Environ. Sci. Technol. 2009, 43, 3905–3912
Oxidative Potential of Semi-Volatile and Non Volatile Particulate Matter (PM) from Heavy-Duty Vehicles Retrofitted with Emission Control Technologies SUBHASIS BISWAS,† VISHAL VERMA,† JAMES J. SCHAUER,‡ FLEMMING R. CASSEE,§ ARTHUR K. CHO,| AND C O N S T A N T I N O S S I O U T A S * ,† Department of Civil and Environmental Engineering, University of Southern California, 3620 South Vermont Avenue, Los Angeles, California 90089, Environmental Chemistry and Technology Program, University of Wisconsin-Madison, 660 North Park Street, Madison, Wisconsin 53706, Center for Environmental Health Research, National Institute for Public Health and the Environment (RIVM), The Netherlands, and Department of Molecular and Medical Pharmacology, School of Medicine, University of California Los Angeles, Los Angeles, California 90095
Received January 12, 2009. Revised manuscript received March 11, 2009. Accepted March 27, 2009.
Advanced exhaust after-treatment devices for diesel vehicles are less effective in controlling semivolatile species than the refractory PM fractions. This study investigates the oxidative potential (OP) of PM from vehicles with six retrofitted technologies (vanadium and zeolite based selective catalytic reduction(V-SCRT,Z-SCRT),Continuouslyregeneratingtechnology (CRT), catalyzed DPX filter, catalyzed continuously regenerating trap (CCRT), and uncatalyzed Horizon filter) in comparison to a “baseline” vehicle (without any control device). Vehicles were tested on a chassis dynamometer at three driving conditions, i.e., cruise, transient urban dynamometer driving schedule (UDDS), and idle. The consumption rate of dithiothreitol (DTT), one of the surrogate measures of OP, was determined for PM samples collected at ambient and elevated temperatures (thermally denuded of semivolatile species). Control devices reduced the OP expressed per vehicle distance traveled by 60-98%. The oxidative potential per unit mass of PM however, was highest for the Horizon followed by CRT, DPX -Idle, SCRTs, and baseline vehicles. Significant reduction in OP (by 50-100%) was observed for thermally denuded PM from vehicles with retrofitted technologies (PM with significant semivolatile fraction), whereas particles emitted by the baseline vehicle (with insignificant semivolatile fraction) did not demonstrate any measurable changes in oxidative activity. This suggests that the semivolatile fraction of particles are far more oxidative in nature than refractory particlessa conclusion further supported * Corresponding author phone: 213 740 6134; fax: 213 744 1426; e-mail:
[email protected]. † University of Southern California. ‡ University of Wisconsin-Madison. § National Institute for Public Health and the Environment (RIVM). | University of California Los Angeles. 10.1021/es9000592 CCC: $40.75
Published on Web 04/10/2009
2009 American Chemical Society
by previous tunnel and ambient studies, demonstrating a decline in PM oxidative activity with increasing atmospheric dilution. Correlation analysis performed between all the species, showed that OP is moderately associated (R ) 0.76) with organic carbon (OC) and strongly associated (R ) 0.94) with the water-soluble organic carbon (WSOC).
