Environ. Sci. Technol. 2006, 40, 155-162
Effects of Dilution on Fine Particle Mass and Partitioning of Semivolatile Organics in Diesel Exhaust and Wood Smoke ERIC M. LIPSKY† AND ALLEN L. ROBINSON* Department of Mechanical Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania, 15213
Experiments were conducted to examine the effects of dilution on fine particle mass emissions from a diesel engine and wood stove. Filter measurements were made simultaneously using three dilution sampling systems operating at dilution ratios ranging from 20:1 to 510:1. Denuders and backup filters were used to quantify organic sampling artifacts. For the diesel engine operating at low load and wood combustion, large decreases in fine particle mass emissions were observed with increases in dilution. For example, the PM2.5 mass emission rate from a diesel engine operating at low load decreased by 50% when the dilution ratio was increased from 20:1 to 350:1. Measurements of organic and elemental carbon indicate that the changes in fine particle mass with dilution are caused by changes in partitioning of semivolatile organic compounds. At low levels of dilution semivolatile species largely occur in the particle phase, but increasing dilution reduces the concentration of semivolatile species, shifting this material to the gas phase in order to maintain phase equilibrium. Emissions of elemental carbon do not vary with dilution. Organic sampling artifacts are shown to vary with dilution because of the combination of changes in partitioning coupled with adsorption of gas-phase organics by quartz filters. The fine particle mass emissions from the diesel engine operating at medium load did not vary with dilution because of the lower emissions of semivolatile material and higher emissions of elemental carbon. To measure partitioning of semivolatile materials under atmospheric conditions, partitioning theory indicates that dilution samplers need to be operated such that the diluted exhaust achieves atmospheric levels of dilution. Too little dilution can potentially overestimate the fine particle mass emissions, and too much dilution (with clean air) can underestimate them.
Introduction Many combustion and other high-temperature sources emit compounds that are semivolatile or volatile at exhaust temperatures but undergo gas-to-particle conversion as the combustion products mix with ambient air. Dilution sampling is a technique developed to simulate these processes in order to better characterize fine particle emissions (1). * Corresponding author phone: (412)268-3657; fax: (412)268-3348; e-mail:
[email protected]. † Present address: Department of Engineering, Penn State McKeesport, McKeesport, PA 15132. 10.1021/es050319p CCC: $33.50 Published on Web 11/30/2005
2006 American Chemical Society
Previous research has shown that fine particle emissions are sensitive to dilution conditions. A major focus has been on nucleation and the particle size distribution, both of which can be extremely sensitive to dilution conditions (2). However, existing emission and ambient standards are based on fine particle mass. Fine particle mass emissions depend on the phase partitioning of semivolatile compounds in the exhaust. For sources with significant organic aerosol emissions, dilution samplers measure higher fine particle mass emission rates compared to filters collected at exhaust temperatures (1, 3). This well-recognized effect is due to gas-to-particle conversion of semivolatile species as the exhaust is cooled during dilution. Cooling reduces the saturation pressures of the semivolatile compounds in the exhaust. Dilution samplers are typically operated at dilution ratios between 20:1 and 200:1. A dilution ratio of 100:1 is generally high enough to reduce the exhaust temperature to ambient levels, but the median dilution ratio of vehicle exhaust in an urban atmosphere is around 10 000:1 (20). Therefore, the concentration of semivolatile species inside a dilution sampler can be orders of magnitude higher than typical atmopsheric conditions. One also needs to account for effects of background pollution on the emissions because dilution samplers mix exhaust with particle- and organics-free air. Dilution reduces the concentrations of semivolatile species; partitioning theory indicates that, under isothermal conditions, this should reduce the amount of semivolatile material in the particle phase (5, 6). However, the significance of this effect is not well understood as illustrated by a recent review concluding that fine particle mass tends to be conserved upon dilution (2). Experiments with diesel exhaust report modest decreases in fine particle mass emissions at higher dilution ratios (3, 7). These changes were attributed to changes in partitioning of semivolatile organics. However, both studies only considered dilution ratios smaller than 100:1. In addition, one study performed the experiments with a constant filter temperature of 52 °C (7) while the other only observed a decrease in emissions at one dilution ratio (3). In contrast, measurements by Hildemann et al. (1) made on a fuel oil boiler suggest increasing emissions with higher dilution ratio. Interpretation of these data is potentially complicated by organic sampling artifacts (8). This paper examines the effects of dilution sampling on the fine particle mass emissions from a diesel engine and a wood stove. Measurements of PM2.5 mass, organic carbon, and elemental carbon emissions were made at dilution ratios between 20:1 and 510:1. Backup and denuded filters were collected to estimate organic sampling artifacts. Partitioning theory is used to discuss the data in the context of real-world dilution.
