An algorithm for the calculation of secondary organic aerosol density combining AMS and SMPS data

An algorithm for the calculation of organic aerosol density in mixed organic-inorganic particles combining measurements by the Aerodyne Aerosol Mass Spectrometer ( AMS) and the Scanning Mobility Particle Sizer ( SMPS) was developed. The approach is applicable to particles with size-dependent composition. The estimated density of secondary organic aerosol ( SOA) formed by alpha-pinene, beta-pinene, and d-limonene ozonolysis was in the range of 1.4-1.65 g cm(-3). However, in two cases the SOA had much lower density ( 0.9-1.0 g cm(-3)) indicating that there may be changes in particle morphology depending on the conditions of SOA formation. The high estimated density for these systems suggests that SOA particles may be solid or waxy. Based on our results, SOA yields in smog chamber experiments may be a lot higher ( up to 50%) than the currently assumed values. Most of the literature results have been calculated by measuring the SOA number distribution with an SMPS and then multiplying the volume concentration with a density equal to 1 or 1.2 g cm(-3).