Introduction A number of population based epidemiological studies as well as recent toxicological and clinical studies indicate a strong association between particulate matter (PM) exposure and adverse health outcomes (1-3). Particles deposit in different parts of the lung according to their aerodynamic diameter, resulting in varying degree of toxic potency. For example, the ultrafine particle fraction, (UFP < 150nm) may be more toxic per mass basis than the accumulation (150 nm-2.5 µm) or coarse mode (2.5-10 µm) particles (3-5) due to their smaller sizes and higher exposed surface area that becomes available for adsorption of potentially toxic organic species. Heavy-duty diesel vehicles (HHDV) are major emission source of ambient UFPs both in terms of number and mass (6-8). In-vitro and in vivo experiments have linked diesel exhaust particle (DEPs) exposure to airway inflammation, mitochondrial damage and lung cancer (3, 9-13). Chronic exposure of DEPs thus may lead to exacerbation of pulmonary diseases such as asthma and bronchitis as well as lung cancer (11, 14). Alveolar macrophages (AM), which are responsible for the defense of susceptible cells, release pro-inflammatory cytokines in the presence of DEP constituents, such as poly aromatic hydrocarbons (PAHs), and may experience alteration in their gene expressions (15). A few studies have also described negative impacts of DEPs on reproductive systems (16), liver functions (17) and brain activity (18). Despite commendable progress in particle-related toxicological research for the last few decades, the exact mechanisms by which PM inflicts health injuries are still largely unknown and constitute a subject of great interest and very active research for the scientific community. It is generally believed that knowing particle chemical composition along with their physical attributes and source of emissions may be the critical starting point in understanding the fraction within PM that drives the health effects. DEPs are complex mixtures of numerous semivolatile (organics, sulfate) and nonvolatile species (metals, elemental carbon (EC)). These semivolatile compounds, depending on their vapor pressures, may be present in gas and/or particle phases. The thermodynamics of gas-to-particle phase partitioning of these species are strong functions of temperature and dilution (19, 20). Therefore, depending on sampling conditions, the compositions of DEPs are likely to change considerably with atmospheric dilution, which may alter their toxic properties. Semivolatile organic compounds, such as PAHs and their derivatives, are known to possess genotoxic and carcinogenic characteristics (21). Some of these DEP constituents are capable of triggering a chain of biochemical reactions in cells, changing its redox state and thus, exerting oxidative stress. A number of bioassays are formulated based on the redox (oxidative) and electrophilic characteristics of PM species to determine the hazard of DEPs, including the dithiothreitol (DTT) consumption rate, ascorbate (reducing agent in lung fluid lining), dihydroxybenzylamine (DHBA) tests (4, 22, 23). These assays estimate the net chemical reactivity of PM without establishing specific mechanistic pathways for VOL. 43, NO. 10, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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toxicity per se. The DTT test measures the ability of PM to catalyze the reduction of oxygen to superoxide in a simple chemical system. Thus, DTT consumption rate is one of the indicators of oxidative potential (OP) and is relevant to the capacity of PM to induce reactive oxygen species in vivo such as superoxide and hydrogen peroxide production (23-25). Researchers believe that polar organic compounds, such as quinones, oxygenated PAHs and aldehydes present in DEPs, induce hemoxygenase-1 expression and are responsible for imparting oxidative stress in macrophages and epithelial cells (24) With the promulgation of stringent PM mass based emission standards (U.S. 2007, 2010; EURO IV, V), new advanced emission control technologies are being inducted in the current fleet. Although these after treatment devices are remarkably efficient in controlling solid particles, they have varied success in removing potentially dangerous semivolatile species. These semivolatile species after passing through the control devices may nucleate to form fresh particles or condense onto existing particles under favorable dilution and temperature conditions (26-28). The primary objective of this study, therefore, was to measure the oxidative potential of these PM fractions from heavy vehicles operating with and without various emissions control technologies. Apart from assessing the potency of semivolatile species to induce oxidative stress, the results reported here will be useful in assessing the efficacy of these control technologies and in establishing future needs for additional measures to control these PM species.
