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First Online Measurements of Sulfuric Acid Gas in Modern HeavyDuty Diesel Engine Exhaust: Implications for Nanoparticle Formation F. Arnold,*,†,‡ L. Pirjola,§,∥ T. Rönkkö,⊥ U. Reichl,† H. Schlager,‡ T. Laḧ de,§,⊥ J. Heikkila,̈ ⊥ and J. Keskinen⊥ †

Max Planck Institute for Nuclear Physics (MPIK), P.O. Box 103980, D-69029 Heidelberg, Germany Deutsches Zentrum für Luft und Raumfahrt (DLR), Oberpfaffenhofen, Germany § Department of Technology, Metropolia University of Applied Sciences, P.O. Box 4021, FIN-00180 Helsinki, Finland ∥ Department of Physics, University of Helsinki, P.O. Box 64, FIN-00014 Helsinki, Finland ⊥ Aerosol Physics Laboratory, Department of Physics, Tampere University of Technology, P.O. Box 692, FIN-33101 Tampere, Finland ‡

ABSTRACT: To mitigate the diesel particle pollution problem, diesel vehicles are fitted with modern exhaust after-treatment systems (ATS), which efficiently remove engine-generated primary particles (soot and ash) and gaseous hydrocarbons. Unfortunately, ATS can promote formation of low-vapor-pressure gases, which may undergo nucleation and condensation leading to formation of nucleation particles (NUP). The chemical nature and formation mechanism of these particles are only poorly explored. Using a novel mass spectrometric method, online measurements of low-vaporpressure gases were performed for exhaust of a modern heavy-duty diesel engine operated with modern ATS and combusting low and ultralow sulfur fuels and also biofuel. It was observed that the gaseous sulfuric acid (GSA) concentration varied strongly, although engine operation was stable. However, the exhaust GSA was observed to be affected by fuel sulfur level, exhaust after-treatment, and driving conditions. Significant GSA concentrations were measured also when biofuel was used, indicating that GSA can be originated also from lubricant oil sulfur. Furthermore, accompanying NUP measurements and NUP model simulations were performed. We found that the exhaust GSA promotes NUP formation, but also organic (acidic) precursor gases can have a role. The model results indicate that that the measured GSA concentration alone is not high enough to grow the particles to the detected sizes.

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sulfuric acid (GSA).6,7 GSA has a very low saturation vapor pressure, and therefore, it may condense and even nucleate in the cooling dilution process of the exhaust. Thus, the existence of GSA can lead to formation and growth of sulfuric acid−water particles, a particular form of nucleation particles (NUP). Due to the small sizes the NUP can intrude the lowest compartment of the human lung.3,8,9 Other possible oxidation products are partially oxidized hydrocarbons. These may include also condensable gases, particularly organic diacids, some of which possess very low vapor pressures and therefore would be potential condensing and eventually even nucleating gases. In fact, organic diacids have been observed in car exhaust.10,11 Additionally, oxidation products may include also carcinogenic compounds like oxygenated polycyclic organic compounds, particularly ones bearing a NO2 group (Nitro-PAH́ s), whose formation may be promoted by NO2, and some of which may

INTRODUCTION Exhaust aerosol particles emitted by traffic, especially by diesel vehicles, represent major air pollutants in cities and near motorways.1−3 In order to minimize these emissions, modern diesel vehicles are fitted with exhaust after-treatment systems (ATS) which decreases efficiently the solid soot particle and gaseous emissions. Typically, the ATS with quasi-continuous regeneration involve a combination of a diesel particle filter (DPF) 4 and a diesel oxidation catalyst (DOC).5 The most efficient DPFs are so-called wall-flow DPFs, which trap more than 95% of the soot particles. However, wall-flow DPFs are subject to relatively rapid clogging by soot; thus, they require active regeneration and, e.g., fuel additives. Nearly continuous soot regeneration is often achieved by NO2-induced soot burn up. The NO2, which acts as an oxidant already at typical heavyduty diesel exhaust temperatures, is generated by catalytic conversion of engine-generated NO using a DOC upstream of the DPF. Unfortunately, the oxidative exhaust after-treatment may also generate undesired oxidation products. A striking example is SO3, which is formed by oxidation of enginegenerated SO2 and reacts with water vapor, leading to gaseous © 2012 American Chemical Society

