Can you add filters directly to the exhaust of a vehicle to reduce roadside air pollution?

Pollution control can, and is, added to cars in order to prevent the vehicle’s pollution from escaping the tailpipe in the first place. This is the idea behind the three way catalyst found on many spark ignition vehicles, and the particle filters and NOx control systems found on many diesel vehicles. Such systems have greatly reduced pollution produced by traffic, but they are not perfect, some pollution is still emitted, in addition to brake dust and dust from road and tire wear. These emissions control systems are optimised for the extreme conditions found in a tailpipe – which range from freezing temperature when a vehicle is started in a cold environment, to hundreds of degrees when the engine is running full out.

Another perspective would be to add an air cleaning system to the car which cleans the ambient air around it. A car manufacturer played with the idea of putting a pollution control catalyst on the radiators of cars in the 1980s, turning the cars into air cleaners that would sponge up air pollution in a city. They ended the project due to the fact that even if the catalyst removed all pollution, it would not have had a significant effect on urban air pollution — there is simply too much polluted air over a city.

A quick example is that the polluted boundary layer over a city could be a kilometer thick. Imagine a car able to remove all pollution from the air going through the engine bay. A one square meter grill moving at 10 m/s can treat 10 cubic meters per second. To remove pollution from the atmosphere over a 1 km by 1 km area of a city would require 100 million seconds for one car; to do it in an hour would require 30,000 vehicles with the perfect system running continuously.

A major manufacturer played with the idea of putting a pollution control catalyst on the radiators of cars in the 1980s, and they ended the project due to the fact that even if the catalyst removed all pollution, it would not have had a significant effect on urban air pollution.

Because of the problem of dilution, most effort has gone into controlling pollution at the source by developing emissions catalysts, de-nox systems, particle filters, or by removing the engine altogether. For example over half of new car sales in Norway today are for electric vehicles. There is still the problem though of road dust, and particles from tires and brakes. One solution to reduce roadside pollution sources ride a bicycle to work as they do in Copenhagen.

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