Chemical and Physical Properties of Ultrafine Diesel Exhaust

Measurements were car ried out on particles produced by the heavy duty diesel ... Substrates with cut sizes outside the 10 nm to 320 nm range were not...
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Environ. Sci. Technol. 2006, 40, 5502-5507

Chemical and Physical Properties of Ultrafine Diesel Exhaust Particles Sampled Downstream of a Catalytic Trap M E L I S S A G R O S E , †,‡ H I R O M U S A K U R A I , § JAKE SAVSTROM,† MARK R. STOLZENBURG,† WINTHROP F. WATTS, JR.,† CHRISTOPHER G. MORGAN,| IAN P. MURRAY,| MARTYN V. TWIGG,| DAVID B. KITTELSON,† AND P E T E R H . M C M U R R Y * ,† Department of Mechanical Engineering, University of Minnesota, 111 Church Street SE, Minneapolis, Minnesota 55455, TSI Inc., 500 Cardigan Road, P.O. Box 64384, St. Paul, Minnesota 55126, AIST, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8563, Japan, Johnson Matthey Catalysts, Orchard Road, Royston, Herts, England SG8 5HE

The chemical and physical properties of exhaust particles produced by a Caterpillar 3176 C-12 heavy duty diesel engine equipped with a catalytic trap (CRT) are reported. The engine was operated at 600 Nm and 1500 rpm, using fuels containing 15 and 49 ppm sulfur. A two-stage dilution tunnel designed to simulate the reactions that occur when hot combustion products mix with cooler atmospheric air was used. Particle size distributions were measured using a scanning mobility particle sizer (SMPS) and nanoscanning mobility particle sizer (nano SMPS); a nanomicro-orifice uniform deposit impactor (nano MOUDI) collected size-resolved samples for gravimetric and chemical analysis. A nanometer tandem differential mobility analyzer (nano TDMA) was used to measure the volatility and hygroscopicity of 4-15 nm particles. These measurements confirm that the particles consisted primarily of sulfates.

Introduction Diesel particulate matter is emitted in three usually distinct but overlapping size modes: the nucleation mode, typically 3-30 nm diameter, containing most of the particle number; the accumulation mode, roughly 30-500 nm, containing most of the particle mass; and the coarse mode consisting of larger particles and usually comprising less than 10% of the mass (1). Lubricating oil is a dominant component of the nucleation mode particles produced without aftertreatment (2-4). The EPA promulgated a lower particulate matter emission standard for diesel engines, which takes effect in 2007. The new heavy-duty, on-road emission standard will be 0.0134 g/kwh, a 90% reduction from the current standard. To meet this standard, diesel engine manufacturers may have to use exhaust aftertreatment such as filtration devices. An excellent review of such devices is given by van Setten et al. (5). These devices remove nearly all of the carbonaceous †

University of Minnesota. Currently affiliated with TSI Inc. AIST. | Johnson Matthey Catalysts. ‡

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agglomerates found in the accumulation mode (1). However, many of these filtration devices employ catalysts to enhance the process of regeneration or to burn up the collected particles. One such device, a Johnson-Matthey continuously regenerating trap (CRT), was recently tested on-road under highway cruise conditions (6, 7). Although the device reduced the concentrations of accumulation mode particles to levels indistinguishable from background, it increased emissions of extremely tiny particles in the nucleation mode whenever the load on the engine was high enough to raise the exhaust temperature above about 300 °C. The CRT is known to increase sulfate emissions at high exhaust temperatures by conversion of SO2 emitted by the engine to SO3 and H2SO4 (8). Thus it was hypothesized that the nucleation mode particles would consist mainly of sulfates. In this study we measured the chemical and physical properties of particles in the exhaust gases downstream of the CRT under simulated on-road conditions.

