Environ. Sci. Technol. 1999, 33, 3730-3736
Investigation of Ultrafine Particle Formation during Diesel Exhaust Dilution JI PING SHI AND ROY M. HARRISON* Institute of Public and Environmental Health, The University of Birmingham, Edgbaston, Birmingham, B15 2TT, U.K.
Measurements of the size distribution of particles emitted from a modern heavy duty diesel engine using fuel with a sulfur content of between 0.03 and 0.05% by mass have been made under constant engine operating conditions, but with variations in the humidity of dilution air and dilution ratio prior to particle size measurement. The results show clearly that the measured size distribution is crucially dependent upon the conditions of dilution, hence creating real difficulties for comparison of data between different investigators. Conditions of high dilution ratio and high relative humidity both tend to favor the production of nanoparticles, especially within the range below 50 nm diameter. Application of homogeneous nucleation theory shows that nanoparticle production during dilution is qualitatively consistent with the production of sulfuric acid, but the predicted nucleation rates are lower than those measured, in common with studies of nucleation in the atmosphere. Chemical analysis of size-fractionated particles shows enhancement of sulfate concentrations in humid dilution conditions and at high dilution ratios consistent with the above mechanism. The possible role of semivolatile organic compounds in these processes has not been investigated.
Introduction Various researchers (1, 2) have investigated the number concentration and size distribution of particles emitted from diesel engines. It is widely accepted that the volume and mass concentration of particles in the exhaust of diesel engines has been reduced steadily over the past 20 years due to the application of new technologies. However, knowledge of particle number concentration, which is strongly influenced by ultrafine particle formation during dilution, is very limited. Moreover, current regulation of road vehicle particle emissions is based on mass concentration, but according to some mechanistic theories the number of particles to which the individual is exposed is more important than their mass (3). For this reason, and the fact that ultrafine particles contribute much to number but little to mass emissions, it is important to evaluate both the mass and size distribution of engine emitted particulate matter. A comprehensive study of fine particles in diesel exhaust has been carried out from 1996 to 1998 using the engine test beds at Perkins Engines Company Limited. More than 14 11-mode (similar to ECE-49) runs as well as numerous fivemode and single mode runs were carried out to investigate the characterization of particles in the exhaust (4). We found * Corresponding author phone: 0121 414 3709; fax: 0121 414 3494; e-mail: R.M.Harrison.
[email protected]. 3730
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 33, NO. 21, 1999
that under specific dilution conditions, a large number of ultrafine particles were formed during dilution. Ultrafine particle formation during dilution processes has also been reported by Abdul-Khalek et al. (5). Humidity, dilution ratio, and residence time were found to affect the particle size distribution. The mechanism of the effect of dilution conditions is not clear, but the implication is that comparison of emissions data between different researchers using differing dilution conditions is problematic. Additionally, if atmospheric dilution processes affect the number and size of particles emitted from diesel engines, there are implications for human exposure and possible consequent health effects. In earlier work, the fuel sulfur level has been found to influence ultrafine particle formation in jet aircraft emissions (6) and in diesel exhaust dilution (7). In the former study, plume age was also found to influence ultrafine particle formation. During the dilution of exhaust the gases cool and the saturation ratio of semivolatile species goes through a maximum, which may lead to nucleation and/or condensation. Few substances are known to be capable of homogeneous nucleation under environmental conditions, and binary nucleation of sulfuric acid with water is by far the best known process. The role played by sulfuric acid in the formation of ultrafine particles was investigated in this study. The methods of investigation included measurement of the size distribution of sulfate particles and a theoretical application of binary nucleation theory. The theory of binary nucleation of H2SO4 and H2O has been developed by many researchers (e.g. refs 8-10) and used to predict nucleation in the atmosphere. The only earlier application of nucleation theory to predict H2SO4-H2O particle formation in diesel exhaust is by Baumgard and Johnson (7), who used Doyle’s theory which is now considered thermodynamically incorrect (11). Revisiting this issue was therefore considered valuable.
