You have to understand that baseline fuel is 5,000 parts per million sulphur. Onroad now is 15. Natural gas traditionally has had a very strong advantage on particulate emissions. It was one of the reasons California encouraged the use of it way back when. I think measurability is maybe part of the issue here, but natural gas definitely has inherently lower particulate and NOx emissions.
On May 17th, 2012. See this statement in context.