Experimental Section Apparatus and Instrumentation. Measurements were carried out on particles produced by the heavy duty diesel engine described in Table 1. Reported measurements were carried out at 32% load and 1500 rpm and under steady-state operating conditions. The engine exhaust was at a temperature of 312(6 °C, which is in the range where a nucleation mode was observed in on-road studies (6, 7). The exhaust flowed through a CRT and was then diluted in a two-stage micro-dilution tunnel (9) prior to measurement. On average, the primary dilution ratio was 13:1 and the secondary dilution ratio was 20:1. Between the two dilution stages a tunnel allowed the aerosol to age ∼1 s. The diluter is intended to simulate interactions between the hot combustion products and the cooler atmospheric air found under typical road conditions. A schematic of the apparatus used in this study is shown in Figure 1. Further details about the aerosol instrumentation and measurement protocols are discussed elsewhere (4, 10). A nano MOUDI with cut sizes of 320, 180, 56, 32, 18, and 10 nm was used to collect samples for chemical and gravimetric analysis. Ungreased aluminum foil was selected as the most suitable substrate for the nano MOUDI collection stages, as it would be stable during the sample preparation process for both liquid chromatography and thermal analysis techniques. Upstream stages having cut sizes of 3.2 and 1.0 µm were coated with Apiezon grease to eliminate the possibility that coarse particles would bounce and be collected on the fine particle stages. Substrates with cut sizes outside the 10 nm to 320 nm range were not analyzed. The aluminum foil substrates were calcined in air at 600 °C for 3 h to remove any volatile or carbonaceous residues. Four samples, collected over 5-6 h each, were analyzed for sulfate, nitrate, and ammonium content using a Dionex ion-exchange highperformance liquid chromatograph (HPLC). A Varian-Markes thermal desorber-gas chromatograph (TD-GC) was used to determine the volatile organic fraction (VOF). Ion-exchange HPLC analysis was carried out in accordance with the Institute of Petroleum test protocol IP416/ 96. Half of each aluminum foil substrate was immersed into 10 mL of a 10% isopropyl alcohol/90% demineralized water solution in a new, cleaned glass vial. The samples were sonicated for 1 h and left overnight (testing had shown that longer soaking times had no effect on the concentrations obtained). The resulting solutions were injected into the HPLC system for analysis. Ions were identified by retention time and quantified by peak area against NIST-traceable reference 10.1021/es052267+ CCC: $33.50

 2006 American Chemical Society Published on Web 07/21/2006

TABLE 1. Engine Parameters for This Study engine model year emission standards cylinders displacement configuration peak power peak torque fuel system

Caterpillar 3176 C-12 1995 updated to meet 1998 post consent decree standards 6 12 L turbocharged and aftercooled 265 kW (355 hp) @ 1800 rpm 1800 Nm @ 1200 rpm electronically controlled

standards. Sulfate results included 1.33× residual water. Analysis of the 10% isopropyl alcohol/90% demineralized water solution gave zero response for nitrate, sulfate, or ammonium ions. For TD-GC analysis, half of each aluminum foil substrate was mounted in a metallic tube in a heating element. The volatile components present were desorbed into an inert gas flow at 350 °C and introduced onto a gas chromatography column. A flame ionization detector (FID) was used to determine the quantity of hydrocarbon species passing through the column as a function of retention time, compared to fuel and oil standards. Samples of the cleaned aluminum foil and the grease used in the nano MOUDI assembly gave negligible response. Experiments were carried out to determine the limit of detection (LOD) for each analyzed component. Analyses were conducted on cleaned aluminum foils doped with different levels of nitrate, sulfate, ammonium, and VOF in the µg range. Measured concentrations were plotted against theoretical concentrations for each PM component and a line of best fit was established. LOD values were determined using a calculation based on Institute of Petroleum and CARB methods: LOD ) (b + (t × s))/m, where b ) y-intercept, t ) factor for 99% confidence in a one-sided Gaussian distribution, s ) standard deviation of low concentration samples, and m ) slope of linear regression. These LOD values are shown in Table 2. For sulfate, nitrate, and VOF, LOD values were below 1.0 µg per foil sample. Average sulfate masses of DEPs collected on the foils during this study ranged from 3.7 to 33.1 µg, which is significantly above the LODs. Average masses of VOF ranged from 0 to 4.1 µg, while the variability from one sample to the next was greater than that observed for the sulfates. Therefore, while measured VOF was often above the LOD, measurement uncertainties are greater than

