Using Portable Emission Measurement Systems for Transportation Emissions Studies

Portable emission measurement systems (PEMS) are increasingly being used in a variety of transportation research projects to determine the impact of real-world vehicle emissions. One of the key questions that remain is how well these systems perform compared with testing that occurs in controlled laboratory conditions. To help answer this question, three PEMS were carefully evaluated for both gasoline and diesel light-duty vehicles in a dynamometer test facility. The evaluation was focused on the systems’ accuracy, time correspondence, and suitability for measuring transient emissions. Both cumulative mass emissions and modal emissions for carbon monoxide (CO), hydrocarbons (HC), oxides of nitrogen (NOx), and carbon dioxide (CO2) were measured for three gasoline and three diesel vehicles on three widely varying driving cycles. All of the PEMS proved to be both reasonably accurate and precise. The CO2 emissions measured by the PEMS were in excellent agreement (within 98%) with measurements from the laboratory system. Other pollutants measured were found to be in reasonable agreement (within 20% or better) for NOx and HC on diesel vehicles and CO on gasoline vehicles. The second-by-second emission rate measured with the PEMS matched well with the corresponding laboratory modal analyzer data for CO2, NOx, and CO under all driving cycles. Transient emissions of all pollutants agreed within 10% of the two systems for more than 6,000 data points from each vehicle. The results suggest that when properly set up and calibrated, PEMS are capable of measuring emissions from both gasoline and diesel vehicles to an accuracy within 20% of conventional laboratory modal analyzer systems.