Podcast: Tackling air pollution

Exhaust fumes

14 April 2014 by Sue Nelson

This week in the Planet Earth podcast, Ranjeet Sokhi and Kevin Douglas of the University of Hertfordshire explain why scientists are calling for greater controls on so-called non-exhaust emissions to combat Britain's air pollution problem.

To assist those who find text-based content more accessible than audio, a transcript of this recording is available below.

Sue Nelson: This time in the Planet Earth podcast, reducing traffic pollution and how to help clean up the air we breathe. The World Health Organisation recently cited air pollution as the world's biggest environmental health risk. It's linked to around 7 million deaths each year, most of them related to the heart or lungs, that's one in eight people on our planet. Sand from the Sahara Desert may have hit the headlines recently in terms of air quality, but one of the biggest causes of air pollution is, as you can hear, traffic. It's estimated to cost the UK economy up to £16 billion every year and that's why I'm in Hatfield by the A1 in Hertfordshire, a road that's over 400 miles long connecting Edinburgh with London. Now you'd assume that what comes out of a car is the main cause of air pollution, but research at the University of Hertfordshire has shown that exhaust fumes are not the main problem and are only responsible for a third of the pollution. Well here to discuss why is Professor Ranjeet Sokhi, director of the Centre for Atmospheric and Instrumentation Research at the University of Hertfordshire and Dr Kevin Douglas. Ranjeet, we're here because your team took air samples from the Hatfield tunnel on the A1, which is very close here, why did you choose it, apart from the fact that it's extremely convenient?

Ranjeet Sokhi: The main reason we chose a tunnel is because we wanted to see what else was coming out from vehicles as they travel on a road. We know that the air pollution from an exhaust is hazardous and that's been for a long time, but there are other sources such as brake wear tyre wear and we felt that if we were within a tunnel environment we could get some higher levels, which could be detectable and measured.

Sue Nelson: So what particles are the main culprits though from exhaust fumes in terms of pollution in the air?

Ranjeet Sokhi: Yeah, the main size of particle that comes from exhaust is what we call fine particles and they tend to be particles of less than about 100 nanometres, but very quickly as they come out of the exhaust they accumulate and get larger in size. So the fraction that we're looking for from an exhaust pipe tends to be what we would classify, in a legislative way, as PM2·5. PM stands for particulate matter, these are particles with an aerodynamic size of less than 25 microns.

Sue Nelson: A micron being a millionth of a metre, a nanometre being a billionth of a metre, so you can see how these particles can affect the lungs and the heart because they're so small.

Ranjeet Sokhi: Yeah, absolutely, and the very finer particles can actually get in to your blood stream as well and it's the toxic components of those particles that are particularly relevant in terms of health studies, and what we were looking at is where do these particles come from? And the fraction we were looking at is the next sized fraction, which is called PM10 and that refers to particle with sizes less than 10 microns, 10 micrometres. And we know from our studies and other studies that roughly two thirds of PM10 really is PM2·5 anyway.

Sue Nelson: So you're in the tunnel taking samples, how did you actually do that?

Ranjeet Sokhi: Logistically it's quite difficult but we set up two stations in the tunnel, it really was a field laboratory that we set up. We had several measuring systems at the entrance of the tunnel and then also a similar set of measurements at the exit and what we are really looking at is the increment, the build up of pollution. And we were sampling on very fine pure filter papers and those filter papers then are analysed in the laboratory to see what the particles are really made of. And it's the increment, that's the build up of the pollution as the vehicles are travelling in to the tunnel, it creates what we can call like a piston effect and you can see an increase in the pollution at the exit. And the difference can be related to the traffic volume and the flow do the traffic and that's what helps us to understand how much pollution is being generated, which then is analysed in laboratories to get a deeper understanding of the constituents.

Sue Nelson: And that brings us to you, Kevin, because you've been working on a similar project for the European Commission. So how do you actually analyse your air samples?

Kevin Douglas: We use a range of different experimental techniques to investigate the different components of the particular matter. Some of the main ones are to look at metals in the particular matter, we would use what's called x-ray fluorescent spectroscopy, so we shine...

Sue Nelson: There's always a noisy motorbike, so carry on.