those found for sulfates. Nitrate levels were below the limit of detection in every sample. Ammonium was only detected in 3 of the 28 samples, in each case at significantly higher levels than the total mass of PM determined using the microbalance. It was, therefore, suspected that the ammonium results were due to contamination or a measurement artifact and they were excluded. A dynamic blank nano MOUDI sample was collected as part of the engine tests. Analysis found that the substrates typically contained 0.5-1 µg sulfate, 3-5 µg VOF, and ammonium and nitrate levels below the respective limits of detection. For analysis of the PM-containing samples, the blank results for the nano MOUDI stages (corrected for collection time) were subtracted from species masses measured on the corresponding stages. Particle size distributions (PSD) were measured using two scanning mobility particle sizers (SMPS) operating in parallel. The long-column SMPS measured particles in the 20-280 nm mobility diameter range, and the nano SMPS measured particles in the 3-30 nm mobility diameter range. DMA penetration (11) and CPC counting efficiencies (12, 13) were taken into account when inverting nano SMPS data to obtain size distributions. Transport and particle detection efficiencies for the long-column SMPS were assumed equal to 1.0, which should be reasonable for particles in that size range. The volatility and hygroscopicity of particles in the 4-15 nm mobility diameter range were measured using a nanometer tandem differential mobility analyzer (nano TDMA) (4, 14). The relative humidity of particles entering the PSD and nano TDMA systems ranged from 5 to 15%. Nano TDMA measurements involved selecting particles of a given size with DMA1 (sizes of 4, 6, 9, 12, and 15 nm were used in this study), conditioning them by either increasing their relative humidity to 85% or heating them to a temperature of 25160 °C within a 6.35 mm o.d., 50 cm long stainless steel tube for 0.23 s, and measuring the resulting size or distribution of sizes with DMA2. Humidification was achieved by passing the size-classified particles sequentially through two MD110-24S Perma Pure dryers; the relative humidity of the air flowing through the outer annular space of these dryers was controlled to ensure that the relative humidity of the sampled particles was nominally 85%. The DMA2 sheath air was also controlled to 85%. Particles were counted downstream of DMA2 using a TSI 3025 UCPC (12). Nano TDMA experiments were also carried out using laboratory-generated nanoparticles of known composition to help with interpreting the

FIGURE 1. Schematic of experimental apparatus. VOL. 40, NO. 17, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. Averaged SMPS measurements of diesel exhaust particle size distributions downstream of the CRT for BP15 and BP50 fuels at 32% engine load. Primary dilution was 13:1 and secondary dilution was 20:1. Note that for both fuel types the modal size for particle number concentrations was below 10 nm, but that the average volume concentration for 15 ppm fuel was just 7% of the volume concentration for 50 ppm fuel.

TABLE 2. Limits of Detection for Speciation Measurements

component

std dev (%)

LOD (µg/mL)

nitrate sulfate ammonium VOF

0.81 0.75 0.34 0.25

0.042 0.052 0.289

LOD (µg/half substrate)

mass on substrate (µg)

