EFFECT OF PARTICLE-SIZE DISTRIBUTION ON THE CUT POINT BETWEEN FINE AND COARSE AMBIENT MASS FRACTIONS

A review of recent studies on the size distribution of ambient particl es reveals considerable variability, both spatially and temporally, de pending on the source of origin, geographical location, meteorological conditions, and various other chemical and physical parameters. Howev er, there is generally a clear separation into fine and coarse modes, with a dividing point between 1.0 and 2.5 mu m where the mass of the t wo modes is at a minimum. In the 1970's, the U.S. Environmental Protec tion Agency (EPA) developed the dichotomous sampler to separate fine a nd coarse particles and choose a cut point at 2.5 mu m. However, recen t data collected with new samplers, such as the size-fractionating MOU DI impactor, reveal that both aged sulfates and particles associated w ith aerosol acidity reside in the range below 1.0 mu m diameter. Indoo r combustion aerosols such as cigarette smoke and kerosene heater emis sions have also been found to reside in the submicrometer range. Befor e embarking on a new and extensive aerosol characterization study and perhaps establishing separate standards for fine and coarse particles, the EPA might wish to reevaluate the choice of a cut size. This artic le discusses the size distribution makeup of ambient aerosols from var ious sources and locations throughout the United States. The effect of sampler cut points and sampling efficiencies on the accuracy of colle ction in eastern combustion-dominated and western wind-blown dust-domi nated settings is examined. The advantages and disadvantages of changi ng the cut point between fine and coarse from 2.5 mu m to something ne arer 1.0 mu m are discussed.