Book Description

Even at low concentrations, the criteria air pollutant particulate matter (PM) is an environmental and public health hazard. Emissions levels legislated for modern diesel vehicles are so low (~90% lower than 2003) that it has become difficult to accurately measure PM by the regulatory metric: the mass of particles collected on a filter (i.e., the gravimetric method). Additionally, gravimetric analysis cannot measure real-time emission rates, and therefore is unable to characterize high-emitting transient events (e.g., engine starts, stop-and-go driving). By an alternate method, PM can be estimated by measuring the number-weighted particle size distribution (PSD) and calculating mass with a combination of theoretical and empirical constants (e.g., particle effective density). This integrated particle size distribution (IPSD) method is capable of high measurement sensitivity and real-time resolution. Real-time measurements by the IPSD method require fast-sizing spectrometers, such as the TSI Engine Exhaust Particle Sizer (EEPS), which sizes (between 5.6-560 nm) and counts particles based on their electrical mobility. The EEPS utilizes a unipolar charger to quickly charge particles for sizing and counting, however this mechanism has been shown to produce a less predictable charge distribution than bipolar chargers used in Scanning Mobility Particle Sizer (SMPS) systems – the gold standard “slow-sizing” spectrometer. Several evaluations have shown deficiencies in EEPS PSD measurements due to charging differences (associated with particle morphology) unaccounted for in the transfer function matrix used to calibrate the EEPS. Specifically, the unipolar charger multiply charges a higher percentage of soot agglomerates (fractal-like particles common in diesel engine exhaust) than bipolar chargers. Because inaccurate PSDs are a primary reason for reported discrepancies between IPSD calculated mass and the gravimetric method, it is important to correct this deficiency in EEPS measurements. Recently, TSI has released additional EEPS calibration matrices (“Soot” and “Compact”) which have shown better agreement with SMPS measurements under preliminary test conditions. This study further evaluates the performance of these new matrices relative to the original “Default” matrix for diesel and biodiesel exhaust particles. Steady-state (75% engine load) emissions were generated by a light-duty diesel engine operating on (1) ultra-low sulfur diesel (ULSD) and (2) 100% soybean biodiesel. Raw EEPS data processed with each matrix were compared to simultaneously collected reference measurements from an SMPS. PSDs were evaluated based on their shape – i.e., multimodal fits of geometric mean diameter (GMD) and geometric standard deviation (GSD) – and concentration at peak particle diameter. For both fuels, all measurements agreed well in terms of the shape of the PSD: primary mode (accumulation) GMD ± 10nm, GSD ± 0.3. For ULSD, EEPS Default, Soot, and Compact concentrations were higher than the SMPS by factors of 1.9, 1.3, and 2.5, respectively. For biodiesel, EEPS Default, Soot, and Compact concentrations were higher than the SMPS by factors of 2.1, 1.7, and 2.4, respectively. Based on these results, the Soot matrix produced acceptable agreement between EEPS and SMPS measurements of ULSD exhaust particles. However, based on the factor of ~2 difference observed here, an additional calibration matrix may be necessary for the EEPS to accurately measure biodiesel exhaust particles. The IPSD method for estimating PM mass was applied to available data sets with corresponding gravimetric measurements (one ULSD transient cycle test and the same biodiesel steady-state test used for PSD evaluation). Real-time PSDs from each of the three EEPS matrices were used in combination with three sets of values assumed for size-dependent particle effective density (representing a range of potential conditions), resulting in nine IPSD estimates of PM mass corresponding to each gravimetric sample (one ULSD, one biodiesel). For the transient ULSD test, a widely used effective density distribution for fractal-like soot agglomerates resulted in good agreement between IPSD estimated mass and the gravimetric measurement (within 9% and 6% for Soot and Compact matrices, respectively). For the steady-state biodiesel test, assuming unit density (1g/cm3 for all particles) resulted in good agreement between IPSD estimated mass and the gravimetric measurement (within 7% and 2% for Soot and Compact matrices, respectively). These results support previous findings that the Soot matrix is currently the best available option for measurement of ULSD exhaust particles by the EEPS and that particle effective density distributions similar to the “fractal-like” one used here are an accurate estimate for ULSD exhaust particles under many conditions. However, based on the discrepancies between the EEPS and SMPS measured biodiesel exhaust PSDs observed here, as well as a current lack of information on the effective density of biodiesel exhaust particles, it is clear that additional research is necessary in order to understand the properties of biodiesel exhaust particles, especially as they relate to electrical mobility measurements and IPSD estimation of PM mass.