Oxidative Potential of Semi-Volatile and Non Volatile Particulate Matter (PM) from Heavy-Duty Vehicles Retrofitted with Emission Control Technologies

Abstract

Advanced exhaust after-treatment devices for diesel vehicles are less effective in controlling semivolatile species than the refractory PM fractions. This study investigated the oxidative potential (OP) of PM from vehicles with six retrofitted technologies (vanadium and zeolite based selective catalytic reduction (V-SCRT, Z-SCRT), Continuously regenerating technology (CRT), catalyzed DPX filter, catalyzed continuously regenerating trap (CCRT), and uncatalyzed Horizon filter) in comparison to a "baseline" vehicle (without any control device). Vehicles were tested on a chassis dynamometer atthree driving conditions, i.e., cruise, transient urban dynamometer driving schedule (UDDS), and idle. The consumption rate of dithiothreitol (DTT), one of the surrogate measures of OP, was determined for PM samples collected at ambient and elevated temperatures (thermally denuded of semivolatile species). Control devices reduced the OP expressed per vehicle distance traveled by 60-98%. The oxidative potential per unit mass of PM however, was highest for the Horizon followed by CRT, DPX -Idle, SCRTs, and baseline vehicles. Significant reduction in OP (by 50-100%) was observed forthermally denuded PM from vehicles with retrofitted technologies (PM with significant semivolatile fraction), whereas particles emitted bythe baseline vehicle (with insignificant semivolatile fraction) did not demonstrate any measurable changes in oxidative activity. This suggests that the semivolatile fraction of particles are far more oxidative in nature than refractory particles-a conclusion further supported by previous tunnel and ambient studies, demonstrating a decline in PM oxidative activity with increasing atmospheric dilution. Correlation analysis performed between all the species, showed that OP is moderately associated (R = 0.76) with organic carbon (OC) and strongly associated (R = 0.94) with the water-soluble organic carbon (WSOC).