Artists impression of the purification tower. SCMP / handout
An air purifying tower the size of a small skyscraper has been built in the city of Xi'an, China. Reports from the scientists behind the project seem at first glance to be rather positive, offering a technological route to clean pollutants out of the air at the sort of scale that could help an entire city. But as there isn’t any hard data publicly available to support the claim, for now a dose of scepticism is likely in order.
This isn’t the first project to use a technological or natural intervention to try and clean up pollution from ambient air. In India water cannons have been used recently in an attempt to wash out particles, while in London NOx-eating paint has been promoted as a solution to harmful nitrogen oxides. And many cities have mooted the planting of more trees and shrubs as a route to cleaning up the air.
All of these, and indeed the purifying tower, are underpinned by at least some plausible science. A tower that filters the air no doubt will take tiny harmful particles out of the air, titanium oxide paints do react with NOx, and trees do act as sinks for air pollution. However, the more important but often neglected question is whether the effects really make a useful difference.
The biggest challenge in trying to take pollution out of air (rather than exhaust pipes) is that it is very diluted. Although humans live predominantly in the bottom couple of metres of the atmosphere, what we breathe is heavily influenced by a much deeper layer of air that runs from the surface up to around 1km in height, referred to in meteorological terms as the planetary boundary layer. Pollution is rapidly mixed in the boundary layer due to turbulence and thermals, and it is this much bigger volume of air that needs scrubbing and cleaning if pollution is really to be reduced on large scales.