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This study characterizes the size distribution and composition of metals in diesel exhaust particulates (DEPs) emitting from four driving conditions. We quantified 17 metals in DEPs (34-1000 nm) with a total concentration ranging from 5.4-7.0 microg/m(3). Depending on driving conditions, ultrafine (<100 nm) and accumulation-mode DEPs carried up to 41% and 75% of the quantified metals, respectively. The size distribution of individual metals consistently indicates that under a medium (60%) engine load, more than three fourths of quantified metals partitioned in accumulation-mode DEPs, indicating prominent heterogeneous condensation. Enhancing the engine load up to 100% significantly increased metals in ultrafine DEPs around 1.8 times, particularly in DEP < 66 nm, suggesting enhanced metal nucleation. Under the maximum engine load, metals and elemental carbon showed an opposite trend in size distribution, providing tailpipe evidence that metals may catalyze oxidation of DEPs during combustion. Among the identified metals, Fe (2.3-3.9 microg/m(3)) was most abundant contributing to more than 43% of quantified metals, followed by Li, V, and Pb. Although As and Cd together contributed to less than 2% of the total quantified metals in DEP (<1 microm), their concentrations peaked in ultrafine DEPs under the maximum engine load, indicating that a decrease in engine loads can reduce amounts and toxicity of ultrafine DEPs.

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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 (Յ400 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 ϫ 10 8 to 4 ϫ 10 8 #/cm 3 ) and more than 3 times (from 2.7 ϫ 10 8 to 0.8 ϫ 10 8 #/cm 3 ), respectively. The same reduction in the engine load significantly decreased persistent free radicals in DEPs up to approximately 30 times (from 123 ϫ 10 16 to 4 ϫ 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/m 3 , and from 17.6 to 9.2 mg/m 3 , respectively. For DEPs smaller than 1 m, 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 m) at least 3 times (0.6 to 0.2 mg/m 3 ) and 2 times (0.4 to 0.2 mg/m 3 ), respectively. Taken together, decreasing the full engine load to a medium (60%) level effectively reduced the number concentrations (Յ400 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. INTRODUCTIONDiesel exhaust particles (DEPs) have been associated with adverse health effects, including cardiovascular diseases, 1 lung cancers 2,3 and asthma. 4 -7 DEPs can also impede atmospheric visibility 8,9 and affect global climate changes. 10,11 Although advanced technologies can reduce mass concentrations of DEPs, population (numbers) of ultrafine particles (UFPs, Յ100 nm) can be consequently increased. [12][13][14] This is of concern because a larger population of UFPs provides a substantial amount of surface area to carry toxic materials, which can cause more serious health effects. 15 To better understand the role of DEPs and their effect on air quality, Kittleson et al. 16 monitored size distribution and number concentrations of DEPs using a mobile emission laboratory traveling along highways. Actual onroad measurements have advantages of monitoring DEPs from various traffic conditions, incorporating a real-world dilution and discriminating proper background interference. Although such data improve our understanding of exposure to on-road DEPs, studies on how driving conditions affect DEPs...