Received: Revised: Accepted: Published: 11227

June 16, 2012 September 11, 2012 September 17, 2012 October 4, 2012 dx.doi.org/10.1021/es302432s | Environ. Sci. Technol. 2012, 46, 11227−11234

Environmental Science & Technology

Article

Table 1. Driving Parameters with Maximum Ranges of Variationa ESC mode

load (%)

10 12 13 11

100 75 50 25

a

engine speed (rpm) 1800 1800 1800 1800

± ± ± ±

5 5 5 5

torque (Nm) 1676 1258 838 420

± ± ± ±

10 2 2 2

power (kW) 316 237 158 79

± ± ± ±

exhaust temperature (°C)

2 2 2 2

430 ∼370 ∼330 ∼305

± ± ± ±

15 15 15 15

air mass flow (kg/h) 1455 1280 1035 725

± ± ± ±

20 30 20 20

fuel consumption (kg/h) 64 49 34 20

± ± ± ±

1 2 1 1

Exhaust temperature was stabilized to the presented values within 10 min.

oxidative ATS nucleation mode formation seems to be a sulfurdriven process but, e.g., without any ATS the initial particle formation seems to be a high-temperature process followed by condensation or heterogeneous nucleation of semivolatile compounds during cooling dilution.20 Due to the small NUP mass, direct NUP composition measurements could so far not be realized, neither quantitative online measurements of NUP precursor gases (GSA and organics). First, this paper reports on the online measurements of acidic exhaust gases, including also top candidates for NUP formation, particularly GSA and possible dicarboxylic acids. To our knowledge, these are the first GSA measurements for heavyduty diesel engine at realistic engine operational conditions and with fuels with different FSCs. Second, we show for one combination of fuel and exhaust ATS how GSA affects nucleation particle formation. For that reason, exhaust particle number concentration and size distribution were measured after the exhaust dilution system. Furthermore, the experimental study was complemented by model studies on particle formation and growth.

be condensable gases. However, although the oxidative exhaust after-treatment can cause the increases of certain undesired emissions like GSA and thus increases of NUP emissions, it should be keep in mind that typically the exhaust aftertreatment decreases the total emissions (e.g., of hydrocarbons) significantly. Exhaust sulfur compounds may experience partial storage in the ATS at low exhaust temperatures and eventually release at higher exhaust temperatures. In principle, this can lead, e.g., to short-time high-concentration emission peaks especially at transient driving conditions. In addition, storage of sulfur compounds in the DOC is highly undesirable since it decreases the DOC oxidation power (“sulfur poisoning”)12 and thus increases emissions of gaseous hydrocarbon compounds. Therefore, the tolerated upper limit of the diesel fuel sulfur content (FSC; hereafter always in mg S/kg fuel) has been reduced in the European Union to 50 (since 1995) and 10 (starting from 2009). In the United States and Japan the FSC limits are presently 15 (from 2006) and 10 (from 2007), respectively. Previous investigations related to the exhaust particle emission of oxidative ATS-fitted diesel vehicles, made in engine test laboratories and during on-road driving,13−15,2,16,17 indicate NUP diameters around 5−15 nm and NUP number concentrations of up to about 108 cm−3 (calculated to raw exhaust). The NUP number concentration, diameter, and volume concentration have been found to decrease significantly when the FSC has been lowered.13,14,18,19 The particles are uncharged,20 they grow upon humidification like sulfates,21 and when heated they evaporate at temperatures from 50 to 250 C.15,21 These findings suggest that a sulfur-bearing exhaust gas, presumably gaseous sulfuric acid (GSA), is involved in NUP formation and growth.22,23 The particle characteristics and the fact that the dilution parameters affect particle formation indicate that particle formation takes place during exhaust cooling and dilution. One indication of the role of GSA in particle formation is reported by Arnold et al.,23 who measured GSA in the diesel exhaust of a passenger car under idling conditions by periodically pumping the pedal to vary the engine speed from 750 to 4000 rpm. They observed that the number concentration of particles larger than 3 nm in diameter was positively correlated with GSA, indicating that GSA could be involved in particle formation if its concentration exceeded a threshold value. However, several questions related to the chemical nature and mechanism of formation of NUP are still open, particularly for modern diesel exhaust when an ATS and low or even ultralow sulfur fuels are used. Besides binary H2SO4−H2O nucleation,24,48,25 other nucleation mechanisms, also relevant under atmospheric conditions, such as activation nucleation, kinetic nucleation, ternary nucleation,44 hydrocarbon nucleation, ion-induced nucleation, and heterogeneous nucleation onto solid core particles,26 have been suggested. It should be noted that the nucleation mode particle formation seems to be strongly dependent on ATS choices; with an

