Among the high-rise apartment blocks of Xi’an, China, there is a 100-metre cylindrical tower. It looks like a chimney, but it has a very different purpose. It is trying to filter the city’s air. Smaller examples of outdoor air filters have been erected in the Netherlands and Poland, and filters have been installed next to roads in Delhi and on buses in Southampton. Other ideas include plants and surfaces that react with the pollution, but cleaning outdoor air is difficult since the pollution is already diluted. It’s like trying to take the milk out of your tea.
Ally Lewis, professor of atmospheric chemistry at York University, likened filtering outdoor air to “trying to air-condition a room with the roof off”. The volumes that we would need to filter are vast. For example, if you drive a typical diesel car made between 2010 and 2015 for a distance of 8km (about 5 miles), it will emit around 9g of nitrogen oxides. To remove this pollution from the atmosphere in central London you would have to scrub around 150,000 cubic metres of air – approximately the volume of St Paul’s Cathedral. It is much easier to stop the air being polluted in the first place.
• With thanks to Anna Font, King’s College London for checking the volume calculation.
