Microphysical and optical properties of organic aerosols and their relevance to cloud condensation nuclei

The present dissertation is motivated by the need to improve our understanding of the indirect effect of aerosols on the atmosphere. This study is part of the ongoing effort to better understand the role of atmospheric aerosols in the formation of clouds. Cloud condensation nuclei (CCN) are those particles of the aerosol population that can activate and grow into cloud droplets under suitable atmospheric conditions. The supersaturation conditions under which a given CCN will activate is dependent on both the particle size and composition; however, details regarding this relationship are not well understood. The present investigation focuses on the microphysical and optical properties of a particular class of organic aerosols, specifically benzene and its derivatives, namely chlorobenzene and nitrobenzene and the aliphatic hexane. Fundamental microphysical properties of interest in this study were the size distribution of the organics, the associated nucleation potential and the optical extinction coefficients relating to water droplets and cloud formation. The size distribution of the aerosols was determined using a Differential Mobility Analyzer and a reliable estimate concerning the onset of nucleation was done theoretically. Optical parameters associated with the aerosols were measured using the Fourier Transform Infra-Red (FTIR) spectroscopy. A theoretical calculation of the frequency shift due to specific phase transitions is presented and compared with the experimental data. A comparison within the above cited class of organic aerosols, which are generated mostly by biomass burning, allow us to compare and assess the importance of this class of aerosols and their significance in condensation and cloud formation. It is anticipated that the results from these studies can provide meaningful initial input parameters for cloud modelers.