Measurements on truck-dominated freeways in southern California have offered a unique opportunity to track emission changes that have occurred due to the implementation of local and state regulations affecting heavy-duty diesel trucks. These regulations have accelerated fleet turnover to cleaner and newer trucks. In this study, a mobile platform was used to measure nitrogen oxides (NOX), black carbon (BC), and ultrafine particles (UFPs) on diesel-dominated southern California freeways. Fleet-averaged fuel-based emission factors were calculated for diesel trucks and the results showed NOX and BC emissions were reduced by 40% or more between 2009 and 2011, but there were no statistically significant reductions for UFP. Technologies associated with these new trucks, mainly diesel particulate filters, have changed the physical characteristics of diesel particulate, shifting the size distribution of such particles to smaller modes (10-20 nm). In addition, integration of 2007 MY trucks into the fleet was also observed in on-road ratios of nitrogen dioxide (NO2) and NOX. NO2/NOX ratios steadily increased from 0.23 ± 0.06 in 2009 to 0.30 ± 0.03 in 2010 but plateaued and declined in 2011.