Frequency-dependent optical constants have been determined from the Fourier transform infrared spectra of laboratory-generated liquid sulfuric acid/water aerosols over a range of temperatures and compositions that are relevant to the upper troposphere and lower stratosphere of Earth. The compositions of the particles were determined in situ using a tunable diode laser to monitor equilibrium water vapor pressures. The infrared complex refractive indices of sulfuric acid are shown to be strongly dependent on temperature and composition, because of changes in the equilibrium between sulfate and bisulfate ions. Results from this study also have implications in understanding the temperature dependence of intermolecular interactions within ionic solutions. The database presented here is the most extensive yet available for the liquid solutions of sulfuric acid.

Abstract. Complex refractive indices have been retrieved for the first time directly from the Fourier transform infrared extinction spectra of supercooled liquid HNO3/H20 aerosols at 220 K. Aerosol compositions were determined by simultaneously monitoring equilibrium HNO3 and H20 vapor pressures with a tunable diode laser. The new optical constants are compared with previous results from room temperature bulk solutions and < 160 K amorphous solid thin films, and the differences are explained primarily in terms of the temperature-dependent dissociation of molecular HNO3 into its constituent ions. Mie scattering analysis of aerosol extinction spectra suggests that using either the room temperature liquid or the amorphous solid refractive indices may result in inaccurate particle size determination. These new data sets are directly applicable to the characterization of polar stratospheric clouds through spectroscopic remote sensing.

Abstract. Complex refractive indices have been retrieveddirectly from the FT-IR extinction spectra of laboratorygenerated, binary sulfuric acid/water aerosols over a range of temperatures and compositions relevant to the atmosphere of Earth. The refractive indices are strongly dependent upon both' temperature and composition, due to changes in the equilibrium between sulfate and bisulfate ions and the resultant change in the corresponding solvation interactions. The present results have important implications for determining the composition of sulfuric acid aerosols by remote sensing applications.

We present here the first measurements of the infrared complex refractive indices for supercooled ternary solutions of H 2 SO 4 , HNO 3 , and H 2 O under stratospheric conditions. The data sets were retrieved directly from the infrared extinction spectra of laboratory-generated aerosols. Comparisons of these results with a previously reported empirical mixing rule model 1 reveal significant differences, which we trace to errors in the binary acid/water thin film refractive indices used in the model. In addition, the model does not properly account for the concentrations of ionic and neutral species in the ternary solutions. The experimentally determined optical constants reported here will be useful for determining polar stratospheric cloud aerosol properties from infrared spectroscopic remote sensing measurements.

Using high-resolution Fourier-transform infrared absorbance and transmittance spectral data for ammonium sulfate (AMS), calcium carbonate (CAC), and ammonium nitrate (AMN), we made comparisons with previously published complex reactive-index data for AMS and CAC to infer experimental parameters to determine the imaginary refractive index for AMN in the infrared wavelength range from 2 to 20 microm. Subtractive Kramers-Kronig mathematical relations were applied to calculate the real refractive index for the three compositions. Excellent agreement for AMS and CAC with the published values was found, validating the complex refractive index obtained for AMN. We performed backscatter calculations using a log-normal size distribution for AMS, AMN, and CAC aerosols to show differences in their backscattered spectra.