ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-12-5511-2012 Absorbing aerosols at high relative humidity: linking hygroscopic growth to optical properties Michel Flores J. 1 2 5 Bar-Or R. Z. 3 Bluvshtein N. 3 Abo-Riziq A. 3 Kostinski A. 4 Borrmann S. 1 2 Koren I. 3 Koren I. 3 Rudich Y. 3 Max Planck Institute for Chemistry, Particle Chemistry Department, Mainz, Germany University of Mainz, Institute for Atmospheric Physics, Mainz, Germany Weizmann Institute of Science, Environmental Science Department, Rehovot, Israel Department of Physics, Michigan Technological University, Houghton, Michigan, USA now at: Weizmann Institute of Science, Environmental Science Department, Rehovot, Israel 25 06 2012 12 12 5511 5521 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/12/5511/2012/acp-12-5511-2012.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/12/5511/2012/acp-12-5511-2012.pdf

One of the major uncertainties in the understanding of Earth's climate system is the interaction between solar radiation and aerosols in the atmosphere. Aerosols exposed to high humidity will change their chemical, physical, and optical properties due to their increased water content. To model hydrated aerosols, atmospheric chemistry and climate models often use the volume weighted mixing rule to predict the complex refractive index (RI) of aerosols when they interact with high relative humidity, and, in general, assume homogeneous mixing. This study explores the validity of these assumptions. A humidified cavity ring down aerosol spectrometer (CRD-AS) and a tandem hygroscopic DMA (differential mobility analyzer) are used to measure the extinction coefficient and hygroscopic growth factors of humidified aerosols, respectively. The measurements are performed at 80% and 90%RH at wavelengths of 532 nm and 355 nm using size-selected aerosols with different degrees of absorption; from purely scattering to highly absorbing particles. The ratio of the humidified to the dry extinction coefficients (<i>f</i>RH<sub>ext</sub>(%RH, Dry)) is measured and compared to theoretical calculations based on Mie theory. Using the measured hygroscopic growth factors and assuming homogeneous mixing, the expected RIs using the volume weighted mixing rule are compared to the RIs derived from the extinction measurements. <br><br> We found a weak linear dependence or no dependence of <i>f</i>RH(%RH, Dry) with size for hydrated absorbing aerosols in contrast to the non-monotonically decreasing behavior with size for purely scattering aerosols. No discernible difference could be made between the two wavelengths used. Less than 7% differences were found between the real parts of the complex refractive indices derived and those calculated using the volume weighted mixing rule, and the imaginary parts had up to a 20% difference. However, for substances with growth factor less than 1.15 the volume weighted mixing rule assumption needs to be taken with caution as the imaginary part of the complex RI can be underestimated.

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