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MATERIALS AND METHODS Measurements were performed with a Euro IV heavy-duty diesel engine on an engine dynamometer. Measurements were conducted at four steady-state driving conditions (modes 10− 13 of European Stationary Cycle, ESC). Driving parameters (engine speed, torque, power, load, intake air mass flow, fuel consumption, and stabilized exhaust temperature) for each mode are presented in Table 1. The order and duration of the modes were not similar in each day. For example, to study the dependence of the nucleation mode particle number on GSA, full load driving dominated the tests (Figure 2). In order to produce different sulfur compounds concentrations to the exhaust, different fuels and exhaust aftertreatments were used. Three diesel fuels were used; low-sulfur standard fuel (FSC = 36 ppm), ultralow sulfur standard fuel (FSC = 6 ppm), and biofuel (FSC < 1 ppm, under detection limit). The fuel referred to here as biofuel consisted of hydrotreated vegetable oils (HVO), having very similar composition like Fischer−Tropsch fuels. The biofuel contained essentially no aromatic compounds, sulfur, or oxygen. Exhaust after-treatment systems were an oxidation catalyst 1 with a wall-flow diesel particle filter (DOC1 + CDPF) and an oxidation catalyst 2 with an open-channel diesel particle filter (DOC2 + ODPF). In addition to the DOC, catalyst material was used also in DPF and ODPF. With ultralow fuel and biofuel some of the measurements were also repeated without any ATS. Before each combination of the ATS and fuel, the engine was warmed more than 0.5 h at ESC 12. Simultaneously, we checked the function of the sampling system and instruments. Exhaust Sampling. The measurement setup is shown in Figure 1. For the GSA measurements the exhaust was sampled and diluted using an ejector-type diluter with heated dilution air 11228

dx.doi.org/10.1021/es302432s | Environ. Sci. Technol. 2012, 46, 11227−11234


Article

reactor. The GSA detection limit is 4 × 105 cm−3, and the time response of the CIMS instrument is 1 min. The uncertainty of the measured GSA concentrations of GSA present in the flow tube reactor is ±30%. Details of the instrument can be found in ref 30. The additional acidic trace gases HX were measured in a way analogous to GSA via the reaction NO3−HNO3 + HX → XHNO3− + HNO3

(R1)

In order to undergo this reaction, a monoacid HX must possess a gas-phase acidity larger than that of HNO3. A dicarboxylic acid may undergo reaction R1, even when its gas-phase acidity is not larger than that of HNO3. We investigated the reactions for some monocarboxylic acids in the laboratory, and the corresponding measured rate coefficients were close to 2 × 10−9 cm3 s−1.32 In the following, we assume a maximum possible rate coefficient of 2 × 10−9 cm3 s−1 for all acids. For H2SO4, this rate coefficient is confirmed. For all other gases it represents an upper limit. Hence, for these other acids, we obtain only a lower limit concentration. Aerosol Particle Measurement Equipment. Number size distributions of the exhaust particles were measured using two scanning mobility particle sizers, SMPS.33 One SMPS (particle diameters 3−60 nm) was equipped with DMA 3085 (TSI Inc.) and CPC 3025 (TSI Inc.), and the other (particle diameters 10−430 nm, used to ensure DPF function) with DMA 3071 (TSI Inc.) and CPC 3375 (TSI Inc.). Electrical Low Pressure Impactor ELPI (Dekati Inc.)34 was used to control the stability of the particle emission during the SMPS scans. Particle losses in the instrument were corrected. Due to evolution of particle size distribution in the dilution system, losses in the primary diluter and aging chamber were not corrected. The length of the sampling line from the secondary diluter to the particle instruments was minimized. Particle volatility was studied with a thermodenuder TD, see, e.g., ref 27, where the diluted sample was heated to 265 °C and after that led into the denuder where the cooled inner wall was covered with active charcoal to collect evaporated compounds. TD was used 1−2 times for each combination of driving mode, fuel, and ATS. The size distributions measured after the TD were corrected for particle losses,35 and particles removed by the TD treatment from size range > 3 nm are considered here as volatile particles. Particle mode geometric mean diameters (GMD) and number concentrations were calculated by fitting log-normal distributions to the measured data. Particle volume concentrations were calculated from fitted number size distributions assuming that particles are spherical. Model Description. Model simulations were performed by an updated version of an atmospheric chemistry and aerosol dynamics box model AEROFOR.36,37 The model includes gasphase chemistry, formation of thermodynamically stable clusters by homogeneous H2SO4−H2O nucleation 38 and by activation/kinetic nucleation,39 condensation of H2SO4, H2O, and an organic vapor onto particles,42 Brownian coagulation of particles,43 temperature and cooling profiles,25 wall losses,45 as well as mixing with the particle-free dry diluted air. To minimize the effect of numerical diffusion, 100 size sections were used in this work. On the basis of the CIMS measurements during this campaign, a condensable organic vapor was assumed to be adipic acid. The set of stiff differential equations describing the time evolution of the particle number concentrations in each section as well as the vapor

