranthene (6), benzo(a)pyrene (7), indeno(1,2,3-cd)pyrene (8), 1-nitronaphthalene (9), 2-nitronaphthalene (10), 3-nitrophenanthrene (11), 9-nitrophenanthrene (12), 9-nitroanthracene (13), 3-nitrofluoranthene (14), 1-nitropyrene (15), 4-nitropyrene (16). Compounds 3-8 are carcinogenic and 14 and 15 are mutagenic according to the WHO (14, 15). Different nitrating species such as NO2 or NO3 radicals, nitrosyl- (NO+), and nitronium (NO2+) ions, or HNO2 and HNO3 may be involved. Hydroxyl radicals (HO) may activate PAHs in a first step, followed by the addition of the nitrating species. This two-step chemistry is observed in ambient air (23-26). In principle, DPFs can support all of these processes. At temperatures up to 500 °C, even larger PAHs may volatilize leading to gas-phase reactions not observed at ambient conditions. Vapor pressures of nitro-PAHs are lower than those of the parent PAHs. Therefore, one expects longer residence times and higher conversion efficiencies, unless nitro-PAHs are formed de novo. Both filter families can support nitration reactions but may also catalyze the nitroPAH degradation. If the NO2 concentration is a limiting factor, one would expect higher nitration potentials in hox-DPFs. Again, also a further oxidation of nitro-PAHs may be catalyzed by higher NO2 concentrations. It remains to be shown, which of these competing processes dominate in DPFs. Both nitronaphthalene isomers (9, 10) are found in engineout exhaust (Figure 3, lower diagrams). With vapor pressures of 3.2 × 10-2 Pa (25 °C), one would expect filtration efficiencies, similar to the one of fluoranthene (1.2 × 10-3 Pa). Whereas 1-nitronaphthalene (9) was formed de novo in lox- (η ) -0.47) and hox-DPFs (η ) -0.84), 2-nitronaphthalene (10) was converted with η ) 0.26 and 0.81 efficiency, which is similar to fluoranthene (η ) 0.31 and η ) 0.91). Similarly, 9-nitrophenanthrene (12) is formed in both filter families (η ) -6.5 and η ) -1.2), whereas 3-nitrophenanthrene (11) is converted in hox-DPFs (η ) 0.62) and formed to some degree in lox-DPFs (η ) -0.11). A third interesting pair is 1- and 4-nitropyrene (15 and 16); the latter is degraded in most DPFs, and the former is obtained in some cases. Certain nitro-PAHs are indeed formed in DPFs, but others are converted. PAH nitration is more pronounced in loxthan in hox-DPFs. Therefore, the NO2 concentration is not

a limiting factor and engine-out levels are sufficiently high to support PAH nitration in DPFs. We conclude that nitration reactions are important pathways to remove PAHs and possibly also nitro-PAHs. Both classes of compounds offer extended π-electron systems, susceptible to electrophiles such as NO2. Most DPFs induced the formation of 1-nitronaphthalene, 9-nitrophenanthrene, and 1-nitropyrene among others. Consequently, the resulting nitro-PAH profiles changed as well. Ratios of 1-/2-nitronaphthalene increased from 0.5 to 0.9 to 4.4 without, with lox-, and hox-DPFs, respectively. Ratios of 3-/9-nitrophenanthrene decreased from 2.5 to 0.4 and 1-/4-nitropyrene ratios increased from 16 to 27 without and with DPFs. Both DPF families lowered 3-nitrofluoranthene (14) emissions with efficiencies of η ) 0.21 and 0.89, whereas 1-nitropyrene (15) was removed in hox-DPFs (η ) 0.66) but formed in some lox-DPFs (η ) -0.63). This is of toxicological relevance because both nitro-PAHs are mutagenic (4). Nitration of PAHs can also occur during sampling. The use of 13C-labeled precursor PAHs is recommended to verify the extent of nitration during sampling and cleanup. In the case of nitronaphthalenes and nitrophenanthrenes, sampling artifacts can be excluded because none or very little of the given 13C6-labeled PAHs were nitrated. Nitration of pyrene is more of an issue. No labeled 2- and 4-nitropyrenes were detected, but 15 ( 16%, 22 ( 28%, and 29 ( 18% of 13C3labeled 1-nitropyrene was found in samples without, with lox-, and hox-DPFs, respectively. We conclude that nitration artifacts can occur and should be investigated but did not bias our findings. Environmental Perspectives. The data set, comprising 14 different DPFs, reveals two important filter families with distinct emission characteristics. High oxidation potential DPFs remove most reductants but support the formation of strong oxidants such as NO2. Lox-DPF exhaust still contains VOL. 44, NO. 3, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. Effects of DPFs on PAH (upper diagrams) and nitro-PAH emissions (lower diagrams). Box plots display median, 10%-, 90%-percentiles, minimum, and maximum emission factors (g/L fuel) of individual aryl hydrocarbons without (ref), with low- (lox), and high-oxidation potential (hox) DPFs. Median conversion efficiencies are given. Negative values indicate a net formation, positive a net conversion. Conversion efficiencies (η) of individual DPFs are plotted versus their CO conversion (ηCO). Coated (]) and fuel-borne catalyst DPFs (•) are distinguished. considerable amounts of reducing agents with little oxidants left. Mixing these exhausts with ambient air will affect atmospheric redox-chemistry differently. Higher NO2 levels enhance photochemical smog formation during daytime and support nitration chemistry at nighttime. Ozone levels are expected to increase. Such effects were already noticed when diesel oxidation catalyst (DOC) vehicles were introduced to the Swiss car fleet (27). With increasing shares of such vehicles, which release considerable amounts of NO2, ambient air ozone was not quenched as efficiently by the exhaust plume as before. Therefore, both O3 and NO2 levels increased over time. It is worthwhile to compare exhaust of diesel- and gasoline-fueled vehicles. The latter are equipped with three1082

