Technical considerations for choosing a metric for vehicle remote-sensing regulations

An increasing number of countries, including mainland China, are considering enforcing emission regulations for in-use diesel vehicles via remote sensing, a promising technology that measures exhaust emissions from a large number of on-road vehicles without interrupting traffic. To improve the utility of using remote sensing for that purpose, this briefing first explains the current lack of accuracy in estimating tailpipe pollutant concentrations in diesel vehicles, notably for nitrogen oxides (NO_x), and then discusses ways to overcome it.

Estimating tailpipe concentration is difficult with diesel engines because combustion almost always happens under conditions of variable excess air. As illustrated in the figure below, the precise optical path length of the pollutant (e.g., NO_x) through the exhaust plume is unknown, and the pollutants disperse rapidly once exiting the tailpipe of a diesel vehicle. While that makes the concentration of an individual species in the plume hard to measure or estimate, remote sensing can precisely measure the ratio between them (NO_x/CO₂), because that remains relatively constant. The authors suggest that converting these pollutant-to-CO₂ ratios to a fuel-specific metric is the best approach for setting regulatory limits, as it means high-emitting vehicles are identified with available remote-sensing technologies and would allow China in particular to keep the stringency of current thresholds in its remote-sensing regulation. Additionally, this approach can be applied to both diesel and gasoline vehicles.

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