Figure 1. Measurement setup. Sample flow control was conducted using pumps (P) and mass flow controllers; sample was diluted using a porous tube diluter (PD) and ejector-type diluters (ED).

(250 °C). In addition, the sample transfer line was heated to 200 °C to prevent particle formation and wall losses of GSA. In order to measure particles, a partial flow sampling system,27 similar to that used in refs 18 and 16, was used to sample and dilute the exhaust. The system has been shown to mimic the real-world NUP formation and growth processes relatively well.28,16 It consisted of a porous tube-type primary diluter followed by an aging chamber and an ejector-type diluter. In the primary diluter, the dilution air temperature was 30 °C, relative humidity was close to zero, and dilution ratio was around 12. The aging chamber was used to ensure adequate residence time for the condensational growth of the NUP in the cooled and diluted aerosol sample. The following ejector diluter (dilution ratio = 8) was used to bring the sample into ambient pressures and ensure that the particle number concentration was in the measurement range of particle instruments, without significant effect on particles formed during exhaust dilution and cooling.29 Final dilution ratio values were calculated from the measured CO2 concentrations of the diluted exhaust sample and raw exhaust. CIMS Instrument. For online detection of gaseous strong acids and strong bases we used a powerful, selective, sensitive, and fast chemical ionization ion-trap mass spectrometry (CIITMS) method originally introduced by MPIK Heidelberg30,31 and improved by a collaboration of MPIK Heidelberg and DLR Institute for Atmospheric Physics. The CIMS (chemical ion mass spectrometer) setup consists of a flow tube reactor through which is passed the diluted exhaust. Reagent ions of the type NO3−HNO3 which are generated in an external ion source equipped with a radioactive alpha-particle emitter or a corona discharge are introduced into the flow tube reactor. In the flow tube reactor the reagent ions undergo an ion molecule reaction with gaseous H2SO4 molecules leading to product ions of the type HSO4−HNO3. Attached to the rear end of the flow tube reactor is an ion trap mass spectrometer which measures the abundance of the different ion species. From the measured abundance ratio of product and reagent ions the GSA concentration in the flow tube reactor can be determined by consideration of the rate coefficient (known from laboratory measurements) of the requisite ion molecule reaction and the time span during which ions are passed through the flow tube 11229



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Figure 2. Time series of the measured gaseous sulfuric acid mole fraction (MF) and engine load for FSC = 6 ppm and ATS = DOC1 + CDPF. Also shown are the variations of GSA due to the load changes and engine stops. Engine stops were to ensure the security during instrument checks and maintenance. CIMS data are lacking at, e.g., 14:45 and 16:30 because of positive ion mode tests with CIMS, not relevant from the viewpoint of this study. It should be noted that the GSA concentrations were relatively high when sampling from the exhaust line was continued during engine stops (e.g., after the full load from 17:30 until 18:00). GSA detection limit was ∼3.6 × 10−14 MF.

studies (e.g., refs 48 and 25) the GSA concentrations are based on indirect approximations, whereas the measured concentrations have been used for the simulations in this paper.

