That is my understanding also. There is a tradeoff between very complete combustion in a lean mixture under high pressure that produces few particulates but lot

That is my understanding also. There is a tradeoff between very complete combustion in a lean mixture under high pressure that produces few particulates but lots of NOx and running a rich mixture at lower temperature that produces less NOx and more particulates. Fortunately modern engines have filters both for NOx and particulates, so that neither is really a problem. At least once the car has warmed up, which in cold climates and city driving can be roughly never...