First gaseous sulfuric acid measurements in automobile exhaust: Implications for volatile nanoparticle formation

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Atmospheric Environment 40 (2006) 7097–7105 www.elsevier.com/locate/atmosenv

First gaseous sulfuric acid measurements in automobile exhaust: Implications for volatile nanoparticle formation F. Arnolda,, L. Pirjolab,c, H. Aufmhoff a, T. Schucka, T. La¨hded, K. Ha¨merib,d a

Max-Planck-Institute for Nuclear Physics, Atmospheric Physics Division, P.O. Box 103980, D-69129 Heidelberg, Germany b University of Helsinki, Department of Physical Sciences, P.O. Box 64, FIN-00014 Helsinki, Finland c Helsinki Polytechnic, Department of Technology, P.O. Box 4020, FIN-00099 Helsinki, Finland d Finnish Institute of Occupational Health, Department of Physics, Topeliuksenkatu 41, FIN-00250 Helsinki, Finland Received 21 February 2006; received in revised form 25 June 2006; accepted 27 June 2006

Abstract Gaseous sulfuric acid (GSA) is thought to represent an important if not the most important nucleating gas present in modern diesel automobile exhaust. It triggers the formation of new aerosol particles, which grow by condensation and coagulation. Here we report on the first measurements of GSA in automobile exhaust. The experiment was made using a modern passenger diesel car equipped with an exhaust after-treatment system composed of an oxidation catalyst and a diesel-particle filter. The diesel fuel used had an ultra-low sulfur mass fraction of only 5  106. Measured GSA number concentrations reached up to 1  109 cm3. Freshly nucleated particles with diameters larger than 3 nm were also measured. The concentrations reached up to 1  105 cm3 and were positively correlated with GSA for GSA exceeding a threshold value in the range of 5  107–2  108 cm3. This suggests that GSA was involved in the formation of new volatile particles. r 2006 Elsevier Ltd. All rights reserved. Keywords: Sulfuric acid measurements; Diesel nanoparticles; Conversion factor; After-treatment system

1. Introduction Atmospheric aerosol particles present in urban environments and near motor ways in large number concentrations originate mostly from automobile exhaust and represent a potential health risk which is of major current concern. They include two major families namely solid soot particles with mean diameters around 80–100 nm (Kittelson et al., Corresponding author. Tel.: +49 622 1516 467; fax: +49 622 1516 324. E-mail address: [email protected] (F. Arnold).

1352-2310/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2006.06.038

2000; Harris and Maricq, 2001; Lehmann et al., 2003) and volatile nanoparticles with mean diameters below 20 nm (Kittelson, 1998; Kittelson et al., 2000; Khalek et al., 2000; Shi and Harrison, 1999; Pirjola et al., 2004; Giechaskiel et al., 2005; Ro¨nkko¨ et al., 2006). Compared to soot particles nanoparticles contribute much to the total particle number and surface but only little to the total particle mass. They intrude particularly deeply into the human lung and therefore may cause adverse health effects (e.g. Pope et al., 2002; Katsouyanni et al., 2001; Donaldson et al., 1998). However, nanoparticles are

