Optical, Physical, and Thermodynamic Properties of Organic Particulate Matter

Abstract

Organic compounds constitute 20 to 90% of the submicron atmospheric particulate matter (PM) mass. The majority of this mass is produced by the oxidation and subsequent condensation of biogenic and anthropogenic gaseous precursors, namely secondary organic material (SOM). The optical, physical, and thermodynamic properties of organic PM can influence many aerosol processes, yet the quantitative knowledge on these properties is still scarce. My doctoral thesis presents results of laboratory experiments using newly developed thin-film based techniques for the characterization of physicochemical properties of different types of SOM. For characterizing optical properties, continuous spectra of the real and imaginary refractive indices (m = n – i k) in the ultraviolet (UV) to near-infrared region were obtained using spectroscopic ellipsometry combined with UV-visible spectrometry. For different types of SOM, the wavelength dependence of the refractive index n generally follows a characteristic curve. The values of absorptive index k, however, were found to vary by orders of magnitude, depending on the types of precursor and the presence of molecular heteroatoms. Physical and thermodynamic properties were characterized by an apparatus based on the quartz-crystal microbalance (QCM) technique. In a series of laboratory experiments, the mass labilities of SOM films were directly determined by QCM measurements by analyzing evaporation rates and vapor mass concentrations of semi-volatile organic compounds. The results highlights a nonlabile-to-labile transition at a threshold humidity for material representative of anthropogenic particulate matter. The behavior differs markedly from materials representing biogenic sources, which lack such a transition. The effective diffusion rate Dorg for the biogenic case is at least 103 times greater than that of the anthropogenic case. In another series of experiments, diffusivities of small molecules were directly measured for several types of SOM. These diffusivities, combined with the Dorg of large organic molecules, were used to interpret the reaction rates between the toluene-derived SOM and gas-phase ammonia. Experiments show that the produced light-absorbing organonitrogen species strongly depended on RH. These RH-dependent behaviors can be well captured by a basic model when diffusivities of both small molecules of guest species and large molecules making up the organic PM were taken into account.