The mechanism of the formation of supercooled ternary H2SO4/H2O/HNO3 solution (STS) droplets in the polar winter stratosphere, i.e., the uptake of nitric acid and water onto background sulfate aerosols at T < 195 K, was successfully mimicked during a simulation experiment at the large coolable aerosol chamber AIDA of Forschungszentrum Karlsruhe. Supercooled sulfuric acid droplets, acting as background aerosol, were added to the cooled AIDA vessel at T = 193.6 K, followed by the addition of ozone and nitrogen dioxide. N2O5, the product of the gas phase reaction between O-3 and NO2, was then hydrolyzed in the liquid phase with an uptake coefficient gamma(N2O5). From this experiment, a series of FTIR extinction spectra of STS droplets was obtained, covering a broad range of different STS compositions. This infrared spectra sequence was used for a quantitative test of the accuracy of published infrared optical constants for STS aerosols, needed, for example, as input in remote sensing applications. The present findings indicate that the implementation of a mixing rule approach, i.e., calculating the refractive indices of ternary H2SO4/H2O/ HNO3 solution droplets based on accurate reference data sets for the two binary H2SO4/H2O and HNO3/H2O systems, is justified. Additional model calculations revealed that the uptake coefficient gamma(N2O5) on STS aerosols strongly decreases with increasing nitrate concentration in the particles, demonstrating that this so-called nitrate effect, already well-established from uptake experiments conducted at room temperature, is also dominant at stratospheric temperatures.