Materials and Methods Dynamometer Set-up. Experiments were carried on a chassis dynamometer at the California Air Resources Board’s (CARB) heavy-duty diesel emission testing laboratory (HDETL) in Los Angeles. The vehicle-testing plan is explained in detail by Biswas et al. (29) and only an excerpt will be provided here. A total of four vehicles retrofitted with six after-treatment devices were evaluated and compared with a baseline vehicle (Supporting Information Table S1). The baseline vehicle (i) was a 1998 Cummins M11 engine (11 L), in a Kenworth truck, operating without any post engine emission control technology. The same vehicle was also tested with three different retrofitted control technologies: (ii) a continuously regenerating technology (CRT: diesel oxidation catalyst (DOC) + an uncatalyzed trap); (iii) a Zeolite; and (iv) a vanadiumbased selective catalytic reduction (SCR) technologies (ZSCRT and V-SCRT: Zeolite or Vanadium SCR+ CRT). The other three selected technologies were (v) a diesel hybrid electric bus equipped with a catalyzed continuously regenerative trap (CCRT: DOC + catalyzed trap), (vi) an International DT466 engine (7.6 L) in a medium heavy duty truck, retrofitted with an Engelhard DPX filter (diesel particulate trap with catalytic wash-coats), and (vii) a school bus, equipped with an electric particle filter (EPF, Horizon). The EPF consists of a noncatalyzed silicon carbide substrate for PM control, coupled with an electric heating element and a small blower. The trap is regenerated periodically using electrical power from the grid (plug-in configuration) during nonoperational periods. Hereafter, the test fleet is referred as baseline, CRT, V-SCRT, Z-SCRT, CCRT, DPX, and Horizon. It should be noted that only CRT and the two SCRTs are directly comparable with the baseline vehicle. The vehicles were operated at three standard simulated driving conditions, i.e., steady state cruise (80 kph), transient (EPA urban dynamometer driving schedule (UDDS) (30)) and idling. Ultra low sulfur diesel (BP) fuel certified to have 90%) in mass as well as most of the chemical constituents was achieved with the introduction of control technologies for all the driving cycles. DTT. Blank-subtracted (filter blank DTT∼0-0.0005 nmole µg-1 min-1) DTT consumption rates, normalized per unit mass of PM are reported in Figure 1a-c for various vehicle configurations. The vehicle with the Horizon trap had the highest per mass OP (0.16-0.19 n-mole µg-1 min-1 for cruise and UDDS) irrespective of driving conditions. The DTT consumption rates from both Vanadium and Zeolitebased SCRTs are on the same order of magnitude (0.01-0.02 n-mole µg-1 min-1). It is interesting to note that when the selective catalytic reduction (SCR) section from SCRT is removed from the exhaust stream and the vehicle is operated with only CRT (i.e., the DOC+ uncatalyzed filter), the oxidative potential increased by a factor of almost 3, both for cruise and UDDS cycles. The CCRT (Catalyzed filter + DOC) is the most efficient among the test fleet with DTT rates as low as 0.006 n-mole µg-1 min-1. PM generated during idling (Figure 1 c) is highly oxidative per unit mass. Of particular note is the elevated level of OP for the DPX and Horizon vehicles. The baseline vehicle emitted PM with similar oxidative characteristics to those of newer vehicles with control technologies. Figure 1 also reports the DTT values for the thermo denuded filters. There is a significant reduction (50-100%) in OP as particles are heated to 150 °C and their semivolatile component is removed. The baseline vehicle, however, did not show any alteration in DTT response between denuded and undenuded exhaust stream, except while idling. This is because of the highly refractory nature (mostly soot) of these particles (29, 31). The increased OP of the semivolatile PM fraction is further highlighted by comparing with their corresponding semivolatile PM mass and number fractions (Figure 2). For vehicles with (soot removing) control devices, the semivolatile fraction contributed roughly 70-100% of the net oxidative activity compared to 20-30% for baseline vehicle. The sole exception is the CCRT vehicle, which hardly produced any oxidative activity from its semivolatile fraction, despite emitting 45% of semivolatile PM by mass and 37% by number. For this vehicle (CCRT) the relatively low level of OP has originated from its residual nonvolatile fractions. In general, particle number-based volatility, which is predominantly driven by the evaporation of the sub-50 nm, so-called nucleation mode particles (35), followed the OP trends better than the PM mass volatility. The DTT results (Figure 3a-c) expressed per unit vehicle distance traveled provide a better idea of the total oxidative load imparted on the environment by these vehicles. They also provide a quantitative assessment of the effectiveness of these after treatment devices. While we observed a
