Review of diesel particulate matter sampling - Department of ...

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Exhaust Sampling Conditions Relative Humidity and Temperature of Dilution Air Recent studies have investigated the impact of the relative humidity and temperature of the air used to dilute diesel exhaust during sampling for particle measurement or in the environment for on-highway studies. Weingartner, Burtscher, and Baltensperger (1997b) performed a fundamental study on the hygroscopic properties of diesel particles. The study included generating diesel particles under different engine conditions, conditioning the particles to change their hygroscopicity, and then varying the relative humidity and measuring the effects on particle size and number. Results included: • When particles were dried, exposed to higher humidity and then re-dried, they shrank with subsequent exposure to higher humidity levels. • When particles were not dried, they grew with increasing relative humidity. • When particles were not dried and the fuel sulfur concentration was increased, more growth of the exhaust particles with increasing relative humidity was seen. • Particles generated under idle conditions shrank with increasing relative humidity even with high sulfur fuel. Investigators attributed this to increased organic fraction of particles generated at idle acting as a barrier to water. • Particles generated under load that were treated with ozone and ultraviolet light formed a bimodal number distribution with the new peak in the nuclei mode at about 30 to 40 nm. • When a filter was used to remove the diesel particles and the remaining gas phases were treated with ozone and ultraviolet light, a nuclei mode of particles appeared with the peak also at about 30 to 40 nm. The investigators believed gaseous constituents were nucleating to form nuclei particles. • When the exhaust was not treated and no filter was used, the exhaust had a single peak at about 90 nm in the accumulation mode size range. These results demonstrate that particle composition including organic fraction and sulfur, particle hygroscopic properties, and the relative humidity of the aerosol in the environment or laboratory dilution conditions may change the measured exhaust aerosol properties. Testing conditions for this study were not representative of dilution conditions of diesel exhaust in an on-highway setting. However, changes in particle number and size measured with changing relative humidity and particle hygroscopic properties indicate humidity in the environment or dilution air in a laboratory setting could play a role in affecting diesel exhaust particle formation and transformation. Shi and Harrison (1997) also reported finding effects of relative humidity of dilution air on particle number concentration and size. Increasing dilution air humidity increased the peak diameter of the particle number concentration and total particle number and volume concentrations. (These results are currently unpublished and were presented at a nanoparticle measurement workshop. No further details were provided.) 01/14/99 Page 48
Kreso, et al. (1998a,b) looked at the effect of relative humidity of the dilution air on the nucleation of condensable sulfur species in two low sulfur (0.031% wt. sulfur) fuel diesel engines. The relative humidity in the sampling dilution tunnel of these tests was varied from 10 to 30%. No significant effect was observed on the particle number and volume concentrations with the varying humidity levels. The authors concluded that the nuclei mode particles from the low sulfur diesel engines tested did not consist primarily of condensable sulfur species (H2O-H2SO4 particles) that would respond to changes in relative humidity. Although there is evidence that the relative humidity of a diesel exhaust aerosol may affect the particle characteristics, these effects have not been conclusively demonstrated. Further work to determine the impact should be performed. Dilution Ratio and Residence Time Other issues should be considered if nanoparticle emissions are a concern. If these particles are primarily composed of volatile materials they are formed as the exhaust dilutes and cools. For most of the engines and fuels tested at the University of Minnesota, nuclei mode particles appear to be mainly volatile. The only exception to this was observed when the use of metallic fuel additives caused the formation of a large, solid nuclei mode. (Du, et al. 1998; Kittelson, et al., 1978) The number formed depends primarily upon the dilution ratio and residence time. Atmospheric dilution should be simulated to obtain measurements representative of real-world conditions. Furthermore, volatile particles may have different health impacts than solid carbonaceous particles. Formation of nanoparticles during the dilution process has been seen to be a highly non-linear function of dilution ratio, temperature and residence time. From figure 9 it can be seen that the time spent in the critical dilution ratio range of 5 - 50:1 is about 0.25 s. 11 This is near the lower end of residence times used in laboratory dilution systems, so that one might expect that laboratory conditions might often tend to overestimate nanoparticle formation. However, many measurements made in the atmosphere over roadways show large concentrations of nanoparticles. This suggests that roadway dilution conditions may lead to nanoparticle formation. Concentrations of nanoparticles over roadways are high enough that coagulation is likely to cause nanoparticles to grow quickly into the accumulation mode size range (Harrison, et al., 1996). The character of these agglomerates will depend upon whether the nanoparticles are solid or volatile. Because of coagulation, the highest number concentrations and exposures to nanoparticles will occur on or near roadways where particles are fresh. 11 From the engine lab survey it can be seen that the residence time from the tailpipe to the primary dilution tunnel varies considerably based upon the engine and duty cycle. Times range from 0.015 - 4 s with longer rather than shorter times being more common. Reported primary dilution ratios are within the critical range of 5 - 50:1. Long residence times within the critical dilution range favor the formation of nanoparticles. 01/14/99 Page 49
Page 1 and 2: REVIEW OF DIESEL PARTICULATE MATTER
Page 3 and 4: ABSTRACT Federal government regulat
Page 5 and 6: INTRODUCTION Federal government reg
Page 7 and 8: • to critically evaluate methods
Page 9 and 10: Table 2. Changes in diesel engine t
Page 11 and 12: AEROSOL SIZE DISTRIBUTION Figure 3
Page 13 and 14: Figure 5. - Schematic illustrating
Page 15 and 16: Heat transfer during sampling and d
Page 17 and 18: • humidity and temperature • ba
Page 19 and 20: Doyle (1961) predicted that homogen
Page 21 and 22: Engine Emission Characteristics Eng
Page 23 and 24: accumulation mode particle diameter
Page 25 and 26: Adverse health effects of respirabl
Page 27 and 28: content of the particle. (Burtscher
Page 29 and 30: Table 7. Instrument summary Miscell
Page 31 and 32: Table 8. Studies Measuring Diesel A
Page 33 and 34: slowly moving diesel trucks, and a
Page 35 and 36: Figure 13. - Volume fraction versus
Page 37 and 38: Table 10. N/VT Ratios (Number of pa
Page 39 and 40: Figure 15. - Relationship between d
Page 41 and 42: Exhaust Constituents The nature of
Page 43 and 44: increased number concentrations of
Page 45 and 46: system, volatile particles evaporat
Page 47: With low carbon emission engines, t
Page 51 and 52: • Treatment of sample dilution ai
Page 53 and 54: somewhat larger, 60 - 70 nm. The ma
Page 55 and 56: Bagley, S. T., et al. 1996. Charact
Page 57 and 58: Doyle, G.J. 1961. Self-Nucleation i
Page 59 and 60: Keady, P.B., F. R. Quant, and G. J.
Page 61 and 62: Seminar on Particle Size Distributi
Page 63 and 64: Watson, J. G. et al. 1998. Northern

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