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not considered by current air-quality regulations which are based on particulate mass rather than number or surface area. Nanoparticles are formed by nucleation in the diluting and cooling exhaust plume downstream of an automobile exhaust pipe but the nature of the nucleating exhaust gases is presently not known with certainty. Indirect evidence from previous investigations (Khalek et al., 2003; Vogt et al., 2003; Ntziachristos et al., 2004; Ro¨nkko¨ et al., 2006; Vaaraslahti et al., 2004, 2005) suggests that the key nucleating exhaust gas is the powerful aerosol precursor gaseous sulfuric acid (GSA) originating from fuel and lubricant oil. It was also suggested that modern exhaust aftertreatment using oxidation catalysts (OXICAT) tend to promote formation of nanoparticle at least with high-sulfur fuel and high engine loadings (Maricq et al., 2002; Vogt et al., 2003; Vaaraslahti et al., 2005). Diesel-particle filters (DFP) are expected to amplify this effect since the absence of the soot mode reduces GSA scavenging by pre-existing particles. For soot removal the OXICAT is used for NO oxidation to NO2 which in turn is used as an oxidant for soot burning in the DPF. Importantly NO2 allows soot burning already at typical exhaust gas temperatures (250 1C). In comparison the use of O2 as an oxidant requires much higher temperatures. However, in the OXICAT also sulfur IV (hereafter SIV) mostly sulfur dioxide formed during combustion undergoes oxidation. It is converted to sulfur VI (hereafter SVI) including mostly SO3 which ultimately undergoes conversion to sulfuric acid (see below). The SVI may lead to OXICAT poisoning resulting in reduced oxidation ability. In turn this results in reduced efficiency of soot removal. This undesired effect of fuel sulfur is a major reason for the recent drastic reduction of automobile diesel fuel sulfur. In 2005, a new automobile diesel fuel regulation came into operation in the European Community which limits the fuel sulfur mass fraction (FSC) to 5  105 or 50 ppmM. In some European countries the FSC is now even below 10 ppmM and the corresponding fuel is termed ‘sulfur-free’. Here the critical question arises whether this ultra-low sulfur fuel really prevents the formation of volatile nanoparticles in the exhaust of modern diesel cars equipped with an OXICAT/DPF system. However, GSA has so far not been measured in car exhaust. In fact, GSA measurements in car exhaust are difficult for several reasons. GSA

concentrations are expected to decrease rapidly due to exhaust dilution but more importantly also due to rapid GSA condensation on aerosol particles and eventually also due to GSA nucleation. Consequently, measurable GSA concentrations can be found only in the very young exhaust close to the car’s exhaust pipe. Ideally, GSA measurements should be made under real driving conditions on a road in the early exhaust plume at a short distance of around 1–3 m from the rear-end of the car’s exhaust pipe using a transfer tube as short as possible and with an inner diameter as large as possible. Currently, we are investigating the experimental setup of a future on-road measurement using a chemical ionization mass spectrometer (CIMS) setup with careful GSA calibration. In the course of these investigations we have recently made a first test with a stationary diesel car (without dynamometer) which yielded the first GSA measurements made in automobile exhaust. Here we report on the results of these first tests. GSA along with nanoparticles were measured in the exhaust of a diesel passenger car combusting ultralow sulfur fuel with a fuel-sulfur content, FSC, of 5 ppmM and which was equipped with an OXICAT and a DPF. Our measurements indicate that the GSA emission was large enough to induce efficient nanoparticle formation although the FSC was 10 times lower than tolerated by the most stringent present automobile-fuel-sulfur regulations (FSC ¼ 50 ppmM in the EU since 2005). Another paper (Pirjola et al., 2006a) reporting on model simulations supporting the interpretation of the present measurements is in preparation. 2. GSA and nanoparticle formation To begin we discuss a working hypothesis for GSA and nanoparticle formation in the exhaust of a sulfur-containing-fuel-combusting diesel car equipped with an OXICAT and a DPF. Fig. 1 shows an simplified schematic including the major processes. During combustion of sulfur-containing fossil fuel sulfur oxidation produces mostly SO2 and only very little SO3. For example, for aircraft gas turbine engines the fraction of fuel-sulfur conversion to SVI (F) was previously found to be only around 0.02–0.03 (see Katragkou et al., 2004; Sorokin et al., 2004). When the exhaust passes through the sufficiently heated OXICAT of a diesel car additional SO2 conversion to SO3 takes place

