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Summary

Remote Emission Sensing

Remote emission sensing is a measurement methodology for measuring real-world emissions from real vehicles on real roads driven by real drivers.  It was developed in America in the late 1980’s to measure carbon monoxide emissions from road vehicles.  It was improved in the mid 1990’s to include hydrocarbon emissions and later to include oxides of nitrogen, including NO and later NO2.  A smoke measurement was also developed to measure particulate matter emissions. Accurate measurements of real driving emissions are important because they allow people to understand the actual emissions a specific vehicle creates rather than relying on estimates based on type approval.  The better the knowledge of a vehicle’s emissions, the more fairly it can be considered in emission reduction policy.  Whilst many may argue that this approach would add even more costs to drivers, it would actually mean that all drivers would be fairly penalised based on only their contribution to the problem.

Concept

Remote vehicle emission sensing operates on the principle of opacity.  Light sources of known starting intensity are shone through vehicle exhaust plumes and the final intensity is measured.  The measured decrease in opacity is linked to the column density of the pollutant in the plume and when multiple pollutants are measured then the ratio between the two can be calculated.  Remote sensing is also usually deployed with a vehicle dynamics sensor so the speed and acceleration can be linked to CO2 emissions.  Emissions of CO2 are much better understood as they are more closely linked to driving dynamics than chemical processes. Simple equations of physics, calculating the work done to move a car along the road, can then be used as a somewhat reliable estimate for converting ratios to absolute values.  

 Remote sensing uses lasers tuned to the specific quantum energy levels of the target molecules to avoid contamination of the measurement.  Electrons in molecules have well known energy level structures and finding an energy transition unique to the molecule allows for very specific measurements to be made.  The energy transitions in oxides of nitrogen are typically higher than in carbon monoxide and hydrocarbons and this required additional development of an ultraviolet laser, rather than an infra-red laser, to measure them.  The energy levels of oxides of nitrogen are also very similar to water vapour.  Water vapour is common in vehicle exhaust plumes and distinguishing between the two has proved very challenging for remote sensing.  The more modern sensors have the ability to resolve nitric oxide and nitrogen dioxide with a useful level of precision and have been demonstrated in numerous field studies over the last decade. 

More about the author

Dr. Chris Rushton is a researcher at the University of Leeds, who is interested in the integration of new and emerging technologies such as sensors and measurement devices to tackle air pollution. He will present some of his work as part of the EU-funded City Air Remote Emissions Sensing (CARES) project, which demonstrates the integration of modern database technology, web applications, and the most up-to-date road vehicle emission sensing technology currently being deployed in the UK and EU.