Aerosol Chamber Study of Optical Constants and N2O5 Uptake on Supercooled H2SO4/H2O/HNO3 Solution Droplets at Polar Stratospheric Cloud Temperatures

Robert Wagner, Karl-Heinz Naumann, Alexander Mangold, Ottmar Möhler, Harald Saathoff, and Ulrich Schurath,
Forschungszentrum Karlsruhe, Institute of Meteorology and Climate Research (IMK-AAF), Karlsruhe, Germany


Abstract:

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 O3 and NO2, was then hydrolyzed in the liquid phase with an uptake coefficient γ(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 (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.