There's a big difference between general "emissions" which a lot of people take colloquially to just mean greenhouse gases (and CO2 in particular with combustion engines), and then the small particulates and other things (like NOx) that create worse local air conditions[1]. Most "modern" diesels in use do still have the latter problem. I think at the very cutting edge, if you maintain a very good filter and operate in favorable conditions you can cut those emissions dramatically, but as far as I know it isn't widespread among the personal diesel fleet.

[1] http://theconversation.com/fact-check-are-diesel-cars-really...

Ehh.... I worked for a long time as a mechanic on diesel engines. The engines are designed to conform with emissions requirements as they are written, not necessarily to be more efficient or cleaner. And of course some auto manufacturers have taken rather "creative" approaches to meeting these requirements.

Even with everything above board, the technology is not really there to make diesel a great solution for autos. First, the emissions equipment in modern diesel engines adds weight and reduces engine airflow, making them less efficient than they could be. In fact, today's small diesel engines are only 10-20% more efficient than gas equivalents, when in the past they have been over 50% more efficient.

There are three primary types of pollution that come from an internal combustion engine--carbon, which causes global warming; nitrous oxides, which cause smog, and particulate matter, which contributes to air pollution and also causes all sorts of nasty health problems. Diesel engines are better than gasoline on carbon emissions by virtue of being more efficient, although as I mentioned this gap is closing. Special catalytic exhaust equipment has been added to newer diesel engines to reduce nitrous oxides, but it's expensive and the environmental benefits are marginal. Particulate matter is addressed by burning the soot that comes out of the exhaust, again using expensive equipment that reduces efficiency, and AFAIK the science is still out on whether and by how much this actually reduces environmental/health impacts.

Some municipalities in the US have perhaps misguidedly banned diesel engines from their roads. The situation is a little different in Europe, where diesel engines are found in a much larger share of passenger vehicles. If Europeans drove as much as Americans did, their cities would all look like LA circa 1975. I'm not sure about car ownership trends in Europe, but I think it would make sense for any city that expects increases in car ownership to start regulating diesel more heavily.

It’s backed by science. Due to the high temperatures in a Diesel engine, the amount of NOx produced is far higher than for petrol cars. NOx can be handled by a catalysator using AdBlue or similar, so in theory Diesel engines could be clean. In practice the standards are routinely ignored by the car makers.

The ruling allows banning cars that do not fulfil current standards, so a recent car is probably unaffected. However, the restriction could be lifted if only banning old cars proves insufficient. Then, newer cars could be affected.

From the article: »Cities will probably target models with older emission controls. Of the 15 million diesel cars on Germany’s roads, only 2.7 million have the latest Euro-6 technology, data by the KBA motor vehicle watchdog showed.«

Diesel cars have been under fire for quite some time because auto makers across the entire industry cheat on emission tests: https://en.wikipedia.org/wiki/Diesel_emissions_scandal

The emission standards in question are per m² in the city, not per car. It doesn't matter whether the emissions come from 1000 clean or 100 dirty cars.

Diesel cars produce less CO2, and I believe that's only because diesel engines use less fuel than gas engines (about 40% less). This was the idea behind "diesel being cleaner".

However, diesel is way more dangerous to human life because of the NOx particles. This is why diesel cars will now be the first to go.

This is about NOx, which is not healthy for humans. In regards to CO2, which is bad for the climate, Diesels are indeed better than petrol cars.

Besides all the other answers here, there's a surprising amount of people that disable DPF units on their diesels making them very dirty.

Modern diesels, sure. Latest offerings from BMW, Mercedes or Audi are actually super clean, especially if equipped with an AdBlue system. But get out on the road and you see people still driving 90s diesels which emit huge plumes of black smoke every time they accelerate, and I know people who buy brand new cars and the first thing they do is cut out the DPF, EGR and pretty much every other emissions reducing system "to keep the running costs low". It's a disgrace, and older diesels should absolutely be straight up banned in cities with very harsh penalties for modifying modern ones(in UK removing the DPF attracts a £5000 fine, but in many EU countries it's still not a problem).

Generally they are not. Newer ones are within limits set acceptable by emission standards, but those standards don't really model real world use where their performance is generally atrocious.

I happen to be (now not very happy) owner of a diesel car.

They emit less of the greenhouse gas CO_2 than gasoline engines, but often more unhealthy nitrogen oxides and particulate matter.

I think cities are only interested in banning older diesels that don't meet Euro-5 or Euro-6.

Yes it is. It is not about CO2 emissions, where Diesel engines are indeed more efficient than petrol engines, but about the "fine dust" emitted by a highly charged turbo setup, like most modern Diesel layouts are...