Effects of driving conditions on diesel exhaust particulates

Four driving conditions were examined to characterize how speeds and loads of a medium-duty diesel engine affect resultant diesel exhaust particulates (DEPs) in terms of number concentrations (< or =400 nm), size distribution, persistent free radicals, elemental carbon (EC), and organic carbon (OC). At the medium engine load (60%), DEPs surged in number concentrations at around 40-70 nm, whereas DEPs from the full engine load (100%) showed a distinctive bimodal distribution with a large population of 30-50 nm and 100-400 nm. Under the full engine load, engine speeds insignificantly affected resultant DEP number concentrations. When the engine load decreased from 100% to the medium level (60%), DEPs of ultrafine size and 100-400 nm decreased at least 1.4 times (from 5.6 x 10(8) to 4 x 10(8) #/cm3) and more than 3 times (from 2.7 x 10(8) to 0.8 x 10(8) #/cm3), respectively. The same reduction in the engine load significantly decreased persistent free radicals in DEPs up to approximately 30 times (from 123 x 10(16) to 4 x 10(16) #spin/g). Decreasing the engine load from 100 to 60% also concurrently reduced both EC and OC in total DEPs around 2 times, from 27.3 to 13.9 mg/m3, and from 17.6 to 9.2 mg/m3, respectively. For DEPs smaller than 1 microm, under the full engine load, EC and OC consistently peaked at 170-330 nm under an engine speed of 1800 rpm or 94-170 nm under an engine speed of 3000 rpm, reflecting processes of nucleation, cluster-cluster agglomeration, and condensation. Decreasing the engine load from 100 to 60% reduced EC and OC in DEPs (smaller than 1 microm) at least 3 times (0.6 to 0.2 mg/m3) and 2 times (0.4 to 0.2 mg/m3), respectively. Taken together, decreasing the full engine load to a medium (60%) level effectively reduced the number concentrations (< or =400 nm), persistent free radicals, EC, and OC of total DEPs, as well as the concentration of EC and OC in ultrafine and accumulation-mode DEPs.