Greening up our cities: tackling air pollution with plants

Interviewer: Andy Tootell (Ideas Lab)
Guest:  Professor Rob MacKenzie
Recorded: 31/07/2012
Broadcast: 06/08/2012

Intro VO: Welcome to the Ideas Lab Predictor Podcast from the University of Birmingham. In each edition we hear from an expert in a different field, who gives us insider information on key trends, upcoming events, and what they think the near future holds.

Andy: Hello, today I’m with Professor Rob MacKenzie who’s Professor of Atmospheric Science in the School of Geography, Earth and Environmental Sciences, here at the University of Birmingham. Hello Rob.

Rob: Hi Andy.

Andy: So, do you want to tell me a little bit about what you do?

Rob:  I’m essentially a Chemist who’s interested in the atmosphere, all different parts of the atmosphere actually. I’ve spent a long time thinking about processes that are important for the ozone layer for instance, but all the way through my career there’s been a thin thread running through it to do with trees and how trees change the atmosphere down at ground level where we are.

Andy:  And that’s what we’re here to talk about today, air pollution and the benefit of greening up our cities.  So do you want to explain a little bit about what that is? What’s your research been looking into?

Rob: It’s become quite common now for quite a number of people who are interested in ‘green things’, particularly green things, living things, in cities to talk about the services that they provide. So we talk about things called ‘ecosystem services’, so the living part of the planet provides services that we make use of as humans and that is true in the rural environment but it’s also true in cities. We tend to think of cities as being entirely made by us of concrete and brick and glass but actually there’s quite a lot of living non-human green stuff in cities.  And one of the things that the green stuff can do is absorb air pollutants, particularly two kinds of air pollutants. There’s a gas called Nitrogen Dioxide and then there are tiny microscopic particles which nowadays we usually call particulate matter or PM. Both of these things are bad for our health. There are literally hundreds of places across the UK where Nitrogen Dioxide has been recognised as a problem so the work that we’ve been doing lately, we’ve thought a little bit harder about how green things interact with the actual structure of the city, so how the city is built up and particularly the fact that a lot of the city is made up of these narrow roads and streets with buildings on either side and we call those ‘street canyons’.  Of course most of the pollution is happening at the bottom of those street canyons because that’s where the traffic is. The air takes several minutes usually circulating around inside the street canyon before slowly it mixes with the air above roof level. That might not sound like a very long time but in that time that little volume of air is contained and it has very high concentrations of pollution in it. If you put an absorbing material into the street canyon then, it has a much better chance of absorbing the pollution. It’s much more efficient than if you try and catch the pollution when it’s been diluted away into a much bigger volume.

Andy: So if you’ve got your leaf and your leaf is acting as a filter to take all of these nasties out of the air, is there a limit to what a leaf can take or can they just sort of take as much pollutants as you can throw at them?

Rob: Now, I think as far as the Nitrogen Dioxide is concerned, it’s taken right into the leaf and as long as it doesn’t kill the leaf, it’s just metabolised, taken away in the metabolism of the plant and it’s gone.  For the particulate matter, the plants act in exactly the same way as your hoover filter. The little particles stack up on the top of the leaf and the leaf is just like an inanimate filter. But of course fortunately, especially in Britain, it rains frequently and so that just washes all the stuff off the leaves and the leaf can start again, essentially.  Any deciduous plant will in any case shed its leaves once a year and of course the new growth the next season will be able to act as a filter again.

Andy: What kind of plants are we looking at here? Do you have any idea of an ideal set of plants that we can green up our cities with?

Rob: Now as an atmospheric chemist who’s not a very good gardener, that’s a really difficult question. It’s one I keep getting asked these days. As far as I’m concerned, in order to make this process work you need plants that can grow in cities that can survive the pollution levels and that have lots of leaves and I guess ideally keep those leaves all the way through the year.

Andy: How would you go about targeting cityscapes and areas that would most benefit?

Rob: We can look at the structure of the cities. Most cities now have a map of where their pollution hotspots are. More and more there are laser maps that have been produced, so you fly a plane over your city and you can actually map out not just the buildings but also the plants, so you actually know what you’ve got. So with all that data overlain on top of each other, first of all you can spot where the green stuff is doing good now but you can also spot where your pollution hotspots are and whether you might want to intervene with putting more vegetation into that area. We wouldn’t argue that green stuff’s going to do everything. It turns out from our calculations that it looks like it does a lot more for us than we give it credit for, but I think we all know just from our day to day experience in busy cities that the air is still pretty crummy often in terms of air pollution and so the single best thing you could do is emit less pollutants and then in those places where we can make an effective intervention with green stuff then yeah, we should go ahead and do that.

Andy: Now your research so far has been based on a lot of computer modelling but I suppose the next phase is to try and put this computer modelling to some kind of real scenario. What does the future hold for your research and the development of this greening of cities?

Rob:  You’re absolutely right. As far as our research is concerned, I think what we’ve done is we’ve made the prediction. We’ve said well, given what we think we know about plants and what we think we know about how air moves in cities, this is what should happen when you put green stuff into street canyons. We should see quite a big effect but people haven’t been putting green walls into street canyons that much so far because when people put green walls up, they usually like them to be highly visible so they tend to be set back from the main road on big signature buildings.   So we are really looking now to see whether either there exists a good example for us to go and put our equipment into, or whether we can persuade someone to put up a test wall for us.

Andy: Have people been generally receptive to the idea, people who you have discussed these findings with and the possibilities?

Rob:  I’ve been inundated with emails and phone calls since we first produced this paper on the effect of plants in street canyons, especially from local Government officers and I put that down to the fact that they are, not to put too fine a point on it, they are desperate. Their job is to improve the quality of the air, so reduce the pollution concentrations in these busy streets and everything that happens, whether it’s something they do at local level to do with changing the parking prices or changing when you allow vans in and out, or things that happen at national level to do with emission controls, none of that has had any significant impact on these two pollutants, NO2 and PM.  So really a lot of people involved in city planning and looking after our environment, our urban environment, are looking out for things that can make a difference and this story I think is attractive to them because it looks like there are some significant gains to be made here and it ties in with a whole load of other possible benefits of putting green things into cities.  So there’s already been a lot of work by other people on how putting green things into cities helps with flooding, how it helps with the very hot temperatures that we can get on summer nights, so there are a lot of mutual benefits here which if we can package them all up together, there’s the potential here to make our cities much more attractive places to be.

Andy: Looking far ahead into the future, do you have any kind of idea what kind of timescale we might be looking at where cities are designed this way?

Rob:  I’m a bit of a Jekyll & Hyde on this one I have to say because there are a lot of people doing very positive things and having brilliant imaginative visions for the future, where the future is perhaps two or three decades hence. So these are all technologies that could be rolled out now and would be in place in a few decades, really making a radical change to our experience of cities. But I just wonder whether we’re as a society, as a race, I just wonder whether human beings are ready for it, whether we’re ready to take that imaginative leap. 

Andy: Well I’m determined to end this podcast on a hopeful note so we’ll look ahead and say that there are better times ahead in our cities. Professor Rob MacKenzie, thank you very much for joining me today.

Rob: You’re very welcome.  Thank you.

Outro VO: This podcast and others in the series are available on the Ideas Lab website: On the website, you can find out how to e-mail us with comments, questions or suggestions for future topics for the podcast. There's also information on the free support Ideas Lab has to offer to TV and radio producers, new media producers and journalists. The interviewer and producer for the Ideas Lab Predictor Podcast was Andy Tootell.