0.157

0.831 1.046 5.789 0.157

exhaust particle data. Volatility measurements were carried out with 30 nm particles produced by nucleating hot (150200 C) vapors of the n-alkanes C28H58 and C32H66 and with 12 nm ammonium sulfate produced using the TSI 3480 electrospray. Hygroscopicity measurements were carried out with ammonium sulfate, ammonium bisulfate, and sulfuric acid particles of 5-15 nm. Details regarding these experiments are discussed by Fink (10). Two different approaches were used for inverting the nano TDMA data. For the humidification experiments it was assumed that the shape of the mobility distribution upstream of DMA2 is similar to the distribution at the inlet to the RH conditioner, but shifted to larger sizes. This is consistent with assumptions of the TDMA data interpretation framework described by Rader and McMurry (15) and with the TDMAFIT algorithm developed by Stolzenburg and McMurry (16). Particle evaporation in the heater, however, typically leads to a transformation in the mobility distribution. In this case the inversion algorithm developed by Sakurai et al. (4), which makes no assumptions about the shape of the aerosol size distribution entering DMA2, was used. Note that both of these approaches account for the variability of the sampled distribution function within the DMA2 mobility window. Diesel Exhaust Particle Measurements. The CRT can operate only with low sulfur fuels because SO2 reduces the effectiveness of the system (17-19). We carried out measurements using British Petroleum fuel with sulfur contents of 15 and 49 ppm (referred to in this paper, respectively, as BP15 and BP50). BP 15 is of particular interest because it meets the requirements of the U.S low sulfur diesel rule taking effect in fall 2006 prior to the implementation of reduced diesel emission standards in 2007. Measurements with BP50 were carried out on three consecutive days. At the end of the third day the fuel was switched to BP15, and two consecutive days of measurements were carried out. Castrol heavy-duty 5504

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FIGURE 3. Speciation of the PM samples collected during testing with BP50 and BP15 fuels. Two determinations were made at each particle size and the average is plotted. diesel lubricating oil (0.521% S) was used. Chemical analyses of the fuel and oil are reported by Savstrom (17). On-road measurements with the CRT have shown that a large nucleation mode is produced under highway cruise conditions, even with BP15 fuel (6, 7). Laboratory studies have shown that nucleation in diluted diesel exhaust is sensitive to the temperature and relative humidity of the dilution air. Also important are the dilution rate or residence time at intermediate dilution ratios, where saturation ratios are high (9, 20, 21). By carefully preconditioning the engine and cooling the primary dilution air, we were able to reproducibly sustain a nucleation mode. Figure 2 shows number distributions downstream of the CRT for BP15 and BP50 averaged over several days of measurements. The error bars represent standard deviations of the daily averages. The average mean size for particles produced from BP15 and BP50 were, respectively, 4.4 ( 0.6 nm and 6.6 ( 0.2 nm, while number concentrations were 1.7 ( 0.7 × 108 and 8.8 ( 1.1 × 108 cm-3, and volume concentrations were 12 ( 8 and 169 ( 46 µm3cm-3. These values were obtained by averaging over the 3-200 nm mobility diameter range. Thus, the mean particle size, the total number concentration and the volume concentrations were all lower for the lower S fuel.

Results and Discussion Speciated mass concentrations measured on the seven nano MOUDI stages are shown in Figure 3. Two samples were acquired for each of the two fuels (BP50 and BP15), and the results shown are the averages of those samples. The total mass collected per substrate was less than about 30 µg, which is orders of magnitude lower than PM levels obtained from typical unfiltered exhaust. Because of the small masses collected, the microbalance measurements include large uncertainties and are not shown. However, for both fuel types the trends in gravimetric mass concentrations with size are the same as those of the total speciated mass shown of Figure 3. Table 3 summarizes nano TDMA measurements that were carried out. Growth factors and volatility were measured for 4-15 nm particles produced by BP50 exhaust particles. In addition, the hygroscopicity of 9 nm particles was tracked in detail between 39% and 91% RH. This size was singled out because it was present in sufficient concentrations for relatively rapid scanning. For BP15 exhaust, 4.5 nm particles were measured for hygroscopicity and volatility. Results of nano TDMA volatility experiments for DEPs and laboratory-generated n-alkane and ammonium sulfate

TABLE 3. Summary of Exhaust Measurements fuel type

DMA1 particle size [nm]

