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Rushton, CE; Tate, JE; Shepherd, SP; Carslaw, DC
Publisher: Taylor & Francis
Languages: English
Types: Article
Emissions of NOX by vehicles in real driving environments are only partially understood. This has been brought to the attention of the world with recent revelations of the cheating of the type approval tests exposed in the dieselgate scandal. Remote sensing devices offer investigators an opportunity to directly measure in-situ real driving emissions of tens of thousands of vehicles. The availability of remote sensing NO2 measurements are not as widely available as would be desirable. The aim of this study is to improve the ability of investigators to estimate the NO2 emissions and to improve the confidence of the total NOX results calculated from standard RSD measurements. The accuracy of the RSD speed and acceleration module was also validated using state-of-the-art onboard GPS tracking. Two RSDs used in roadside vehicle emissions surveys were tested side by side under off carriageway conditions away from transient pollution sources to ascertain the consistency of their measurements. The speed correlation was consistent across the range of measurements at 95% confidence and the acceleration correlation was consistent at 95% confidence intervals for all but the most extreme acceleration cases. VSP was consistent at 95% confidence across all measurements except for VSP ≥ 15kW t−1 which show a small underestimate. The controlled distribution gas nitric oxide measurements follow a normal distribution with 2σ equal to 18.9% of the mean compared to 15% that was observed during factory calibration indicative of additional error introduced into the system. Systematic errors of +84ppm were observed but within the tolerance of the control gas. Inter-instrument correlation was performed with the relationship between the FEAT and the RSD4600 being linear with a gradient of 0.93 and an R2 of 0.85 indicating good correlation. A new method to calculate NOX emissions using fractional NO2 combined with NO measurements made by the RSD4600 was constructed, validated and shown to be more accurate than previous methods. Implications: Synchronised remote sensing measurements of NO were taken using two different remote sensing devices in an off-road study. It was found that the measurements taken by both instruments were well correlated. Fractional NO2 measurements from a prior study, measurable on only one device, were used to create new NOx emission factors for the device that could not be measured by the second device. These estimates were validated against direct measurement of total NOx emission factors and shown to be an improvement on previous methodologies. Validation of Vehicle Specific Power was performed with good correlation observed.
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