On the Physicochemical Processes Controlling Organic Aerosol Hygroscopicity

Aerosol particles in the atmosphere can influence air quality and climate through their interaction with water. Aerosols are an important factor in cloud formation because they serve as cloud condensation nuclei (CCN). Organic compounds contribute a large fraction of the atmospheric aerosol mass but their ability to serve as CCN is less certain relative to inorganic compounds. Limitations of the measurement techniques and theoretical gaps in understanding have prevented agreement between predicted and measured CCN. One way to quantify a compound's CCN activity is by the hygroscopicity parameter, kappa. This dissertation presents research towards constraining the variability of organic aerosol kappa at the process level using three approaches: developing a measurement technique; measuring the dependence of kappa on molecular functional groups; and measuring the effect of surface active molecules on kappa for mixtures. Chapter 2 presents a Hygroscopicity Tandem Differential Mobility Analyzer (HTDMA) instrument to measure aerosol water uptake at high relative humidity (RH). Measurements up to 99% RH were achieved by improving the precision of aerosol sizing, actively controlling temperature, and calibrating RH between measurements. Osmotic coefficients were obtained within +/-20% for organic aerosols sized between 30 and 200 nanometers. These results may improve water uptake models by providing accurate data at high RH. Chapter 3 presents a study of the sensitivity of kappa to changes in molecular functional group composition for pure compounds. Molecules were synthesized via gas and liquidphase reactions varying the type and location of functional groups, purified by High Performance Liquid Chromatography (HPLC), and routed for CCN measurement. The hydroxyl (-OH) and carbon chain length (-CH2-) changed kappa most, where hydroxyl groups increase kappa and longer carbon chains decrease kappa. This suggests that hydroxyl groups and molecular size dominate the hydrophobic-to-hydrophilic conversion of organic matter in the atmosphere. The results of this study contribute to models seeking to estimate kappa from chemical composition. Chapter 4 presents a study of the effect of strongly surface active molecules on kappa. Three strong surfactants and eleven two-component surfactant + nonsurfactant systems were tested. Results support the theory that large increases in kappa due to surface tension reduction are compensated by surface partitioning of surfactant molecules, which prevents them from taking up water. A constraint on the surface layer thickness of surfactant molecules improved the performance of an existing theoretical framework to predict CCN activity. Results also suggest that solution non-idealities can reduce kappa up to 40% relative to ideal mixing for some mixtures. This study constrains the effect of organic surfactants on CCN activity. The studies presented in this dissertation contribute to the understanding of organic CCN activity at the molecular level and may be generalized to facilitate the prediction of the atmospheric CCN spectrum based on existing models of chemical composition or reaction mechanisms.