Understanding the relationship between aerosols and clouds: field investigations and instrument development

The research presented in this thesis is part of the ongoing effort to better understand the role of atmospheric aerosols in the development of clouds. Cloud condensation nuclei (CCN) are the subset of the aerosol population that can activate and grow into cloud droplets under suitable atmospheric conditions. The supersaturation at which a given CCN will activate is dependent on the particle's size and composition, but the details of the relationship are not completely understood. CCN observations from the CRYSTAL-FACE (Cirrus Regional Study of Tropical Anvils and Cirrus Layers- Florida Area Cirrus Experiment) field campaign are presented in Chapter 2. These measurements are compared to predictions based on measured aerosol size distributions with an assumed chemical composition to determine whether activation theory is sufficient to describe what is observed. The analysis indicates that, in cases like those included in the study, CCN concentrations can be accurately predicted from the size distribution even in the absence of detailed chemical compositional data. A case study is described in Chapter 3 to demonstrate the potential importance of anthropogenic aerosols in the development of clouds. During a CRYSTAL-FACE flight, an aerosol plume was encountered in the boundary layer near the base of a large mixed-phase convective cloud. Evidence suggests that an oil-burning power plant south of Miami was the likely source of the plume. The convective cloud was probed at higher altitudes, and a spatial gradient was observed in the ice particle concentrations. The evidence linking the plume in the boundary layer to the upper-level trends is inconclusive, but worthy of further study. The measurement of CCN in the atmosphere is difficult, and improved instrumentation would significantly improve our ability to obtain the detailed information necessary to understand the relationship between aerosols and clouds. The concept for an improved CCN spectrometer is outlined in Chapter 4; this new design would expand the resolvable range of supersaturations for which data can be obtained. The dependence of the instrument's performance on various design parameters is evaluated, and a configuration is proposed that would be a significant improvement over currently available instrumentation.