concentrations was solved using Numerical Algorithms Group, Ltd. library FORTRAN-routine D02EJF.46 Only the results from the model simulations for FSC = 6 ppm at 100% engine load (T = 659 K) and with DOC1 + CDPF are shown here. The raw exhaust diluted rapidly by dry air (T = 303 K). On the basis of the raw and diluted CO2 measurements the final dilution ratio in this case was 15. The cooling time constant was chosen to be 0.03 s based on the temperature measurements, and the dilution time constant was assumed to be 0.12 s. The total simulation time was 2.7 s, from which 2.6 s was in an aging chamber. The initial raw exhaust GSA concentration varied in the range from 4.2 × 109 to 3 × 1011 cm−3 adopted from our measurements. The condensable organic vapor concentration is a free parameter. Simulations were also repeated when no condensable organic vapor existed. Due to the use of CDPF the initial particle number concentration was set to zero. Nucleation was assumed to occur by activation nucleation, i.e., nucleation is thought to happen as activation of small clusters containing one sulfuric acid molecule via, e.g., heterogeneous nucleation or heterogeneous chemical reactions; organic vapors are typically needed as condensing agents.47 The nucleation rate of critical clusters (diameter ≈ 1.5 nm) can be assumed to depend linearly on sulfuric acid J = A[H2SO4]; the activation coefficient A = (1−5) × 10−4 s−1 was chosen so that the particle number size distribution at the end of the aging chamber matched with the measured one. This A value is an extreme upper limit of the A values presented in ref 39 based on atmospheric measurements in urban areas. Note that the pre-exponential value A includes the effect of H2O concentrations and temperature during nucleation as in ref 40. The nucleation mechanism for diesel exhaust is not yet known, and selecting activation nucleation does not rule out other mechanisms. For example, Du and Yu48 suggest based on their improved kinetic quasi-unary nucleation model simulations that binary homogeneous nucleation is the main source of nanoparticles for vehicles equipped with continuously regenerating diesel particle filters. Note that in other model

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RESULTS AND DISCUSSION Exhaust GSA. Figure 2 presents the time series of the measured gaseous sulfuric acid mole fraction for the engine equipped with DOC1 and CDPF. The ultralow sulfur fuel (6 ppm) was used. It can be seen that the GSA mole fraction is not stable, although the engine is operated at steady driving mode; this is particularly emphasized during the 100% engine load periods when the exhaust GSA mole fraction increased even more than 2 orders of magnitude. The lowest GSA values indicate that only an extremely small fraction of fuel sulfur was converted to GSA or that the majority of sulfur is stored in ATS and exhaust line. At the end of the three high-load test periods the GSA mole fractions were 5.0 × 10−9, 1.8 × 10−8, and 2.8 × 10−8, respectively (Figure 2). Because of the variations in the GSA mole fractions due to storage and release effects, only the values at the end of the test periods are used below when the effects of the driving conditions, FSC, and ATS on GSA are shown. In summary, the storage and release effects in ATS are important issues. However, to study formation of nucleation mode particles and their dependence on GSA concentration, it is enough to have simultaneously measured high-resolution temporal data for the GSA concentrations and particle number size distributions. Figure 3 shows the engine load versus the GSA mole fraction (converted to the undiluted raw exhaust). All three fuel/ATS combinations are shown, in addition to the reference measurement without exhaust after-treatment (see the legend in Figure.3). The fuel, practically the fuel sulfur content, clearly affects the GSA if the engine is equipped with an oxidative ATS; lowest GSA values were measured for the sulfur-free biofuel and highest for the fuel with FSC = 36 ppm. For all fuels, GSA increases with increasing engine load. This reflects the combined effect of the decreasing air-to-fuel ratio and the increasing exhaust temperature. The decreased air-to-fuel ratio 11230



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Figure 3. Variation with engine load of experimental GSA mole fraction. Data refer to the undiluted raw exhaust and the conditions when GSA has reached a near constant value during the corresponding engine load phase. Four different measurement runs with different FSC and ATS scenarios were used; a wall-flow diesel particle filter with an oxidation catalyst 1 (DOC1 + CDPF) and an open diesel particle filter with an oxidation catalyst 2 (DOC2 + ODPF). FSC < 1 (ppm) refers to the biofuel, whereas FSC = 36 (ppm) and FSC = 6 (ppm) refer to the standard diesel.

