ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-3-1147-2003 A quantitative test of infrared optical constants for supercooled sulphuric and nitric acid droplet aerosols Wagner R. 1 Mangold A. 2 Möhler O. 1 Saathoff H. 1 Schnaiter M. 1 Schurath U. 1 Forschungszentrum Karlsruhe, Institute of Meteorology and Climate Research, Karlsruhe, Germany Forschungszentrum Jülich, Institute of Chemistry and Dynamics of the Geosphere I (ICG-I): Stratosphere, Jülich, Germany 08 08 2003 3 4 1147 1164 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/3/1147/2003/acp-3-1147-2003.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/3/1147/2003/acp-3-1147-2003.pdf

In situ Fourier transform infrared (FTIR) extinction spectra of supercooled H<sub>2</sub>SO<sub>4</sub>/H<sub>2</sub>O and HNO<sub>3</sub>/H<sub>2</sub>O solution droplets were recorded in the large coolable aerosol chamber AIDA (Aerosol Interactions and Dynamics in the Atmosphere) of Forschungszentrum Karlsruhe for a range of aerosol compositions and at temperatures extending down to 192 K. The measured spectra were quantitatively analysed in terms of aerosol composition and mass concentration by using Mie theory in combination with published refractive index data as input parameters. Simultaneously, total sulphuric acid and nitric acid mass concentrations from filter analysis and total water concentrations measured with the Lyman-<font face="Symbol">a</font> hygrometer of Forschungszentrum Jülich were used to calculate the aerosol composition at thermodynamic equilibrium inside the aerosol chamber. By comparing these measured aerosol parameters with those retrieved from the analysis of the FTIR spectra, the accuracy of the literature data sets of refractive indices could be assessed. In summary, four data sets were tested in the H<sub>2</sub>SO<sub>4</sub>/H<sub>2</sub>O system as well as two data sets in the HNO<sub>3</sub>/H<sub>2</sub>O system, partly revealing significant discrepancies in the retrieved aerosol properties. Potential explanations for these differences are discussed in this paper.