Experimental determination of the H2SO4/HNO3/H2O phase diagram in regions of stratospheric importance

We have investigated the region of the H2SO4/HNO3/H2O ternary liquid/solid phase diagram bounded by ice, nitric acid trihydrate (NAT), and sulfuric acid tetrahydrate (SAT) using differential scanning calorimetry and IR spectroscopy of thin films. We report measurements and analysis of the eutectic melting curves in the ternary system of the hydrates mentioned as well as the temperature of the eutectics ice/SAT/NAT, ice/sulfuric acid hemihexahydrate (SAH)/NAT, and SAT/NAT. We report for the first time an analysis of the content of the solid phase of completely frozen samples and find that sulfuric acid octahydrate (SAO) is often present in frozen ternary samples and can be a significant portion of the solid phase. We provide a description of how the melting path of a frozen ternary sample can be predicted using the ternary phase diagram. We have parametrized our melting point data and provide equations to generate the ternary melting surface. Finally, we compare our results to the historic work of Carpenter and Lehrmann (Carpenter, C. D.; Lehrman, A. Trans. AIChE 1925, 17,35) to other more recent work and to the output of the Aerosol Inorganics Model.