I must confess that I was at least a little surprised that the hybrids have higher particle emissions than their conventional counterparts, the same maybe, but did not expect higher.
Toyota explicitly stated in a press release in 2010 that its Lexus CT 200h had "almost no NOx or particulate emissions" and it "generates significantly less NOx and particulate emissions than an equivalent diesel engine vehicle" (
http://pressroom.lexus.com/article_display.cfm?article_id=2082). I was skeptical of the significantly-less-particle-emissions-than-an-equivalent-diesel claim (assuming they were referring to new-technology diesels engines) since diesel with DPF emits PN similar to HEPA-filtered background air even according to CARB except during regeneration, but I expected hybrid version to have less particle emissions, or at least no more, than conventional gassers.
Here are a few more references regarding PN emissions from hybrids...
...As explored earlier in this study, a large variation in frequency and duration of engine-off events occur (Robinson, 2010), and the resulting spiking of total particulate emissions resulting from the engine “restart” events leads to potential hot-spot locations where excessive emissions are observed....
Matt Conger, Britt A Holmen, "Impact of Road Facility Attributes, Congestion and Temperature upon Changes in Particulate Emission Rates Resulting From Hybrid Engine Re-start Events." American Association for Aerosol Research 30th Annual Conference, 2011,
http://aaarabstracts.com/2011/viewabstract.php?paper=489
...Overall, PN concentrations, at the run level, were significantly higher under hybrid ICE-on operation compared to the conventional operation....
Matt Conger, Britt A Holmen, "Real-World Engine Cold Start and “Restart” Particle Number Emissions from a 2010 Hybrid and Comparable Conventional Vehicle." 2011,
http://www.uvm.edu/~transctr/research/grad/12-4570.pdf
The link to the reference in Post #14 is
http://amonline.trb.org/12k8g9/3
It should be pointed out that the PN emissions of the hybrids tested were still low (in the low 10^11 particles/km range which means that it would meet the diesel PN limit in Europe (6X10^11 particles/km)), but they aren't really "near zero" nor "significantly less than equivalent diesel engine vehicles".
On a related note...I wonder what the PN emissions of GDI with start/stop technology would be?