The second part of our feature in which Dr James Allen explained the workings of the urban background air quality monitoring ‘supersite’ in Manchester, which is run by the University of Manchester
Click here to read the first part of this article
PM Question Time
As you might imagine, there are plenty of screens flashing up complex information and for the most part I chose not to investigate these arcane instruments, but one stood out, a machine that was reporting levels of PM4. It turns out this is the main metric used in occupational health research for workplace exposure limits. So when people talk about inhalable dust, they’re actually talking about PM4.
On more familiar ground, James acknowledges that PM2.5, NO2 and ozone are the three main pollutants we all care about but everything on top of that is useful to add extra dimensions to the data. ‘And then we get things like this,’ he says, affectionately (or did I imagine that?) patting a particular piece of equipment. ‘This is the really fun stuff from a research perspective. This is a metals analyser. Metals don’t necessarily contribute a big fraction of PM2.5 but they may have their own individual toxicological properties, or they may tell us something interesting about where it’s coming from.
‘So as certain sources of pollution are starting to diminish, like the particulate pollution from vehicles exhausts, do we need to worry about other sources, for instance, the tyres or the brakes on those vehicles? Do we need to worry about wood burning? Do we need to worry about industrial sources that we previously didn’t care that much about?’
The metals analyser examines both PM10 and PM2.5, making measurements of the metals present using a technique called X-ray fluorescence.
‘What we look for is things like lead, which obviously not good for you,’ James explains. ‘Chromium, arsenic, things like that are very interesting from a toxicological perspective. And then you have other things which are going to tell you where stuff’s come from such as potassium or iron.’
What would that tell you? ‘We know that wood burning emits a lot of potassium, just from potassium present in the wood. With brake dust, for instance, you’d expect things like iron, but brakes also have a lot of copper and barium in them as well, so you can really identify that one. There’s other kind of more esoteric ones, like emissions from ship’s engines which may have traces of vanadium in them.’
Past particulates
As we briefly mentioned earlier, the Supersites don’t just measure air pollution, their meteorological measurements are also important. There is an instrument looking at the structure of the atmosphere above us, using a laser, measuring the boundary layer height to see how well mixed the atmosphere is. There is also got a rainfall radar and equipment to take measurements of sunlight to see how much that’s influencing the chemistry of the atmosphere.
‘We also monitor greenhouse gasses’ says James, ‘and that’s of interest because they’re traditionally measured in the middle of nowhere, away from the pollution sources. But there is a lot of interest in the greenhouse gas emissions that you see within a city.
‘We’re also measuring ammonia which is a big contributor to PM2.5 and also to eutrophication, providing excess nutrients to ecosystems. Most of the ammonia which is emitted to the atmosphere is from agriculture but there is a lot interest in whether there are significant sources in urban areas that are less well characterised.’
It’s true, we don’t think of ammonia as being emitted from cities, so what are the sources? ‘You have things like municipal waste handling, household kitchen waste where you have rotten plant matter, even just some parkland with a duck pond – these are the more obvious things you might think of.
‘But on top of that there is the use of selective catalytic reduction to reduce NOx emissions from diesel engines. Those catalysts use AdBlue. The idea was that you put the minimum amount possible in there, but they found that that wasn’t working well enough, so now they tend to overdose them, which increases the danger of ammonia slip, where the AdBlue gets converted into ammonia and comes out of the exhaust. There are various estimates of how big a problem that is, and we’re here to measure it.’
A burning question
We recently covered the effect of Bonfire Night on air quality in the UK, so I wondered how the event would be perceived by people using equipment this sophisticated, is it an annoyance? ‘If I’m getting students to analyse data, the first thing I tell them to do is ignore Bonfire Night. Or not, if they want to see something really interesting. Fireworks use metals for colouration and stuff and so yes, the measurements just go absolutely crazy on Bonfire Night.
‘There was one year in particular, 2014 so before this site came into existence, but we got a load of our existing instruments and set them all up measuring, just on the off chance we’d get something. But as it turned out that year saw a massive temperature inversion and the PM2.5 was massive in Manchester. We got multiple research papers out of it.’
Certainly, the data acquired here is nonpareille but it is this research that really elevates the supersites above the other monitoring stations in the UK. Multiple PhDs have used measurements from the Manchester site and in addition to his own students, James also welcomes other universities to the site. ‘Later this summer, we’re hosting some people from Leeds University. They’ve got this big instrument for studying peroxy radicals which is really important in atmospheric chemistry and is indirectly important for the air quality metrics.’
Future findings
So what next for Manchester’s Supersite? In terms of collecting data, is there anything that’s agonisingly out of reach right now? Of course there is: “There’s been quite a few developments in terms of ultrafine particles. It’s not that the measurement of ultrafine particles is new but some of the newer instruments are interesting.”
I suggest that in the future the Supersite will justify its existence by measuring different things to those measured now, but I wasn’t reading the room. ‘To be frank, you can get a lot of stuff just by making the same measurements for an extended period of time,” James counters. “You look at long-term trends and you can get more reliable statistics… It’s worth mentioning that the biggest source of variability in air pollution is the weather and that’s constantly varying. So if you want good statistics you’ve got to make measurements for a long period of time.’
This article originally appeared in the July issue of Air Quality News magazine.
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