Experimental Setup and Procedure Experiments were performed to measure the effects of dilution on fine particle mass emissions from a diesel engine and a wood stove. Figure 1 shows a schematic of the experimental setup. Filter samples were collected using three completely independent dilution sampling systems operated simultaneously at different dilution ratios. This approach of simultaneous sampling with multiple samplers minimizes the effects of temporal variations in emissions on the results. Detailed descriptions of the design, characterization, and operation of the dilution samplers can be found in Lipsky and co-workers (9, 10). Briefly, each sampler isokinetically collects exhaust through separate, heated inlet lines that are maintained at a temperature slightly above the exhaust temperature to minimize thermophoretic losses. The sampled VOL. 40, NO. 1, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Aerosol Characterization. Filter samples were collected using a sampling train that consists of a sharp-cut PM2.5 cyclone operating at 24 liters per min (lpm) (Figure 1). Downstream of the cyclone the flow is split and passed through two filter packs each operating at 12 lpm. One filter pack contains a single quartz filter (Bare-Q); the second filter pack contains a Teflon filter followed by a backup quartz filter (quartz behind Teflon or QBT). The QBT filter is used as an estimate of the positive sampling artifact from gas adsorption of semivolatile organic material (8). Identical filter trains were attached to the end of each dilution tunnel.
FIGURE 1. Experimental setup of simultaneous sampling with three dilution tunnels. Each sampler has a separate heated inlet line. exhaust is then rapidly mixed by turbulence with filtered (HEPA and activated carbon) dilution air inside a dilution tunnel. Filter trains are connected to the end of the dilution tunnel. Each tunnel provides about 2.5 s of residence time after mixing but before filter collection. Previous research has shown that additional residence time beyond that provided by the tunnel does not affect filter measurements under the conditions of these experiments (10). All samplers are constructed out of stainless steel with Teflon gaskets to minimize sample contamination. Two different dilution tunnel designs were used for these experiments. To establish the comparability of the different designs, separate experiments were conducted while operating all three samplers simultaneously at the same dilution ratio (10). These intercomparison experiments show excellent agreement between the different samplers across the range of dilution ratios considered here; for example, the average relative bias in the PM2.5 mass emissions measured using the two designs is 1% ( 17% (average ( standard deviation). The dilution ratio within each sampler is determined by simultaneously measuring exhaust and dilution tunnel CO2 levels:
DR )
(CO2)ex - (CO2)bck (CO2)tun - (CO2)bck
(1)
where (CO2)ex, (CO2)bck, and (CO2)tun are the CO2 mixing ratios in the exhaust, dilution air, and dilution tunnel, respectively. Separate CO2 monitors are used to continuously monitor (CO2)tun at the end of each tunnel. The dilution air CO2 mixing ratio is measured both before and after each day of tests; the small variations in (CO2)bck, are insignificant compared to the relatively high tunnel and exhaust CO2 levels. No dilution corresponds to a dilution ratio of one. To compare measurements made at different dilution ratios, emissions are reported as fuel-based emission factors (e.g., g PM2.5/kg fuel):
EF )
[P] C [C] f
(2)
where [P] and [C] are the background-corrected pollutant and carbon concentrations inside the dilution tunnel, respectively, and Cf is the mass fraction of carbon in the fuel. [C] is determined from the measured exhaust gas composition, assuming all of the fuel carbon is emitted as CO and CO2. Small amounts of fuel carbon are emitted as either gasor condensed-phase organic compounds; however this organic material contributes negligibly to the overall carbon balance. We use a value of Cf of 0.87 for the diesel fuel and 0.40 for the wood fuel. Unless otherwise noted, the word “emission” or “emission rate” refers to fuel-based emissions. 156
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The Teflon filter is used for quantifying total PM2.5 mass emissions. Teflon filters are weighed before and after sampling using a microbalance in a temperature and relative humidity controlled environment (30-40% RH at 21-23 °C). The filters are equilibrated for 24 h before gravimetric analysis. Samples on quartz filters are used to quantify organic and elemental carbon (OC and EC) emissions using a Sunset Laboratories laboratory thermal-optical transmission OC/ EC analyzer. The temperature protocol is a modified version of the NIOSH 5040 protocol (11). Quartz filters are prepared before sampling by baking them at 500 °C in air for at least 6 h to remove any residual carbon. Both quartz and Teflon filters are stored in a freezer (-18 °C) between sample collection and analysis. All filter measurements are blank corrected (10). A second filter train was occasionally operated in parallel with the first train to further investigate organic sampling artifacts. This train consists of a sharp-cut PM2.5 cyclone, a carbon monolith denuder (MastCarbon Ltd., U.K.), followed by a filter