The case of the large scrubber farm

Every now and then people get "creative" with their solutions about air pollution. Back in Santiago (Chile) I saw a number of ideas being floated, with various levels of "seriousness" and I always thought that the responses from the authorities and the researchers were either dismissive or incomplete, keeping the door open for these ideas to come back and not clarifying what the good things were about those ideas and what the problems were so they were again received with the same responses.

One of those ideas was the use of what basically amounted to large outdoor air purifiers (scrubbers or filters). The first time I heard about that was back in the 1990's when the air pollution in Santiago started to be taken more seriously by the population (after other much more urgent issues were addressed as practical) and hence the elected were pushed to try and solve the problem.

View of Santiago on a winter's day.
By Jorge Barrios Riquelme
[CC BY-SA 4.0  (https://creativecommons.org/licenses/by-sa/4.0)],
from Wikimedia Commons

The plan was (and it may have actually been attempted with one device in a large urban park) to have many large wet scrubber sucking dirty air and blowing clean air. Now, I'm a chemical engineer and I'm trained to like these sort of things but what's often missed is that even though the underlying technology works (scrubbers and filters are very good at cleaning gases), the implementation details are not just difficult (I love a challenge) but impractical.

Let's put some numbers on this and we'll see where things get "implausible". The key parameter in this is the volume of air that needs to be treated for this to work. So ... how much air should we pass through these devices?

Let's assume that, on an annual scale, the city is in equilibrium so that the ambient concentration is "stable" in time, in other words, there is a balance between the sources (emissions) and sinks (deposition, transformation, etc). If there was no balance, then the concentrations would either be decreasing (not a problem then) or would be increasing (a worst-case scenario). Now, our goal would be to remove all man-made emissions from the air and leave clean background air.

How much does Santiago emit in a year? Quite a lot as it turns out. The Registro de Emisiones y Transferencias de Contaminantes (Pollutant Release and Transfer Register) indicates that for 2011 the total $PM_{10}$ emissions were around 300 $g/s$ (more than 9000 $ton/year$).

So ... we only need to remove less than half a kilogram of material from the air, that doesn't sound too bad! Well, it isn't until you realize that all this material is not lumped together ready to be scrubbed but it is diluted throughout the city in concentrations of a few micrograms (millionths of a gram) per cubic meter. The WHO guideline for long term $PM_{10}$ exposure is 20 $\mu g/m^3$ as an annual average. For Santiago this value is 69 $\mu g/m^3$  which means that to extract those 300 $g/s$ of pollution from the air we'd need to process more than 4 billion $m^3/s$ ($4.3*10^9 m^3/s$).

Let's push the point home a little deeper. To process that amount of air we'd need quite large fans to push the air through the scrubber. Well, I found this place that has a 4.3 m wide wind tunnel that pushes air up to 275 $km/hr$ and even though it sounds awesome it turns out that we'd need over 2 million of those and even if you find a power source large enough to supply those turbines ... we're left with the decision of whether to have a city or a turbine farm.

In conclusion, it's not practical to clean the air of a city with scrubbers. Now, as I said in the beginning, these things work wonders when dealing directly with the emissions so instead of trying to put them all over a city, put them where the emissions are!