1Department of Chemistry & Biochemistry, University of Colorado, Boulder, CO 80309, USA
2Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO 80309, USA
3Department of Atmospheric and Oceanic Science, University of Colorado, Boulder, CO 80309, USA
4Faculty of Chemistry, Bielefeld University, 33615 Bielefeld, Germany
5Department of Chemistry, The Pennsylvania State University, University Park, PA 16802, USA
6NASA Ames Research Center, Moffett Field, CA, USA
Abstract. Sea-salt aerosol particles (SSA) are ubiquitous in the marine boundary layer and over coastal areas. Therefore SSA have ability to directly and indirectly affect the Earth's radiation balance. The influence SSA have on climate is related to their water uptake and ice nucleation characteristics. In this study, optical microscopy coupled with Raman spectroscopy was used to detect the formation of an NaCl hydrate that could form under atmospheric conditions. NaCl(s) particles deliquesced at the well established value of 75.7 ± 2.5 % RH. NaCl(aq) particles effloresced to a mixture of hydrated and non-hydrated particles at temperatures between 236 and 252 K. The aqueous particles effloresced into the non-hydrated form at temperatures warmer than 252 K. At temperatures colder than 236 K all particles effloresced into the hydrated form. The deliquescence relative humidities (DRH) of hydrated NaCl(s) particles ranged from 76.6 to 93.2 % RH. Based on the measured DRH and efflorescence relative humidities (ERH), we estimate crystalline NaCl particles could be in the hydrated form 40–80 % of the time in the troposphere. Additionally, the ice nucleating abilities of NaCl(s) and hydrated NaCl(s) were determined at temperatures ranging from 221 to 238 K. NaCl(s) particles depositionally nucleated ice at an average Sice value of 1.11 ± 0.07. Hydrated NaCl(s) particles depositionally nucleated ice at an average Sice value of 1.02 ± 0.04. When a mixture of hydrated and anhydrous NaCl(s) particles was present in the same sample, ice preferentially nucleated on the hydrated particles 100 % of the time. While both types of particles are efficient ice nuclei, hydrated NaCl(s) particles are better ice nuclei than NaCl(s) particles.