Comparison of strategies for the measurement of mass emissions from diesel engines emitting ultra-low levels of particulate matter

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.