Measuring the Size Distribution of Diesel Exhaust and Mine Dust Aerosol Mixtures with the Microoriflce Uniform Deposit Impactor

"Under contract to the U.S. Bureau of Mines, a study was conducted to investigate the feasibility of using the microorifice. uniform deposit impactor (MOUOI) to measure the size distribution of aerosols containing various mixtures of coal dust and diesel exhaust particles. The project objective was to determine if the mass concentration of diesel exhaust particles, in an airborne mixture of coal dust and diesel exhaust particles, could be determined on the basis of the size distribution of the two aerosols. In general, the mass of diesel exhaust particles is in the submicrometer size range while that of the coal particles lie in the larger size ranges.The MOUDI is a cascade impactor with small diameter nozzles to obtain small cut sizes and rotating impaction plates to avoid particle overloading of the deposit. For this study, a 7 stage version was used. At the sample flow rate of 30 L/min, the cut sizes were 0.10, 0.23, 0.70, 1.0, 2.6, 4.9 and 10 pm.Size distributions of aerosols generated in the laboratory and in several underground coal mines were measured. The primary emphasize of the laboratory tests was to ascertain if the diesel exhaust particles could be separated from the coal dust, regardless of mass fraction of the diesel exhaust particles in the total aerosol mixture. Diesel exhaust, from a Caterpillar Type 3304 NA diesel engine, was mixed with coal dust generated with a· TSI Model 3400 fluidized bed dust generator. Experiments showed the size distributions of the mixed aerosols exhibited two definite modes with the minimum between the two modes in the 0.7 to 1.0 um size range. Distinct modal separation was obtained as the mixture fraction of the diesel exhaust aerosols was varied from 3% to 98%.The field study was conducted in three underground coal mines, two utilizing diesel-powered equipment and the third exclusively using electric equipment. Sampling sites were selected to obtain a wide variety in the diesel exhaust and dust aerosol concentrations. Size distribution data showed these aerosols are bimodal. The mass median aerodynamic diameters of the diesel particulate mode and the coal dust mode were approximately 0.15 µm and 3 to 10 un, respectively. The results, in all cases, clearly show a separation between the two modes in the 0.7 to 1.0 µm size range with the minimum in the vicinity of 0.8 pm. The separation was found to be independent of mass fraction of the diesel exhaust particles, total particle concentration and sampling site."