pack with a quartz fiber filter and a carbon impregnated glass-fiber (CIG) filter (47 mm, Schleicher & Schuell, GF 3649) (11). The flow rate through the denuded filter train was 12 lpm to match the filter face velocity of the standard train. The CIG filter was analyzed with the Sunset OC/EC analyzer in a helium atmosphere using a stepped temperature profile ramping to 330 °C (11). Sources. Emissions tests were performed using a small diesel generator and a wood stove. The diesel generator is a single-cylinder Yanmar L70AE air-cooled diesel engine connected to a 4.5 kW generator. The engine is EPA and CARB exhaust emission compliant. Experiments were performed at constant load: low load (25% of rated capacity) or medium load (55% of rated capacity). Exhaust temperatures at the sampling location were in the range of 230-260 °C for low-load tests and 260-290 °C for medium-load tests. The engine was operated at the specified load setting for a minimum of 45 min before filter sampling to allow all components to achieve a steady-state temperature. Diesel fuel was purchased at a local gas station. The wood stove is an EPA approved Jøtul “602 CB Classic” wood stove. The wood fuel was a mixture of oak, cherry, and some ash. Wood-burning experiments involved starting the fire with a small amount of wood. After the fire was established, the stove was loaded to capacity and the wood was allowed to burn down for 45-75 min until the combustion stabilized and the exhaust temperature at the sampling location was in the range of 130-150 °C. Sampling was then started, and stable flaming combustion conditions were maintained by adjusting the vents on the door of the stove. Exhaust temperatures and O2 and CO levels were reasonably constant during sampling.
Results Fuel-based emissions of PM2.5 mass, total carbon (TC), and organic and elemental carbon (OC & EC) as a function of dilution ratio are shown in Figure 2 for low- and medium-
FIGURE 2. Fuel-based emissions from low-load diesel (left column), medium-load diesel (middle column), and wood smoke (right column) experiments. Plots a1-a3 show PM2.5 mass emission rate; plots b1-b3 compare organic carbon emission factors measured with the Bare-Q and Q - QBT approaches; plots c1-c3 show particulate organic carbon (OC) and elemental carbon (EC) emission factors measured using the Q - QBT approach. Bare-Q is carbon measured with a quartz filter; Q - QBT is carbon measured with a backup-corrected quartz filter. Q - QBT is particulate OC, while Bare-Q has substantial positive artifact, as discussed in the text. In plots c1-c3 symbols indicate total carbon emissions and shading indicates the contribution of OC and EC to the emissions. Plots c1-c3 also show the carbon fractions from the OC/EC analysis: He1 OC is the OC that evolves at 340 °C, and He2-He4 OC is the OC that evolves at temperatures greater than 340 °C during the OC/EC analysis. The diesel plots combine data from back-to-back experiments conducted on the same day. Lines are intended as a visual aid. Vertical bars in (a) are experimental uncertainties determined from intercomparison experiments (10). load diesel operations and a wood smoke experiment. The emissions are presented on a fuel basis in order to compare measurements made at different dilution ratios on a consistent basis; fuel-based emissions of inert pollutants such as EC should be independent of dilution ratio, appearing as a horizontal line in Figure 2. The fuel-based PM2.5 mass emissions decrease with increasing dilution ratio during low-load diesel and wood smoke experiments. For example, Figure 2 shows that increasing the dilution ratio from 20:1 to 350:1 decreased the PM2.5 mass emissions from the diesel engine operating at low load by 55%. Increasing the dilution ratio from 20:1 to 120:1 decreased the PM2.5 mass emissions from the wood stove by over 60%. Within experimental uncertainty, the PM2.5 mass emissions measured during medium-load operation of the diesel engine are constant (Figure 2). Trends similar to those shown in Figure 2 were observed across the entire set of experiments; however, combining results from different experiments is difficult because of day-to-day variability in engine emissions and experiment-to-experiment variation in wood smoke emissions. Changes in the phase partitioning of the semivolatile organic material with dilution is the likely explanation for the changes in the PM2.5 mass shown in Figure 2. Although dilution does not alter the total (gas + particle) emission rate of semivolatile species, dilution does affect the phase partitioning of this material. Gas-particle partitioning occurs via absorption with an organic solution or adsorption to soot and mineral surfaces (5, 6). Therefore, phase partitioning depends on the concentration and saturation pressure of the semivolatile species and the concentration and composition of the sorptive material (5, 6). Under the conditions of these experiments, changes in concentration of the semivolatile and sorptive material with dilution likely cause
