Leonidas Ntziachristos

Performance Evaluation of a Novel Sampling and Measurement System for Exhaust Particle Characterization

This paper presents a novel partial flow sampling system for the characterization of airborne exhaust particle emissions. The sampled aerosol is first conditioned in a porous dilutor and then subsequent ejector dilutors are used to decrease its concentration to the range of the instrumentation used. First we examine the sensitivity of aerosol properties to boundary sampling conditions. This information is then used to select suitable sampling parameters to distinguish both the nucleation and the accumulation mode. Selecting appropriate sampling parameters, it is demonstrated that a distinct nucleation mode can be formed and measured with different instruments. Using these parameters we examine the performance of the system over transient vehicle operation. Additionally, we performed calculations of particle losses in the various components of the system which are then used to correct signals from the instruments. Several quality characteristics are then discussed, such as the repeatability and reproducibility of the measurements and the potential to derive total emission rate with a partial flow sampling system. Comparisons in different laboratories show that repeatability (intra-laboratory variability) is in the order of 10% for accumulation mode particles and 50% for nucleation mode ones. Reproducibility (inter-laboratory variability) values are in the range of ±20-30%. Finally, we compared laboratory size distributions with ambient samples obtained chasing a vehicle. This demonstrated that the sampling system accurately reproduced the accumulation mode particles as well as the potential for nucleation mode formation. This sampling system has been used in the framework of a European project for measurement of emissions of a number of light duty vehicles and heavy duty engines.

Application of a Diffusion Charger for the Measurement of Particle Surface Concentration in Different Environments

Particle surface area has recently been considered as a possible metric in an attempt to correlate particle characteristics with health effects. In order to provide input to such studies, two Nanoparticle Surface Area Monitors (NSAMs, TSI, Inc.) were deployed in different urban sites within Los Angeles to measure the concentration levels and the diurnal profiles of the surface area of ambient particles. The NSAMs principle of operation is based on the unipolar diffusion charging of particles. Results show that the particle surface concentration decreases from ∼150 μ m2 cm−3 next to a freeway to ∼ 100 μ m2 cm−3 at 100 m downwind of the freeway, and levels decline to 50–70 μ m2 cm−3 at urban background sites. Up to 51% and 30% of the total surface area corresponded to particles < 40 nm next to the freeway and at an urban background site, respectively. The NSAM signal was well correlated with a reconstructed surface concentration based on the particle number size distribution measured with collocated Scanning Mobility Particle Sizers (SMPSs, TSI, Inc.). In addition, the mean surface diameter calculated by combination of the NSAM and the total particle number concentration measured by a Condensation Particle Counter (CPC, TSI, Inc.) was in reasonable agreement with the arithmetic mean SMPS diameter, especially at the urban site. This study corroborates earlier findings on the application of diffusion chargers for ambient particle monitoring by demonstrating that they can be effectively used to monitor the particle surface concentration, or combined with a CPC to derive the mean surface diameter with high temporal resolution.

Calibration and modelling of ejector dilutors for automotive exhaust sampling

Ejectors are widespread dilutors for automotive exhaust sampling. Their dilution ratio depends on their geometrical characteristics, the properties of the sample and the dilution gas, and the conditions at the ejector dilutor outlet. In this paper we present a detailed calibration of a typical ejector dilutor, and we model its operation under steady-state conditions. Experiments showed that the dilution ratio can shift by 10–20% when changing the sample or outlet pressure by only 5 kPa or the dilution gas pressure by 50 kPa. Increase of the sample temperature by 140 °C also leads to a dilution ratio increase of 20%. Diluting pure CO2 instead of ambient air may increase the DR by 20%, due to the different CO2 properties. These are typical changes in the operating conditions when sampling automotive exhaust. Hence, in order to predict and correct for these changes, we developed a model to calculate the actual value of the dilution ratio as a function of the ejector operating parameters. The model assumes compressible flow which reaches supersonic velocity at a variable cross-section duct. This terminates with a normal shock wave occurring at the mixing area. The model can be used with any calibrated ejector dilutor to predict changes in the dilution ratio in actual field measurements.

Comparability of particle emission measurements between vehicle testing laboratories: a long way to go

The characterization of automotive exhaust aerosol today involves determination not only of the mass emission rate but additional particle properties such as number and surface concentration and size distribution. However, despite the fact that several measurement campaigns are conducted world-wide utilizing various sampling systems and measurement devices, there is no consistent information on the uncertainty range for such measurements. This paper presents the within laboratory and between laboratories variability expected for measurements of exhaust particle properties, processing the data of several instruments and driving cycles collected in a specifically designed interlaboratory study. Results show that within laboratory variability is in the range of 20% of the mean value, excluding outliers. However, between laboratories variability can regularly reach or exceed 50% when small deviations from the sampling protocol occur, bringing significant implications to the comparability of the results. However, continuation of the study demonstrated that it is possible to decrease the between labs uncertainty range below 20% when laboratories get more experienced in the strict application of the measurement protocol. The study concludes that the comparability of exhaust aerosol measurements can only be verified with enforcement and strict application of common sampling, measurement and calibration protocols.

Particle emissions characteristics of different on-road vehicles

Due to the stringent emission standards set worldwide, particulate matter (PM) emissions from diesel vehicles have been significantly curtailed in the last decade, and are expected to be reduced even further in the future. This evolution has brought forward two main issues: whether PM emissions should only be regulated for diesel vehicles and whether gasoline powered vehicles can be further neglected from PM emission inventories. This paper addresses these issues comparing the characteristics of particle emissions from a current diesel passenger car, a gasoline one and two small two-wheelers. It is shown that the gasoline car is a negligible source of particle emissions while the two-wheelers may be even more significant particle sources than the diesel car.