Yesterday, analyses of the air in several parts of France showed it contained high levels of ultrafine particles emitted by road vehicles.
Among the guiltiest culprits were older diesel vehicles with minimal emission-control systems, particularly those built under the Euro 4 standard in effect through 2005.
Over the last year, Paris has instituted increasingly strict curbs on older cars in an effort to ban those with the worst exhaust emissions.
Now we know how authorities in the French capital will enable police officers and others to distinguish among different years of otherwise identical cars that may have engines fitted with less-capable emission-control systems.
Drivers will put in their windshields, according to a report today by Reuters, a color-coded sticker that indicates a group of model years.
The city rolled out the plan today, giving the oldest diesel cars registered between January 1997 and December 2000 a grey sticker.
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Those models are banned from entering the city altogether under the "Crit'Air" system; they represent roughly 6 percent of the 32 million vehicles on the country's roads as a whole.
The next group, those registered from 2001 through 2005, get a brown sticker.
They make up 14 percent of the country's vehicles, and Paris mayor Anne Hidalgo has said she hopes to extend the ban to them as well.
That would ensure that the only cars that can legally be operated within the French capital are those that fall under the Euro 5 emission standards, plus a very small number of pre-1997 antique or collector vehicles.
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With the European Union now able to fine cities for air quality that doesn't meet health standards, the issue of emissions from the dirtiest diesels has risen to front and center for those cities' mayors.
European diesel emission standards have lagged considerably behind those in the United States.
The EPA's so-called "Tier 2, Bin 5" standards apply to all cars manufactured after January 1, 2008, that are to be sold in the U.S.
European standards to bring down emissions to that level did not come into effect until this year, under the Euro 6b standards.
Those essentially require costly urea injection systems, also known as selective catalytic reduction.
That's the technology Volkswagen declined to use on many of its so-called "clean diesel" vehicles, which led its engineers to insert "defeat device" software into the car's control systems to cheat on the emission tests.