Experimental Section A Perkins diesel engine was tested in this study. It is a vehicle specification model year 1995, turbocharged intercooled engine, emissions certified at that time for the U.S. market. Parameters of the engine are bore stroke injection system compression ratio displacement per cylinder cylinder number
100 mm 127 mm Bosch MW RS 1520 17.5:1 1L 4
The properties of fuel used for the test are shown in Table 1. The fuel analysis certificate was issued in November 1996. A schematic diagram of the sampling system is shown in Figure 1. Measurements were carried out on two similar engine test beds (bed 51 and bed 57). The engine exhaust pipe diameter was 63.5 mm for the first unlagged 1.8 meters which connected between the engine and the test bed permanent installation. After this, the permanently installed pipe diameter was 101.6 mm, over a length of about 5.2 m (lagged), connecting to the tunnel. The dilution tunnel measurements were 450 mm o.d. and 4850 mm from entry of the exhaust to the sampling ports for positions 1 and 2. The sampling port for position 3 was in the middle of the lagged pipe. A TSI scanning mobility particle sizer (SMPS [It has been found that there can be a significant difference in particle size distribution between SMPS software version 2.1 and 2.4 at size range smaller than 15 nm. All SMPS data in this paper 10.1021/es981187l CCC: $18.00
1999 American Chemical Society Published on Web 09/16/1999
FIGURE 1. Sampling system at Perkins engine test bed.
TABLE 1. The Properties of the Test Diesel Fuel
TABLE 2. Comparison of Ratios of Sulfate and Nitrate Mass to Total Particle Mass at Position 3 (Dry Air Dilution) and Position 2 (Humid Air Dilution)
specification property (units) cetane index cetane no. viscosity at 37.8 °C, mm2 s-1 density @15 °C, kg/L sulfur content, ppm distillation, °C IBP 10% vol point % @240 °C 50% vol point 90% vol point FBP flash point, °C aromatics, % v/v
min
max
result
40 48 47.69 40 48 46.1 2 3.2 2.46 0.84 0.855 0.842 300 500 427 171.5 204
204 237.5
243.5 287.5 293.5 332 321.5 365 54.5 27
194.5 215 31 257.5 309 345 79 27.74
method D976 D613 D445 D1298 D4294 D86
D93 D1319
were analyzed using software version 2.4.], measuring range 9.6-352 nm) was used for particle sizing, and an electrical low-pressure impactor (ELPI, Dekati, measuring range (50% cut size) 30-10 000 nm) was used as a cascade impactor for the collection of particles with the electrical power turned off. There are 12 stages inside the ELPI, and the last stage collects particles in the smallest size range (30-73 nm). Particles were collected on the aluminum foil placed on each stage of the ELPI at positions 1 and 3 of the test bed (see Figure 1). The aluminum foils were then removed into vials, and 5 mL of distilled deionized water was added together with a small drop of hydrogen peroxide solution. The vials were shaken for 20 min on a Gallenkamp Flask Shaker. Anions were determined using a Dionex LC20 chromatography enclosure, fitted with an AS40 automated sampler, an IonPac AS11 4 mm column, a GP40 gradient pump with sodium hydroxide eluent, and a CD20 conductivity detector. The Dionex has an interface with a computer using software Dionex PeakNet (release 4.30). A Teflon filter pack accommodating a 47 mm Whatman PTFE and a Na2CO3 coated Whatman paper filter was used to collect particles at positions 1 and 3; two 70 mm filters (Teflon coated, T60A20, Pallflex) were placed in a steel filter holder at position 2 which is a standard arrangement for heavy duty engine emission testing. The anions (SO42- and NO3-) were analyzed by the method above except when using 20 mL of distilled deionized water.
position 3
position 2
ratio (%) engine speed and load
dilution ratio
NO3-
1600 rpm 50% 1600 rpm 50% 1600 rpm 25% 1600 rpm 25%
18 72 5 36
0.4 1.5 0.0 0.4
ratio (%)
SO42-
dilution ratio
NO3-
SO42-
0.9 2.7 0.3 0.6
47 47 51 51
1.2 1.2 1.6 1.5
5.0 5.3 2.6 2.2
Results of Engine Emissions Testing. In our engine test bed measurements, it was found that the measured particle size distribution and number concentration changed with dilution conditions, e.g. the dilution ratio and humidity of dilution air. Typically, a higher humidity of dilution air and a higher dilution ratio (particularly the first stage dilution ratio, discussed later) favored higher particle number concentrations, as shown in Figure 2. The difference in sampling positions 1 and 3 is that dryer dilution air was used at position 3, and humid dilution air was used for first stage dilution (the wind tunnel) at position 1. Dryer dilution air was compressed air with a filter and water trap in the line; its relative humidity was 5.4% at 22 °C. Humid dilution air was room air passing through a high efficiency filter with 5865% relative humidity at 22 °C. The effects of humidity of dilution air and dilution ratio on measured particle number concentration were found repeatable in the dilution conditions of this study. Transmission electron microscope (TEM) analysis showed that there are very fine particles (
