ACPD Atmospheric Chemistry and Physics Discussions ACPD 1680-7375 Copernicus GmbH Göttingen, Germany 10.5194/acpd-11-24813-2011 Aerosol hygroscopic growth parameterization based on a solute specific coefficient Metzger S. 1 Steil B. 1 Xu L. 2 Penner J. E. 2 Lelieveld J. 1 3 Max Planck Institute for Chemistry, Mainz, Germany University of Michigan, Ann Arbor, Michigan, USA The Cyprus Institute, Nicosia, Cyprus 05 09 2011 11 9 24813 24855 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-discuss.net/11/24813/2011/acpd-11-24813-2011.html The full text article is available as a PDF file from http://www.atmos-chem-phys-discuss.net/11/24813/2011/acpd-11-24813-2011.pdf

Water is a main component of atmospheric aerosols and its amount depends on the particle chemical composition. We introduce a new parameterization for the aerosol hygroscopic growth factor (HGF), based on an empirical relation between water activity (<i>a</i><sub>w</sub>) and solute molality (&mu;<sub>s</sub>) through a single solute specific coefficient &nu;<sub>i</sub>. Three main advantages are: (1) wide applicability, (2) simplicity and (3) analytical nature. (1) Our approach considers the Kelvin effect and covers ideal solutions at large relative humidity (RH), including CCN activation, as well as concentrated solutions with high ionic strength at low RH such as the relative humidity of deliquescence (RHD). (2) A single &nu;<sub>i</sub> coefficient suffices to parameterize the HGF for a wide range of particle sizes, from nanometer nucleation mode to micrometer coarse mode particles. (3) In contrast to previous methods, our analytical <i>a</i><sub>w</sub> parameterization depends not only on a linear correction factor for the solute molality, instead &nu;<sub>i</sub> also appears in the exponent in form <i>x</i> · <i>a</i><sup>x</sup>. According to our findings, &nu;<sub>i</sub> can be assumed constant for the entire <i>a</i><sub>w</sub> range (0–1). Thus, the &nu;<sub>i</sub> based method is computationally efficient. In this work we focus on single solute solutions, where &nu;<sub>i</sub> is pre-determined with the bisection method from our analytical equations using RHD measurements and the saturation molality &mu;<sub>s</sub><sup>sat</sup>. The computed aerosol HGF and supersaturation (Köhler-theory) compare well with the results of the thermodynamic reference model E-AIM for the key compounds NaCl and (NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub> relevant for CCN modeling and calibration studies. The equations introduced here provide the basis of our revised gas-liquid-solid partitioning model, i.e. version 4 of the EQuilibrium Simplified Aerosol Model (EQSAM4), described in a companion paper.

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