Figure 4. Experimental total mole fractions (lower limits) of acidic gases HX (other than GSA) versus measured GSA at high engine load (100%). Data refer to the undiluted raw exhaust: (open circle) FSC < 1, no ATS; (filled black circle) FSC = 6, no ATS; (filled gray triangle) FSC < 1, DOC1 + CDPF; (filled black triangle) FSC = 6, DOC1 + CDPF; (open diamond) FSC = 36, DOC2 + ODPF. For comparison, the background value (black cross) is also shown.

results in an increased SO2 abundance in the exhaust, and the increased exhaust temperature and therefore also the increased oxidation catalyst temperature results in an increased SO2 oxidation. For standard fuel with FSC = 6, the maximum ratio of the GSA-sulfur and total exhaust-sulfur is 0.3% (without ATS) and 13% (with ATS). Hence, oxidative ATS induces very substantial additional sulfur oxidation and GSA formation. This conversion ratio is in good agreement with Arnold et al.,23 who found the apparent ratio of fuel-sulfur conversion to GSA mostly in the range of 1−7%. However, when these conversions are compared to reported conversions of SO2 in oxidative exhaust after-treatment,49 the conversion ratios here are substantially lower. This indicates that the fuel sulfur was stored in the exhaust after-treatment or the sulfur is included in other compounds than GSA. However, it should be noted that the GSA and the effect of engine load on it was also observed in this study when the sulfur-free biofuel was used. This can be seen as an indication of the role of lubricant oil (here the lubricant oil sulfur content was 0.23%) in the GSA formation. In previous studies the link between lubricant oil sulfur content and so-called sulfur-driven nucleation has been observed,15,18,19 but the exact particle formation mechanism has been an open question due to the lack of GSA measurements. Besides GSA, lower limit mole fractions of numerous additional gaseous acids (hereafter termed HX) were detected in the exhaust. Several HX species could be identified by fragment ion analysis as dicarboxylic acids, including oxalic, malonic, succinic, glutaric, and adipic acid; however, other detected HX species we could as yet not identify unambiguously. Figure 4 shows the upper limit of the total mole fraction of the measured acidic gases other than GSA versus the mole fraction of the stabilized GSA, measured at the 100% engine load tests and at the end of the test period. The mole fraction of these acids ranged between about 10−9 and 10−7. Without aftertreatment systems, the concentration of HX was four times higher for standard fuel (FSC = 6) compared to biofuel. This difference must be due to the sulfur and/or aromatics content of standard fuel. With ATS, the mole fraction of HX is correlated with GSA for all three fuels (dotted line in Figure 4).

Here, the mean molar ratio HX/GSA is about 1, compared to 28 without ATS. For biofuel, the mole fraction of HX is 16 times smaller when an ATS is used. Probably, this reflects oxidative HX destruction (HX burn up) by the ATS. In general, it seems that an elevated abundance of GSA (or its precursor SO3) would promote an elevated HX abundance. It is at least conceivable that storage of sulfur compounds may decrease HX losses associated with burn up or storage of HX. Another even more speculative possibility is an eventual role of sulfur compounds in HX formation. Still another possibility, which cannot entirely be excluded, is chemical conversion of HSO4− cluster ions in the flow reactor. This conversion may involve very acidic exhaust gases. GSA and Particles. The engine fueled by the ultralow sulfur standard fuel (FSC = 6 ppm) and equipped with the wall-flow diesel particle filter represents well the present vehicle technologies. Therefore, this combination was chosen for nucleation particle studies. Clear correlation was observed, when the data of the GSA measurements was compared to the particle concentrations and size distributions measured after cooling dilution and (short) aging of exhaust. Figure 5 shows this correlation for the measurement at high 100% load with FSC = 6 and DOC1 + CDPF system. Due to the wall-flow DPF, the soot particles were efficiently removed from the exhaust and the aerosol number size distribution consisted exclusively of NUP (diameters = 4.0−5.5 nm). These NUP were removed when the thermodenuder was used; thus, they were volatile at T = 265 °C. Figure 5 shows the total number concentration of NUP (converted to raw exhaust) versus GSA (raw exhaust). An example of the measured particle size distribution is in Figure 6. Figure 5 shows that NUP increases linearly with increasing GSA, and the slope in the log−log scale is close to unity. Due to the short residence time of 2.7 s and relatively rapid dilution, the effect of coagulation is low, and due to the linear growth rate of the nucleated particles, the slope of unity in Figure 5 might indicate that also the nucleation rate (i.e., formation rate of 1.5 nm particles) for these cases is proportional to GSA at power one.39,41 Verification of this statement will be addressed by our future work. However, a 11231