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way catalysts today, producing exhausts without oxidants such as NO2 but still containing reductants such as CO, hydrocarbons, H2, NO, and NH3 (28, 29). Mixing such exhausts with those of diesel vehicles equipped with DOCs and hoxDPFs likely support the secondary formation of ammonium nitrate particles, typically found in ambient air particulate matter. Nitration reactions are important transformation processes in DPFs and engine-out NO2 concentrations are sufficiently high to support such chemistry. Most DPFs supported the formation of certain nitro-PAHs but converted others. Therefore, one cannot exclude that some additional toxic compounds not studied herein were formed. In some

cases, we compared the biological activity of filtered and unfiltered exhaust with different in vitro reporter gene bioassays (14, 30). One assay is based on rat liver cancer cells, which express the aryl hydrocarbon receptor (AHR). The other assay relies on human breast cancer cells, expressing the estradiol receptor (ER) sensitive to hormonelike compounds. In all cases, AHR and ER responses were lowered by 80-90% and 55-66% when DPFs were applied, which is comparable to efficiencies reported herein. In addition to these assays, which responded on both PAHs and nitro-PAHs, other bioassays such as the Ames test are interesting alternatives to probe for other toxic compounds. Although we cannot exclude that other genotoxic compounds, not yet searched for and identified, may form in DPFs, we conclude that all tested filters converted all examined PAHs and most nitro-PAHs. The de novo formation of some nitro-PAHs did not outrange the overall removal of aromatic hydrocarbons as indicated by the AHR bioassay. Wall-flow DPFs have evolved to a mature environmental technology. Further improvements are needed with respect to the secondary formation of NO2. Considering the efficient removal of many genotoxic compounds and the almost complete elimination of solid nanoparticles, DPFs are now considered as effective technologies to detoxify diesel exhaust. More than 30 different DPFs are now VERT-approved (13). Most of the evaluated filters fulfilled all criteria. They remove nanoparticles by more than 98%, lower emissions of genotoxic compounds, and do not support the formation of new pollutants to a large extent. On the basis of these findings, approved DPFs are now considered as best available technology to lower the genotoxicity of diesel exhaust. Consequently, Swiss occupational health authorities (SUVA) monitoring respiratory air quality at workplaces, forced the use of DPFs for construction engines (31) and the Swiss clean air act limits particle number emissions of construction machinery to 1012 particles/kWh, which can be fulfilled with efficient DPFs only (32). New machines have to meet this standard by 2009 (>37 kW) and 2010 (18-37 kW) respectively, and in-use machines of the years 2000-2008 (>37 kW) have to be retrofitted by May first, 2010. A particle number-based emission limit of 6 × 1011 particles/km becomes effective at the Euro 5 and Euro 6 stage for all categories of diesel vehicles (33).

Acknowledgments We would like to acknowledge the support and good collaboration with all filter suppliers. We also thank the Swiss Federal Office for the Environment for financial support.

Supporting Information Available Figures displaying the ISO 8178/4 C1 cycle, pre- and postDPF exhaust temperatures, and the sampling device. Tables reporting DPF-, fuel-, and emission data on major combustion products, PAHs, and nitro-PAHs. This material is available free of charge via the Internet at http://pubs.acs.org.

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