Bimodal character of accumulation mode aerosol mass distributions in Southern California

Size-resolved measurements of fine particle chemical composition and physical measurements of fine particle size distributions obtained during the Southern California Air Quality Study (SCAQS) are compared. Number distributions of the ambient aerosols were measured using optical particle counters and electrical aerosol analyzers. Optical counter data are reduced using an ambient-based calibration. Mass size distributions are inferred from the sum of size-resolved chemical composition as measured by impactors. Optical counter data reduced with an ambient-based calibration compare well to impactor measurements. Both sets of data show that the accumulation mode of the total mass size distribution may be bimodal. Condensation and droplet modes previously identified in chemical species size distributions are frequently apparent in the total mass size distribution.