Laboratory Investigations of Soot Optical Properties and the Hygroscopicity and Production of Sea Spray Aeroso

Aerosols impact climate directly by scattering and absorbing solar radiation and indirectly by acting as cloud condensation nuclei (CCN). The effect of aerosols is one of the largest uncertainties in global radiative forcing (RF). The laboratory studies described here connect the physical and chemical properties of aerosols to their ability to absorb and scatter light and their ability to activate into cloud droplets. Additionally, laboratory experiments were conducted to examine the temperature dependence of sea spray aerosol (SSA) production.;In the study described in chapter 2, optical and physical properties of flame-generated soot particles were characterized to investigate the ability of various commonly used optical models to reproduce observations. An increase in the mass absorption coefficient (MAC) for the soot particles was observed for small particles with size parameters ( x = pi x diameter/wavelength) < 0.9, but were relatively constant for x > 0.9. Wavelength-specific refractive indices for the sampled particles were derived by fitting the observed absorption cross-sections to spherical particle Mie theory and Rayleigh-Debye-Gans theory, which assumes soot is composed of an aggregate of spheres. These observations indicate that Mie theory systematically predicts lower MACs relative to observations for larger particles with x > 0.9. In contrast, Rayleigh-Debye-Gans theory, which does not assume spherical particle morphology exhibited good agreement with the observations for x > 0.9. We hypothesize that the increase in MACs with size for x < 0.9 is likely due to changes in soot maturity. It is recommended that climate models use a constant, wavelength-specific MAC at all sizes for uncoated soot particles. Additionally, it was found that soot particle scattering is sensitive to shape, while absorption is not.;In chapters 3 and 4, experiments investigating the impact of marine-derived organic matter on the subsaturated and supersaturated water uptake of SSA particles are described. Concurrent measurements of dry and wet (relative humidity = 85%) particle scattering coefficients were used to determine growth factors (GFRH=85%) of SSA particles generated from two microcosm experiments in which two phytoplankton blooms were induced. Significant depressions in GFRH=85% relative to pure sea salt were caused by the large observed organic matter fractions in the SSA particles. These measurements demonstrate the sensitivity of hygroscopic growth (and therefore light scattering) to SSA particle chemical composition. In chapter 4, experiments examining the impact of model surfactants on the surface tension (sigma) of laboratory-generated SSA particle proxies near and at the point of activation are described. Results from this study indicate reductions in sigma relative to water are significant at relative humidities just below activation. At activation, however, reductions in sigma are small. This is due to the large droplet sizes present at activation, which lead to molecular areas that are too large to significantly reduce sigma. Thus, the surfactants tested here do not significantly impact CCN activation efficiency of SSA particles. Kinetic limitations to droplet growth were observed for NaCl particles coated with palmitic acid. However, NaCl particles coated with oleic acid-palmitic acid mixtures did not exhibit these kinetic limitations, suggesting that mixed oleic acid-palmitic acid films pack less densely than pure palmitic acid films.;In chapter 5, a set of experiments investigating the impact of temperature on SSA particle production is described. SSA particles for these experiments were generated from wave breaking analogs that reproduce bubble and size distributions of breaking waves. Observations from these experiments indicate that particle number concentrations exhibit a positive dependence on temperature for reef salt and NaCl water. This behavior contrasts with previous literature that used other types of wave breaking analogs. Additionally, observed number concentrations for experiments conducted with non-autoclaved filtered seawater exhibited substantial variation that did not depend on temperature. It was found that these variations, which are hypothesized to be due to changes in chemical composition, are potentially more important than variations driven by changes in temperature.