City pollution stories mostly focus on emissions: traffic, vehicles, fuel, industrial sources and background pollution. These are relevant, but they do not tell the whole story, because once emissions enter the city, they are subject to transport, recirculation, dilution and redistribution by the ambient airflow.
Tracer experiments are one method used to study this. A known quantity of a substance, the tracer, is deliberately released into the environment, then detected and measured. It is not designed to replicate pollution from ordinary sources exactly, but to follow the dispersion of a tracer under realistic or simulated conditions.
This is what DAPPLE’s field and laboratory work did. Alongside DAPPLE wind measurements, traffic data, exposure measurements, wind tunnel modelling and numerical simulations, tracer release experiments were a major component of the programme.
Why Are Tracer Experiments Useful?
The problem with urban pollution studies is that the environment is not well controlled. Traffic patterns change; the weather changes; background pollution changes; pedestrians, vehicles and cyclists use the same streets in different ways.
By carrying out a tracer experiment, we know where, when and what was released. We know how much was released. This helps us to measure how it spreads.
It can help us understand:
- how far away from the point of release the tracer disperses;
- how quickly it moves away from the source;
- how close to the point of release it remains;
- what the effect of different wind directions may be;
- where we expect high concentrations;
- how junctions, buildings and canyons influence air flow patterns;
- if and how well we can replicate tracer behaviour in our models.
In other words, tracer experiments allow us to link the way air physically moves to policy issues of exposure.
From Emissions to Exposure Pathways
Perhaps one of the major outcomes of the DAPPLE tracer work is that we move away from the idea of emissions alone and towards the idea of pathways of exposure. A tracer released at road height does not always simply blow away. It can travel further along the road, down into a side street, stay in the road for a while, or even go above the canopy depending on prevailing conditions. It can behave very differently in different places in a city.
The original DAPPLE-EPSRC project included tracer release measurements in the field and laboratory in order to investigate and characterise these pathways of exposure.
Why does it matter? Because what we care about with pollution is not only how much is being emitted, but where it goes and who it affects. The release at ground level could impact pedestrians at a road junction, pedestrians waiting for a bus, cyclists passing through a lane of cars, or building occupants. Tracer experiments can be used to assess which urban paths are significant.
What Tracers Tell Us About Street Canyons
Street canyons provide one of the clearest examples of the value of tracer experiments. Where buildings line either side of the street, air movement at ground level may differ very substantially from that at roof height. There may be some retention of pollutant within the canyon and dilution into the ambient air only later on.
A tracer can indicate whether the material is retained within the street, crosses the street to the other side of the canyon, how rapidly it exits at roof height, or how much the street-to-street variation can be.
This is linked with the broader issue of why street-level air pollution is so hard to predict: how buildings distort the flow field, how traffic is a moving source, and how the meteorology can change the behaviour of the same street at different times.
Tracer experiments give evidence of those processes rather than just treating them as modelling assumptions.
Wind Direction and Turbulence
Wind direction is a major reason why dispersion differs for a street. A wind aligned with the street will potentially carry material along the street corridor. A wind across the street may lead to recirculation within the street canyon. Near junctions, around building corners and gaps, the pattern is likely to be more complex.
Turbulence also plays a part. It is not just the atmospheric turbulence that matters; buildings create turbulence, street geometry creates turbulence, and sometimes traffic movement does too. This can lead to enhanced mixing in some places, and pockets of poor ventilation in others.
Tracer experiments can indicate whether the tracer plume is simple or complex, whether it follows a particular pattern or has an irregular shape. Measurements of concentration taken at several points can reveal whether concentrations decrease rapidly at first and then less so further from the source, for example.
This matters for policy and planning because a location close to the source is not necessarily the only location of concern. It may be transported to other locations of interest under particular wind directions and speeds.
Fixed Sources and Moving Sources
A fixed source is useful for demonstrating dispersion from a single, known location or for providing a test case of model results. However, not all urban emissions are fixed in this way; traffic emissions are carried as the vehicle moves along its route.
DAPPLE’s later fieldwork involved testing tracer dispersion from both fixed and moving sources. The distinction matters. The moving release can help us look at the movement of vehicle exhausts as they travel down the streets of a city. The fixed release can provide more controlled information on how well material is dispersed around the urban flow field from a single point.
They answer different questions. The fixed release can tell us about local dispersion from a specific source, while the moving release can show how emissions from along a route might affect exposure through a wider area of the street network. Together they provide a better sense of how pollution behaves in urban environments.