the changes in partitioning. The changes are not caused by changes in saturation pressure because the temperature of the diluted exhaust at the filter holder was essentially constant across this entire set of experiments (27 ( 2 °C). (Note that mixing inside the dilution tunnel is not adiabatic; at the lowest dilution ratios there was some heat transfer to the surroundings). Dilution reduces the concentration of both semivolatile species and the sorptive material which, under constant temperature conditions, requires semivolatile species to transfer from the particle to the gas phase to maintain phase equilibrium. This reduces the PM2.5 mass emission rate. A complication is the fact that dilution samplers are operated using cleaned air, while real-world dilution mixes emissions with polluted air. This issue is addressed in the discussion section. If changes in phase partitioning of semivolatile organics are responsible for decrease in PM2.5 mass, then these changes should be apparent in measurements of particulate OC. The simplest approach for measuring OC emissions is to use a quartz filter (Bare-Q). Figure 2 shows that the trends in Bare-Q OC with dilution are different than the changes in PM2.5 mass. For example, at low load the Bare-Q OC increase with dilution ratio while the PM2.5 mass decreases. However, OC measurements can be significantly impacted by sampling artifacts; these artifacts are carefully considered in the next section. Dilution-ratio-dependent losses are another potential explanation for the trends shown in Figure 2. Fuel-based EC emissions measured by the different samplers were within (12% for a given experiment, indicating consistent collection of a nonvolatile component of PM2.5 (Figure 2c). Furthermore, there were no consistent trends in the EC emission factor with dilution ratio. VOL. 40, NO. 1, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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consistent with a significant positive artifact on the Bare-Q filter. For the wood smoke experiments (Figure 3c), the TC measured by a Bare-Q filter is smaller than the Teflon mass. Assuming wood smoke is dominated by carbonaceous species, an organic-mass-to-organic-carbon ratio (OM/OC) of 1.3 is required to close the mass balance between the Teflon and Bare-Q filters. This value is smaller than expected given the relatively polar composition of wood smoke (12), indicating that the wood smoke also creates a net positive artifact on a quartz filter. Other researchers report that the positive artifact is dominant when sampling both diesel exhaust and wood smoke (13-17).
FIGURE 3. Average ratio of total carbon (OC + EC) to PM2.5 mass for (a) low-load diesel, (b) medium-load diesel, and (c) wood smoke experiments. Three different estimates of OC are shown: bare quartz filter (Bare-Q), backup-corrected quartz filter (Q - QBT), and denuded quartz filter (DenQ). There are two different estimates of EC (Q QBT EC and Bare-Q EC are the same). Error bars are standard deviations of the ratio of OC + EC to PM2.5 mass across the set of experiments. This ratio is expected to be less than one because of the contribution of elements other than carbon to the PM2.5 mass. Organic Sampling Artifacts. Both positive and negative artifacts can significantly affect quartz-filter measurements of particulate OC (8). Positive artifact is caused by organic vapors adsorbing onto the quartz filter resulting in an overestimate in the amount of organic aerosol. Negative artifact is caused by particle-phase organic compounds volatilizing after collection resulting in an underestimate of the organic aerosol mass. Organic sampling artifacts are commonly accounted for using a combination of denuders and/or backup filters (8). We employed both approaches in this study. The OC collected on a backup quartz filter behind a Teflon filter (QBT) is a standard estimate of the positive artifact (8). The approach assumes that the inert Teflon filter only collects particles, allowing gas-phase organics to pass through to the backup quartz filter. The particulate OC emissions are then estimated by subtracting the OC collected on the QBT from that collected on the Bare-Q filter (Q - QBT). The denuded filter train provides a second estimate of the particulate OC emissions. The denuder removes most of the organic vapor from the sample upstream of the quartz filter, minimizing the positive artifact. However, the depletion of semivolatile organic vapor from the air stream may cause volatilization of semivolatile organic material collected on the quartz filter resulting in a negative artifact (8). This negative artifact is estimated using the carbon impregnated glass-fiber (CIG) filter downstream of the denuded quartz filter. Therefore, the particulate OC emissions are estimated by adding the OC measured on the CIG filter and the denuded quartz filter (DenQ + CIG). Figure 3 compares the average OC emissions measured using the three different approaches: Bare-Q OC, backupcorrected (Q - QBT) OC, and denuded OC. To combine data from different experiments, the OC and EC data are normalized by PM2.5 mass before averaging. The ratio of TC to PM2.5 mass should be less than 1 because non-carbon organic components and inorganic species also contribute to PM2.5 mass. Figure 3 combines data from filters collected at different dilution ratios. When the diesel engine is operated at low and medium loads (Figure 3, parts a and b), the TC measured with a Bare-Q filter is, on average, slightly greater than the PM2.5 mass 158
