Air pollution is less ubiquitous than it appears. Even at the city scale, it can vary from street to street.
But certain models are unable to account for that variation. And in “canyon cities,” — where street width closely parallels building height (and where cars tend to file in close rank) — like Barcelona, Spain, those variations can be marked, creating pockets of intense air pollution.
Researchers at the Barcelona Supercomputing Center, Dr. Albert Soret and Dr. Oriol Jorba, spoke to us about the challenges and import of modeling nitrogen oxide pollution at the street level and their new high-resolution model for doing so.
What is CALIOPE-Urban? How does it calculate emissions, and does it do this differently from other modeling systems?
Soret: So 40 years ago we started to think about air quality more in detail, at the street level. Because, up until now, we were working with what we call mesoscale models — those that can go up to a one-kilometer resolution. This is interesting to analyze what's happening on the original level. But when you go to the city, to understand what's happening street by street, you need higher resolution.
That's why we were evaluating different models.
For four years, the objective was to work together with those models, to calibrate them, to use a massive amount of observations — all the observations that were available — to adjust and calibrate these models. And to understand the whole workflow from their original air quality model to the CALIOPE-Urban. This is how it started.
And then once we had the expression of the machine, we started to use the system to analyze different questions, like the ones that we cover in this paper.
Jorba: It's a measure of the gradient flow for pollution. In the city, we have huge gradients. And mesoscale models cannot reproduce that.
Take an elevator and go to the roof of a building and measure the concentration right there. But if you go to the street next to the cars – well, things change a lot. And gradients are huge.
"[Europe has what] we call a compact city. It's quite different from the typical US cities that are huge open spaces. In Europe, we have more of this street canyon environment." ~ Dr. Jorba
Administrators, especially in the health community, are extremely interested in that information. What's the concentration close to the street? For residential neighborhoods? They really need these types of information.
That's the main aim of the CALIOPE-Urban model. It’s the module to cover the street scale.
You were asking about emissions. We try to use as much information as possible. For instance, traffic counters, number of cars that cross a specific street — different cities have a lot of points where they track that.
We gather this information from the municipality. We also tried to characterize the fleet composition for that street. How many petrol engines, diesel engines, light duty vehicles, and heavy duty vehicles are circulating every day?
It's a huge work trying to characterize that. That's the first step. If you don't have good emissions, forget about the rest. That's our main weakness in our field. Emissions are uncertain and difficult to characterize.
Light-duty vehicles were central to this paper. Can you discuss them, especially considering the fact that the paper concluded that 20% of NO2 emissions in Barcelona came from diesel engines?
Oriol: That's something I would say is particular to Barcelona. It’s a city with a huge density of traffic. We target light duty vehicles because they’re a huge share of the total vehicles circulating across the city.
For this final number of the 20 percent, we used the current knowledge of how much different types of engines emit from our lab studies. And then we also take the legislation that says [a specific] type of engine should emit a certain amount of nitrogen oxides.
Then we run through a scenario, one assuming the real driving conditions and another one assuming the standards.
Just comparing both scenarios, we realize that there are an excess of around 30% of NOx from the diesel vehicles. This excess is what they emit in real driving conditions compared with what they should emit following the legislation.
This results in an impact of 20% of NO2 in the atmosphere [in Barcelona].
This is an estimation. We use a lot of information. We tried to use as much detail and relevant, curated information as possible. But always, in our field and in modeling studies like this where you try to model a scenario, there are uncertainties.
The paper concludes that cleaner LDVs are “not enough” to meet NO2 limits, and instead recommends fewer vehicles. Can you expand on that?
Oriol: Well, mainly because there are too many cars in the city. That’s the simple answer. Barcelona is a city with a lot of vehicles moving around. You can try to reduce emissions. [Europe has what] we call a compact city. It's quite different from the typical US cities that are huge open spaces.
In Europe, we have more of this street canyon environment. This ratio of height of buildings and width of the street is quite one-to-one. Under these conditions, a lot of cars will end up creating issues regarding air quality episodes.
Soret: For us, this message is also important because we know in Barcelona, in Madrid, in Paris, in many cities in Europe, there are huge problems of air quality. And every two years, it seems like a new technology development will help to improve air quality – like reduction of emissions [from engines] or electric vehicles.
It’s like thinking “Ok, we can continue to do the same, but with technological developments the problem will be solved.”
This was not the point. Changing our way of moving is needed. To change our modes of transport, to use bicycles or to go walking to work — all this has to be on the table to improve air quality. If it’s just some technological developments, it will not be possible at all.
Oriol: We really thought that diesel vehicles were going in the right direction, but now we realize they are emitting a lot. What’s happening right now? We are moving to super vehicles that are larger, they weigh more, they emit more.
So again, new technologies or new solutions don’t mean that we are going in the right direction.
Is it too late to seriously reduce emissions in cities like Barcelona?
Oriol: I don't think it's too late. It's a matter of political willingness and also a social willingness to move to better cities, better public commuting. I think there's a lot of benefits to changing the way cities work currently.
Soret: That’s why it’s so important that the message here is to reduce traffic. This is the only way to make the targets.
"Changing our way of moving is needed. To change our modes of transport, to use bicycles or to go walking to work—all this has to be on the table to improve air quality. If it’s just some technological developments, it will not be possible at all." ~ Dr. Soret
Oriol: In the end, air quality is just an issue of using proper technologies. Now we have a lot of knowledge of what we should tackle to try to improve that. As for the impact on climate, that’s another story. We are quite late on that. But at the urban scale, discussing about air quality…
For instance, the pandemic has shown us how shockingly and how strongly air quality reacts to changes in different socioeconomic activities. We never thought that we would live through that kind of period in our lives. We need to learn from that. It’s not impossible to change the way we live in cities.
Soret: For a long time, we have had the evidence that we need to improve air quality. We know that air pollution has an impact on our health, and not just on premature diseases — on everything.
But it's difficult to really link someone dying from air quality and pollution —because it's not like when you have an accident and there’s a direct link.
When talking about air quality [for people in cities], we have time. But in terms of climate, maybe…
Oriol: We’re at a mitigation scenario in climate.
Soret: We are at a certain tipping point, so now we have to take many, many measures to solve anything. It is how it is.