UCLA

The refractive index is the fundamental property controlling aerosol optical properties. Secondary organic aerosols (SOA) constitute a large fraction of aerosols in the atmosphere, and yet the optical properties of this complex material are just beginning to be understood. They appear to be much more variable than expected. We explore the factors controlling the real refractive indices (mr) of SOA under conditions that are as close to atmospheric conditions as possible. SOA were generated from the alpha-pinene, beta-pinene, limonene and toluene using several oxidation chemistries including ozonolysis with and without scavenger and photooxidation at different HC/NOx ratios, different mass concentration and experimental temperatures. Mr were retrieved from polar nephelometer measurements using parallel and perpendicular polarized 532 or 670 nm light where there is no evidence of significant absorption by the particles investigated here. Retrievals were performed with a genetic algorithm method using Mie-Lorenz scattering theory and measured particle size distributions. Overall examination of the SOA data shows that SOA mr ranges from 1.34 to 1.62, reflecting the factors to control the chemical composition; decreases as the HC/NOx ratio increases, decrease at lower temperature (< 20 ºC) and toluene SOA has higer mrs than biogenic SOA. Aerosol mass spectrometer (AMS) measurement reveals that laboratory SOA is less oxidized than ambient aerosol and the increase in refractive index is most correlated with the H:C ratio, indicating that condensation of semivolatile species increases the refractive index, although all other combined factors relate to the mrs. In an effort to measure more atmospheric relevant SOA, high volatile species are evaporated using thermodenuder (TD) at 65 - 110 ºC temperature. Retreived mr become narrower but still has range of 1.41-1.62 which has a significant impact on radiative transfer calculations of SOA direct effects. We showed that changing the mrs from 1.4 to 1.5 can produce an increase in the radiative forcing by at least 12-19 % for non-absorbing particles. There is more work to be done before recommendations can be made for atmospheric applications, but this thesis clearly highlights a single value for SOA mr will not be sufficient to accurately model radiative transfer.