Simultaneous Real-Time Measurements of NO and NO2 in Medium Duty Diesel Truck Exhaust

The goal of the present work was to investigate the ability of the SEMTECH^®-D Portable Emissions Measurement System (PEMS) to provide simultaneous, accurate, real-time (1Hz) measurements of NO and NO₂ in vehicle exhaust. Extensive chassis dynamometer laboratory evaluation studies of the SEMTECH^® system were conducted. The instantaneous (1Hz) NOx emissions were measured using a conventional chemiluminescence analyzer (CLA) and were compared to the sum of the instantaneous NO and NO₂ measurements from the SEMTECH^®-D. The sum of the NO and NO₂ emissions measured by the SEMTECH^® were in excellent agreement (within 95% in most cases) with the total NOx measurements from the conventional CLA.

During the laboratory evaluation studies, several Federal Test Procedure (FTP) drive cycles were conducted. Examples of the NO and NO₂ concentration and mass emissions measured using the SEMTECH^®-D are presented along with the corresponding SEMTECH^®-D detection limits. From the data it is clearly evident that the NO and the NO₂ levels measured were, in most cases, well above the detection limits of the SEMTECH^®-D. The results also indicate that for the vehicles tested in this study, the SEMTECH^® system can easily distinguish differences between NOx, NO, and NO₂ emissions measured during cold start transient phase (Bag 1) and the hot start transient phase (Bag 3) of the FTP.

The SEMTECH^®-D PEMS utilizes a sample gas conditioning unit prior to the analyses of NO and NO₂. Therefore, studies were also conducted to evaluate potential losses of NO₂ via adsorption, condensation, and reactions with the water that may accumulate in the condenser of the sample conditioning unit. The results of these studies indicate that for NO₂ concentrations of less than ∼300 ppm (the range of NO₂ typically emitted from the vehicles tested) the losses in the SEMTECH^® system were comparable to those losses of NO₂ that may be attributable to NOx converter efficiencies (> 95% efficient, as specified in the Code of Federal Regulations) in conventional laboratory analyzers (1).