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The effects of fuel composition on diesel exhaust particulate emissions have been studied at several steady-state operating conditions using a heavy-duty laboratory engine. Particulate emissions were influenced by three primary fuel factors: sulfur content, aromatics content, and volatility. At all but lightly loaded operating conditions, fuel sulfur was the dominant fuel factor in particulate formation. Fuel sulfur affects particulate emissions primarily by formation of sulfate (as sulfuric acid) and associated “bound” water and, to a lesser degree, by increasing the amount of soluble organic material collected. Aromatics and volatility influence the amount of carbonaceous material formed. Five polynuclear aromatic hydrocarbon (PAH) compounds were measured in the soluble organic fraction. Engine operating conditions greatly affected PAH levels; however, no strong influence of fuel composition on PAH concentrations was observed in the normal range of No. 1 - No. 2 diesel fuels.