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Figure 3 indicates that the artifact-corrected estimates of particulate OC are less than the PM2.5 mass for all three experimental conditions. For diesel exhaust, the backup and denuder approaches measure the same OC and EC emission rates and the majority of the Bare-Q OC appears to be positive artifact. On average, particulate OC contributes only 35% and 20% of the Bare-Q OC at low and medium loads, respectively. In contrast, other papers report that particulate OC contributes a majority of Bare-Q OC (13-15). Differences in engine technology and operating conditions likely influence sampling artifacts; in addition, the larger positive artifacts reported here may be due, in part, to the effects of dilution on artifacts discussed below. The CIG filter indicates that there was little negative artifact from the denuded quartz filter. For wood smoke, the EC emissions measured by the different approaches are comparable, but OC emissions measured using the denuded filters are somewhat smaller than the Q - QBT OC. However, the wood smoke emissions are highly variable as indicated by the large error bars in Figure 3. Positive artifact is estimated to contribute between 25% and 50% of wood smoke Bare-Q OC, which is consistent with results from Fine et al. (16, 17). The Teflon mass minus EC divided by the artifactcorrected, particulate OC can be used as a crude consistency check for the artifact corrections. Under the assumption that carbonaceous materials dominate the fine particle mass, this value is the OM/OC ratio. For wood smoke, the Q - QBT OC requires an OM/OC ratio of 1.8 for mass balance closure, which is within the range of expected values (12) suggesting the Q - QBT approach provides a reasonable estimate of particulate OC. However, this approach yields estimated OM/ OC ratios that are much larger than expected for diesel exhaust; for example, the low-load data require a ratio of 2.6 for mass balance closure versus expected values in the range of 1.2-1.4 (12). This indicates that, for the diesel exhaust, the Q - QBT (or denuder) approaches are underestimating the particulate OC emissions and/or that a significant fraction of the emissions are inorganic. Mass balance problems between sum of aerosol species and Teflon mass are not uncommon for measurements of diesel exhaust (13). A linear regression of the Teflon mass minus EC versus Q - QBT OC yields a slope of 1.4 µg/µg-C and an intercept of 0.5 g/kg fuel with an R2 value of 0.85 (4). This suggests an OM/OC ratio of 1.4, in line with expectations, and that there are significant inorganic emissions from the diesel engine (water or sulfate seem to be the most obvious candidates). Effects of Dilution on Organic Particulate Emissions. Figure 2 indicates that the trends in artifact-corrected, particulate OC match those of the PM2.5 mass, which is consistent with the changes emissions with dilution being caused by changes in phase partitioning of semivolatile organic compounds. Increasing the dilution ratio from 20:1 to 120:1 decreased the Q - QBT OC by 75% during the wood smoke experiment shown in Figure 2. At low load, the diesel particulate OC decreased by almost 70% when the dilution
FIGURE 4. Evolution of particulate carbon during OC/EC analysis of quartz filters collected at three dilution ratios during a low-load diesel experiment. Particulate carbon is defined as backupcorrected carbon (Q - QBT). The first four groups of bars indicate the amount of carbon that evolves from the four different temperature steps of the He mode of the analysis (340, 500, 700, and 870 °C). The final group of bars indicates the amount of EC that evolves during the analysis. Increasing the dilution ratio substantially reduces the amount of carbon that evolves at the lowest temperature step of the He mode, while the carbon at the higher temperature steps remains relatively constant. Backup and Bare-Q levels at the He3700C peak were the same at DR ) 200. ratio was increased from 20:1 to 350:1. For the medium-load diesel experiments, the Q - QBT OC is constant. Dilution also alters the composition of the particle-phase emissions. The ramping temperature protocol used for OC/ EC analysis crudely classifies the OC by volatility; the most volatile species evolving in the lowest temperature step of the helium mode (340 °C) and progressively less volatile compounds evolving in the higher temperature steps. Figure 4 plots a thermogram of particulate OC (Q - QBT) measured during a low-load diesel experiment. Increasing the dilution ratio dramatically decreases the amount of OC evolving in the lowest temperature step of the OC/EC analysis, while the amount of OC evolving in the higher temperature steps (500 °C or higher) remains relatively constant. At low dilution ratios, carbon that evolves at 340 °C contributes the majority of the particulate OC emissions; at high dilution ratios the higher temperature (less volatile) fractions dominate the particulate emissions. Figure 2, part c3, indicates that similar trends with dilution are observed in the wood smoke particulate OC with dilution. The modest decrease in particulate wood smoke OC with dilution in the higher temperature steps may be related to pyrolysis of semivolatile organics during OC/EC analysis. Low- versus Medium-Load Diesel Operations. Although the overall PM2.5 mass emission factors of the diesel engine at low and medium loads are similar, at low load the PM2.5 mass emission factor is a strong function of dilution ratio, but independent of dilution ratio at medium load. This difference appears related to the composition of the emissions. At low load the PM2.5 emissions are dominated by OC (average OC/EC ratio of 7) while at medium load emissions are dominated by EC (average OC/EC ratio of 0.4). This likely influences the relative importance of the different sorption mechanisms controlling gas-particle partitioning. At higher EC loadings adsorption to EC is likely more important than absorption in organic matter. This effect is illustrated by changes in thermal desorption profiles of diesel nanoparticles with soot loading (18). Another difference between the low- and medium-load diesel exhaust appears to be the composition of the OC emission. Carbon that evolves in the 340 °C temperature
