Characteristics of Individual Particles at a Rural Site in the Eastern United States

To determine the nature of aerosol particles in a rural area of the eastern United States, aerosol samples were collected at Deep Creek Lake, Maryland, on various substrates and analyzed by a scanning electron microscope (SEM) and a transmission electron microscope (TEM). SEM analysis of particles larger than 2.5 μm collected on Nuclepore filters revealed the following: clay minerals, quartz, gypsum, and calcite comprised 50 percent of the particles analyzed; spores, pollen, and plant debris comprised 25 percent; 9 percent were fly ash; 11 percent were sulfates; 5 percent were unidentified. Particles ranging from 0.3 to 2 μm were collected in a cascade impactor on grid-supported carbon films and analyzed by TEM for decomposition rate as well as for reaction with the barium chloride and nitron (C₂₀H₁₆N₄) films that were applied after sampling. The TEM analyses indicated that as much as 95 percent of the particles in the 0.3- to 2-μm diameter range were pure ammonium sulfate or acidic ammonium sulfate; they contained essentially no insoluble or nonvolatile matter. About 5 percent of the particles were fly ash spheres. When replicas of particles collected on Nucleopore filters were analyzed by TEM, we observed agglomerates of particles smaller than 0.1 μm.