PURPOSE AND SCOPE With the growing attention on atmospheric pollution@ it has become increasingly desirable to obtain a detailed technical understanding of the chemical nature of those components of diesel exhaust which relate to odor. Coordinating Research Council@ and the National Air Pollution Control Administration. U.S. Public Health Service have had an interest in this problem for many years@ as shown through their support of a variety of research programs. Most of the work to date has been oriented toward determining the chemical composition of diesel exhaust and studying the odor using olfactory techniques.1-3 Recently@ it became apparent that a greater understanding of the chemistry of diesel exhaust particularly related to odor was needed and research efforts in this direction were undertaken. Part of this effort has been in support of the present program at Arthur D. Little. Inc.@ the purpose of which is to chemically identify the odor components of diesel exhaust. The objectives of the total program@ a part of which is described in this report@ are: (1) to determine the relative contribution of individual chemical species in diesel exhaust to the intensity and character of the exhaust odor; and (2) to describe these species in terms of chemical parameters. Broken down into several steps@ the program becomes: Step I Using olfactory techniques@ characterize the odor components in diesel exhaust with respect to their type@ intensity@ threshold values@ character@ and possible contribution to human irritation@ Step 2 Separate the odor components from the exhaust@ Step 3 Chemically identify specific compounds of diesel exhaust which relate to odor components and correlate these species with their odor character@ and Step 4 Determine the influence of various operating conditions (e.g.. fuel@ engine type. load) on the presence of specific odor components in the diesel exhaust. In this first year's efforts@ our emphasis has been on Steps 1 and 2 with some work on Step 3. The odor profile of diesel exhaust at a 1/600 dilution in air describes three odor character notes - oily@ burnt@ and kerosene ?? and two feeling sensations - nose irritation and eye irritation. As might be expected@ these five elements appear to vary in intensity as a function of diesel exhaust level in the test room. To characterize the chemical odor species we concentrated and then separated the components from diesel engine exhaust to permit their identification by high-resolution mass spectrometry. The system developed ?? silica liquid column chromatography (LCC) followed by silicone and carbowax gas chromatography (GC)@ monitoring the effluent components by odor as well as by instruments - demonstrates the variety of odor types and complexity of the chemical mixture involved in diesel exhaust. Several individual components with odors relating to the burnt complex are recognizable and in some cases are present in sufficient concentration to permit identification@ particularly with regard to the lower boiling- temperature fractions. A How scheme summary@ representing the overall handling of the exhaust through to the isolated smoky-burnt fractions@ is shown in Figure 1. The effectiveness and necessity of this approach can be seen from the data in Table 1 and a consideration of instrumental detection limits. From the 6000-liter aliquot of Sample 25@ which was carried through to the final GC analysis@ we have just managed to achieve resolution of some components and have sufficient material with which to obtain chemical identification. At threshold dilution of diesel exhaust@ individual odorous components may be present at a part-per-trillion (10-12). 1. Diesel Exhaust Composition and Odor@ Scott Research Laboratories@ Final Report on Contract No. PH 86-67-134@ and earlier references cited therein. 2. An investigation of Diesel Powered Vehicle Odor and Smoke@ Southwest Research Institute@ Final Report on Contract No. 86-67-72 and earlier references cited therein. 3. Composition of Diesel Exhaust@ Reports to PHS from Bartlesville Petroleum Research Center@ Bureau of Mines@ U.S. Department of the Interior.