Modes in the size distributions of atmospheric inorganic aerosol

Abstract

Ambient aerosol in the size range 0.075–16 μm was sampled with Berner cascade impactors during the summer and fall intensive sampling periods of the Southern California Air Quality Study (SCAQS) of 1987. Deposits on the greased Tedlar stage substrates were extracted with deionized water and analyzed for inorganic anions and cations by ion chromatography. Stage mass data were inverted by a modified version of the Twomey nonlinear iterative algorithm and modes in the inverted size distributions were fitted with lognormal functions. Nine hundred size distributions of ionic species were obtained. Three modes, two submicron and one coarse, were sufficient to fit all of the size distributions. The smallest mode, at 0.2±0.1 μm, aerodynamic diameter, is probably a condensation mode containing gas phase reaction products. A larger mode, at 0.7±0.2 μm, is identified as a droplet mode. Evidence was obtained that the droplet mode grew out of the condensation mode by the addition of water and sulfate. During SCAQS, most of the inorganic particle mass was in the droplet mode except during a period of exceptionally low relative humidity. Nitrate was internally mixed with sulfate in the droplet mode. Since most of the aerosol fine mass is included in the ions analyzed, the observed condensation and droplet modes characterize the overall size distribution in the 0.1–1 μm range, previously described by Whitby as a single accumulation mode.