Some researchers in New Delhi have observed paradoxical phenomena near the smog towers. The air closest to the tower should be cleanest, but the device recorded the opposite in several instances.

What are Smog Towers?

• Smog towers are structures designed to work as large-scale air purifiers. They are fitted with multiple layers of air filters and fans at the base to suck the air.
• After the polluted air enters the smog tower, it is purified by multiple layers before being re-circulated into the atmosphere.

Structure of the Delhi smog tower

• The structure is 24 m high, about as much as an 8-storey building — an 18-metre concrete tower, topped by a 6-metre-high canopy. At its base are 40 fans, 10 on each side.
• Each fan can discharge 25 cubic metres per second of air, adding up to 1,000 cubic metres per second for the tower as a whole. Inside the tower in two layers are 5,000 filters.
• The filters and fans have been imported from the United States.

How does it work?

• The tower uses a ‘downdraft air cleaning system’ developed by the University of Minnesota.
• Polluted air is sucked in at a height of 24 m, and filtered air is released at the bottom of the tower, at a height of about 10 m from the ground.
• When the fans at the bottom of the tower operate, the negative pressure created sucks in air from the top.
• The ‘macro’ layer in the filter traps particles of 10 microns and larger, while the ‘micro’ layer filters smaller particles of around 0.3 microns.
• The downdraft method is different from the system used in China, where a tower uses an ‘updraft’ system — air is sucked in from near the ground, and is propelled upwards by heating and convection.
• Filtered air is released at the top of the tower.

Likely impact

• Computational fluid dynamics modelling suggests the tower could have an impact on the air quality up to 1 km from the tower.
• The actual impact will also determine how the tower functions under different weather conditions, and how levels of PM2.5 vary with the flow of air.

Issues with smog towers

• Many experts say that the smog towers are not a viable method to clean city’s air.
• The government had talked about 80% pollution reduction at inlet and outlet of the tower but never mentioned about the effect of distance from the tower.
• Instead of spending ₹40 crore on two towers, the government could have spent the funds on several other options such as replacing the small and polluting industrial boilers or chimneys etc.

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