FIGURE 5. Changes in fuel-based PM2.5 and organic mass emissions of low-load diesel exhaust as a function of dilution ratio. Organic mass is defined as OC multiplied by 1.4. To easily compare the changes in the different measurements with dilution ratio, the quartz filter data have been shifted to match the PM2.5 mass data at a dilution ratio of 20. This is done by adding the number indicated in the legend to each measurement. The changes in particulate organic mass (Q - QBT) match those of the overall PM2.5 mass. The backup-Q filter provides a measure of the gas-phase semivolatile organic carbon which, on a fuel basis, increases with dilution due to changes in partitioning. step of the OC/EC analysis contributes 60-70% of the lowload Bare-Q OC versus only 25-35% at the medium load. Therefore, less semivolatile carbon may be emitted under medium load. Sampling Artifacts and Dilution. Data presented in Figure 5 suggest that the individual quartz filters provide a surprisingly consistent picture of the changes in phase partitioning of the semivolatile organics. The measurements are from a low-load diesel experiment; to make direct comparisons with PM2.5 mass, organic mass and not OC is plotted using an OM/OC ratio of 1.4. Figure 5 indicates that the changes in particulate organic mass, (Q - QBT) × 1.4, with dilution ratio exactly match the decrease of PM2.5 mass. Compared to the changes observed with the other filters, the Bare-Q OC reported on a fuel basis is relatively constant, varying by less than (25% across the range dilution ratio for the set of filters collected during a given experiment (Figure 2b and Figure 5). The Bare-Q OC modestly increased with dilution ratio during the diesel experiments and decreased modestly during the wood smoke experiments. In contrast, for experiments in which there was a decreasing trend in PM2.5 mass with dilution, the OC measured by the QBT filter increases dramatically with dilution ratio. Figure 5 illustrates this trend for a low-load diesel experiment. One explanation of the Bare-Q and QBT OC data in Figure 5 is that the quartz filters provide a consistent measure of the gas-phase semivolatile organics. The Bare-Q OC emissions are relatively constant because the filter is collecting both gas- and particle-phase OC and the total OC emissions (gas + particle) do not change with dilution. At low dilution ratios the Bare-Q filter collects the majority of the semivolatile organics as particles while at higher dilution ratios it collects them as gases. In contrast, the QBT OC emissions increase with dilution because these filters are only exposed to the gas-phase compounds whose relative concentration increases at higher dilution ratios due to changes in partitioning. This interpretation of the quartz-filter data depends on the filters not being in equilibrium (or saturated) with the gas-phase organics (19). Figure 6 plots the mass of OC collected on the QBT filter as a function of the volume of exhaust passed through the filter during low-load diesel VOL. 40, NO. 1, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 6. Organic carbon collected on a backup quartz filter behind a Teflon filter (QBT) as a function of volume of sampled exhaust. Data are from the low-load diesel experiments. Volume of sampled exhaust is used as a proxy for the total mass of organics to which the filter is exposed. The data show that quartz filters do not reach equilibrium with the gas-phase organics under the conditions of the experiments. The roll over at high volumes of sample exhaust may indicate the filters are approaching equilibrium but also may be caused by changes in gas-to-particle partitioning of semivolatile organics. Higher exhaust volumes correspond to lower dilution ratio experiments. experiments. The volume of exhaust is a proxy for the mass of gas-phase organics to which the filter is exposed. Except for a couple of low dilution ratio points, the amount of carbon collected by the backup quartz filter increases with the volume of sampled exhaust indicating the quartz filters are not in equilibrium with the gas phase. At the highest volumes of sampled exhaust, there is evidence of the mass of carbon collected starting to roll over consistent with the quartz filters approaching equilibrium. However, Figure 6 is based on measurements made at different dilution ratios. At low dilution ratios a larger fraction of the semivolatile OC exists in the particle phase; therefore some of the roll over shown in Figure 6 is likely due to changes in partitioning and not equilibrium effects. Although Figure 6 indicates