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Tailpipe emissions from road transport have fallen dramatically over the last 30 years due to the combinedeffect of increasingly stringent regulations and technological improvements. However, the air pollutionburden due to road vehicle emissions remains the dominant source of many air pollutants in urbanareas. Policies such as Low Emission Zones (LEZs) have become increasingly popular as a method of reducingon-road emissions by restricting access to the oldest and most polluting vehicles. Most modern vehiclesare fitted with sophisticated exhaust aftertreatment systems, which should lead to significantly reducedemissions of pollutants such as nitrogen oxides (NOx= NO+ NO2). However, the performance of suchsystems is non-uniform across all driving conditions. Urban driving conditions are among the most challenging, where vehicle speeds are often low and congestion results in a considerable amount …