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Figure 7. Total particle volume concentration (undiluted raw exhaust) versus GSA concentration (undiluted raw exhaust). Also shown are the modeled values for different GSA concentrations in the range from 4 × 109 to 3 × 1011 cm−3 with a condensable organic vapor concentration of 0 (open triangle) and 4 × 1011 cm−3 (open circle), respectively. In this case, DOC1 + CDPF was used at EL = 100% and FSC = 6 ppm.

Figure 5. Measured NUP concentration by nano-SMPS (converted to undiluted raw exhaust) versus measured GSA concentration by CIMS (converted to undiluted raw exhaust). Also shown is the line with a slope of unity. Open triangles and open circles refer to the modeled values for GSA from 4 × 109 to 3 × 1011 cm−3 along with a condensable organic vapor concentration of 0 and 4 × 1011 cm−3, respectively. In this case, DOC1 + CDPF was used at EL = 100% and FSC = 6 ppm.

Figure 5. V increases linearly with the GSA with a slope somewhat higher than unity. This suggests that the NUP grow by an uptake of GSA and/or other condensable acidic gases whose total concentration is proportional to GSA (Figure 4). Also shown in Figure 7 are the modeled results for the engine load of 100% and different GSA concentrations. These results again indicate that GSA alone is not sufficient to grow the particles to the observed volume; however, if the condensable organic vapor concentration is ∼4 × 1011 cm−3 (model 2) the modeled particle volume agrees with the experimental value. The major conclusions drawn from our present investigations are as follows. • The modern diesel vehicle ATS promotes formation of gaseous sulfuric acid. Simultaneous particle and GSA measurements indicate that the GSA is a key nucleating gas involved in the NUP formation for the engine equipped by a DPF. • The ATS does not prevent totally formation of organic condensable gases. These contribute markedly to the NUP growth. • The NUP precursor gases can experience strong storage in and release from the ATS. • The modern ATS-fitted diesel vehicle, even when combusting ultralow sulfur fuel with an FSC = 10, produces at high load (when catalyst temperature is high), roughly as much sulfuric acid as the same vehicle without ATS combusting formerly used fuel with an FSC = 300. However, decreasing FSC might increase the contribution of lubricant oil. NUP are not directly regulated, not even by the most stringent of the presently operational air-quality and emission regulations, since these are particle mass rather than particle size and number oriented. Considering their strong lung intrusion efficiency,53 their role as potential carriers of highly acidic carcinogenic compounds, and their significant role in urban aerosol, NUP deserve increased future attention.

Figure 6. Measured and modeled number size distribution of exhaust particles after the aging chamber when GSA concentration is 3 × 1011 cm−3 and no condensable organic vapor is present and when the condensable organic vapor concentration is 4 × 1011 cm−3. x axis refers to wet diameter.

similar behavior has been found for nucleation in ground-level air in several places.50−52 Also shown in Figure 5 are four AEROFOR36,37 model simulations for GSA concentrations from 4 × 109 to 3 × 1011 cm−3. Simulations were performed for an engine load of 100%, with ultralow fuel and DPF. Model simulations were made to study more details of the role of condensable vapors in growing the particles. If no condensable organic vapor is present (model 1), the nucleated particles are not able to grow large enough (Figure 6); however, if the condensable organic vapor concentration is 4 × 1011 cm−3 (model 2), the modeled results match very well with the measurements (Figures 5 and 6). The mean mode diameters in these two cases are 3.8 and 5.6 nm, respectively, whereas the measured one is 5.5 nm. The number concentration of the particles larger than 3 nm is 4.2 × 106 and 7.9 × 106 cm−3, respectively, whereas the measured value is 6.1 × 106 cm−3. Figure 7 shows the NUP volume concentration V (raw exhaust) versus GSA (raw exhaust) for the same points as in

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Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Max Planck Society, DLR, Ecocat Oy, Neste Oil Oyj, and the Finnish Funding Agency for Technology and Innovation (TEKES). F.A. was also funded via a Max-Planck-Research award (Physics).

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