Comparison of Strategies for the Measurement of Mass Emissions from Diesel Engines Emitting Ultra-Low Levels of Particulate Matter

Abstract

Regulatory methods for the measurement of particulate matter (PM) mass emissions have traditionally been gravimetric. Modern diesel engines equipped with aftertreatment systems, especially Diesel Particulate Filters (DPFs), however, emit much smaller amounts of particulate matter as compared to traditional diesel engines and emit particulate matter with variable compositions. These changes have led to difficulties in measuring PM emissions rates from modern diesel engines using gravimetric methods. Issues associated with diesel PM mass measurement, such as the semi-volatile nature of PM, the interactions with components in the dilution air such as water and ammonia, and the possibility of sampling artifacts, have counteracted a singular focus on mass measurements. These inherent problems may warrant some alternative approaches to characterizing emissions, using methods related to mass and impacts of emissions that can be more accurately defined. The present study provides a comparison and relative precision of several alternative mass measurement methods employed to measure the mass emissions of particulate matter from diesel engines with low and ultra-low levels of emissions. The methods of measurement reviewed in this study include two gravimetrically based methods, a chemically reconstructed mass method, and an integrated particle size distribution (IPSD) method. The mass measurements were consistent at low emission levels but the chemical speciation and IPSD methods achieved closer agreement and were more precise at ultra-low emission levels. Although mass measurement is a NIST-traceable quantity, alternative methods may present a new paradigm that better characterizes engine emissions in an atmospherically relevant manner.

Comparison of Vehicle Exhaust Particle Size Distributions Measured by SMPS and EEPS During Steady-State Conditions

“…The IPSD method was initially defined by Liu et al (2009), and more recently evaluated using emissions from LDVs by Li et al (2014) and Quiros et al (2015b), although the concept of using size distribution to estimate PM mass was discussed much earlier by Maricq and Xu (2004). Briefly, the IPSD method estimates total particle mass by integrating the fractional mass obtained by multiplying the particle effective density and volume concentration for each size bin from the measured PSD as follows:…”

“…One of the alternative PM measurement methods that CARB (2012) is evaluating to support the LEV III PM standards is integrated particle size distribution (IPSD) (Liu et al 2009), which estimates real-time suspended particle mass from particle size distribution (PSD) and an effective density function. Since the method does not use filter media, IPSD is unaffected by gaseous adsorption artifact or particle evaporation after collection onto filters, and therefore has been explored to evaluate whether the method offers greater sensitivity for measuring PM mass at low emission levels (Liu et al 2009;Li et al 2014;Quiros et al 2015a). Furthermore, measuring PSD provides not only the ability to estimate PM mass but also other important characteristics of vehicle particle emissions such as total particle number and surface area.…”

Effective Density and Volatility of Particles Emitted from Gasoline Direct Injection Vehicles and Implications for Particle Mass Measurement

“…It can also be used to convert particle size distributions to particle mass distributions from which total particle mass concentration can be determined. The integrated particle size distribution (IPSD) method is seen as an option for particle mass emission factor measurement for the modern vehicles (Liu et al 2009(Liu et al , 2012, which produce very few particles and where gravimetric measurement is difficult due to the low mass collected and the significance of errors due to sampling artifacts (Chase et al 2004). The effective density function for GDI particles has been measured by Maricq and Xu (2004), Symonds et al (2008), and Quiros et al (2015).…”

Alternatives to the Gravimetric Method for Quantification of Diesel Particulate Matter near the Lower Level of Detection

“…97 Liu et al 106 suggested that eff ϭ 1.2378e Ϫ0.0048d m best fit data given by Maricq and Xu. 97 The exponential relationship used by Liu et al 106 smoothly accounts for transition in the particle size distribution from the nucleation mode, in which particles have a fractal dimension of 3 and a density near 1 g/cm 3 , to the accumulation mode aggregate particles that have a fractal dimension between 2 and 3 and a size-dependent effective density. Furthermore, they experimentally compared this relationship with filter mass measurements taken from a heavy-duty diesel engine without emission control devices and found agreement.…”

“…106 However, the primary size standards discussed above relate to sizing spherical particles of known morphology and composition, as opposed to the complex chain aggregates common in diesel aerosol. Effective density techniques are used to account for the fact that mobility diameter is not a geometric measure for aggregate particles because of their fractal-like nature.…”