Gasoline engines use a pre-mixed combustion process which produces negligible levels of soot, so particulate emissions have not been an issue for gasoline engines, particularly with modern port fuel injected (PFI) engines which provide excellent mixture quality.
Most of the particles emitted by a gasoline-driven car are superfine "meaning they are really really tiny" which keeps the total mass easily below the limits imposed even on diesel engines. The number of particles emitted, however, is actually pretty high.
Superfine particles are believed to penetrate deeper into the lungs and to stay there longer than larger particles. The mass of one particle with a diameter of 3 micrometer is equal to the mass of 1 million particles with a diameter of 0.03 micrometer. Most particles from gasoline engines fall below 0.1 micrometer. This is why they contribute little to the mass of particles collected during a conventional emissions test.
However, by increasing the combustion efficiency of gasoline, perhaps the number of these particles can also be reduced. More research is required.