Diluted exhaust from a selection of Air Force ground support vehicles was subjected to gravimetric, carbon, and size distribution analyses in September 1999. The vehicles operated on diesel and JP-8 fuels. In most cases, the engines involved were similar to civilian counterparts. The tests involved "low" and "high" idle settings but no external loads were imposed. Particle size distribution data, obtained over the 10 to 352 nanometer diameter range using an SMPS instrument, showed that the relative number count of accumulation mode particles increased with respect to nucleation mode particles as the engine rpm increased. The SMPS distributions often explained the main variations in the integrated PM2.5 gravimetric mass data. Particulate mass derived from the SMPS data and from cascade impactor measurements were well correlated (regression slope 1.02). Empirically determined "elemental" carbon (EC) and "organic" carbon (OC) were the main constituents of the PM2.5 gravimetric mass (regression slope 0.89). EC contributed less, and OC contributed more to the PM2.5 mass than was found in some recent studies of exhaust from vehicles operated under external loads. The observed particle nucleation modes were attenuated by coagulation with accumulation mode particles, but it does not appear that artifact particle formation was operative in these experiments. The estimated ±1σ measurement precisions range from about ±4% for the largest impactor mass concentration determinations to ±24% for some of the SMPS mass concentration estimates.
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