Y. J. Rigg^{1,*}, P. A. Alpert^{1,*}, and D. A. Knopf^{1}
^{1}Institute for Terrestrial and Planetary Atmospheres/School of Marine and Atmospheric Sciences, Stony Brook University, Stony Brook, New York, USA

^{*}These authors contributed equally to this work.

Received: 01 Feb 2013 – Published in Atmos. Chem. Phys. Discuss.: 20 Feb 2013

Revised: 10 Jun 2013 – Accepted: 11 Jun 2013 – Published: 12 Jul 2013

Abstract. Immersion freezing of water and aqueous (NH_{4})_{2}SO_{4} droplets containing leonardite (LEO) and Pahokee peat (PP) serving as surrogates for humic-like substances (HULIS) has been investigated. Organic aerosol containing HULIS are ubiquitous in the atmosphere; however, their potential for ice cloud formation is uncertain. Immersion freezing has been studied for temperatures as low as 215 K and solution water activity, *a*_{w}, from 0.85 to 1.0. The freezing temperatures of water and aqueous solution droplets containing LEO and PP are 5–15 K warmer than homogeneous ice nucleation temperatures. Heterogeneous freezing temperatures can be represented by a horizontal shift of the ice melting curve as a function of solution *a*_{w} by Δ*a*_{w} = 0.2703 and 0.2466, respectively. Corresponding hetrogeneous ice nucleation rate coefficients, *J*_{het}, are (9.6 ± 2.5)×10^{4} and (5.4 ± 1.4)×10^{4} cm^{−2} s^{−1} for LEO and PP containing droplets, respectively, and remain constant along freezing curves characterized by Δ*a*_{w}. Consequently predictions of freezing temperatures and kinetics can be made without knowledge of the solute type when relative humidity and ice nuclei (IN) surface areas are known. The acquired ice nucleation data are applied to evaluate different approaches to fit and reproduce experimentally derived frozen fractions. In addition, we apply a basic formulation of classical nucleation theory (α*(T)*-model) to calculate contact angles and frozen fractions. Contact angles calculated for each ice nucleus as a function of temperature, α*(T)*-model, reproduce exactly experimentally derived frozen fractions without involving free-fit parameters. However, assigning the IN a single contact angle for the entire population (single-α model) is not suited to represent the frozen fractions. Application of α-PDF, active sites, and deterministic model approaches to measured frozen fractions yield similar good representations. Furthermore, when using a single parameterization of α-PDF or active sites distribution to fit all individual *a*_{w} immersion freezing data simultaneously, frozen fraction curves are not reproduced. This implies that these fitting formulations cannot be applied to immersion freezing of aqueous solutions, and suggests that derived fit parameters do not represent independent particle properties. Thus, from fitting frozen fractions only, the underlying ice nucleation mechanism and nature of the ice nucleating sites cannot be inferred. In contrast to using fitted functions obtained to represent experimental conditions only, we suggest to use experimentally derived *J*_{het} as a function of temperature and *a*_{w} that can be applied to conditions outside of those probed in laboratory. This is because *J*_{het}*(T)* is independent of time and IN surface areas in contrast to the fit parameters obtained by representation of experimentally derived frozen fractions.

**Citation:** Rigg, Y. J., Alpert, P. A., and Knopf, D. A.: Immersion freezing of water and aqueous ammonium sulfate droplets initiated by humic-like substances as a function of water activity, Atmos. Chem. Phys., 13, 6603-6622, doi:10.5194/acp-13-6603-2013, 2013.