number of measurements

conditioner

conditioner range

BP50 BP50 BP50 BP15 BP15

4, 6, 9, 12, 15 9 4, 6, 9, 12, 15 4.5 4.5

2 of each size 38 10 of each size 24 12

hygroscopicity hygroscopicity volatility hygroscopicity volatility

RH ) 86% 41% < RH < 91% 18 °C < T < 110 °C 7% < RH < 89% 18 °C < T < 125 °C

FIGURE 4. Decrease in particle size as a function of heater temperature for laboratory-generated aerosol standards (C28 and C32; ammonium sulfate) and for diesel exhaust particles produced from BP15 and BP50 fuel and sampled downstream of the CRT. particles are shown in Figure 4. In this figure, the decrease in mobility size is shown as a function of the heater temperature. Theory predicts that when particles evaporate only partially, as occurred in these measurements, size change is independent of particle size provided that curvature effects can be neglected (3). Note that 12 nm DEPs produced from BP50 and 4.5 nm DEPs produced from BP15 have a volatility that is similar to that of 12 nm ammonium sulfate. The C28 and C32 n-alkanes, however, are much more volatile. We conclude that the volatility of DEPs downstream of the CRT is similar to that of ammonium sulfate. This is in contrast to the earlier work of Sakurai and co-workers, who found that the volatility of diesel exhaust nanoparticles produced in a modern heavy-duty diesel engine that was not equipped with a CRT was similar to that for C24-C32 n-alkanes (2, 3). These observations provide evidence that the CRT effectively oxidizes the high molecular weight semivolatile organics and are consistent with the conversion of SO2 to sulfate reported by Cooper and Thoss (8). Figure 5 shows diameter growth factors for 6, 9, and 12 nm laboratory-generated ammonium sulfate particles as a function of relative humidity. Theoretical curves showing the expected equilibrium water uptake of ammonium sulfate after accounting for the effect of curvature are also shown (10). For relative humidities below the 80% deliquescence relative humidity of ammonium sulfate (22), the theory applies to metastable solutions. The deliquescence relative humidity observed in our study for 12 nm particles is close to 80%. However, we found that for 9 and 6 nm particles the deliquescence humidities were, respectively, about 75% and 60-65%, indicating that as particle size decreases, the deliquescence relative humidity decreases. Previous experimental and theoretical work has shown that the deliquescence relative humidity for sodium chloride increases with decreasing size (22-24), and we confirmed that result with our apparatus (10). Russell and Ming predict that the

