| Aerosols participate in many atmospheric processes, including global climate forcing and polar ozone depletion. However, questions remain concerning the magnitude of their impact, because the chemical and physical properties of most atmospheric aerosol systems are not well known. Attempts to characterize atmospheric aerosols are made by analyzing remotely based infrared spectroscopic measurements with Mie scattering theory. This requires that the real and imaginary complex refractive indices, or “optical constants,” are known for the proposed aerosol constituents. For stratospheric aerosols, these components are primarily H2SO4, HNO3 and H2O, but the exact compositions and phases are not known. Unfortunately, the optical constant data that has been available for these systems were produced at room temperature, and do not likely reflect the physical properties of the aerosols existing in the stratosphere.; The goal of this research project is to investigate the temperature and composition dependence of the aerosol optical constants, and to provide the scientific community with new data sets that are relevant for modeling the infrared spectra. Experimentally, this is accomplished by forming aerosols in a flow cell, and analyzing them with a unique combination of spectroscopic techniques. Broadband Fourier transform infrared (FT-IR) spectra are recorded which exhibit molecular absorption bands as well as infrared light scattering, allowing the intimately related real (scattering) and imaginary (absorbing) components of the complex refractive indices to be deconvolved. A high resolution infrared tunable diode laser (TDL) is employed to scan rovibrational absorption lines attributed to the vapor in equilibrium with the aerosols, thus providing vapor pressures that are used to calculate solution concentrations based on a thermodynamic relationship.; In these experiments, aerosols have been studied over a wide range of stratospherically relevant conditions. Results have shown that the optical constants of aerosols composed of H2SO4, HNO3 and H2O are often strongly temperature dependent, due primarily to the complex dissociation chemistry of each acidic species that shifts significantly when the aerosols are cooled to temperatures representative of Earth's upper atmosphere. Such findings have made it clear that only refractive index data produced under relevant experimental conditions are suitable for characterizing actual stratospheric aerosols. |
