Observations and Characteristics of Particles in the 0.5 to 2.5 micron Size Range

The size distributions of particles with diameters between ~0.5 and 2.5 microns are difficult to characterize accurately. However, these particles frequently constitute large fractions of the PM2.5 or "fine mode" aerosol mass concentrations; contribute non-negligibly to aerosol extinction; and may represent key cloud-active particles that include giant cloud condensation nuclei and ice nucleating particles. Some fraction of these particles is derived from sources in common with true coarse mode particles (diameters between ~2.5 and 10 microns), while the remainder are derived from sources that dominate submicron particles, including cloud processing. In this presentation we review measurements by our group in various U.S.-based field studies to demonstrate the variability in this mode and in its contributions to the aforementioned characteristics of atmospheric relevance. Size-resolved composition data demonstrate the complexity of the aerosol sources contributing to this size range.