BIMODAL CHARACTER OF ACCUMULATION-MODE AEROSOL MASS DISTRIBUTIONS IN SOUTHERN CALIFORNIA

Size-resolved measurements of fine particle chemical composition and p hysical measurements of fine particle size distributions obtained duri ng the Southern California Air Quality Study (SCAQS) are compared. Num ber distributions of the ambient aerosols were measured using optical particle counters and electrical aerosol analyzers. Optical counter da ta are reduced using an ambient-based calibration. Mass size distribut ions are inferred from the sum of size-resolved chemical composition a s 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 distributi on may be bimodal. Condensation and droplet modes previously identifie d in chemical species size distributions are Frequently apparent in th e total mass size distribution. Copyright (C) 1996 Published by Elsevi er Science Ltd