Using Mobile Monitoring to Understand Vehicle Emissions in Urban Areas
Authors: Wilde Shona , Farren Naomi , Wagner Rebecca , Lee James , Wilson Samuel , Padilla Lauren , Slater Greg , Peters Daniel , Alvarez Ramon , Carslaw David 
Published: February 2023

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 of stop-startdriving with repeated accelerations and decelerations. Under such conditions some aftertreatment systemscannot reach the high temperatures required to operate efficiently, which may limit the effectiveness ofpolicies that target vehicle type alone. Therefore, to develop effective air quality management strategies itis necessary to understand the relative importance of factors that influence vehicle emissions, such as fleetcomposition, traffic state, driver behaviour and ambient temperature.In this work we present results from a mobile monitoring campaign in London, UK. Measurements weremade in two unique locations (central and outer London) in order to provide a quantitative understandingof the main drivers for concentrations in terms of traffic conditions. We show that there is a significant lowspeed penalty for NOx concentrations in central London, where there is a high proportion of diesel vehicles,which are predominately taxis and buses. This effect arises due to the near constant congestion andslow average moving speed of only 12 km h-1, resulting in the non-optimal performance of aftertreatmenttechnologies fitted to diesel vehicles. Moreover, despite the heavy restrictions imposed by the Ultra LowEmissions Zone, which requires all diesel vehicles in the zone to be Euro 6/VI (light/heavy vehicles) compliant,we find that the mean emissions intensity (&#916;NOx/&#916;CO2) in central London was 0.0039 ppb ppb-1, afactor of 2 higher than outer London (0.0021 ppb ppb-1). For context we compared the measured emissionsintensity to an &#8220;urban average" value (0.0018 ppb ppb-1) derived from 135,000 remote sensing measurementsmade directly at the tailpipe. Whilst good agreement was found for outer London, central Londonwas twice as high, suggesting there is a highly unfavourable mix of technology and conditions, which mayhinder the improvements due to current policies. This work aims to quantify the unique effect of congestionon different vehicle types and to provide policy makers with the information needed to better understandthe benefits of congestion control, given that restrictions on technology alone may not always be enough toreduce emissions.