Using Mobile Monitoring to Understand Vehicle Emissions in Urban Areas

Tailpipe emissions from road transport have fallen dramatically over the last 30 years due to the combined
effect of increasingly stringent regulations and technological improvements. However, the air pollution
burden due to road vehicle emissions remains the dominant source of many air pollutants in urban
areas. Policies such as Low Emission Zones (LEZs) have become increasingly popular as a method of reducing
on-road emissions by restricting access to the oldest and most polluting vehicles. Most modern vehicles
are fitted with sophisticated exhaust aftertreatment systems, which should lead to significantly reduced
emissions of pollutants such as nitrogen oxides (NO_x = NO + NO₂). However, the performance of such
systems 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-start
driving with repeated accelerations and decelerations. Under such conditions some aftertreatment systems
cannot reach the high temperatures required to operate efficiently, which may limit the effectiveness of
policies that target vehicle type alone. Therefore, to develop effective air quality management strategies it
is necessary to understand the relative importance of factors that influence vehicle emissions, such as fleet
composition, traffic state, driver behaviour and ambient temperature.
In this work we present results from a mobile monitoring campaign in London, UK. Measurements were
made in two unique locations (central and outer London) in order to provide a quantitative understanding
of the main drivers for concentrations in terms of traffic conditions. We show that there is a significant low
speed penalty for NO_x 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 and
slow average moving speed of only 12 km h^-1, resulting in the non-optimal performance of aftertreatment
technologies fitted to diesel vehicles. Moreover, despite the heavy restrictions imposed by the Ultra Low
Emissions 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 (ΔNO_x/ΔCO₂) in central London was 0.0039 ppb ppb^-1, a
factor of 2 higher than outer London (0.0021 ppb ppb^-1). For context we compared the measured emissions
intensity to an “urban average" value (0.0018 ppb ppb^-1) derived from 135,000 remote sensing measurements
made directly at the tailpipe. Whilst good agreement was found for outer London, central London
was twice as high, suggesting there is a highly unfavourable mix of technology and conditions, which may
hinder the improvements due to current policies. This work aims to quantify the unique effect of congestion
on different vehicle types and to provide policy makers with the information needed to better understand
the benefits of congestion control, given that restrictions on technology alone may not always be enough to
reduce emissions.