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COMBUSTION FUEL SULFUR

SO2 OXICAT

ENGINE AND EXHAUST LINE SO3

H2O NUCLEATION H2SO4

SAW

COM

SOOT

COAT-SOOT AP

COM-SAW AP

ATMOSPHERIC PARTICLES

YOUNG EXHAUST PLUME

AGED EXHAUST PLUME

Fig. 1. Simplified schematic of GSA and nanoparticle formation by a modern sulfur-containing-fuel-burning diesel automobile equipped with an exhaust after-treatment system including an OXICAT and a DPF. It includes the gaseous molecules SO2, SO3, H2SO4, and COM. Aerosol particles (marked by black boxes) are composed of sulfuric acid/water (SAW), COM and soot as well as internal mixtures of these. COAT-SOOT denotes soot particles coated by SAW and COM. AP denotes atmospheric aerosol particles. Processes taking place inside the cars engine and exhaust line, in the young exhaust plume, and in the aged exhaust plume are marked by large black, dark gray, and light gray boxes, respectively. When the OXICAT would be removed, SO2 conversion to SO3 would be reduced and when the DPF would be removed, soot processes (dotted arrows) would be increased. Both changes would reduce gaseous H2SO4 and thereby would reduce H2SO4-induced nanoparticle formation. For details see text.

and F might be much higher. However, F may be quite variable depending on the OXICAT temperature and the degree of OXICAT poisoning by sulfur. In the OXICAT and further downstream in the DPF a major fraction of the SVI and possibly also its precursor SO2 may become scavenged. The SO3 leaving the DPF undergoes conversion to GSA by reacting with water vapor molecules which are abundant in the exhaust. Additionally, CO2 is the most abundant primary combustion product. After leaving the exhaust pipe the exhaust cools very rapidly allowing the GSA saturation ratio to increase rapidly and eventually to become larger than unity. If so, GSA may undergo bimolecular H2SO4–H2O homogeneous nucleation leading to fresh sulfuric acid–water particles with initial diameters around 1 nm. Nucleation is expected to take place mainly at short distances from the exhaust pipe within a narrow distance interval

where the GSA supersaturation is largest (see Pirjola et al., 2006a). At smaller distances temperature is too large and at larger distances the GSA concentration becomes too small mostly due to exhaust dilution and also due to GSA scavenging by aerosol particles. Whether the GSA concentration reaches the threshold value required for efficient nucleation depends on the SVI emission at the rearend of the car’s exhaust pipe. The SVI emission increases with increasing FSC and increasing fraction F of SO2 converted to SO3. In turn F increases with OXICAT temperature and therefore with exhaust temperature. The latter increases with increasing fuel consumption. In the early exhaust plume the GSA concentration is also influenced by GSA condensation onto particles including soot particles formed by the engine, entrained atmospheric aerosol particles, and freshly formed nanoparticles. Use of a DPF very markedly reduces the number of soot particles and therefore GSA condensation is mostly due to

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entrained atmospheric particles and the newly formed fresh particles. Hence the absence of engine soot allows the built up of larger GSA supersaturations which tends to very markedly increase GSA nucleation leading to fresh nanoparticles. The fresh nanoparticles will grow by uptake of condensable gases including GSA and probably also condensable organic exhaust molecules (COM) whose chemical nature and concentrations are presently not known with certainty. The more nanoparticles are formed and the faster these nanoparticles grow, the larger will be the fraction of condensable gases which are taken up by nanoparticles rather than by soot particles or entrained atmospheric particles. Particularly for large nucleation rates additional nanoparticles’ growth may be caused by self-coagulation. Ultimately, the nanoparticles will become scavenged by larger particles (soot particles and atmospheric particles). The life time of 10 nm particles with respect to coagulation with larger particles depends on their concentration. However, the coagulation life time increases if the nanoparticles grow preferably by secondary COM condensation with growth rates of 1–20 nm h1 (Kulmala et al., 2004; Fiedler et al., 2005). This in turn allows for very substantial additional growth on the following sunny day. Thus the atmospheric residence time of nanoparticles increases with increasing particle diameter and decreases with increasing large-particle concentration. The above hypothetical scheme contains major uncertainties including particular highly uncertain GSA concentrations as well as COM concentrations and properties. A major open question is whether the new stringent fuel-sulfur regulations (FSCo50 ppmM; since 2005 in the EU and use of FSCo10 ppmM in some EU countries) are suffi-