FIGURE 5. Nano TDMA measurements of water uptake by 6, 9, and 12 nm ammonium sulfate aerosol standards. The theoretical curves show expected equilibrium water uptake after accounting for the effect of curvature. deliquescence RH for ammonium sulfate should also increase with decreasing size (24), and experimental work by Han and co-workers supported this prediction (25). However, measurements by Ha¨meri and co-workers (26) found that the deliquescence relative humidity for ammonium sulfate decreased with decreasing size, similar to our observations. We hypothesize that as particle size decreases, ammonia evaporation leads to particles that are not fully neutralized, and that this is the reason that water uptake occurs at lower relative humidities as size decreases. Further work is required to confirm this. Figure 6 compares measured size-dependent growth factors for DEPs with theoretically predicted values for ammonium sulfate and ammonium bisulfate after accounting for the effect of curvature. The theoretical curve for ammonium sulfate assumes that particles entering the relative humidity conditioner downstream of DMA1 are crystalline solids. This is reasonable since the relative humidity in DMA1 is well below the efflorescence relative humidity for ammonium sulfate, which is approximately equal to 32% (27). As another limiting case, the ammonium bisulfate curve was calculated assuming that the particles entering the relative humidity conditioner were metastable aqueous ammonium bisulfate solutions at 10% RH. Both theories show that the functional form of the measured size-dependent water uptake is in reasonable agreement with our measurements, but the results for metastable ammonium bisulfate are more nearly in quantitative agreement. Our hypothesis that ammonia evaporation could explain the observed dependence of deliquescence relative humidity on particle size is consistent with these results. If particles were less than fully neutralized they might retain some water at the inlet to the RH conditioner downstream of DMA1. Furthermore, growth factors for 6 nm laboratory-generated ammonium sulfate particles were indistinguishable from those found for the diesel exhaust particles. These results, together with those in Figure 4 showing that volatilization of the DEPs was similar to that VOL. 40, NO. 17, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 6. Size-dependent diameter growth factors upon humidification from ∼10% to 85% RH for diesel exhaust particles produced from BP50 and BP15 fuels downstream of the CRT, and for 6 and 12 nm laboratory-generated ammonium sulfate particles. Note that the single point from BP15 follows the trend observed for the BP50, suggesting that the compositions of particles produced by these fuels were similar. Theoretical predictions for DMA1-classified particles that consist of crystalline ammonium sulfate and metastable ammonium bisulfate are also shown. for ammonium sulfate but not C28 and C32 n-alkanes, shows that nanoparticles downstream of the CRT behave similarly to sulfates that have been neutralized to some extent. The nano MOUDI substrates showed no evidence for ammonium. We hypothesized that this might have occurred because deposited ammonium sulfate may have interacted with the aluminum foil, loading the formation of aluminum sulfate and the evolution of ammonia gas. To test this hypothesis, experiments were carried out to determine whether ammonium sulfate was stable when deposited onto cleaned aluminum foil substrates. A 1 M ammonium sulfate solution was prepared and 100 µL samples were sprayed onto aluminum foil and glass fiber (Whatman GFA) substrates using a microsyringe. The samples were left overnight for any interaction to take place, before being analyzed for ammonium and sulfate in the same way as for the nano MOUDI substrates. All experiments were carried out in duplicate. We found no evidence that ammonium was lost from the aluminum substrates. This result provided support for our conclusion that the DEPs were composed primarily of sulfuric acid. In contrast, the nano TDMA measurements showed that particles behaved more like ammonium bisulfate than ammonium sulfate. Nano TDMA volatilization measurements with 12 nm laboratory-generated sulfuric acid, ammonium bisulfate, and ammonium sulfate particles are shown in Figure 7. These measurements show that the volatilization behaviors of ammonium bisulfate and ammonium sulfate are indistinguishable. Sulfuric acid, however, exhibited a bimodal behavior: some particles evaporated significantly while others behaved more like the neutralized sulfates. We conclude from these observations that a fraction of the pure sulfuric acid particles become neutralized in our apparatus, presumably due to ammonia contamination. Therefore, it is not possible to distinguish unambiguously between sulfuric acid and its neutralized counterparts with the nano TDMA that was used in these experiments. However, the fact that the 12 nm exhaust particles all behaved like the neutralized sulfate (Figure 4) and contained none of the more volatile particles observed with 12 nm sulfuric acid (Figure 7) suggests that the DEPs were neutralized to some extent. 5506

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FIGURE 7. Decrease in particle size versus temperature for laboratory generated 12 nm ammonium bisulfate, ammonium sulfate, and sulfuric acid particles. Thus, we remain uncertain about the extent to which the exhaust particles were neutralized by ammonia. However, even if particles consisted of pure sulfuric acid when they were emitted, they would soon become neutralized in the atmosphere, since the reaction rate of ammonia with sulfuric acid particles is close to collision-controlled (28). The time required for a 30 nm pure sulfuric acid particle to become 50% neutralized at a typical ambient urban ammonia concentration of 1 ppb is ∼10 s. Therefore, in the immediate vicinity of the roadway particles may be acidic, but they would become neutralized a short distance downwind. Previous research suggests that at typical ambient concentrations, ammonium sulfate particles may be nontoxic (29, 30). Our observations, therefore, provide support for the argument that particulate emissions from diesel vehicles equipped with advanced particulate control devices might be less toxic than typical uncontrolled diesel emissions, which contain high concentrations of organic compounds. However, due to the complexity of diesel exhaust, toxicology studies are required to confirm this.

Acknowledgments This work was supported primarily by the IGERT Program of the National Science Foundation under Award DGE0114372. This research was also supported by the Office of Science (BER), U.S. Department of Energy, Grant DE-FG02-05ER63997, and by a gift from Johnson Matthey. The engine and dynamometer and support for engine operation were provided by Caterpillar and the Engine Manufacturers Association. The fuel and lubricating oil were donated by BP and Castrol. We also thank the reviewers for their insightful comments.

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Received for review November 10, 2005. Revised manuscript received June 7, 2006. Accepted June 12, 2006. ES052267+

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