that the quartz filters are clearly not in equilibrium with the exhaust, we attribute the increases in dilution-corrected OC measured by the Bare-Q filters in the diesel experiments to the effects of equilibrium. As a filter approaches equilibrium the collection efficiency of gas-phase OC decreases, reducing the amount of gasphase OC collected per unit volume of exhaust. Under the conditions of these experiments (variable dilution ratio coupled with fixed filter sampling times), this change in collection efficiency will cause an apparent increase in the fuel-based OC emissions as a function of dilution ratio consistent with the diesel Bare-Q OC data shown in Figure 2. The fact that the ratio of PM2.5 mass to the backup-corrected OC (Q - QBT) remains constant across an experiment indicates that the Bare-Q and QBT filters are approaching equilibrium at the same rate. An interesting consequence of the changes in phase partitioning with dilution ratio coupled with the efficient collection of gas-phase semivolatile organics by quartz filters is that the positive artifact on a Bare-Q filter increases with dilution ratio. This effect is illustrated in Figure 7 which plots the ratio of TC to PM2.5 mass as a function of dilution ratio for the two diesel load conditions. Across the entire range of dilution ratios, the ratios of Q - QBT and the DenQ TC to PM2.5 mass are essentially constant indicating that both approaches provide a consistent correction of the positive 160
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FIGURE 7. Ratio of total carbon (OC + EC) to PM2.5 mass measured using three different filter configurations while sampling from the diesel engine operating at (a) low and (b) medium load. The results show a substantial increase in the positive artifact on a Bare-Q filter with dilution ratio. Both the denuded quartz and backupcorrected quartz approaches provide a consistent correction for this positive artifact. artifact. In contrast, the ratio of Bare-Q TC to PM2.5 mass increases with dilution ratio indicating increasing positive artifact with dilution ratio. This increase is particularly dramatic for low-load operations where the Bare-Q TC to PM2.5 mass ratio increases from 0.8 at a dilution ratio of 20:1 to a value of 1.7 at a dilution ratio of 350:1. The trends in Figure 7 can be explained by a combination of changes in partitioning and effects of the quartz filter approaching equilibrium. Under low-load conditions, the relative amount of gas-phase semivolatile organics increases with dilution due to changes in phase partitioning, which, in turn, increases the positive artifact. Under medium-load conditions, changes in partitioning were not observed and the increase in the positive artifact is likely associated with the previously discussed equilibrium effects. At high dilution ratios, the positive artifact dominates the OC measured with the Bare-Q filter. At lower dilution ratios, the relative amount of positive artifact reported here is consistent with results of previous studies of diesel exhaust (13-15).
Discussion The major finding of this paper is that increasing dilution after the temperature of the exhaust has reached ambient levels can dramatically reduce the PM2.5 mass emission rate from a diesel engine and wood stove. For example, the measured PM2.5 mass emission factor for the diesel engine operating under low load is 2.9 g/kg fuel at a dilution ratio of 20:1 versus 1.4 g/kg fuel at a dilution ratio of 350:1. This decrease is caused by changes in partitioning of semivolatile organic compounds in the emissions. From a scientific perspective, there is not a unique value for the fine particle mass emission rate for sources that emit semivolatile species because the partitioning of these species varies continuously with dilution and temperature. One needs to understand these effects in order to correctly interpret measurements made with dilution samplers. For the two sources considered
here, measurements at dilution ratios less than 100:1 likely substantially overestimate the emitted fine particle mass relative to the more dilute conditions encountered in the real world. This has obvious implications for using emission measurements in air quality models that mix emissions directly into large grid cells or use source profiles with species concentrations that are normalized by OC or PM2.5 mass. Partitioning theory provides a context for understanding the results of these experiments. Following Pankow (5, 6), the concentration of a semivolatile compound in the aerosol phase is as follows:
Ca,i )
1 1+
1 Kp,iCPM
Ctot,i
(3)
