And just as much a problem for direct injection petrol engines, apparently. Tests carried out on some Euro 6 DI petrol cars found particulate emissions worse than Euro 5 diesels & if you look at the small print of the Euro 6 regs now you will find particulate levels set for DI petrols as well as diesels. A modern Euro 6 diesel is not that much (if any) "worse" than a petrol in reality, but that doesn't matter to the politicos.
More widespread use of LPG would help, as there are no particulate or unburned hydrocarbon issues with LPG - plus it is a byproduct of oil refining & we don't use all that we produce. Maybe if Tony Bliar's lot hadn't pissed the car & fuel distribuition industries off with broken promises about LPG we would still have the choice (like they do in Italy, for example).
Hydrogen most likely won't come to anything (too many safety issues) so hybrids the most likely short term solution, with all electric in the longer term.
(veteran of two LPG converted cars)