Using Mobile Monitoring to Understand Vehicle Emissions in Urban Areas

<p>Tailpipe emissions from road transport have fallen dramatically over the last 30 years due to the combined<br />effect of increasingly stringent regulations and technological improvements. However, the air pollution<br />burden due to road vehicle emissions remains the dominant source of many air pollutants in urban<br />areas. Policies such as Low Emission Zones (LEZs) have become increasingly popular as a method of reducing<br />on-road emissions by restricting access to the oldest and most polluting vehicles. Most modern vehicles<br />are fitted with sophisticated exhaust aftertreatment systems, which should lead to significantly reduced<br />emissions of pollutants such as nitrogen oxides (NO_x = NO + NO₂). However, the performance of such<br />systems is non-uniform across all driving conditions. Urban driving conditions are among the most challenging,<br />where vehicle speeds are often low and congestion results in a considerable amount of stop-start<br />driving with repeated accelerations and decelerations. Under such conditions some aftertreatment systems<br />cannot reach the high temperatures required to operate efficiently, which may limit the effectiveness of<br />policies that target vehicle type alone. Therefore, to develop effective air quality management strategies it<br />is necessary to understand the relative importance of factors that influence vehicle emissions, such as fleet<br />composition, traffic state, driver behaviour and ambient temperature.<br />In this work we present results from a mobile monitoring campaign in London, UK. Measurements were<br />made in two unique locations (central and outer London) in order to provide a quantitative understanding<br />of the main drivers for concentrations in terms of traffic conditions. We show that there is a significant low<br />speed penalty for NO_x concentrations in central London, where there is a high proportion of diesel vehicles,<br />which are predominately taxis and buses. This effect arises due to the near constant congestion and<br />slow average moving speed of only 12 km h^-1, resulting in the non-optimal performance of aftertreatment<br />technologies fitted to diesel vehicles. Moreover, despite the heavy restrictions imposed by the Ultra Low<br />Emissions Zone, which requires all diesel vehicles in the zone to be Euro 6/VI (light/heavy vehicles) compliant,<br />we find that the mean emissions intensity (&#916;NO_x/&#916;CO₂) in central London was 0.0039 ppb ppb^-1, a<br />factor of 2 higher than outer London (0.0021 ppb ppb^-1). For context we compared the measured emissions<br />intensity to an &#8220;urban average" value (0.0018 ppb ppb^-1) derived from 135,000 remote sensing measurements<br />made directly at the tailpipe. Whilst good agreement was found for outer London, central London<br />was twice as high, suggesting there is a highly unfavourable mix of technology and conditions, which may<br />hinder the improvements due to current policies. This work aims to quantify the unique effect of congestion<br />on different vehicle types and to provide policy makers with the information needed to better understand<br />the benefits of congestion control, given that restrictions on technology alone may not always be enough to<br />reduce emissions.</p>