Kevin Douglas: So x-ray fluorescent spectroscopy we have our filters and we shine an x-ray on these and it excites different elements in there, which then fluoresce and by looking at the florescent spectrum you can work out how much of different elements is in the particular matter, so this is mainly for looking at different types of metals. We also use ion chromatography, this is for looking at different inorganic ions. So you've got the positive metal ions such as potassium, magnesium, calcium and also inorganic ions such as sulphates, nitrates, phosphates and other inorganic. Another technique we use is to look at the organic part of the particular matter and we use GCMS, so gas chromatography, mass spectrometry, and what we're mainly looking at here is polycyclic aromatic hydrocarbons, so things that come from combustion of organic substances.

Sue Nelson: So how did you find out then, Ranjeet, that not all, what we take for granted in terms of traffic pollution, was caused by what came out of an exhaust, which most people think of as being carbon monoxide?

Ranjeet Sokhi: Yeah, as Kevin has explained we use markers, chemical markers, which is why we have to do such a large range of analysis. Certain metals that are within the particles can be related to certain sources, for example copper can be associated with brake wear, vanadium, for example, can be associated with fuel, so that comes from exhaust. However that's not enough because the problem with metals is that they're everywhere. So what we do is we use the other techniques that Kevin has mentioned, for example GCMS, and that gives us the organic markers. Organic markers are more specific, you can get specific compounds that can tell us that the material is coming from an exhaust and whether it's petrol or diesel. And there are other compounds that you can use in combination with the metals to tell us whether they're coming from tyre wear, for example, road wear or brake wear. And then on top of that, once you've got the data you then need to relate it to traffic volume, traffic flows and do, through what we call a receptor modelling technique you can then identify the main sources that explain as much of the data as you possibly can.

In our study we were able to explain more than 80% of the variation in the data through the different sources. So we identified contribution from diesel, obviously, from exhaust and from petrol, which we knew, and as you mentioned at the beginning that roughly accounted for about a third of the total PM10. The rest of it, nearly half of the PM10 was attributed to sources like brake wear, road surface wear and also re-suspension, as the car goes over or a vehicle goes over dust it re-suspends the dust, and that made up 49%, which was a surprising effect because usually you tend to think it's only about 10%, but actually it was quite large. This doesn't mean that exhaust emissions are not important, it simply means that we have to pay more attention to the other emissions, which are the non exhaust, as we term them.

Sue Nelson: It certainly does, does this mean that policymakers are effectively not necessarily concentrating on the most important issues here?

Ranjeet Sokhi: Well, what's happened is it was obvious that we had to focus on exhaust emissions, I mean, you can see it, you can smell it, so that was the way to do it. And then in time, as the exhaust emissions have dropped because of more stricter technological measures and controls, other pollutants, other sources, have become more important. So over the last decade or so it's been recognised there must be other pollutants and what studies like ours are showing is that we're trying to quantify those non exhaust emissions. And in terms of policy those are the most difficult to regulate, whereas exhaust you can measure the exhaust emissions and say, and you can set a limit that it must be below that limit. In road wear, surface wear, tyre wear, brake wear, they're much harder to control, so that poses a policy challenge as well.

Sue Nelson: Now I know you are calling for greater control of non exhaust pollution, but when Britain has just failed to meet clean air targets that have been set by the European Union, do you think anyone's actually listening?

Ranjeet Sokhi: I think they are listening, I think the problem is being appreciate by the government, especially Defra, who are in charge of a national policy on air quality, I think the issue really is what do we do about it. Now there are two things first of all, one is to get a better base of knowledge on non exhaust particles. Our study is a local study, local in the sense it was conducted in a tunnel, we did that for scientific reasons because we needed to make sure were able to measure those pollutants. However are those data values that we've got, are they applicable generally to the urban area? Well, we don't know, we need to do more studies on that. And secondly what we need to do is to have a great dialogue with the manufacturers of, for example, brakes and tyres and see what opportunities there are to try and reduce those kind of emissions. I know in Europe there are studies, for example, trying to put certain materials to try and keep the particles on the surface, they do washing of pavements, for example, on roads. But most of these techniques are not that effective so we do have to look at some technological and perhaps non technological measures to have a combined approach.

Sue Nelson: Professor Ranjeet Sokhi and Dr Kevin Douglas, thank you both very much indeed for joining me. And that's the Planet Earth podcast from the Natural Environment Research Council. You can follow us on Facebook and Twitter where we'll post a picture of our interviewees from today's recording. I'm Sue Nelson, thanks for listening.