Are dust filters for cars an efficient way of reducing particulate matter pollution?

In order to meet the emission norms for particulate matters (Directive 98/70/EC, EURO4), car constructers have equipped their vehicles with dust filters. The filter eliminates a large part of the particles in exhaust gases of diesel cars, by means of a physical filtration on a ceramic substrate.

To guarantee the efficiency of the filtration and to avoid obstruction and loss of engine performance, it is necessary that the filter is regenerated during approximately 10% of the driving time. During this regeneration phase, the temperature in the filter has to be approximately 500°C, so it is possible to burn and destroy the accumulated particles. This temperature is much higher than the normal temperatures of exhaust gases, which is about 200°C. The solution is to raise the temperature ahead of the filter, by injecting an amount of fuel into the catalyst, where it will be ignited. In some filters, the addition of a catalyst additive to the fuel will allow a decrease of the regeneration temperature.

In absence of an additive, the regeneration is less effective, certainly under urban traffic conditions. In real driving conditions, the regeneration phase implies an increased emission of polluting gases (NO2). In the test cycles for emission levels (EURO norms) as of today, this is not taken into account. Additionally, the effectiveness of the filters decreases with the size of the particles. Even if only 1 to 5% of the total mass of particles escapes the filter, this number is very important, seeing as these particles often have a diameter below 1 µm and thus belong to the fraction having the largest impact on health.