Effect of Particle Size Distribution on the Cut Point Between Fine and Coarse Ambient Mass Fractions

Abstract

A review of recent studies on the size distribution of ambient particles reveals considerable variability, both spatially and temporally, depending on the source of origin, geographical location, meteorological conditions, and various other chemical and physical parameters. However, there is generally a clear separation into fine and coarse modes, with a dividing point between 1.0 and 2.5 μmlm where the mass of the two modes is at a minimum. In the 1970's, the U.S. Environmental Protection Agency (EPA) developed the dichotomous sampler to separate fine and coarse particles and choose a cut point at 2.5 μmlm. However, recent data collected with new samplers, such as the size-fractionating MOUDI impactor, reveal that both aged sulfates and particles associated with aerosol acidity reside in the range below 1.0 μmlm diameter. Indoor combustion aerosols such as cigarette smoke and kerosene heater emissions have also been found to reside in the sub-micrometer range. Before 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 article discusses the size distribution makeup of ambient aerosols from various sources and locations throughout the United States. The effect of sampler cut points and sampling efficiencies on the accuracy of collection in eastern combustion-dominated and western wind-blown dust-dominated settings is examined. The advantages and disadvantages of changing the cut point between fine and coarse from 2.5 μmlm to something nearer 1.0 um are discussed.