cient to avoid GSA-induced nanoparticle formation in modern diesel automobile exhaust. 3. Experimental conditions and instrumentation Our present GSA and particle measurements were made in April 2005 in the exhaust of an ultra-low sulfur fuel (FSC ¼ 5 ppmM) combusting passenger diesel car (Peugeot 607, year 2004) which was equipped with an OXICAT and a DPF. The car was never operated with fuel whose FSC exceeded 10 ppmM. The measurements took place at Hyytia¨la¨ (Finland) in a remote location with only little atmospheric GSA pollution. Measurements were made while the car was stationary and the gas pedal was periodically moved down and up with a period around 12 min1. This pumping of the gas pedal continued for about 5 min and thereafter was stopped for about 3 min. The whole procedure was repeated many times during a total time span of about 3 h. By the gas pedal pumping the engine frequency was varied from 750–4000 rounds min1. The car’s exhaust was streaming into an open-end constant-volume sampling tunnel composed of a cylindrical tube (inner diameter 20 cm and length 10 m) through which ambient atmospheric air was passed by the action of an air blower (Fig. 2). The experiments were made in light windy conditions (wind speed 1–2 m s1 and wind direction mostly 2601 from the Peugeot’s heading) so that the wind carried away the exhaust gases and particles leaving the sampling tunnel not allowing these to accumulate at the test site. Inside the sampling tunnel the volume flow was 1100 m3 h1 and the gas flow velocity was 9.7 m s1 (35 km h1). The entrance cross-section of the sampling tunnel was nearly entirely open and therefore accessible to inflowing atmospheric air with the exception of the small CO2 and aerosol measurement

CIMS instrument

inner diameter 20 cm

air flow

d

exhaust tube P0

P1

2m

P2

8m

P3

Fig. 2. A schematic of the sampling setup. The distance d between the sites P0 and P1 was either 1 m or 0 m.

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central area occupied by the car’s exhaust pipe. The distance d between the exhaust pipe exit plane (position P0) and the entrance plane of the sampling tunnel (position P1) was either 0 m or 1 m (Fig. 2). The position P2 at which the GSA measurements took place was located 2 m downstream of P1. The position P3 at which the CO2 and aerosol measurements took place was located 10 m downstream of P1. For d ¼ 0 m radial expansion of the exhaust plume and therefore dilution were limited and the approximate exhaust age was 0.2 s (at P2) and 1 s (at P3). For d ¼ 1 m, the exhaust after leaving the exhaust pipe experienced rapid dilution and cooling due to mixing with the ambient air (temperature 285 K and relative humidity 40%). The exhaust ages at P2 and P3 are difficult to estimate since the gas flow velocities between P0 and P1 are not well known. The GSA measurements were made using a CIMS method developed by MPIK Heidelberg which has previously been used for the first GSA measurements in aircraft engine exhaust (Katragrou et al., 2004; Sorokin et al., 2004) and for measurements of atmospheric GSA (Fiedler et al., 2005; Aufmhoff et al., 2006). The CIMS setup consists of a flow-tube reactor, through which is passed the exhaust containing air from the exhaust plume. Reagent ions of the type NO 3 HNO3 which are generated in an external ion source equipped with a radioactive alpha-particle emitter 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 HSO 4 HNO3. Attached to the rearend of the flow-tube reactor is an ion-trap mass spectrometer which measures the abundances 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 reactor. The GSA detection limit is 4  105 cm3 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 Aufmhoff et al. (2006). Particle and gas measurements were performed by the instrumentation installed in a mobile laboratory Sniffer (Pirjola et al., 2004, 2006b) which