Why Repeat Experiments Matter
Single tracer releases are valuable but, taken on their own, they do not capture the range of possibilities that might occur. The urban atmosphere is not always the same and, over time, we might expect to see some variation between different scenarios.
That is the thinking behind the DAPPLE-HO project. Here there was a strong focus on short-range dispersion, urban canopy flow, repeat experiments and conditions that could lead to extremes of exposure.
Repeated tracer experiments help us understand how variable the situation can be in similar, but not identical, conditions. But repetition is difficult at full scale, because wind and weather conditions can change before an experiment can be completed. This leaves a set of field releases which provide evidence, but also carry an element of uncertainty.
This is another reason why DAPPLE combined field work with wind tunnel simulations. The field provides realism; the wind tunnel provides control and repeatability. Both are essential because, on their own, they cannot answer all the questions we might want to ask about dispersion behaviour.
How Tracer Experiments Support Model Validation
Models have to be validated if they are to prove useful. In this respect, tracer experiments play an important role. Urban dispersion models are often applied where no measurements of concentration can be obtained, and are also used to assess planning scenarios, emergency response assumptions or policy interventions.
If the material released is supposed to stay on a particular street and the tracer data show that it moves into other streets, it makes a difference. If the predicted peak values at a junction are too low, this may have implications for exposure and health. If the model works well for one wind direction but less well for others, then we know where it may need improvement.
The DAPPLE technical work included numerical and analytical modelling, and here the models spanned everything from CFD to more empirical modelling approaches. In essence, the value of the tracer experiment is that it provides an evidence base against which the performance of such models can be measured.
That takes model validation out of the purely scientific and technical domain and places it firmly in policy terms. Models used in planning, transport management and emergency response contexts should not only produce reliable results; they must also have been shown to reproduce correctly the dispersion patterns observed in reality.
Implications for Emergency Response
Tracer experiments are also of value when considering accidental, as well as deliberately placed, releases of dangerous materials. Here the issue is no longer just average air quality, but rather the movement of a pollutant over very short timescales and its impact on people at risk.
DAPPLE-HO focused on short-range dispersion in cities, which was directly relevant to planning for emergency incidents. Tracer studies provide important support for this work by demonstrating how quickly and irregularly material can disperse in the built environment.
The DYCE project, which followed later, dealt with a similar issue from the point of view of sensors: how to deploy and redeploy a limited amount of sensors following chemical incidents in outdoor urban or industrial environments.
Tracer studies and sensor deployment are distinct approaches and tools, yet both are focused upon a similar practical problem. Both seek to understand a changing plume in the built environment. For planning in emergency situations, this implies a need for models and monitoring that take into account local heterogeneity, as opposed to more generic assumptions on the prevailing wind direction and distance.
The Policy Relevance of Tracer Studies
The policy relevance of tracer studies is about understanding uncertainty. Tracer studies show how pollution does not disperse through the built environment in an even and uniform way. They show how the configuration of the street network, the configuration of street-side buildings, direction of the wind, location of a source and other factors influence how a person may be exposed.
They show how a monitor may not accurately reflect the levels downwind in a street of interest. They also show why models need to be validated against real data or data derived from the laboratory.
In terms of the implications for urban air quality policy, this means:
- Local exposure should not be viewed simply as a local reflection of city-wide concentrations. Local exposure depends on local features.
- Traffic and urban planning policies should take into account the influence of street configuration on the movement of air. Emissions matter, but so does the way air moves through the built environment.
- Policies for emergency response should be based on evidence that explains short-term movement. Long-term averages are not enough for incidents that unfold over short timescales.
- Monitoring networks should be designed with urban heterogeneity in mind. A good monitoring network should not just be about collecting data; it is also about asking whether the network is collecting data in the right place and under the right conditions.
A Method for Understanding Movement
Tracer studies are about movement. Unlike standard pollution monitoring techniques, they are not only interested in whether pollutants are present, but about why. Tracer studies explain how pollution is transported from its source through the urban environment.
For the DAPPLE project, the tracer work represented one strand of a wider set of activities carried out in the course of understanding short-range dispersion. The study relied on a combination of field monitoring, exposure studies, wind tunnel experiments and computational fluid dynamics.
The combination of monitoring and models is crucial to an understanding of how pollution moves in our built environment and is a necessary component of evidence-based urban air quality policies.
Urban air quality policies should not rely on emission levels alone. A good set of policies should also help us understand how pollution travels in our streets, which are where we live, work and travel.
Tracer studies help us to understand how pollutants move and provide details that may not always be provided by standard monitoring techniques. They turn an otherwise invisible process into something we can measure and test, and use it as the basis for making more informed decisions.
