I don't think particulates was an issue (but I couldbe wrong). The main issue was Nitrogen Oxide emissions.

Basically, when not in emissions testing mode, the engine was running at higher temperatures which improves fuel efficiency but generates more NO from atmospheric Nitrogen. Also, the vehicles had insufficently sized or simply missing diesel exhaust fluid (mostly urea and blue coloring) resevoirs, so not enough was injected to react with the NO to hit targets; injecting DEF into the engine also reduces fuel efficiency. There was also something about not properly cycling the special catalytic converter for NO, which would probably show an efficiency difference on long trips.

I sold back my VW diesel and stopped paying attention, it would be interesting to see what the measured real life fuel efficiency is before and after the changes required by regulators.