On the other hand, I wonder if anyone's ever built a 100m placebo before. It could be a really interesting university study on the placebo effect in disguise.
Let's do the math.
Guidelines state that the limit on PM2.5 particulates is approximately 10 micrograms per cubic meter for an annual average. Xian, where the tower was installed, is currently (http://aqicn.org/city/xian/) under 231 ug/m^3 of pollution. The tower is supposed to process 10 million cubic meters of air per day.
10^7 m^3 x 231 x 10^-6 g/m^3 x 10^-3 g/kg x 0.15 = 231 x 0.15 x 10^-2 =>
Let's also check the amount of air it should be processing. It's about 100 meters tall, and intended to cover 10 square kilometers. We'll conservatively assume that this volume represents the total quantity of air it needs to process. Ihe volume is:
10 km^2 x 10^3 m/km x 10^3 m/km x 10 ^ 2 m = 10^8 m^3
The math still seems low. Micrograms per cubic meter are hard to intuit.
The microgram is a very small unit :) I mean, a gram is already pretty dinky, a millionth of it is even tinier! The whole point of these particles is that they're miniscule and can pass through tissue other particles can't, so maybe it's not so surprising that it's not a massive amount when you pile them up.
As for the volume of air, I'm most curious where the 15% measurement was taken and how consistent it was. How far from the tower? How many different places were measured? How many days are in the sample? What's the P-Value?
But still, thanks. Makes this interesting to think about.
I agree, this is very surprising. Before I saw LeifCarrotson's calculation, I would have thought "tons" was the right unit as well. There's so much polluted air, you'd think that it would add up to some significant amount of matter, but apparently not.
EDIT: In 2016, the United States burned roughly 728 million tons of coal, enough to fill a typical railroad car every 4 seconds
So it's still a lot of pollution, just much less actual mass than I would have guessed.
There's the issue that it's probably removing nearly all of the particulates from the air it actually processes, say over 90%, which works out to 2.1 kg. It doesn't reduce the PM2.5 level in the area by 90%, but only by 15%, because it's only able to process some 16% of the air in its area during whatever the relevant time interval is, which seems like a good match to your 10%-per-day.
However, you got the volume of that area wrong: it's 10⁹ m³, not 10⁸ m³. That suggests that the natural lifetime of PM2.5 particulates is closer to a week than a day.
As for jhugg's question about how the filters are cleaned, a common way to remove particulates from flue gas or indoor spaces is using electrostatic precipitators, which are flat plates at a high voltage. The voltage sticks the particulates to the plates, and when the coating is thick enough, you close the valves to stop the gas flow and vibrate the plates to unstick the dust, and it falls out the bottom of the filter into a bucket (or, say, a truck). Other systems clean the plates with water (either spray or immersion) and possibly soap or lye.
That's what I get for interweaving a mantissa of 10 with scientific notation! Too late to edit now, but people should see this comment thread.
Three are alternative chemical processes to clean up such filters.
See the protests that erupt over mobile(Cell) phone towers. People claiming towers that weren't even on made them sick etc.
https://idle.slashdot.org/story/10/01/15/1516245/tower-switc...