Diesel engines are now the largest source of nitrogen oxides (NOx) and fine particulate black carbon (soot) emissions in California. The California Air Resources Board recently adopted a rule requiring that by 2014 all in-use heavy trucks and buses meet current (2007) exhaust particulate matter (PM) emission standards. Also by 2023 all in-use heavy-duty vehicles will have to meet current NOx emission standards, with significant progress in achieving the requirements for NOx control expected by 2014. This will require retrofit or replacement of older in-use engines. Diesel particle filters (DPF) reduce PM emissions but may increase the NO2/NOx emission ratio to ∼35%, compared to ∼5% typical of diesel engines without particle filters. Additionally, DPF with high oxidative capacity reduce CO and hydrocarbon emissions. We evaluate the effects of retrofitting trucks with DPF on air quality in southern California, using an Eulerian photochemical air quality model. Compared to a 2014 reference scenario without the retrofit program, black carbon concentrations decreased by 12 ( 2% and 14 ( 2% during summer and fall, respectively, with corresponding increases in ambient ozone concentrations of 3 ( 2% and 7 ( 3%. NO2 concentrations decreased by 2-4% overall despite the increase in primary NO2 emissions because total NOx emissions were reduced as part of the program to retrofit NOx control systems on in-use engines. However, in some cases NO2 concentrations may increase at locations with high diesel truck traffic.

Diesel exhaust is now one of the largest sources of nitrogen oxide (NOx) emissions, and also contributes significantly to emissions of fine particulate black carbon (soot). The California Air Resources Board (CARB) recently adopted a rule to requiring that all inuse trucks operating in California meet current (2007) exhaust PM emission standards by 2014. This will require retrofit or replacement of older in-use engines, with emission reductions occurring on an accelerated schedule compared to what would be achieved by fleet turnover alone. A side effect of using diesel particle filters (DPF) to control exhaust PM emissions is that the NO2/NOx ratio in exhaust emissions may increase to ~35%, in comparison to lower values (~5%) typical of engines without particle filters. We evaluate the effects of increased deployment of DPF systems as well as NOx controls on air quality in southern California using an Eulerian air quality model. Compared to a 2014 reference scenario without the retrofit program, we found black carbon concentrations decreased by 12±2% and 14±2% during the summer and fall, respectively, with corresponding increases in ambient ozone concentrations of 3±2% and 7±3%. NO2 concentrations decreased by 2-4% despite the increase in primary NO2 emissions – because total NOx emissions were reduced as part of a parallel but more gradual program to retrofit NOx control systems on in-use engines. However, in some cases, NO2 concentrations were found to increase at locations with high diesel truck traffic. Increases in fine particulate nitrate of 3-7% were predicted near downtown Los Angeles.