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was stationed at the rear-end of the sampling tunnel. Particle-size distributions were measured by the electrical low-pressure impactor (ELPI, Dekati Ltd.) with flow rate of 10 lpm (Keskinen et al., 1992). The ELPI with the electrical filter stage (Marjama¨ki et al., 2002) enables real-time particlesize distribution in the size range of 7–10 mm (aerodynamic diameter) with 12 channels. Additionally, the total number concentration of particles larger than 3 nm was detected by an ultra-fine condensation particle counter CPC 3025 (TSI, Inc.). CO2 was monitored by VA-3100 (Horiba) and NOx by Model APNA 360 (Horiba). Although the stationary experiment does not represent a simulation of real driving conditions it may contribute to a better understanding of exhaust processes involved in nanoparticle formation and growth. The major difference is a low engine load during the stationary pumping experiment resulting in only moderate exhaust temperatures Tex (o460 K). The latter tend to facilitate GSA nucleation by reduced thermal dissociation of sub-critical H2SO4/H2O clusters. On the other hand a low Tex also implies a low F which tends to reduce the GSA concentration in the early exhaust plume and thereby tends to reduce nucleation. A low Tex also tends to decrease the efficiency of COM destruction by the OXICAT which in turn tends to increase the COM concentration in the exhaust leaving the exhaust pipe. The latter may tend to increase particle growth by COM condensation. 4. Exhaust measurement results Fig. 3 depicts time-series of the measured excess CO2 concentration (DCO2 ¼ total CO2 minus ambient atmospheric CO2), the measured GSA concentration, the abundance ratio GSA/DCO2, the measured number concentration N3 of particles with diameters larger than 3 nm, and the abundance ratio N3/GSA. Note that before 14:15 LT the distance d was 0 m and after 14:46 LT d was 1 m. The measured CO2 mole fraction ranges between the atmospheric background of 370 ppmV and 2580 ppmV. Elevated carbon dioxide (DCO2) indicates an elevated fuel flow. Assuming rapid mixing in the sampling tunnel the variation of the measured CO2 reflects the variation of the exhaust flux to the total gas flux in the wind tunnel. This variation results from the variation of fuel flow induced by the pumping of the gas pedal (see above). For d ¼ 1 m the peak DCO2 concentrations

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Fig. 3. Time-series of parameters measured during the stationary experiment. Shown are the measured excess DCO2 concentration, the GSA, the abundance ratio GSA/DCO2, the measured number concentration of particles with diameters larger than 3 nm (N3), and the abundance ratio N3/GSA of excess N3 and excess GSA. Note that before 14:15 LT the distance d was 0 m and after 14:46 LT d was 1 m. See details in text.

are on average about 3–4 times lower compared with d ¼ 0 m. This reflects a 3–4 times larger exhaust dilution. The measured GSA varies between 1  106 cm3 (also ambient atmospheric background was measured by another CIMS instrument) and 1  109 cm3. It approximately co-varies with the CO2 concentration. However, the abundance ratio GSA/DCO2 varies substantially between 4  1010

and 2  106. Interestingly, GSA/DCO2 is larger during the final phase of the experiment when the temperature of the OXICAT/DPF system is expected to be the largest. An apparent conversion factor F (the ratio GSA/ St where St is the total exhaust sulfur) may be estimated from our data as follows. Considering FSC ¼ 5  106 one obtains a sulfur emission index of 9.4  1016 S-atoms per gram fuel burnt. With a

