Specifically, any light-duty diesel vehicle initially registered on or before 31 December 2006 will enjoy a one-time credit of €330 (US$439) on their vehicle license fee on the day its owner proves to the authorities that a DPF was retrofitted between 1 January 2006 and 31 December 31 2009. For qualifying retrofits executed before the law goes into effect, the credit becomes effective on the day that it does.

Note that there are two types of qualifying filters: wall-flow filters, which are ~98% effective and reduce PM emissions below the level of the future Euro 5 regulations today. This type requires periodic regeneration and hence, an engine control unit featuring software to execute that. Given the high thermal stresses that can occur in the monolith while burning off the soot, the software must be fine-tuned to each engine and exhaust design. Quite a few current-generation vehicle models are already shipping with wall-flow filters as standard equipment.

For many other models, it is now an available factory-installed option that prescient buyers have already shelled out for. For these—and only these—models, a wall-flow filter can also be retrofitted, but at €600 the price tag is substantially higher than the factory-installed option.

The other type of DPF is called flow-through and is only 30-50% effective. However, it requires no regeneration cycles and is therefore also suitable for legacy vehicles. For example, the PM emissions of a vehicle originally certified at Euro 1 will be lowered to roughly the Euro 2 level.

Aftermarket suppliers already offer suitable devices for some but far from all makes and models, priced at €550-700 installed depending on vehicle model. Unfortunately, in many older underbody designs, there is simply no room to install any DPF.

The price differential is why the tax credit applies only to retrofits and not to vehicles shipped with factory-installed DPFs. To fund it, the proposed law will impose an annual penalty of €1.20 per 100 cubic centimeters of engine displacement through Dec 31, 2011 on the following two classes of diesel vehicles:

1. Those initially registered on or before December 31, 2006 but still without a filter of either type; and

2. New vehicles that do not meet the future Euro 5 limit of 5 mg/km particulate emissions. Presumably, "new" in this context refers to an initial registration date after Jan 1, 2007.

Since 1 January 2005, Austria has offered a one-time €300 credit for new vehicle registrations featuring a DPF. This is funded by a commensurate penalty for those that do not. However, financial support for DPF retrofits is currently only offered in three regions and several cities.

Note that the EU also requires all local governments to take remedial action if their ambient PM level exceeds a threshold level for more than 35 days in a given year.

Respected mostly in the breach last winter, there is now a proposal in Germany to require all owners of light-duty diesel vehicles to obtain a official decal advertising which level of the Euro emissions regulations they were certified/retrofitted to with respect to PM. On days with poor air quality, diesel vehicles below the minimum certification level defined for that day would not be permitted to operate at all in that municipality. It’s not immediately clear if or when this additional red tape will go into effect and, how it will mesh with the DPF retrofit law. (Earlier post.)

According to the Austrian automobile association OeAMTC (Unsere Luft, ISBN 3-900235-84-8, in German), only 6.6% of PM2.5 emissions in 2005 could be traced back to LDV diesels and another 8.4% to HDVs. The rest came from space heating, industry, power plants, off-road diesels and agriculture. However, these values reflect averages across the country. In city centers, on-road diesels presumably play a bigger role.