FIGURE 1. (a-c) Oxidative potential (DTT consumption in n-moles min-1 µg-1 of PM) of thermo-denuded and undenuded PM. comparable or lower oxidative response of the baseline vehicle per unit mass of PM, its net DTT consumption (per km) is substantially higher (>3 times) than retrofitted vehicles. The OP reduction is more visible for the UDDS than for the cruise cycle. The DTT response from the idling (Figure 3c) registered the highest mitigation (reduction >98%) results. VOL. 43, NO. 10, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Relationship between oxidative activity and semivolatile PM fraction at 150 °C (Showing good correlation of DTT (filled circles) with particle number (hollow diamonds) and poor correlation with PM mass (bars)). Water-Soluble Organic Carbon and DTT. Figure 4 shows the WSOC (% of PM mass) and DTT consumption (per unit PM mass) for vehicle-configurations. DTT and WSOC followed the same trend, irrespective of driving condition or control devices. The school bus (Horizon), which exhibited the highest oxidative activity per unit mass, also emitted the highest fraction of soluble organics (35-83%). Similarly higher WSOC content was found for the higher DTT values of PM produced by the DPX-idle cycle. Limited number of data points (Z-SCRT, Baseline) is available for thermo-denuded (*) WSOC (Figure 4). In general, we notice a slight decrease in the solubility of organics as the aerosol is heated and its semivolatile fraction is removed. Correlation Between Chemical Species and Oxidative Potential. Pearson correlation coefficients for different selected PM species (normalized with PM mass, similar to the reported DTT consumption rate) are shown in Table 1 for undenuded samples. Although regression analysis was performed among all the measured PM chemical constituents, we have reported only the relevant species (dominating the diesel exhaust) including ions (sulfate, nitrate, ammonium etc.), EC, OC, WSOC, PAHs, alkanes (Alk.) and organic acids (OA). Species with similar characteristics were grouped (for example, alkanes, PAHs, organic acids) as our regression focused on relationships with classes of organic compounds that are present in the exhaust. PAHs, which are generally adsorbed onto soot particle surfaces, are highly correlated with EC (R ) 0.98). Good correlations (R ) 0.88) exist between sulfate and ammonium ions and between OC and WSOC (R ) 0.86). Table 1 also shows that the organic acids (sum, limited data) are well correlated with WSOC (R ) 0.98). To assess the contribution of various species to OP, the R values and the associated significance levels (p-value) have been calculated and shown in Table 2 for the correlation of different chemical constituents with DTT values. Detailed list of correlations for various organic species are provided in SITable S3. The DTT was significantly correlated with WSOC (R ) 0.94, p < 0.01) and organic acids (sum, R ) 0.91; p < 0.01) and moderately correlated with organic carbon (R ) 0.76, P ) 0.02). All other species, including inorganic ions, PAHs, EC, alkanes either have low or negative correlation. No correlation analysis was performed between metals 3908
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FIGURE 3. (a-c) DTT consumption per unit distance (in n-moles min-1 km-1) traveled by vehicles for Cruise and UDDS cycles and per hour (in n-moles min-1 hr-1) for Idle. and DTT. Although some of the transition metals (such as Cu) can actively participate in DHBA assay, their contributions are insignificant for DTT tests (23). Moreover, metals and trace element emissions estimated in dynamometer studies do not represent real-world highway driving conditions, since they exclude prominent sources of these species, i.e. tire and engine wear and resuspended road dust.
Discussion This study demonstrates that while reducing a vehicle’s overall oxidative burden, the OP of particles emitted by diesel vehicles may vary drastically with retrofit types. From our previous work (29) we found the school bus (Horizon) to be an efficient test vehicle in controlling both particle mass and number emissions (>90%). However, particles downstream of its trap possess the highest OP on a per PM mass basis. It is important to note that the school bus, equipped only with a noncatalyzed filter and lacking a diesel oxidation catalyst, may be less efficient in controlling semivolatile
FIGURE 4. Oxidative potential (DTT consumption in n-moles min-1 µg-1 of PM) relation to the water-soluble organic carbon (WSOC) of PM from the exhaust.
TABLE 1. Pearson Correlation Coefficient (R) between Chemical PM Constituentsa
EC OC NO3SO42NH4+ K+ ClWSOC Alk. PAHs OA a
EC
OC
NO3-
SO42-
NH4+
K+
Cl-
WSOC
Alk.
PAHs
OA
1.00 0.18 -0.29 -0.48 -0.47 -0.40 -0.42 -0.27 0.84 0.98 -0.40
1.00 -0.19 -0.61 -0.48 0.33 0.44 0.86 0.25 0.22 0.18
1.00 0.05 -0.02 0.13 -0.01 -0.01 -0.37 -0.39 -0.15
1.00 0.88 -0.21 0.04 -0.36 -0.38 -0.43 -0.20
1.00 -0.12 0.32 -0.39 -0.32 -0.41 -0.24
1.00 0.33 0.49 -0.47 -0.41 0.17
1.00 0.60 -0.25 -0.37 -0.32
1.00 0.08 -0.13 0.98
1.00 0.64 -0.22
1.00 -0.34
1.00
Note: OA: Organic acids; Alk: alkanes.
TABLE 2. Correlation Coefficient (R) and Significance Level (P) for Oxidative Potential (Measured by DTT) and Selected Chemical Species species
R
P
EC OC NO3SO42NH4K+ ClWSOC alkanes (Alk.) PAHs organic acids (OA)
-0.35 0.76 -0.09 -0.32 -0.25 0.43 0.34 0.94 0.03 -0.26 0.91
0.37 0.02 0.77 0.27 0.26 0.20 0.15
