The number of sea-salt, sulfate, and carbonaceous particles in the marine atmosphere: EM measurements consistent with the ambient size distribution

Abstract

To evaluate the number of sea-salt, sulfate, and carbonaceous particles associated with the fine and coarse mode aerosol in the marine boundary layer and from the marine/continental interface, the elemental composition of individual aerosol particles was determined with EM. Samples were collected from the Pacific marine boundary layer during a research cruise along 140°W from the southern to northern hemisphere, and from a coastal station in NW Washington. Consistently, the most dominant aerosol types found were ammonium sulfate and acidic sulfate comprising 52 to 96% of the total number at a median diameter of 0.14 μm. Sea-salt particles were 4 to 13% of the total number, with modes at 0.2 and 0.6 μm. Carbonaceous particles, mostly as organic compounds, made up the remainder of the submicrometer aerosol at a few % to as much as 31% of the total number for continentally influenced periods. Silica-rich minerals and potassium and calcium salts were observed during such periods, representing a measurable, but small fraction of the total number. Sea-salt particles were 86 to 100% of the number of supermicrometer particles with a mass median diameter of 1.5 μm. A less abundant, and variable fraction of mineral particles made up the remainder of the coarse aerosol number concentration identified primarily as aluminosilicates, mass median diameter of 0.9 μm. Very little soot was observed in either the coarse or fine mode. Particle number concentrations were consistently low and airmass trajectories suggest the air originated over the open ocean for the majority of the sampling periods. Overall, normalized EM distributions agreed fairly well with in-situ measurements of the aerosol size distribution.