Particle effective density and mass during steady-state operation of GDI, PFI, and diesel passenger cars

• We measured effective density of five passenger cars meeting LEV III standards.
• Gasoline PFI emissions have greater density and “fractal dimension” than GDI emissions.
• TSI EEPS and SMPS size distributions are compared quantitatively over steady-state cycles.
• Particles larger than 560 nm as measured by a TSI APS contribute substantially to total mass.
• Time-integrated suspended mass agrees well with gravimetric measures well below 1 mg/mi.

Particle effective density is an important physical property of vehicle exhaust, and is required for estimating particulate matter (PM) mass emissions using the Integrated Particle Size Distribution (IPSD) method. In this study, we measure particle effective density of five light-duty vehicles with PM emissions below the Low Emission Vehicle (LEV) III PM standards of 1 or 3 mg/mi (0.62 and 1.86 mg/km) using the Differential Mobility Analyzer (DMA) – Centrifugal Particle Mass Analyzer (CPMA) approach. Test vehicles included two gasoline direct injection (GDI) vehicles without particulate filters, and for the first time reported in the literature, two port-fuel injected (PFI) vehicles and a turbocharged direct injection (TDI) light-duty diesel vehicle with a diesel particulate filter (DPF). The particle effective density functions generally resemble previous work on GDI and diesel engines without particulate filters but, for many size ranges, the PFI and TDI vehicles produced emissions with higher particle effective densities than GDI vehicles. Good linear correlation was found between the gravimetric and IPSD methods when applying the new particle effective density functions to size distribution measured by the TSI Engine Exhaust Particle Sizer (EEPS, 5.6–560 nm, R2=0.84); however, the IPSD method underestimated gravimetric mass by 64%. When using a TSI Aerodynamic Particle Sizer (APS, 0.54–2.5 µm) to measure the contribution of larger particles, underestimation bias was virtually eliminated and the correlation improved dramatically (R2=0.96). Even stronger correlation between IPSD and gravimetric methods was achieved when using a Scanning Mobility Particle Sizer (SMPS, 8.7–365 nm) and the APS (R2=0.97). A procedure for correcting EEPS measurements using the SMPS is presented and evaluated.