where Kp,i is the partitioning coefficient for compound i, CPM is the concentration of particulate matter (a proxy for the amount of sorptive material), and Ctot,i is the total (gas + particle) concentration of species i in the system. Kp,i depends on the saturation pressure of the species i, the type of sorptive process (adsorptive vs absorptive), and the properties of sorptive material (5, 6). Under the conditions of these experiments (isothermal dilution), Kp,i will be independent of dilution ratio (if one assumes it does not change as the composition of the particle phase changes with dilution). Therefore, CPM and Ctot,i determine the partitioning of the emissions; these concentrations are determined by the overall emission rate, the composition of emissions, and the amount of dilution. To predict the overall change in semivolatile organic mass one needs to apply eq 3 to each semivolatile compound in the system. This is a challenging task given the complexity of the emissions. Dilution samplers are typically operated with particleand organics-free air. This means that, inside the dilution sampler, CPM and Ctot,i will both scale as 1/DR (CPM may actually decrease faster than 1/DR due to changes in partitioning). Increasing the dilution ratio will reduce the semivolatile mass in the particle phase. However, in the real world, emissions are mixed with polluted background air, not particle- and organic-free air. This background pollution contributes to CPM and Ctot,i. As the exhaust becomes very dilute, the background concentrations will strongly influence the partitioning of the semivolatile material. To measure the partitioning that occurs under atmospheric conditions, eq 3 indicates that one needs to operate a dilution sampler such that CPM and Ctot,i of the diluted exhaust are at typical atmospheric levels. If the concentrations inside the dilution sampler are significantly greater than atmospheric levels, then too much of the semivolatile material will partition into the particle phase and the measurements will overestimate fine particle emissions relative to atmospheric levels of dilution. Conversely, if the diluted concentrations are significantly smaller than atmospheric levels, then one can underestimate fine particle emissions. These ideas are quantitatively explored in a companion paper that applies absorptive partitioning theory to the low-load diesel and wood smoke data (4). To assist in interpretation of measurements made with dilution samplers, researchers should report the PM or OC concentration (and temperature) of the diluted exhaust. The amount of dilution required to reach atmospherically relevant concentrations depends on the emission rate of semivolatile and sorptive material. For the sources considered here, the minimum OC concentrations of the diluted exhaust were between 100 and 200 µg-C/m3 at a dilution ratio of 350:1 for the low-load diesel and 510:1 for the wood smoke experiments. These concentrations are an order of magnitude higher than typical ambient OC levels, indicating that a dilution ratio of ∼5000:1 is needed to approach typical
atmospheric levels for these sources. The amount of fine particle mass at higher levels of dilution depends on the composition of the emissions. Figure 2 indicates that the changes in fine particle mass are most pronounced at low dilution ratios but that the PM2.5 mass emission rate may still be decreasing at a dilution ratio of 350:1. The fine particle emission rate of our diesel engine emissions is somewhat higher than other engines (13, 15), while our wood combustion emissions were somewhat lower (15-17). Dilution ratios less than 100:1 may be adequate to reach atmospheric levels for low-emitting sources such as natural gas combustion; conversely dilution ratios significantly larger than 10,000:1 may be needed for smoking cars (4). Although not considered in these experiments, the diluted exhaust also needs to be at ambient temperatures. Emission measurements at typical atmospheric concentrations are experimentally challenging; a more attractive approach may be to characterize the composition of the semivolatile species and then use a partitioning model to predict emissions at atmospheric conditions (4). The effects of dilution on partitioning also depend on the composition of the emissions. The exhaust must have significant levels of semivolatile material in order for partitioning to be important. For the sources considered here, the highest fine particle concentrations were several thousand micrograms per cubic meter at a dilution ratio of 20. Given these levels of emissions, eq 3 indicates that compounds with partitioning coefficients such that 1/Kp,i is in range of 1 and 5000 µg/m3 at ambient temperature will undergo phase transition as the exhaust is diluted to typical atmospheric concentrations. C-17 through C-29 n-alkanes are compounds commonly found in combustion exhaust with partitioning coefficients in this range; however, the vast majority of the semivolatile material has never been identified on a compound-by-compound basis (13). In order for the changes in partitioning to measurably alter the overall fine particle mass emissions, the total concentration of these semivolatile species (ΣCtot,i) must be comparable to the fine particle mass. Composition effects likely explain why dilution did not effect the fine particle mass emissions of the diesel engine operating under medium load even though the concentrations of PM in the diluted exhaust were a factor of 10 or more greater than typical ambient levels.
Acknowledgments The authors acknowledge Emily Weitkamp, Andy Grieshop, Mark Prack, Jessica Chiu, Neal Shyam, and R. Subramanian at Carnegie Mellon University for their assistance in conducting this research and an anonymous reviewer whose comments greatly improved the manuscript. This research was supported by the U.S. Department of Energy National Energy Technology Laboratory under Contract DE-FC2601NT41017.
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Received for review February 16, 2005. Revised manuscript received August 5, 2005. Accepted October 14, 2005. ES050319P
