Supplemental Information

Comparison of vehicle exhaust particle size distributions measured by SMPS and EEPS during steady-state conditions

Jian Xuea,b, Yang Lia,b, Xiaoliang Wangc, Thomas D. Durbina, Kent C. Johnsona, Georgios Karavalakis, Akua Asa-Awukua, Mark Villelaa, David Quirosd, Shaohua Hud, Tao Huaid, Alberto Ayalad and Heejung S. Junga,b,*

aCenter for Environmental Research and Technology (CE-CERT), Bourns College of Engineering, University of California, Riverside, Riverside, California, USA

b Department of Mechanical Engineering, University of California Riverside, Riverside, California, USA

cDesert Research Institute, Reno, Nevada, USA

d California Air Resources Board (CARB), 1001 I Street, Sacramento, California, USA

This Supplemental Information contains the following information:

Section S1. Variations in particle mass emissions due to uncertainties in particle density for the nucleation mode.

Section S2. Size distribution of SMPS and EEPS with Mazda 3 GDI at 30 mph.

Table S1. Particle mass emission estimation using two different particle effective density profiles.

Figure S1. Schematic of the test setup.

Figure S2. Particle mass distribution by EEPS and SMPS with a GDI vehicle at steady state speed of 30 mph.


Section S1: Variations in particle mass emissions due to uncertainties in particle density for the nucleation mode.

In the main text, a constant density of hydrated sulfuric acid (1.46 g/cm3) was assumed for dp 30 nm, based on data presented by Zheng et al. (2012). However, this particle density did not consider the condensation of organics and growth of nucleation-mode particles, which may be significant in some engine operation conditions and fuels. Therefore, we assumed a different particle density somewhat arbitrarily to assess the extent of the influence of particle effective density on the total suspended particle mass. For dp 30 nm, the second density profile (denoted as density2) share the same values of particle effective density with the primary density profile (denoted as density1) in the main text. However, dp 30 nm, the soot effective density of 1.2 g/cm3 approximated by Maricq and Xu (2004) was applied. The IPSD mass emission rates calculated with two sets of particle effective densities were calculated and listed in Table S1.

The most significant change (~15%) was observed with the emissions of TDI vehicle operated at 60 mph and 4% load based on SMPS measured particle size distributions. This is expected because 99.9% the measured particles number was from nucleation mode particles. For the emissions from the diesel generator, the GDI vehicle at 30 mph, and PFI vehicle at 25 mph, only very small changes in MIPSD were observed (varying from 0-0.8%). For the emissions with the vehicle at 60 mph and 2% load, a change of 7% was observed as shown in Table S1.

Section S2: GDI vehicles at 30 mph (Overlapping bi-modal distribution)

Figure S2a compares particle size distributions measured by the EEPS and SMPS for the GDI vehicle operated at 30 mph using the EEPS Default and Soot Matrices. Larger uncertainties observed during this condition compared to the 60-mph test condition. The SMPS reported a bimodal size distribution with GMDs of 8 and 52 nm for the nucleation and accumulation modes, respectively. Based on the relative contributions of the bimodal lognormal fit, the nucleation mode contributed 82% of total particle number, a substantially larger fraction than the nucleation mode observed during the test of the diesel generator using bio-fuel (12%). It is also larger than the fraction of the nucleation mode observed during the test of the same GDI vehicle at a higher speed of 60 mph (54%). The EEPS Default Matrix reported a slightly lower particle size distributions for the accumulation mode particles compared to SMPS (GSD= 1.84 vs. GSD=1.97), as shown in Figure S2 and Table 3. Both the EEPS Default and Soot Matrices reported GMDs of accumulation mode much different from the of the reference SMPS measurement.

The total particle number emission rates measured with the EEPS Default Matrix was 2.28 ± 0.88 × 1011 /mile, 28% lower than that estimated by the SMPS (3.18 ± 0.26 × 1011 /mile). MIPSD estimated by the EEPS Default Matrix was 0.012 ± 0.004 mg/mile, 33% lower than that estimated by the SMPS (0.018 ± 0.006 mg/mile). These results indicate that the EEPS Default Matrix underestimates both particle number and mass emissions relative to the SMPS when measuring light-duty GDI emissions at a lower speed. It is very consistent with the results using the same vehicle under higher speed of 60 mph. It is also consistent very well with our previous evaluation of the IPSD method over the transient FTP cycle1.

When the EEPS Soot Matrix was applied, the agreement in particle sizing given by the two instruments was improved for nucleation particles and particles larger than 100 nm. Total particle number emission rate was estimated to be 3.46 ± 1.32 × 1011 /mile, ~9% higher than those measured by the SMPS. The mass (MIPSD) emission rate was estimated to be 0.016 ± 0.006 mg/mile, agreeing well with the emission rate estimated with SMPS (0.018 ± 0.006 mg/mile), but still underestimating the emissions in the mass distribution by around 12%. Compared to the comparison of the emissions of this vehicle operating at 60 mph and a 2% simulated road grade, the agreement of the EEPS Soot Matrix is inferior. The Soot Matrix showed some difficulty reproducing exact bimodality the reference measured showed. We speculate this might be due to difference in sizing resolution between the EEPS and SMPS. This could suggest that for a given characteristic of aerosol especially with bimodal size distirbutions, there may be a limitation of the performance of the EEPS Soot Matrix. Nevertheless, over a range of particle concentrations elicited by the light-duty GDI vehicle, particle number and mass distributions agreed with the EEPS Soot Matrix within a reasonable margin.

Table S1. Particle mass emission estimation using two different

particle effective density profiles.

Tests / SMPS / EEPS Default / EEPS Soot
Density1 / Density2 / Density1 / Density2 / Density1 / Density2
Diesel-Reg (mg/bhp-hr) / 672±28 / 670±28 / 417±37 / 417±37 / 630±62 / 628±62
Diesel-Bio (mg/bhp-hr) / 236±6 / 234±6 / 162±10 / 160±10 / 240±17 / 238±16
GDI-60 (mg/mile) / 0.15±0.03 / 0.14±0.03 / 0.10±0.02 / 0.10±0.02 / 0.15±0.03 / 0.15±0.03
GDI-30 (mg/mile) / 0.018±0.006 / 0.018±0.006 / 0.012±0.004 / 0.011±0.004 / 0.016±0.006 / 0.016±0.006
TDI-60 (mg/mile) / 1.17±0.14 / 1.00±0.12 / 0.12±0.01 / 0.12±0.01 / 0.17±0.01 / 0.17±0.01
PFI-25 (mg/mile) / 0.46±0.32 / 0.46±0.31 / 0.34±0.38 / 0.34±0.38 / 0.52±0.56 / 0.52±0.56

Figure S1. Schematic of the test setup. Other instruments include a series of gas analyzers that measure the concentrations of CO, CO2, NOx and etc.

Figure. S2 (a) Particle size distribution by EEPS and SMPS with a GDI vehicle at a speed of 30 mi/hr and 0% road grade. Black solid and dashed lines present lognormal fitted size distribution of nucleation and accumulation mode particles respectively by SMPS. The same results corrected by dilution ratio of sampling system were presented on the right y-axis. (b) Particle mass distribution by EEPS and SMPS.

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