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CO2 emission index of 3 g CO2 per gram fuel burnt or 4.1  1022 CO2 molecules per gram fuel burnt, one obtains St/DCO2 ¼ 2.3  106 in the raw exhaust gas. In comparison the measured GSA/ DCO2 ranges mostly between 2  108 and 1.5  107. Hence, one obtains an apparent F ¼ (GSA/DCO2)/(St/DCO2) of about 0.01–0.07, but also higher values of up to 0.9 were obtained occasionally when GSA/DCO2 2  106. The highest apparent F corresponds to the final phase of the stationary experiment when the OXICAT temperature is expected to be highest and therefore F should be highest or possible desorption takes place from the OXICAT/DPF system. It is conceivable that for low OXICAT/DPF temperatures a significant fraction of SVI may be trapped by the OXICAT/DPF system. This SVI may be released as the OXICAT/DPF temperature increases. If so, this release may mimic additional SVI formation by the OXICAT. The above-inferred F may be underestimations if upstream of P2 a major fraction of GSA has already experienced conversion to particle sulfuric acid (PSA). The measured particle concentration N3 (particles larger than 3 nm) ranges between 1300 (ambient atmospheric background) and 1  105 cm3 (Fig. 3). It is positively correlated with GSA after GSA has exceeded a threshold value (see below). However the ratio N3/GSA varies markedly reaching up to 0.0013. Interestingly the largest N3/GSA are found in conditions when CO2 had just decreased from a local maximum to a local minimum. This may reflect lower exhaust temperatures favoring GSA nucleation. If so this would also imply that in these conditions when SVI production decreases due to a fall of the OXICAT temperature still sufficient SVI desorbs from the OXICAT/DPF system. The scatter plot of N3 versus GSA reveals pronounced threshold behavior (Fig. 4). As GSA rises beyond a critical level N3 rises very steeply by nearly a factor of 100 from about 1300 to nearly 1  105 cm3. The threshold GSA values range between 5  107 and 2  108 cm3. Probably, this behavior reflects nanoparticle formation by GSA nucleation leading to fresh particles with an approximate diameter of 1 nm which experience further growth to 3 nm preferably by condensation of supersaturated trace gases (GSA and COM) present in the exhaust. At first we investigate whether GSA is sufficient for growth. Fig. 5 shows an average particle-size distribution with the standard deviations as error

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Fig. 4. Scatter plot for the stationary experiment of N3 versus GSA with d ¼ 1 m.

Fig. 5. Mean size distribution with standard deviations of aerosol particles measured during a pumping period in the present experiment.

bars over a pumping period during the stationary experiment. The size distribution is dominated by very small particles with Dpo 40 nm and decreases steeply with increasing Dp by about three orders of magnitude. Due to the DPF the soot particles were filtered away. Assuming the measured particles to be composed exclusively of H2SO4/H2O solution with an expected H2SO4 mass fraction of 45% we estimate based on the measured particle-size distribution (Fig. 5) for the concentration of PSA ¼ 1.4  1010 cm3. In comparison for P2 the measured GSA often reached 5  108 cm3. However, according to model simulations (Pirjola et al.,

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2006a) the time span of only 0.8 s available between P2 and P3 is not sufficient for GSA depletion by the measured particles. Considering a mean particle diameter of 7 nm the lifetime of a GSA molecule with regard to collision with nanoparticles (i.e., the inverse number of the condensation sink, see e.g., Pirjola et al. 1999; Kulmala et al., 2001) becomes about 1000 s which is much larger than the above 0.8 s. It therefore seems that particle growth must have been due to condensation of one or several primary COM gases. Then a COM concentration much larger than the measured sulfuric acid would be required. Importantly, the primary COM in question did not undergo nucleation. Otherwise much larger N3 should have been observed (Pirjola et al., 2006a). 5. Conclusions The major findings obtained from our present experiments may be summarized as follows: (1) Measured GSA concentrations range from 1  106 to 1  109 cm3 and were only crudely correlated with the primary combustion product gas CO2. (2) The apparent fraction F of fuel-sulfur conversion to GSA inferred from the measurements ranges mostly from 0.01 to 0.07. This indicates that the OXICAT temperature was only temporarily high enough to allow the OXICAT to substantially contribute to SVI formation. (3) Maximum concentrations of detectable particles with diameters larger than 3 nm N3 reach up to about 1  105 cm3. (4) Measured concentrations N3 increase steeply with increasing GSA after GSA exceeds a threshold value in the range of 5  107–2  108 cm3. This behavior strongly suggests that primary GSA is the key nucleating gas. (5) Growth of fresh particles with an initial diameter around 1 nm to measurable sizes is not due to GSA but requires far more abundant condensable gases, probably primary COM. Future measurements should include high-time resolution measurements of total sulfuric acid and involve diesel car engine operation with engine loads and OXICAT temperatures typical of roaddriving conditions. Attempts should also be made to

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