Evaluation of an annular denuder system for carbonaceous aerosol sampling of diesel engine emissions

Abstract

Sampling of particle-phase organic carbon (OC) from diesel engines is complicated by adsorption and evaporation of semivolatile organic carbon (SVOC), defined as positive and negative artifacts, respectively. In order to explore these artifacts, an integrated organic gas and particle sampler (IOGAPS) was applied, in which an XAD-coated multichannel annular denuder was placed upstream to remove the gas-phase SVOC and two downstream sorbent-impregnated filters (SIFs) were employed to capture the evaporated SVOC. Positive artifacts can be reduced by using a denuder, but particle loss also occurs. This paper investigates the IOGAPS with respect to particle loss, denuder efficiency, and particle-phase OC artifacts by comparing OC, elemental carbon (EC), SVOC, and selected organic species, as well as particle size distributions. Compared to the filter pack methods typically used, the IOGAPS approach results in estimation of both positive and negative artifacts, especially the negative artifact. The positive and negative artifacts were 190 µg/m³ and 67 µg/m³, representing 122% and 43% of the total particle OC measured by the IOGAPS, respectively. However, particle loss and denuder break-through were also found to exist. Monitoring particle mass loss by particle number or EC concentration yielded similar results ranging from 10% to 24% depending upon flow rate. Using the measurements of selected particle-phase organic species to infer particle loss resulted in larger estimates, on the order of 32%. The denuder collection efficiency for SVOCs at 74 L/min was found to be less than 100%, with an average of 84%. In addition to these uncertainties the IOGAPS method requires a considerable amount of extra effort to apply. These disadvantages must be weighed against the benefits of being able to estimate positive artifacts and correct, with some uncertainty, for the negative artifacts when selecting a method for sampling diesel emissions.

Implications:

Measurements of diesel emissions are necessary to understand their adverse impacts. Much of the emissions is organic carbon covering a range of volatilities, complicating determination of the particle fraction because of sampling artifacts. In this paper an approach to quantify artifacts is evaluated for a diesel engine. This showed that 63% of the particle organic carbon typically measured could be the positive artifact while the negative artifact is about one-third of this value. However, this approach adds time and expense and leads to other uncertainties, implying that effort is needed to develop methods to accurately measure diesel emissions.

Supplemental Materials: Supplemental materials are available for this paper. Go to the publisher's online edition of the Journal of the Air & Waste Management System.

Acknowledgments

The authors thank Dr. Elisabeth Galarneau in AQRD, Environment Canada, for her helpful insight on PAH partitioning.